Other possible synapomorphies

ACCTRAN: The basal tubera and basipterygoid processes are closely situated to each other (character 25-1); maxillary teeth are ovate, but not strongly mediolaterally compressed in occlusal view (character 34-1); maxillary teeth crown is bulbous and partly recurved, has a concave anterior edge, and straight posterior edge (character 35-1); co-ossified sacral vertebrae (character 106-1). DELTRAN: none.

Calyptosuchus Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985

Maxilla

PEFO 49321 preserves both maxillae, but the anatomy of the maxilla is best observed on the right element (Fig. 3.1, 3.2). The right maxilla is well preserved, nearly complete, and not coated with the iron mineralization. The maxilla is triradiate in shape with a main body and three processes (Desojo et al., Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013). In lateral view, the anterior process is dorsoventrally tall, and its dorsal margin forms the posterior half of the ventral margin of the external naris (Fig. 3.1). Anteriorly, the dorsolateral margin is crushed. It is evident that the anterior margin is not fully preserved in the maxillae (Figs. 3.1, S2), so it is likely that the anterior process tapers farther anteriorly, underlapping the lateral process of the premaxilla as typically observed in aetosaurs (e.g., Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021, fig. 6). The anteroventral margin of the dorsal process bears a posterodorsally inclined embayment in lateral view (Fig. 3.1, 3.2) at the junction between the anterior and dorsal processes. This embayment marks the lateral insertion point of the ventral process of the nasal, which comprises the posterior margin of the external naris as observed in most aetosaur taxa that preserve these elements (except Desmatosuchus smalli, TTU-P 9024, Small, Reference Small2002).

The dorsal process is acutely inclined posteriorly in relation to the main body of the element (Fig. 3.1, 3.2), but more so than the condition observed in Paratypothorax andressorum Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985 (SMNS 19003, Schoch and Desojo, Reference Schoch and Desojo2016) where the orientation is more of a right angle. Overall, it resembles the condition observed in Stagonolepis spp. (S. robertsoni, NHMUK PV R 4787, Walker, Reference Walker1961; S. olenkae, ZPAL AbIII/1995, Sulej, Reference Sulej2010) and Aetosauroides scagliai (MCN-PV 2347, Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). Posterolaterally, the dorsal process is dorsoventrally tall and tapers into a posteriorly oriented triangular process near its dorsal border that wedges itself between the lacrimal and nasal in lateral view, resembling the condition in Stagonolepis robertsoni (Walker, Reference Walker1961) and Aetosaurus ferratus (SMNS 5770, Schoch, Reference Schoch2007). Additionally, the posteroventral margin of the dorsal process forms the anterodorsal border of the antorbital fenestra. The posterior process of the maxilla is nearly half the anteroposterior length of the entire element with a dorsal margin that is anteroposteriorly straight in lateral view and forms the ventral border of the antorbital fenestra. The anterior portion of the fenestra between the dorsal and posterior processes is not dorsoventrally tall (Fig. 3.1, 3.2), suggesting that the antorbital fenestra of Calyptosuchus wellesi is subtriangular in shape rather than semicircular or elliptical, resembling that of Aetosauroides scagliai (UFSM 11505, Biacchi Brust et al., Reference Biacchi Brust, Desojo, Schultz, Paes-Neto and Da-Rosa2018; MCN-PV 2347, Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b) and Typothorax coccinarum Cope, Reference Cope and Wheeler1875 (NMMNH P-12964, Heckert et al., Reference Heckert, Lucas, Rinehart, Celeskey, Spielmann and Hunt2010; PEFO 38001/YPM VP.58121, Reyes et al., Reference Reyes, Parker and Marsh2020). The posterior extent of the maxilla is not preserved. Thus, the posterior process does not provide information regarding the contact between the maxilla, lacrimal, and jugal.

The ventral margin of the maxilla is anteroposteriorly straight in lateral view (Fig. 3.1, 3.2), resembling that of Coahomasuchus kahleorum Heckert and Lucas, Reference Heckert and Lucas1999 (TMM 31100-437) and Kocurypelta silvestris Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021 (ZPAL V.66/4). The antorbital fossa is deep and well demarcated on the lateral surface of the maxilla (Fig. 3.1). On the dorsal process the fossa expands onto most of the lateral surface of the maxilla, but it is evident that it does not continue onto the lateral extent of the nasal; instead, it continues posteriorly onto the lacrimal. On the posterior process, the fossa is restricted to the dorsal two-thirds of the lateral surface and is ventrally bordered by a well-developed transverse ridge (Fig. 3.1); a condition also described in the two maxillary fragments UCMP 195193 and UCMP 195194 referred to Calyptosuchus wellesi from UCMP A269 (the Placerias Quarry, Parker, Reference Parker2018a). In most aetosaurs with described maxillae this ridge is prominent except in both species of Desmatosuchus (D. spurensis Case, Reference Case1920, UMMP 7476, Case, Reference Case1922; D. smalli, Small, Reference Small2002), Longosuchus meadei (TMM 31185-98, Sawin, Reference Sawin1947; Parrish, Reference Parrish1994), Typothorax coccinarum (Heckert et al., Reference Heckert, Lucas, Rinehart, Celeskey, Spielmann and Hunt2010; Reyes et al., Reference Reyes, Parker and Marsh2020), and Kocurypelta sylvestris Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021, in which the ridge is poorly developed or absent. Additionally, the transverse ridge in PEFO 49321 continues anterodorsally, confining the antorbital fossa, and fades distally on the lateral surface of the dorsal process (Fig. 3.1) similar to the condition observed in Aetosauroides scagliai (Biacchi Brust et al., Reference Biacchi Brust, Desojo, Schultz, Paes-Neto and Da-Rosa2018; Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b), Paratypothorax andressorum (Schoch and Desojo, Reference Schoch and Desojo2016), Aetosaurus ferratus (Schoch, Reference Schoch2007), and Stenomyti huangae Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013.

A row of five foramina is present on the ventrolateral surface of the maxilla that extends parallel to the ventral margin, just dorsal of the alveolar row (Fig. 3.1). This row of foramina begins 1 cm posterior of the anterior margin of the anterior process and terminates just posteroventral to the anterior margin of the antorbital fenestra, indicating that there is not a one-to-one correlation with the alveoli. In life, these foramina likely transmitted fibers of the superior alveolar nerve, an extension of the maxillary branch of the trigeminal nerve (CN V₂) that innervates the alveoli and soft tissues surrounding the maxilla (George and Holliday, Reference George and Holliday2013; Lessner and Holliday, Reference Lessner and Holliday2020, fig. 10). As in Stenomyti huangae (DMNH V.34565; Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013), the alveolar row of the maxilla of PEFO 49321 contains nine alveoli with three teeth still in situ (Fig. 3.1, 3.2; described below). Based on current understanding, only Neoaetosauroides engaeus (PVL 4363, Desojo and Baez, Reference Desojo and Báez2007; Taborda et al., Reference Taborda, Desojo and Dvorkin2021), Aetosaurus ferratus (Schoch, Reference Schoch2007), Typothorax coccinarum (Reyes et al., Reference Reyes, Parker and Marsh2020), and Calyptosuchus wellesi (based on PEFO 49321) exhibit fewer than 10 tooth positions in their maxillae. The alveolar row extends far posteriorly onto the posterior process terminating at the midline of the antorbital fenestra, unlike Neoaetosauroides engaeus (Desojo and Báez, Reference Desojo and Báez2007) and Kocurypelta sylvestris (Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021), where the alveolar row terminates more anterior to it.

In medial view, the maxilla bears a medial shelf that runs the length of the element parallel to and dorsal to the alveolar row (Fig. 3.2). Anteriorly, on the anterior process, the shelf is bifurcated by a deep longitudinal groove that terminates dorsal to the third alveolus (from anterior to posterior). That groove marks the articulation point for the posteromedial process of the premaxilla (Fig. 3.2) as observed in Desmatosuchus smalli (Small, Reference Small2002) and Stagonolepis olenkae (Sulej, Reference Sulej2010). Dorsally, the medial ridge exhibits a shallow choanal recess (Witmer, Reference Witmer1997) at the junction of the anterior and dorsal processes. The medial surface of the dorsal process exhibits a triangular-shaped articular surface for its contact with the nasal and lacrimal dorsal to the choanal recess (Fig. 3.2). The anterior process of the lacrimal fits anteroventrally into a slot while the nasal truncates this process dorsally; this articulation is best observed in Longosuchus meadei (TMM 31185-97, Parrish, Reference Parrish1994, fig. 3). On the posterior process, the medial shelf is concave ventrally and expands mediolaterally just posterior to the last alveolus for the contact between the maxilla and palate.

PEFO 49321 shows no evidence of a pneumatic accessory cavity at the junction of the dorsal and posterior processes of the maxilla, posterior to the choanal recess (Fig. 3.2) (Witmer, Reference Witmer1997), similar to Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). A pneumatic accessory cavity is documented in Stenomyti huangae (Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013), Stagonolepis (Walker, Reference Walker1961; Sulej, Reference Sulej2010; Parker, Reference Parker2018b), Longosuchus meadei (Parrish, Reference Parrish1994), and Desmatosuchus (Case, Reference Case1920; Small, Reference Small2002), suggesting that this character may be phylogenetically informative, as originally proposed by Small (Reference Small2002). In medial view, the nine alveoli are equally spaced from each other and are divided by ventrally directed, well-spaced, sub-triangular interdental plates (Fig. 3.2), as also observed in Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). Additionally, the anterior wall of the first alveolus is not preserved (Fig. 3.2), further indicating that the anterior extent of the maxilla is not fully preserved.

Jugal

The right jugal of PEFO 49321 is only missing portions of the mid-body and its anterior margin (Fig. 3.3, 3.4). The element is ~60% the anteroposterior length of the maxilla, which is unlike Aetosaurus ferratus (Schoch, Reference Schoch2007) in which the jugal is shorter in length, being ~40% of the length of the maxilla. However, we note that the morphology of Aetosaurus ferratus is based on an aggregate of hatchling specimens (SMNS 5220; Teschner et al., Reference Teschner, Konietzko-Meir, Desojo, Schoch and Klein2023), suggesting that proportion between the jugal and maxilla may be influenced by its skeletal maturity. Anteriorly, the anterodorsal margin of the jugal is not preserved. Thus, we are unable to determine if the jugal separates the lacrimal and maxilla posteriorly and participates in the posterior margin of the antorbital fenestra, a variable character within the Aetosauria (Parker, Reference Parker2016a). The anteroventral margin of the jugal is partially preserved with a posteroventral incision (Fig. 3.3, 3.4). This indicates that the posterior process of the maxilla underlapped the jugal in lateral view as it would have tapered posteroventrally, exhibiting a sinuous, wedge-shaped contact with the jugal as described for Paratypothorax andressorum (Schoch and Desojo, Reference Schoch and Desojo2016). That condition differs from the prong-like bifurcating contact exhibited by the non-aetosaur aetosauriform Revueltosaurus callenderi (PEFO 34561, Parker et al., Reference Parker, Nesbitt, Irmis, Martz, Marsh, Brown, Stocker and Werning2021) and the early-branching aetosaur Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). Dorsally, the margin is concave and forms the ventral border of the orbit.

The main body of the jugal is anteroposteriorly oriented with a straight ventral margin (Fig. 3.3, 3.4), as observed in Stenomyti huangae (Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013) and both species of Coahomasuchus (C. chathamensis, Heckert, Fraser, and Schneider, Reference Heckert, Fraser and Schneider2017, NCSM 23618; C. kahleorum, TMM 31100-437). That condition differs from the strongly downturned jugal observed in Desmatosuchus (Case, Reference Case1920; Small, Reference Small2002) and Longosuchus meadei (Parrish, Reference Parrish1994). Posteriorly, the jugal bifurcates into two triangular-shaped processes that participate in the margins of the infratemporal fenestra. The posterodorsal process exhibits an anteroposterior length that is ~50% that of the posteroventral process. The posterodorsal process does not participate in the margins of the orbit, but its inclination indicates that the ventral process of the postorbital tapered anteroventrally along the posterior margin of the orbit (Fig. 3.3, 3.4), a condition commonly observed in aetosaurs except in both species of Desmatosuchus (D. spurensis, Case, Reference Case1922; D. smalli, Small, Reference Small2002). However, we are unable to ascertain if the postorbital contributes to the margin of the infratemporal fenestra as observed in Typothorax coccinarum (Reyes et al., Reference Reyes, Parker and Marsh2020), Paratypothorax andressorum (Schoch and Desojo, Reference Schoch and Desojo2016), and Aetosaurus ferratus (Schoch, Reference Schoch2007) but does not in Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). The posteroventral process is inclined posteroventrally indicating that the jugal wedged and underlapped the anterior process of the quadratojugal in lateral view (Fig. 3.3–3.5).

The lateral surface of the jugal is ornamented by a continuation of the transverse ridge described in the maxilla (Fig. 3.3). This condition is observed in several aetosaur taxa but not in Desmatosuchus smalli (TTU-P 9024, Small, Reference Small2002), Typothorax coccinarum (PEFO 38001/YPM VP.58121, Reyes et al., Reference Reyes, Parker and Marsh2020), or Paratypothorax andressorum (SMNS 19003, Schoch and Desojo, Reference Schoch and Desojo2016). The transverse ridge terminates just anterior to the posterior bifurcation of the jugal and does not extend onto the posteroventral process, in contrast to the condition observed in Stenomyti huangae (Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013) and Coahomasuchus chathamensis (Heckert et al., Reference Heckert, Fraser and Schneider2017). Additionally, there is no evidence of pitting or foramina on the external surface. In medial view, the anterior end of the jugal exhibits a posteriorly oriented, narrow, triangular surface for the medial reception of the ectopterygoid (Fig. 3.4). Ventral to the orbit, the surface is longitudinally depressed with well-delineated dorsal and ventral borders, as observed in Erpetosuchus (AMNH 29300, Foffa et al., Reference Foffa, Butler, Nesbitt, Walsh, Barrett, Brusatte and Fraser2021, fig. 5i), although there is no evidence of pneumatization (Fig. 3.4).

Quadratojugal

The left quadratojugal and quadrate of PEFO 49321 are still in articulation but are coated with an iron-rich mineralization, which inhibits our ability to fully differentiate the two elements and evaluate the nature of their articulation (Fig. 3.5, 3.6). The quadratojugal appears to exhibit an overall L-shape (Fig. 3.5), a condition that aetosaurs share with other pseudosuchians (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). Anteriorly, it is evident that the quadratojugal overlaps the posteroventral process of the jugal, as observed in Stenomyti huangae (Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013), but unlike that taxon the quadratojugal inhibits the jugal from framing the entire posteroventral margin of the cranium. Thus, the contact observed in PEFO 49321 resembles that of Stagonolepis robertsoni (Walker, Reference Walker1961, fig. 2). As a result of the overlapping contact between the quadratojugal and jugal, the dorsal margin of the anterior process of the quadratojugal forms part of the ventral margin of the infratemporal fenestra, and the posterodorsal process of the jugal participates in the inclined anterior margin of the opening. Based on this information, the infratemporal fenestra most likely had an ovate shape in lateral view.

The lateral surface of the anterior process bears a shallow fossa around the margin of the infratemporal fenestra, which is separated from a deep pit by a ridge (Fig. 3.5). Because of the preservation of the element, it is difficult to ascertain whether the pit and ridge are true anatomical features or a result of taphonomic alteration. The dorsal process is dorsoventrally short, less than 50% of the anteroposterior length of the anterior process. The lateral surface is slightly depressed near the dorsal tip (Fig. 2.11, 2.12), marking the point of contact between the quadratojugal and ventral process of the squamosal. If this interpretation is accurate, then the squamosal would have participated in the margins of the infratemporal fenestra, a condition documented in most aetosaurs except Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b), Coahomasuchus chathamensis (Heckert et al., Reference Heckert, Fraser and Schneider2017), Paratypothorax andressorum (Schoch and Desojo, Reference Schoch and Desojo2016), and Aetosaurus ferratus (Schoch, Reference Schoch2007).

Quadrate

The left quadrate is articulated with the left quadratojugal. The element is only represented by its ventral half, so the head of the quadrate is not preserved (Fig. 3.6). The lateral wing (sensu Walker, Reference Walker1961) is co-ossified to the medial surface of the quadratojugal, but the medial wing is not preserved. The quadrate crest, which divides both wings, is discernible but faint, unlike the well-developed strut in Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b, fig. 12). The crest is situated medially, dorsal to the medial condyle of the quadrate. Lateral to the quadrate crest, there is a shallow pit marking the perforation of the quadrate foramen. The margins of the foramen appear to be formed by both the quadratojugal and the lateral wing of the quadrate (Fig. 3.6), a condition observed in most aetosaurs (Parker, Reference Parker2016a; Schoch and Desojo, Reference Schoch and Desojo2016) except Coahomasuchus kahleorum (TMM 31100-437) and Aetosaurus ferratus (Schoch, Reference Schoch2007) in which the foramen is restricted to the lateral wing of the quadrate. However, there is no evidence of a fossa located on the external surface of the lateral wing just posterior to the quadrate foramen. Ventrally, the convex quadrate condyles are separated by an anteroposteriorly oriented groove. The lateral condyle is larger than the medial condyle and expanded anteromedially, similar to that of Desmatosuchus smalli (Small, Reference Small2002). In posterior view, the medial condyle is positioned more dorsally than the lateral condyle (Fig. 3.6).

Laterosphenoid

PEFO 49321 preserves the first unambiguous braincase element that is referred to Calyptosuchus wellesi. The left laterosphenoid is partially preserved, missing its anterior extent, and is coated in the same iron-rich mineralization present on some of the cranial bones of PEFO 49321 (Fig. 3.7, 3.8). Laterally, the surface is smooth but exhibits a distinct dorsoventrally oriented cotylar crest (Fig. 3.8) similar to Stagonolepis olenkae (ZPAL ABIII/466/17, Sulej, Reference Sulej2010). Two shallow foramina are present anterior to the cotylar crest (Fig. 3.8). The dorsal foramen marks the exit for the anterior cerebral vein (Walker, Reference Walker1990; Sulej, Reference Sulej2010; = ophthalmic artery, Small, Reference Small2002; Clark et al., Reference Clark, Welman, Gauthier and Parrish2010; = transversotrigeminal vein, von Baczko et al., Reference von Baczko, Desojo, Gower, Ridgely, Bona and Witmer2021), and the ventral foramen marks the exit of the trochlear nerve (CN IV). The ventral margin of the laterosphenoid exhibits a well-incised notch for the exit of the of the optic nerve (CN II) anteroventral to the cotylar crest.

Medially, the laterosphenoid exhibits a rugose surface with shallow fossae that are separated by curved ridges (Fig. 3.7). This texture is reminiscent of the contact between the anterior wall of the braincase and the dural envelope, which encompasses the meninges, encephalon, associated nerves, bloods vessels, and sinuses (Witmer et al., Reference Witmer, Ridgely, Dufeau, Semones, Endo and Frey2008). The medial foramina for the exits of the trochlear nerve (CN IV) and anterior cerebral vein are positioned within a dorsoventrally oriented fossa just dorsal of the notch for the optic nerve (CN II) (Fig. 3.7). Dorsally, the capitate process of the laterosphenoid, which contacts the parietal anteroventrally, is short and mediolaterally compressed. The anterior and posterior portions of the laterosphenoid are not preserved. Thus, we are unable to determine if the element participated in the margin of the foramen marking the external exit of the trigeminal nerve (CN V), a feature that is variable across the Aetosauria. The external exit of the trigeminal nerve can be enclosed by both the prootic and laterosphenoid, as observed in the early-branching aetosaur Aetosauroides scagliai (MCP-3450-PV; Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021c) and Desmatosuchus smalli (TTU-P 9420, Small, Reference Small2002; UCMP 27410, von Baczko et al., Reference von Baczko, Desojo, Gower, Ridgely, Bona and Witmer2021), a condition that is shared with the aetosauromorphs Parringtonia gracilis von Huene, Reference von Huene1939 (NMT RB426, Nesbitt et al., Reference Nesbitt, Stocker, Parker, Wood, Sidor and Angielczyk2017, figs. 4, 5) and, hypothetically, Revueltosaurus callenderi (PEFO 34561, Parker et al., Reference Parker, Nesbitt, Irmis, Martz, Marsh, Brown, Stocker and Werning2021, fig. 6e). Alternatively, the foramen can be completely enclosed by the prootic as observed in Scutarx deltatylus (PEFO 34616, Parker, Reference Parker2016b) and Stagonolepis olenkae (ZPAL AbIII/466/17, Sulej, Reference Sulej2010), a condition shared with the rauisuchid Postosuchus kirkpatricki (TTU-P 9002, Weinbaum, Reference Weinbaum2011, fig. 24).

Figure 4. UMMP 13950, holotype specimen of Calyptosuchus wellesi. (1) Articulated dorsal carapace in dorsal view; (2) associated vertebral column with pelvis in dorsal view in approximate anatomical position to the dorsal carapace in (1); (3) anteriormost caudal vertebrae with articulated ischia; (4) schematic of the aetosaurian dorsal carapace in dorsal view, where gray indicates regions preserved in UMMP 13950. Anatomical interpretations based on this study are labeled in black and previous interpretations based on Parker (Reference Parker2018a), following those of Case (Reference Case1932), are labeled in red, arrows also follow color labels, accordingly. ac = anterior caudal region; amt = anterior mid-trunk region; at = anterior trunk region; c = caudal region; co = caudal osteoderm; cv = caudal vertebra; isc = ischia; mc = mid-caudal region; mt = mid-trunk region; ns = neural spine; pc = posterior-caudal region; pmt = posterior mid-trunk region; pt = posterior-trunk region; s = sacral region; so = sacral osteoderm; sv = sacral vertebra; t = trunk region; to = trunk osteoderm; tv = trunk vertebra. Arrows indicate anterior direction.

Figure 5. PEFO 46222, associated trunk vertebrae. (1–5, 11) Trunk vertebra #6; (6, 7, 12, 13) trunk vertebrae #10–12; (8–10, 14) trunk vertebrae #15–16; (11–14) lateral expansions of neural spines range. (1, 7, 8) Anterior views, (3, 6, 10) posterior views, (11–14) dorsal views, (5, 9) ventral views, (2, 9) lateral views. acdl = anterior centrodiapophyseal lamina; cen = centrum; dip = diapophysis; frg = bone fragment; len = lateral expansion of neural spine; ns = neural spine; pap = parapophysis; pcdl = posterior centrodiapophyseal lamina; pp = posteriorly projecting process; poz = postzygapophysis; prz = prezygapophysis; spof = spinopostzygapophyseal fossa; spol = spinopostzygapophyseal lamina; sprf = spinoprezygapophyseal fossa; sprl = spinoprezygapophyseal lamina; tp = transverse process; vb = venral bar. Arrows indicate anterior direction.

Figure 6. PEFO 46222, associated sacrum and ribs. (1, 2) Co-ossified sacral vertebrae; (3) lateral expansion of sacral vertebra #2 neural spine; (4, 5) sacral ribs; (6–14) trunk ribs. (1, 4) Ventral views, (2) posterior view, (3) dorsal view, (5) lateral/distal view, (7, 9, 11, 13) medial views, (6, 8, 10, 12, 14) anterior/posterior views. cen = centrum; cos = co-ossified sacral centra; den = distal end; frg = bone fragment; grv = groove; ila = ilium articulation; len = lateral expansion of neural spine; ns = neural spine; poz = postzygapophysis; spof = spinopostzygapophyseal fossa; spol = spinopostzygapophyseal lamina; sr = sacral rib; sv = sacral vertebra; str = strut. Arrows indicate anterior direction for vertebrae and proximal direction for ribs.

Surangular

The surangular is missing a large portion of its anterodorsal process (Fig. 3.9, 3.10). As typically observed in aetosaurs (Desojo et al., Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013), the surangular arches posteriorly over the external mandibular fenestra and becomes dorsoventrally tall. The anterior process of the surangular is anteroposteriorly long, with a concave ventral margin that forms the dorsal border of the external mandibular fenestra. The process is mediolaterally compressed in contrast to the condition observed in Revueltosaurus callenderi (Parker et al., Reference Parker, Nesbitt, Irmis, Martz, Marsh, Brown, Stocker and Werning2021) and erpetosuchids (Benton and Walker, Reference Benton and Walker2002) in which it is expanded (= surangular shelf). A fragment of this process preserves a prominent anteroposteriorly broad tubercle on the dorsal margin of the element near the midline of the external mandibular fenestra (Fig. 3.9, 3.10), a feature document in most taxa with a preserved surangular except Aetosaurus ferratus (SMNS 5770, Schoch, Reference Schoch2007), Desmatosuchus smalli (TTU-P 9024, Small, Reference Small2002), and Coahomasuchus kahleorum (TMM 31100-437, Parker, Reference Parker2016a).

The lateral surface is smooth, unlike the ornamented surface observed in Acaenasuchus geoffreyi Long and Murry, Reference Long and Murry1995 (UCMP 293853, Marsh et al., Reference Marsh, Smith, Parker, Irmis and Kligman2020), and it lacks the surangular ridge that is present in Revueltosaurus callenderi (Parker et al., Reference Parker, Nesbitt, Irmis, Martz, Marsh, Brown, Stocker and Werning2021) and other archosauriforms (Ezcurra, Reference Ezcurra2016). As the surangular becomes dorsoventrally broad, a small process descends anteroventrally (Fig. 3.9). Although it is not fully preserved, it likely participated in forming the posteroventral margin of the external mandibular fenestra, as observed in Stagonolepis (Walker, Reference Walker1961; Sulej, Reference Sulej2010). The angular is not preserved, however the surangular indicates that these two elements exhibited an anteroposteriorly long (Fig. 3.10), straight contact as the angular tapered posteriorly similar to that of Longosuchus meadei (TMM 31185-97, Parrish, Reference Parrish1994). Ventrally, the contact wraps medially on the surangular and terminates at the retroarticular process. The posterior portion of the surangular bears a posteroventrally inclined elliptical surangular foramen. This foramen likely transmitted fibers of the inferior alveolar nerve, a derivative of the mandibular branch of the trigeminal nerve (CN V₃) that innervates the lower jaw (Desojo and Báez, Reference Desojo and Báez2007; Lessner and Holliday, Reference Lessner and Holliday2020). The surangular foramen exits ventromedially on the medial surangular shelf. Medially, that shelf is anteroposteriorly oriented, dorsoventrally broad, and laterally concave. Posteriorly, the surangular exhibits a dorsoventrally curved contact with the articular in lateral view (Fig. 3.9). The surangular only contributes slightly to the retroarticular process, as observed in Typothorax coccinarum (Reyes et al., Reference Reyes, Parker and Marsh2020).

Articular and prearticular

The right articular and posterior portion of the prearticular are co-ossified in PEFO 49321 (Fig. 3.9, 3.10). The articular is complete. The posterior portion comprises most of the retroarticular process and is anteroposteriorly longer than dorsoventrally tall, as observed in Typothorax coccinarum (Reyes et al., Reference Reyes, Parker and Marsh2020) and Desmatosuchus spurensis (MNA V9300, Parker, Reference Parker2008a). Posterodorsally, the articular exhibits a small dorsally oriented triangular process (Fig. 3.10). The glenoid of the articular is mediolaterally broad for its articulation with the quadrate. The articulation surface for the lateral condyle of the quadrate is concave, mediolaterally compressed, and anteroposteriorly elongate, with a medial inclination in dorsal view. The medial articular surface of the glenoid is concave, circular in dorsal view, and is separated from the lateral articulation surface by a low ridge. Medially, the articular exhibits an anteroposteriorly oriented curved contact with the posterior process of the prearticular (Fig. 3.10). This contact terminates ventral to the glenoid, thus does not extend the entire length of the retroarticular process. Ventrally, the prearticular exhibits an elongate contact with the surangular and contacts the posteriorly tapering process of the angular medially, as observed in Longosuchus meadei (Parrish, Reference Parrish1994).

Maxillary teeth

The maxillary teeth of PEFO 49321 are bulbous, exhibit thecodont implantation (sensu Bertin et al., Reference Bertin, Thivichon-Prince, LeBlanc, Caldwell and Viriot2018), and are uniform in their anatomy, but become proportionately smaller distally in the tooth row. Unlike Coahoamasuchus kahleorum (TMM 31100-437, Parker, Reference Parker2016a), the base of the teeth are slightly constricted (Fig. 3.13) but not to the extent observed in Desmatosuchus smalli (TTU-P 9420, Small, Reference Small2002) or Neoaetosauroides engaeus (PULR 108, Desojo and Báez, Reference Desojo and Báez2007; Taborda et al., Reference Taborda, Desojo and Dvorkin2021). The mesiodistal width of the teeth is relatively unchanged until halfway between the base and apex. Apical to that point, the distal margin remains straight with a slight mesial inclination until it reaches the apex, while the medial margin curves distally (Fig. 3.11). This results in the crown exhibiting a curved, distally oriented apex. However, the teeth are not truly recurved like those of Aetosauroides scagliai (UFSM 11505, Biacchi Brust et al., Reference Biacchi Brust, Desojo, Schultz, Paes-Neto and Da-Rosa2018; MCN-PV 2347, MCP-3450-PV, Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b) and Coahomasuchus kahleorum (TMM 31100-437, Parker, Reference Parker2016a) because the apex is not positioned distal to the distal basal margin; instead, it is positioned just mesial to it (Fig. 3.11, 3.12). The curvature of the teeth in PEFO 49321 is most similar to the condition observed in Aetosaurus ferratus (Schoch, Reference Schoch2007). There is no evidence of vertical fluting on the enamel surface of the teeth, which is observed in the dentition of Stagonolepis olenkae (ZPAL AbII/1995, Sulej, Reference Sulej2010) and Stenomyti huangae (DMNH V.60708, Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013). At the point that the mesial and distal margins become confluent, the margins exhibit fine serrations (Fig. 3.14) composed of small triangular-shaped denticles similar to those of Stagonolepis olenkae (ZPAL AbII/1995, Sulej, Reference Sulej2010), but contrary to those observed in Revueltosaurus callenderi (PEFO 34561, Parker et al., Reference Parker, Nesbitt, Irmis, Martz, Marsh, Brown, Stocker and Werning2021) and Acaenasuchus geoffreyi (PEFO 43699, Marsh et al., Reference Marsh, Smith, Parker, Irmis and Kligman2020) in which the denticles are large and round. The serrated edge forms the distal margin of the crown. However, on the mesial margin, the serrations originate on the lingual surface and curve onto the mesial margin of the crown. The crown exhibits a lingually oriented concave curvature near the apex similar to that of Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021b). The labial surface of the crown apex is relatively more curved than the lingual surface (Fig. 3.13). In cross-section the teeth are mesiodistally ovate, but not to the extent observed in ziphodont teeth, which are labiolingually compressed. There is no evidence of tooth replacement in this specimen.

Trunk vertebrae

PEFO 46222 preserves a total of 11 trunk vertebrae (= dorsal vertebrae; Parker, Reference Parker2008a; Heckert et al., Reference Heckert, Lucas, Rinehart, Celeskey, Spielmann and Hunt2010; Desojo et al., Reference Desojo, Ezcurra and Kischlat2012, Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013). The discovery of partial or relatively complete vertebral columns of various aetosaur taxa indicates that on average the trunk series in aetosaurs is composed of 16 vertebrae and exhibits a 1:1 ratio with the dorsal carapace osteoderms (Parker, Reference Parker2008a). The holotype specimen (Fig. 4.2; UMMP 13950) preserves 14 trunk vertebrae (i.e., tv #3–16), but most of the vertebrae (i.e., tv #3–11) are poorly preserved and are only represented by their centra (Fig. 4.2). PEFO 46222 preserves most of the trunk vertebral series (Fig. 5); the most anterior vertebra represents tv #6 from the anterior middle trunk region (Fig. 5.1–5.5). The middle trunk vertebrae (i.e., tv #6–12) are well preserved with little taphonomic distortion, unlike those from the posterior trunk (i.e., tv #13–16), which are missing their respective transverse processes. Thus, PEFO 46222 allows us to assess the anatomy of trunk vertebrae that are missing or poorly preserved in UMMP 13950.

In general, the centra are amphicoelous with circular articular faces. The centra become proportionally larger towards the sacral region and exhibit a more ovate shape in the posteriormost trunk vertebrae (Fig. 5.8–5.10; tv #15–16). The faces of the centra exhibit well-developed rims and are positioned roughly in the same longitudinal plane (Fig. 5.2), unlike Stagonolepis robertsoni (Walker, Reference Walker1961) and Scutarx deltatylus (Parker, Reference Parker2016b) in which the posterior face is more dorsally offset than the anterior one in the more posterior vertebrae. As observed in UMMP 13950, the body of the centrum is transversely compressed and concave with a smooth surface lacking a ventral keel at the midline (Fig. 5.5, 5.9). These features give the centra a spool shape, which is characteristic among aetosaurs (Desojo et al., Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013). Laterally, the body of the centra are smooth, (Fig. 5.2), as observed in UMMP 13950, UMMP 7470, and several referred vertebrae from UCMP A269 (UCMP 139694, UCMP 139796, UCMP 139702; Parker, Reference Parker2018a). There is no evidence of an ovate fossa on the lateral surface of the centrum in PEFO 46222, which is a feature that is subject to intraspecific variation and can become more developed through skeletal maturity, as observed in Aetosauroides scagliai (Paes Neto et al., Reference Paes Neto, Desojo, Brust, Ribeiro, Schultz and Soares2021c). As observed in UMMP 13950, the trunk centra become more anteroposteriorly compressed in lateral view from the middle trunk to the posterior trunk in which the transverse width of the centrum becomes proportionately larger than the anteroposterior length of the body of the centrum (Fig. 5.2, 5.9). Anteriorly, the neural canal is transversely wider than it is dorsoventrally tall, giving the opening a quadrangular shape (Fig. 5.1). The inverse condition is exhibited by the posterior end of the neural canal in which the opening is dorsoventrally taller than it is transversely wide, giving it an ovate shape (Fig. 5.3).

The trunk vertebrae of PEFO 46222 exhibit both anterior and posterior centrodiapophyseal lamina (sensu Wilson, Reference Wilson1999; = infradiapophyseal lamina, Desojo and Ezcurra, Reference Desojo and Ezcurra2011), as observed in UMMP 7470 (Parker, Reference Parker2018a), Aetosauroides scagliai (Desojo and Ezcurra, Reference Desojo and Ezcurra2011; Paes-Neto et al., Reference Paes-Neto, Desojo, Brust, Schultz, Da-Rosa and Soares2021a), Scutarx deltatylus (Parker, Reference Parker2016b), and Desmatosuchus spurensis (Parker, Reference Parker2008a). This condition is considered plesiomorphic within Archosauromorpha (Ezcurra, Reference Ezcurra2016). In PEFO 46222 the posterior centrodiapophyseal lamina is weakly developed in the middle trunk region (Fig. 5.2) and becomes more prominent in the posterior trunk vertebrae (Fig. 5.8), which also appears to be the case in UMMP 13950. The parapophyses are situated on the transverse process in all the known trunk vertebrae, indicating that no transitional cervical/trunk vertebrae are preserved in PEFO 46222 (Fig. 5.1–5.7). The positioning of the parapophyses on the transverse process is typical of aetosaurians (Desojo et al., Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013), a condition shared with the trunk vertebrae in extant crocodyliformes such as Alligator mississippiensis (Daudin, Reference Daudin1802) Frey, Reference Frey1988, figs. 6, 7) and Alligator sinensis (Fauvel, Reference Fauvel1879) (Cong et al., Reference Cong, Hou, Wu and Hou1998, fig. 80). In general, the transverse processes project laterally, nearly perpendicular to the neural spine. They are anteroposteriorly broad with flat dorsal surfaces and transversely wide, around 2.0–2.5 times the transverse width of the centrum (Fig. 5.6, 5.7). The posterior trunk vertebrae #15–16 (Fig. 5.8–5.10) do not preserve enough of their transverse processes, so we are not able to confirm whether the ribs of PEFO 46222 are co-ossified to the transverse processes as described in Scutarx deltatylus (PEFO 34045, Parker, Reference Parker2016b), Desmatosuchus spurensis (MNA V9300, Parker, Reference Parker2008a), and the holotype of Calyptosuchus wellesi (UMMP 13950, Case, Reference Case1932; Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a). The postzygadiapophyseal lamina has deep associated fossae on the dorsal and ventral surfaces of the transverse process in the middle trunk vertebrae of UMMP 7470 (Parker, Reference Parker2018a, fig. 7). However, postzygadiapophyseal lamina is absent in the trunk vertebrae of PEFO 46222 and there is no clear evidence of its presence in the holotype specimen (UMMP 13950) due to the poor preservation of the trunk vertebrae.

Figure 7. PEFO 46222, associated pelvic elements. (1–5) Right half of disarticulated pelvic girdle; (6) left pubis. (1, 6) Lateral views, (3) medial view, (2) dorsal view, (4, 5) proximal views. ace = acetabulum; fos = fossa; il = ilium; ila = ilium articulation; is = ischium; isa = ischium articulation; isp = ischiac peduncle; obf = obturator foramina; prp = preacetabular process; pop = postacetabular process; pu = pubis; pua = pubic apron; puf = pubic fossa; pun = pubic notch; pup = pubic peduncle; sac = supraacetabular crest; sra = sacral rib articulation. Arrows indicate anterior direction.

The vertebrae of PEFO 46222 lack hyposphene-hypantrum accessory articulations similar to other specimens of Calyptosuchus wellesi (Parker, Reference Parker2018a). Hyposphene-hypantrum accessory articulations are documented within Desmatosuchus (Parker, Reference Parker2005, Reference Parker2008a) and Aetobarbakinoides brasiliensis (Desojo et al., Reference Desojo, Ezcurra and Kischlat2012). The prezygapophyses are inclined ~40° from horizontal and are connected at the midline by a small transversely flat shelf (= ventral bar, Parker, Reference Parker2018a). This shelf marks the floor of the spinoprezygapophyseal fossa (sensu Wilson et al., Reference Wilson, D’Emic, Ikejiri, Moacdieh and Whitlock2011) at the base of the neural spine (Fig. 5.1). The postzygapophyses are also inclined ~40° from the horizontal and project slightly beyond the posterior face of the centrum. At the midline between the postzygapophyses there is a posteriorly oriented projection that rests on top of the ventral bar of the succeeding vertebra as in Scutarx deltatylus (Parker, Reference Parker2016b). Additionally, there is a deep spinopostzygapophyseal fossa (sensu Wilson et al., Reference Wilson, D’Emic, Ikejiri, Moacdieh and Whitlock2011) between the postzygapophyses at the base of the neural spine, dorsal to the posteriorly oriented process.

The main body of the neural spine is transversely compressed and anteroposteriorly broad. It becomes proportionally broader sequentially in the vertebral column until it surpasses the anteroposterior length of the centra in trunk vertebrae #15–16 (Fig. 5.8–5.10). In the middle trunk region, the neural spines are 1.41–1.68 times taller dorsoventrally than the centrum (Fig. 5.1–5.7), while in the posterior trunk region they are 1.30–1.33 times taller. The neural spine exhibits both spinoprezygapophyseal (Fig. 5.1, 5.8) and spinopostzygapophyseal laminae (Fig. 5.3, 5.10), as observed in UMMP 7470 (Parker, Reference Parker2018a) and UMMP 13950. Dorsally, the neural spine expands into a transversely broad and flat table-like structure. In dorsal view the ‘table’ exhibits a heart-shaped outline with the tapered end pointing posteriorly. The transverse width of the tops of the neural spines proportionately increase relative to the transverse width of the centrum down the vertebral column (Fig. 5.11–5.14). Overall, the trunk vertebrae become more robust towards the sacrum.

Sacral vertebrae

The previously known sacral anatomy of Calyptosuchus wellesi is based on UMMP 7470 and UMMP 13950, although the elements are distorted in the latter specimen (Case, Reference Case1922, Reference Case1929, Reference Case1932). The sacral vertebrae of PEFO 46222 are not well preserved (Fig. 6.1) and were recovered in series with the posterior trunk vertebrae (Figs. 5.8–5.10, S1). The body of the centrum is crushed and distorted anteriorly in sacral vertebra #1 (Fig. 6.1), but its anterior face is still mostly intact. The slightly ovate anterior face of sacral vertebra #1 is completely exposed and disarticulated from the posteriormost trunk vertebra (Fig. 5.8–5.10, S1). The neural arch of the anterior sacral vertebra is not preserved, and only a disarticulated sacral rib remains (Fig. 6.4, described below).

Sacral vertebra #2 of PEFO 46222 (Fig. 6.1, 6.2) is robust and well preserved, preserving the neural arch with an articulated sacral rib (described below) and a relatively uncrushed centrum with a slightly ovate articular face. Additionally, the centrum body is anteroposteriorly longer than the transverse width of the articular surface, unlike the more truncated posterior trunk vertebrae (Fig. 5.8, 5.9). The prezygapophyses are not preserved but the postzygapophyses are inclined approximately 30° from the horizontal and project slightly beyond the posterior face of the centrum (Fig. 6.1, 6.2). No posteriorly oriented ventral projection is discernable at the midline of the postzygapophyses. The neural spine is robust and exhibits a posterior spinopostzygapophyseal lamina and spinopostzygapophyseal fossa at its base (Fig. 6.2; sensu Wilson et al., Reference Wilson, D’Emic, Ikejiri, Moacdieh and Whitlock2011). Although the distal half of the neural spine is broken into a separate piece, when articulated the neural spine is 1.6 times taller than the centrum, a proportion similar to those of some middle trunk vertebrae. The dorsal transverse expansion of the neural spine (Fig. 6.3) is broad, robust, and more ovate than those in the trunk vertebrae, with a transverse width that is half that of the articular face of the centrum.

The centrum of sacral vertebra #1 (Fig. 6.1) is completely disarticulated from that of the posteriormost trunk vertebra (i.e., tv #16) (Fig. 5.8–5.10) in PEFO 46222, which is also observed in UMMP 13950 and UMMP 7470 (Case, Reference Case1932; Parker, Reference Parker2018a). This indicates that these two vertebrae were not co-ossified to each other in PEFO 46222. Co-ossification of the posterior trunk vertebra with the first sacral vertebra (= dorsosacral, Griffin et al., 2017) has only been reported in taxa within the more inclusive clade Desmatosuchini (Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2008a, Reference Parker2016a; Nesbitt, Reference Nesbitt2011; Griffin et al., Reference Griffin, Stefanic, Parker, Hungerbühler and Stocker2017). Additionally, incorporation of the posteriormost trunk vertebra into the sacrum does not occur in the early-branching aetosaur Aetosauroides scagliai (Casamiquela, Reference Casamiquela1967; Heckert and Lucas, Reference Heckert and Lucas2002a), which suggests that the incorporation of the dorsosacral in desmatosuchins is a derived state. In PEFO 46222 the two sacral vertebrae are ankylosed at the centra faces (Fig. 6.1). Among aetosaurs, co-ossification of the sacral vertebrae has only been documented in Desmatosuchus smalli (TTU-P 9419, Parker, Reference Parker2005), Desmatosuchus spurensis (MNA V9300, TTU-P 10008, Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2008a), and Longosuchus meadei (TMM 31100-236).

Trunk ribs

Several trunk ribs were intermixed with the semi-articulated trunk vertebrae recovered with PEFO 46222. The ribs are neither complete nor do they preserve the capitulum or tuberculum. We describe the general anatomy of the trunk ribs of Calyptosuchus wellesi based on PEFO 46222 and UMMP 13950. Proximally, the capitulum and tuberculum are differentiated by a transversely oriented shallow groove in dorsolateral view (Fig. 6.7). In proximal cross-section, the bone thickens dorsoventrally just anterior to the groove. Based on comparisons to Scutarx deltatylus (PEFO 34045, Parker, Reference Parker2016b), this thickening is associated with the capitulum. The ribs flare out transversely from the transverse process, gradually descend ventrally, and arch more medially towards the distal end until they terminate at an ovate tip (Fig. 6.6). We note that there is one rib fragment with a strongly arched proximal end (Fig. 6.10). This suggests that the ribs may become strongly arched posteriorly in the trunk, as observed in Typothorax coccinarum (PEFO 42506, Parker et al., Reference Parker, Reyes and Marsh2023), however we cannot confirm this based on the preserved material of PEFO 46222.

The ribs are anteroposteriorly broad proximally with a compressed dorsolateral surface throughout the shaft. Ventromedially, there is variation between the preserved rib fragments near the proximal end. In the more complete fragment, the medial surface exhibits a well-developed strut near the proximal end with thin, gracile flanges extending towards the margins (Fig. 6.7). It is evident that the flanges and strut are confluent near the distal end of the rib, where the surface is flat, and the rib body becomes mediolaterally compressed (Fig. 6.11–6.14). This thin flaring of the bone near the proximal end of the rib is more pronounced in other fragments (Fig. 6.9, 6.10), suggesting that this happens sequentially towards the posterior region of the trunk. This condition is also observed in UMMP 13950 and is documented in Desmatosuchus spurensis, where the trunk (= dorsal) ribs become more expanded and thinner near the proximal end in the more posteriorly positioned trunk ribs (Parker, Reference Parker2008a).

Sacral ribs

The anatomy of the sacral ribs in Calyptosuchus wellesi was briefly described by Case (Reference Case1932) and Parker (Reference Parker2018a), however PEFO 46222 preserves both sacral ribs, allowing for a more thorough description of their anatomy. The right sacral rib of sacral vertebra #1 (= anterior) is preserved but not attached to the centrum (Fig. 6.4, 6.5), but the left sacral rib of sacral vertebra #2 (= posterior) is still in articulation. Based on the articulated sacral rib, the sacral ribs are not shared between the two sacral vertebrae, but rather they are each restricted to a single vertebra (Nesbitt, Reference Nesbitt2011, character 208). Because of poor preservation, we cannot determine how the anterior sacral rib contacts the main body of an anterior sacral vertebra. However, based on the in-situ articulated sacral rib and UMMP 7470, the ribs were attached dorsolaterally on the centrum and partially on the ventral portion of the neural spine (Fig. 6.1). The anterior sacral rib was damaged during excavation, resulting in partial loss of the thin flange near the proximal end (Fig. 6.4; PEFO 46222). In dorsal view, the surface is flat and dorsoventrally broad. In ventral view, there is a well-developed and robust strut running the transverse width of the rib (Fig. 6.4). The strut is concave, giving the rib a strongly bowed appearance that opens ventrally in anterior/posterior view. In lateral view there is a deep groove that separates the ventral strut from the anteroposteriorly expanded dorsal sheet of bone (Fig. 6.5). Distally, the anterior sacral rib (for sacral #1) exhibits a transversely ovate articulation surface that is dorsoventrally thickened (Fig. 6.5). Thus, the anterior sacral rib has a large, robust contact with the medial surface of the ilium. This contact occurs just dorsal to the pubic peduncle, ventral to the preacetabular process, and is slightly shared with the posterior sacral rib. Additionally, there is a thin sheet of bone that dorsally overhangs the articulation surface and contacts the ilium (Fig. 6.4).

The left posterior sacral rib is complete and well preserved. The rib is anteroposteriorly expanded distally giving it a subtriangular shape in dorsal/ventral view (Fig. 6.1). Additionally, it is dorsoventrally thickest anteriorly and gradually thins posteriorly (Fig. 6.5), as seen in UMMP 7470 (Case, Reference Case1929). In the distal cross-section, the articulation surface is dorsoventrally thick anteriorly, where it partially shares the articulation surface of the anterior sacral rib (Fig. 6.5). This was described and well figured by Walker (Reference Walker1961, fig. 9) for Stagonolepis robertsoni. Because the posterior sacral rib becomes anteroposteriorly expanded distally, it exhibits a broad articulation with the ilium. This articulation extends posterodorsally on the medial surface of the ilium, terminating ventral to the postacetabular process. The posterior sacral rib is not bowed like the anterior sacral rib, nor does it exhibit a well-developed strut in ventral view. Overall, the anatomy of the sacral ribs and their contact with the ilium in Calyptosuchus wellesi resembles that of Stagonolepis robertsoni (Walker, Reference Walker1961, fig. 9).

Ilium

PEFO 46222 preserves both ilia. The right ilium is well preserved and serves as the main reference for the anatomical description of the element (Fig. 7). The left ilium is partly preserved and was accidently damaged during excavation (i.e., found unexpectedly with a pickaxe while removing overburden). This resulted in the acetabular rim, ischiac peduncle, and part of the postacetabular process to be broken, and the breaks are delineated by fresh surfaces. The preacetabular process extends anteriorly but not beyond the anterior margin of the pubic peduncle, similar to the condition in Scutarx deltatylus (Parker, Reference Parker2016b) and UMMP 7470 (Case, Reference Case1929). In the referred ilium UCMP 25941 from UCMP A269, the preacetabular process is positioned just dorsal to the pubic peduncle (Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a), unlike the preacetabular process of Neoaetosauroides engaeus, which extends far anteriorly of the pubic peduncle (Desojo and Báez, Reference Desojo and Báez2005). The preacetabular process is a finger-like projection that hooks anteroventrally (Fig. 7.1). This resembles that of UMMP 7470 (Case, Reference Case1929) but is unlike that of UCMP 25941, the specimen from UCMP A269 (Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a), which is more triangular and robust in lateral view. The finger-like anatomy of PEFO 46222 is also shared with the early-branching Aetosauroides scagliai (PVL 2073, Casamiquela, Reference Casamiquela1967; Heckert and Lucas, Reference Heckert and Lucas2002a), Aetosaurus ferratus (Schoch, Reference Schoch2007), and ?Lucasuchus hunti Long and Murry, Reference Long and Murry1995 (TMM 31100-1). In lateral view, the outline of the ilium between the preacetabular process and pubic peduncle is U-shaped (Fig. 7.1), as seen in UMMP 7470 (Case, Reference Case1929), but unlike that of UCMP 25941, which is more V-shaped. Although crushed, the ilium in the holotype specimen of Calyptosuchus wellesi (UMMP 13950, Case, Reference Case1932; Parker, Reference Parker2018a) exhibits a reminiscent U-shape outline between the pubic peduncle and preacetabular process (Parker, Reference Parker2018a, fig. 9).

Anteroposteriorly, the dorsal margin of the iliac blade ascends gradually from the preacetabular process to the postacetabular process and becomes slightly concave dorsal to the iliac body. The dorsal surface of the blade is rugose (Fig. 7.1, 7.2), presumably marking the attachment of the M. iliotibialis (Romer, Reference Romer1956; Hutchinson, Reference Hutchinson2001; Schachner et al., Reference Schachner, Manning and Dodson2011, Reference Schachner, Irmis, Huttenlocker, Sanders, Cieri and Nesbitt2019), although it is not as well defined as seen in UCMP 25941 (Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a). In dorsal view, the iliac blade is mediolaterally compressed and becomes expanded posteriorly (= squared off, Parker, Reference Parker2016a, Reference Parker2018a) at the postacetabular process (Fig. 7.2), as observed in UMMP 13950 and UMMP 7470. As typically seen in aetosaurs, the postacetabular process extends well beyond the posterior margin of the ishiac peduncle. In lateral view, the postacetabular process is knob-like with the ‘neck’ exhibiting a concave posterior margin between it and the acetabulum, a condition shared with UMMP 7470 (Case, Reference Case1929) and UCMP 25941 (Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a). The acetabulum is deeply concave and broad, being anteroposteriorly longer than it is dorsoventrally tall (Fig. 7.1), but not to the extent observed in UCMP 25941 (Parker, Reference Parker2018a). As observed in UMMP 7470, the acetabulum of PEFO 46222 would have faced ventrolaterally due to the articulation between the ilium and sacral vertebrae (Parker, Reference Parker2018a).

The ilium exhibits a well-developed supraacetabular crest, similar to that of Scutarx deltatylus (PEFO 31217, Parker, Reference Parker2016b), Stagonolepis (NHMUK PV R4789, Walker, Reference Walker1961; UOPB00150, Desmet et al., Reference Desmet, Antczak and Bodzioch2022), and Aetosauroides scagliai (PVL 2073, Casamiquela, Reference Casamiquela1967; Heckert and Lucas, Reference Heckert and Lucas2002a). Just dorsal to the supraacetabular crest, the body of the ilium exhibits a shallow fossa (Fig. 7.1), as observed in UMMP 13950. However, it is not deep as described in Scutarx deltatylus (Parker, Reference Parker2016b). This region is partially distorted in UMMP 7470. Ventrally, the margins of the ilium form a V-shaped ventrally projecting point at the midline of the acetabulum. The pubic peduncle is mediolaterally expanded with a flat surface in anterior view and exhibits a comma-shaped outline in ventral view. In contrast, the ishiac peduncle is also comma-shaped in ventral view but exhibits a sharp ridge in posterior view due to it being mediolaterally compressed. The medial surface exhibits prominent scars/attachment sites for the sacral ribs (Figs. 6.5, 7.3). There is a large ovate surface medial to the acetabulum, just dorsal to the pubic peduncle, which represents the attachment site of the anterior sacral rib. Posterodorsal to this ovate surface there is a crest that terminates at the postacetabular process. This serves as the main attachment site for the transversely expanded posterior sacral rib.

Pubis

The right pubis of PEFO 46222 is mostly complete although it is slightly crushed (Fig. 7.1). In contrast, the left pubis preserves the proximal two-thirds of the element (Fig. 7.6) but is not as crushed as the right pubis. Together, the preservation of the pubes allows for a description of the entire element, which is only proximally preserved in UMMP 7470 and partially obscured in UMMP 13950. The pubis of PEFO 46222 exhibits a dorsoventral height that is less than the anteroposterior length of the iliac blade as observed in UMMP 13950, but unlike the condition exhibited by Desmatosuchus spurensis (MNA V9300, Parker, Reference Parker2008a) and ?Lucasuchus hunti (TMM 31100-313, Long and Murry, Reference Long and Murry1995), in which the height of the pubis exceeds the length of the ilium. Proximally, the pubis exhibits a comma-shaped surface that matches the contact surface of the pubic peduncle on the ilium (Fig. 7.4). This is also the case in the referred pubis of Calyptosuchus wellesi (UCMP 23195, Parker, Reference Parker2018a) from UCMP A269. More posteriorly, this surface expands ventrally for its contact with the anterior margin of the ischium. The acetabular portion of the pubis is strongly concave with a well-developed rim (Fig. 7.1), as seen in UCMP 23195 (Parker, Reference Parker2018a) and UMMP 7470 (Case, Reference Case1929). Proximally, the shaft of the pubis is robust anteriorly and becomes more gracile posteriorly where it composes part of the pubic apron, as observed in UMMP 13950. On the lateral surface, the pubis exhibits a deep notch that originates just anterodorsal to the acetabular rim and runs anteroventrally on the anterior surface of the shaft as described in UMMP 7470 (Case, Reference Case1929; Parker, Reference Parker2018a). There is a shallow fossa on the shaft posterior to that groove that extends parallel to the anterior margin.

The surface is smooth on the pubic apron portion of the element and exhibits faint rugosities near its medial margin (Fig. 7.3). This rugose surface presumably marks the attachment site of the M. puboischiofemoralis externus 1–3 (Hutchinson, Reference Hutchinson2001; Schachner et al., Reference Schachner, Manning and Dodson2011). The perforate region of the pubic apron is often poorly preserved or obscured from view in taxa with a preserved pubis (i.e., Aetosauroides scagliai, Casamiquela, Reference Casamiquela1967; Heckert and Lucas, Reference Heckert and Lucas2002a; ?Lucasuchus hunti Long and Murry, Reference Long and Murry1995; Aetosaurus ferratus, Schoch, Reference Schoch2007; Desmatosuchus spurensis, Parker, Reference Parker2008a; Typothorax coccinarum, Heckert et al., Reference Heckert, Lucas, Rinehart, Celeskey, Spielmann and Hunt2010; Stenomyti huangae Small and Martz, Reference Small, Martz, Nesbitt, Desojo and Irmis2013), fortunately this region is well preserved in PEFO 46222. It is evident that the pubis in PEFO 46222 is perforated by two foramina (Fig. 7.1, 7.2, 7.6).

Currently, the presence of two foramina on the pubic apron in aetosaurs has only been unambiguously described in Scutarx deltatylus (PEFO 31217, Parker, Reference Parker2016b) and Stagonolepis robertsoni (NHMUK R4793, MCZD 4, Walker, Reference Walker1961). This is unlike the condition observed in the aetosaurs Neoaetosauroides engaeus (PVL 3525, Desojo and Báez, Reference Desojo and Báez2005) and Desmatosuchus smalli (TTU-P 9419, Martz, Reference Martz2008) where the pubis is perforated by only one foramen, the obturator foramen. In life, the single obturator foramen conveys the obturator nerve and associated blood vessels that partially innervate and vascularize the hindlimb (Romer, Reference Romer1956; Hutchinson, Reference Hutchinson2001; Claessens and Vickaryous, Reference Claessens and Vickaryous2012).

In PEFO 46222 the dorsally positioned foramen is ovate with a dorsomedial inclination, whereas the ventral foramen is significantly larger and semicircular (Fig. 7.6). The two foramina are separated by a thin plate of bone. Ventral to the foramina, the pubic shaft twists medially, and becomes broad and mediolaterally compressed with a similar thickness as the dorsal pubic apron portion (Fig. 7.1). Distally, the pubis exhibits a curved, mediolaterally compressed ventral margin. It lacks the distal expansion (= ‘pubic boot’, Gauthier, Reference Gauthier1986; Nesbitt, Reference Nesbitt2011) present in Desmatosuchus spurensis (MNA V9300, Parker, Reference Parker2008a), Typothorax coccinarum (TTU P-9214, Martz, Reference Martz2002), and Aetosauroides scagliai (PVL 2073, Casamiquela, Reference Casamiquela1967; Heckert and Lucas, Reference Heckert and Lucas2002a).

Ischium

The left ischium of PEFO 46222 is well preserved with few signs of taphonomic distortion, unlike its right counterpart, which is slightly crushed with a distal process that is preserved as a separate piece. The element is anteroposteriorly longer than it is dorsoventrally tall. The main body of the ischium is broad and thickest mediolaterally near the acetabulum. Proximally, the ischium exhibits a comma-shaped cross-section (Fig. 7.5) matching that of the ishiac peduncle on the ilium. Unlike the pubis, the acetabulum is concave and shallow (Fig. 7.1). The acetabular rim is prominent near the proximal contact with the ilium (Fig. 7.5). Posteroventrally, the ischium tapers to a blunt, posteriorly oriented process unlike that of Desmatosuchus spurensis (Parker, Reference Parker2008a) in which the posterior process hooks ventrally. In lateral view, it exhibits a concave posterior margin between its contact with the ilium and posterior process. The ischium exhibits a uniform mediolateral thickness posteroventral to the acetabulum. This is unlike the referred ischia of Calyptosuchus wellesi (UCMP 32153, UCMP 32148, Long and Murry, Reference Long and Murry1995; Parker, Reference Parker2018a) from UCMP A269, which exhibit a ‘rod-like’ body posteriorly and fan out ventrally becoming more gracile.

Laterally, the expanded surface is generally flat and smooth but becomes more rugose near the anteroposteriorly straight ventral margin (Fig. 7.1). This rugose texture presumably marks the attachment site of M. puboischiofemoralis externus 3 (Hutchinson, Reference Hutchinson2001; Schachner et al., Reference Schachner, Manning and Dodson2011). The medial contact between the ischia is well exposed; this contact runs the length of the ventral margin and is composed of a series of inclined ridges and grooves, indicating a strong interlocking contact between the ischia (Fig. 7.3). Previously, the presence of a notch was described on the anterior margin of the ischium in the referred specimens UCMP 32148 and UMMP 7470 (Parker, Reference Parker2018a), however those notches are artifacts of preservation.

Middle trunk region

The middle trunk region encompasses trunk vertebrae and osteoderm rows numbering 5–12 (Figs. 1, 4). It is further subdivided into the anterior middle trunk region, encompassing rows 5–8 (Figs. 1, 4), and the posterior middle trunk region, which encompasses rows 9–12 (Figs. 1, 4). In PEFO 46222 the middle trunk region is composed of several complete paramedian osteoderms (Fig. 8) with three associated lateral osteoderms, which are characterized by a ‘wing’-like lateral flange. In dorsal view, the plates are rectangular with some exhibiting a slight anterolateral curvature. The paramedian osteoderms of this region exhibit the largest width-to-length ratios (W:L) and anteroposterior length of any of the preserved paramedian osteoderms, with an average W:L of 2.7 and anteroposterior length of 8.5 cm. In posterior view and lateral to the dorsal eminence, the paramedian osteoderms exhibit varying degrees of ventral arching at the center of ossification, causing the bone surface to be dorsally convex (Fig. 8.3, 8.4). This is exemplified by a right paramedian osteoderm that is strongly arched (Fig. 8.4), similar to an isolated osteoderm of Adamanasuchus eisenhardtae (PEFO 34638, Lucas et al., Reference Lucas, Hunt, Spielmann, Lucas and Spielmann2007a, fig. 3). This is likely associated with constriction of the trunk into a gracile ‘waist’ (Desojo et al., Reference Desojo, Heckert, Martz, Parker, Schoch, Small, Sulej, Nesbitt, Desojo and Irmis2013), which occurs anterior to the sacral region in the middle to posterior trunk transition and has been documented in aetosaurs such as Coahomasuchus kahleorum (Heckert and Lucas, Reference Heckert and Lucas1999).

Figure 8. PEFO 46222, associated mid-trunk paramedian osteoderms. (1–6) Isolated osteoderms showing the variation in anatomy within the mid-trunk region; (7, 9) four imbricated paramedian osteoderms that include osteoderms derived from the posterior trunk and/or sacral regions; (8, 10, 11) isolated paramedian osteoderm with possible pathology that is observed across the carapace of PEFO 46222. (1, 2, 8) Dorsal views, (5–7, 11) ventral views, (3, 4, 9, 10) posterior views. ab = anterior bar; alp = anterolateral process; amp = anteromedial process; de = dorsal eminence; grv = groove; le = lateral edge; me = medial edge; mto = mid-trunk osteoderm; ptso = posterior-trunk / sacral osteoderm; sc = scalloped; sw = swelling; vs = ventral strut. Arrows indicate lateral direction.

The paramedian osteoderms of the middle trunk region exhibit a pronounced anterolateral triangular process of the anterior bar (Parker, Reference Parker2018a), however this process varies in its inclination in dorsal view throughout this region. In the anterior middle trunk region, the process projects parallel to the horizontal plane in dorsal view (Fig. 8.1) and becomes anterolaterally inclined in the posterior middle trunk region (Fig. 8.2). The sigmoidal nature of the lateral margin of the paramedian osteoderms vary in the middle trunk region; it ranges from being weakly to moderately sigmoidal (Fig. 8.1, 8.2, 8.7, 8.8). Additionally, the lateral margin exhibits an embayed surface positioned just posterior to the anterolateral process for overlap of the adjacent lateral osteoderm. The anterior bar also exhibits an anteromedial process that projects anteriorly. It has been noted that the anterior margin of the anterior bar is ‘scalloped’ in the paramedian osteoderms of Calyptosuchus wellesi (UMMP 7470, Parker, Reference Parker2018a). PEFO 46222 indicates that this scalloped margin is not consistent across all the paramedian osteoderms of this region; it is weak in the anterior trunk region (Fig. 8.1), whereas in the more posteriorly positioned osteoderms it is moderately to well incised (Fig. 8.2, 8.8). The medial edge is roughly anteroposteriorly straight, although on some of the osteoderms the edge curves slightly medially, just posterior to the anterior bar. The medial edge lacks the interlocking tongue-and-groove articulation surface that is seen in Desmatosuchus (Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985; MNA V9300, Parker, Reference Parker2008a). In the middle trunk region, the dorsal eminence is positioned more medially on the posterior border of the plate. It is weakly developed, being a relatively low boss with a slight keel that originates from the posterior margin of the anterior bar (Fig. 8.1, 8.2). In posterior view, the eminence exhibits a mound-like curved outline (Fig. 8.4, 8.10).

The dorsal ornamentation is composed of well-incised pits and grooves radiating from the eminence (Fig. 8.1). However, in several of the preserved paramedian osteoderms, the dorsal ornamentation is broken by large, deep, anteroposteriorly oriented grooves with straight margins (Fig. 8.2, 8.8). Ventrally, there is a ridge and rounded swelling of the bone associated with the groove on the dorsal surface (Fig. 8.10, 8.11). It is possible that this is associated with predation or scavenging (i.e., bite traces), which has also been documented in the osteoderms of Typothorax coccinarum (PEFO 34869, Drymala et al., Reference Drymala, Bader and Parker2021). Alternatively, this damage may be pathological in origin, although testing that hypothesis requires further analysis outside the scope of the present study. In the absence of this abnormal swelling and damage of the bone, the ventral surface is smooth with a weak, transversely oriented ventral strut (sensu Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985; Fig. 6.5) and an embayed posterior margin (Fig. 8.5, 8.6), which overlaps the anterior bar of the succeeding paramedian osteoderm, as described by Parker (Reference Parker2018a).

Posterior trunk and sacral region

The posterior trunk region encompasses trunk osteoderm rows #13–16 and the succeeding sacral region is composed of two sacral osteoderm rows (Figs. 1, 4). UMMP 13950 indicates that the paramedian osteoderms in the posterior trunk and sacral regions are similar in anatomy to those of the mid trunk region. These regions can be differentiated from each other based on the anatomy of the respective lateral osteoderms. The articulated carapace of UMMP 13950 indicates that the lateral osteoderms in the posterior trunk and sacral regions lack the ‘wing’-like lateral flange that is characteristic of the middle trunk region (Fig. 4.1), instead they exhibit a square-like outline in dorsal view with a quadrangular lateral flange.

Two paramedian osteoderms referable to the posterior trunk region are imbricated and diagenetically cemented to two other osteoderms from the middle trunk region (Fig. 8.7, 8.9). Linear measurements of these imbricated osteoderms indicate that they exhibit an average W:L of 2.5 and an anteroposterior length of 7.9 cm. These osteoderms are slightly arched from the medial edge to the dorsal eminence and transversely flat lateral to the eminence in posterior view (Fig. 9.3). Additionally, they exhibit a slight anterolateral curvature in dorsal view (Fig. 9.1), as observed in the posterior middle trunk region (Fig. 8.8). The preserved elements indicate that the triangular anterolateral process of the anterior bar gradually becomes less pronounced moving posterior in the trunk series (Fig. 8.7). The orientation of this process is predominantly inclined anterolaterally. The anterior margin of the anterior bar is moderately ‘scalloped’, unlike those of the posterior middle trunk region where the scalloping of the anterior bar is well developed. A complete left paramedian osteoderm articulates with an associated square-like lateral osteoderm, indicating that the lateral margin is moderately sigmoidal in the posterior trunk and sacral regions (Fig. 9.1), a condition also observed in those osteoderms that are imbricated and diagenetically cemented (8.7). This particular osteoderm exhibits a slightly lower W:L of 2.3 and an anteroposterior length of 7.5 cm, indicating that it succeeds the imbricated osteoderms described above. The medial margins of the paramedian osteoderms are as described for those of the middle trunk region (Fig. 8). The dorsal eminence in the posterior trunk and sacral region is taller and more transversely expanded than those of the middle trunk region. The eminence is a more prominent boss that has a weak keel anteriorly and is rounded or ‘mound-shaped’ in posterior view (Fig. 9.3). The eminence itself is not medially situated; instead, it is positioned at the midline of the posterior border (Fig. 9.1). This aligns with the sequential transverse shortening of the paramedian osteoderms in the carapace (Case, Reference Case1932), which gradually positions the dorsal eminence closer to the medial edge of the osteoderm. The gradual change of the dorsal eminence from the mid trunk to sacral region in PEFO 46222 is best observed in the imbricated osteoderms (Fig. 8.9). The dorsal ornamentation is characterized by well-incised grooves and ridges radiating from the eminence. The ventral surface is smooth and exhibits a weak, transversely oriented ventral strut.

Caudal region

PEFO 46222 preserves three caudal paramedian osteoderms from the right side (Fig. 9.2, 9.6, 9.8). Two of these elements are not fully preserved, missing the lateral extent of the element, which prohibits calculation of their respective width-to-length ratios. However, it is evident that caudal osteoderms are transversely shorter than the paramedian osteoderms of the trunk and sacral regions. The key feature that supports their referral to the caudal region is the presence of a tall, robust, blunt pyramidal dorsal eminence with an anterior keel (Fig. 9.4, 9.9, 9.10). The holotype specimen of Calyptosuchus wellesi (UMMP 13950, Case, Reference Case1932) preserves most of the caudal carapace, and it is evident that only in this region do the dorsal eminences become this prominent (Fig. 4.1; Parker, Reference Parker2018a). Additionally, the caudal paramedian osteoderms become more flexed distally on the tail. Thus, the anatomy of the eminence along with the flexure of the osteoderm provide the best proxy in determining the anatomical position of these caudal elements. The paramedian caudal osteoderms are referable to middle caudal rows #5–10 (Figs. 1, 4). Two of these elements are rectangular in dorsal view (Fig. 9.2, 9.6), whereas the other exhibits a more square-like outline (Fig. 9.8). Comparison with UMMP 13950 indicates that the square-like osteoderm is positioned more posteriorly in the caudal region near caudal row #10 (Figs. 1, 4).

The preserved osteoderms are anteroposteriorly shorter than those from the trunk and sacral region with an average anteroposterior length of 7.15 cm. This shortening occurs gradually from the mid trunk to mid caudal region, as observed in UMMP 13950. The general anatomy of the dorsal eminence is the same in the preserved elements, only varying in their respective heights. The height of the eminence is directly correlative to the transverse shortening of the paramedian osteoderm in the caudal region, thus the eminence becomes taller and more robust in the more caudally positioned elements within the middle caudal region of Calyptosuchus wellesi (Figs. 4.1, 6.4, 9.9, 9.10). In the posteriormost caudal paramedian osteoderms, the eminence exhibits a height that is equivalent to the anteroposterior length of the osteoderm (Fig. 9.8, 9.9). Additionally, the dorsal eminence is positioned closer to the medial edge than those in the trunk and sacral regions. The dorsal ornamentation is composed of grooves and ridges radiating from the eminence. However, in the more posteriorly positioned osteoderm the dorsal surface is dominated by the eminence rather than the ornamentation (Fig. 9.8). Thus, the ornamentation is not as well incised as those from the trunk and sacral regions. One of the caudal paramedian osteoderms exhibits several shallow channels incised faintly onto the bone surface forming irregular loops that are inconsistent with the ornamentation on the dorsal surface (Fig. 9.6, 9.7). We hypothesize that these looped channels may represent osteophagous invertebrate traces similar to those described from other Triassic assemblages (Paes Neto et al., Reference Paes Neto, Parkinson, Pretto, Soares, Schwanke, Schultz and Kellner2016). Alternatively, they may represent evidence of root etching on the bone surface. However, those patterns tend to exhibit a randomized and/or irregular pattern with channels that are interconnected on the bone surface (Francischini et al., Reference Francischini, Lucas, Dentzien-Dias, Schultz, Martínez, Rojas and Cabrera2020). Ventrally, the preserved elements lack a ventral strut and exhibit a smooth surface with a more deeply embayed posterior margin for the overlap of the succeeding osteoderm.

Middle trunk region

PEFO 46222 preserves several lateral osteoderms that are associated with paramedian osteoderms of the mid trunk region. These osteoderms exhibit a significantly larger lateral flange compared to the dorsal flange (Fig. 10.1, 10.2). The outline of the dorsal flange is triangular in dorsal view. The medial edge of the dorsal flange is sigmoidal along its contact with the lateral margin of the adjacent paramedian osteoderm. The anteromedial edge of the anterior bar is incised for reception of the anterolateral process of the anterior bar in the adjacent paramedian osteoderm. The angle of incision varies between the lateral osteoderms (Fig. 10.1, 10.2), which matches the variation observed in the projection of the anterolateral process of the paramedian osteoderms throughout the dorsal carapace. The lateral flange of these lateral osteoderms is triangular in dorsal view and exhibits an anterolateral ‘wing-like’ appearance (sensu Parker, Reference Parker2018a), in which the posterior margin curves anteriorly until it meets the anterior margin at a rounded tip (Fig. 10.2; Parker, Reference Parker2018a). This anatomy is associated with lateral osteoderms positioned in the middle trunk region of UMMP 13950 (Figs. 1, 4). When measuring from the dorsal eminence, the transverse width of the lateral wing varies between these lateral osteoderms. The lateral flange exhibits a larger width the more anteriorly located it is in the middle trunk region and becomes gradually reduced posteriorly in the sequence.

There is also variation in the dorsal eminence on the middle trunk lateral osteoderms. The more anteriorly located elements exhibit a low pyramidal eminence with an anteroposteriorly elongate keel at the midline (Fig. 10.4, 10.5; Parker, Reference Parker2018a). However, unlike the condition observed in an isolated lateral osteoderm (UCMP 136744) referred to Calyptosuchus wellesi from UCMP A269, the dorsal eminence does not project beyond the posterior margin (Parker, Reference Parker2018a). The flexure between the dorsal and lateral flange varies between these lateral osteoderms. The more anteriorly positioned lateral osteoderms exhibit a flexure of nearly 90° between the dorsal and lateral flanges in posterior view (Fig. 10.4), whereas the succeeding lateral osteoderms exhibit a more obtuse flexure in comparison (Fig. 10.5). This variation in the flexure of the lateral osteoderm is also well documented in Desmatosuchus spurensis (MNA V9300, Parker, Reference Parker2008a, fig. 27), in which the osteoderms become more acutely flexed in the middle trunk region.

Posterior trunk and sacral regions

PEFO 46222 preserves a left lateral osteoderm (Fig. 10.3, 10.6) that articulates with a paramedian osteoderm referable to the posterior trunk or sacral region of the dorsal carapace. The element exhibits a sigmoidal medial edge similar to the middle trunk lateral osteoderms, although the anterior bar is not well incised due to the decreased prominence of the anterolateral process of the adjacent paramedian osteoderm (Fig. 10.3). The dorsal flange no longer exhibits a triangular outline in dorsal view, instead it is more rectangular. The lateral flange lacks the triangular-shaped ‘wing’ (sensu Parker, Reference Parker2018a) that is characteristic of the middle trunk laterals. Instead, it exhibits a transverse width similar to the dorsal flange, giving the element a squared outline in dorsal view with a wide obtuse flexure in posterior view (Fig. 10.3, 10.6). The dorsal eminence is partially eroded near the apex, but it is evident that it was moderately developed and exhibited an anteroposteriorly oriented keel on the dorsal surface similar to those from the trunk.

Caudal region

PEFO 46222 preserves a left lateral caudal osteoderm (Fig. 10.7, 10.8) that is referable to the anterior caudal region based on comparisons to UMMP 13950 (Fig. 4.1). It exhibits well-incised radial ornamentation, a low pyramidal eminence that is anteriorly keeled, and a transversely short triangular lateral flange that is slightly wider than the dorsal flange with an anterolateral inclination (Fig. 10.7). Although the lateral flange is similar to those from the middle trunk region, the osteoderm exhibits a wide, obtuse flexure of nearly 180° (Fig. 10.8). The dorsal flange exhibits a rectangular outline in dorsal view similar to those in the posterior trunk and sacral regions. However, the medial edge is weakly sigmoidal in comparison to the more anteriorly positioned lateral osteoderms.

New referred PEFO specimens

PEFO 46222, an associated skeleton preserving elements from the trunk through caudal region including paramedian and lateral osteoderms, vertebrae, ribs, and a disarticulated pelvic girdle; PEFO 49321, an associated skeleton preserving paramedian and lateral osteoderms as well as skull elements including the left and right maxillae with teeth, the right jugal, the left quadratojugal, the left quadrate, the left laterosphenoid, the right surangular, the right prearticular, and the right articular.

Rationale for referral of PEFO specimens

PEFO 46222 and PEFO 49321 preserve osteoderms that are rectangular in dorsal view, indicating they are paramedian osteoderms derived from two medial anteroposterior columns of the dorsal carapace (Parker and Martz, Reference Parker and Martz2010). The paramedian osteoderms lack a beveled edge on the posterodorsal margin, unlike Tecovasuchus chatterjeei Martz and Small, Reference Martz and Small2006 (TTU-P 545), Paratypothorax sp. (TTU-P 09169, Martz et al., Reference Martz, Mueller, Nesbitt, Stocker, Parker, Atanassov, Fraser, Weinbaum and Lehane2013; PEFO 3004, Lucas et al., Reference Lucas, Heckert, Rinehart, Harris, Lucas, Spielmann, Lockley, Milner and Kirkland2006), Kocurypelta silvestris (ZPAL V.66/1, Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021), and Venkatasuchus armatum Haldar, Ray, and Bandyopadhyay, Reference Haldar, Ray and Bandyopadhyay2023 (ISIR267/1-7), which exhibit this anatomy in the trunk and caudal regions. The dorsal ornamentation of the paramedian osteoderms in PEFO 46222 and PEFO 49321 is composed of a strong radial pattern of ridges and grooves radiating from the eminence, unlike Typothorax coccinarum (TTU-P 9214, Martz, Reference Martz2002; NMMNH P-12964, Heckert et al., Reference Heckert, Lucas, Rinehart, Celeskey, Spielmann and Hunt2010), Redondasuchus rineharti Spielmann et al., Reference Spielmann, Hunt, Lucas and Heckert2006 (NMMNH P-43311), Kryphioparma caerula (UCMP 165173, Reyes et al., Reference Reyes, Parker and Heckert2023), and Sierritasuchus macalpini Parker, Stocker, and Irmis, Reference Parker, Stocker and Irmis2008 (UMMP 60817), which exhibit well-incised oblong to circular pits across the dorsal surface, or Apachesuchus heckerti Spielman and Lucas, Reference Spielmann and Lucas2012, which is primarily devoid of ornamentation. Anteriorly, the trunk paramedian osteoderms exhibit a thickened well-developed anterior bar unlike paratypothoracins (e.g., Rioarribasuchus chamaensis Zeigler, Heckert, and Lucas, Reference Zeigler, Heckert and Lucas2003), which exhibit a weakly developed anterior bar (Parker, Reference Parker2007; Martz et al., Reference Martz, Mueller, Nesbitt, Stocker, Parker, Atanassov, Fraser, Weinbaum and Lehane2013; Czepiński et al., Reference Czepiński, Dróżdż, Szcygielski, Tałanda, Pawlak, Lewczuk, Rytel and Sulej2021; Haldar et al., Reference Haldar, Ray and Bandyopadhyay2023; Reyes et al., Reference Reyes, Martz and Small2024), Desmatosuchus spurensis (UMMP 7476, Case, Reference Case1922; MNA V9300, Parker, Reference Parker2007, Reference Parker2008a), which exhibits a depressed surface (= anterior lamina, sensu Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985; Parker, Reference Parker2005, Reference Parker2008a), or Garzapelta muelleri (Reyes et al., Reference Reyes, Martz and Small2024), which exhibits presacral osteoderms with a flat and smooth anterior margin. The anterior bar exhibits both strongly projecting anterolateral and anteromedial projections, similar to Scutarx deltatylus (Parker, Reference Parker2016b). The anterolateral process on the paramedian osteoderm overlaps the anterior surface of the adjacent lateral osteoderm, unlike desmatosuchins (Parker, Reference Parker2007). More specifically, the trunk paramedian osteoderms of PEFO 46222 and PEFO 49321 exhibit a ‘scalloped’ anterior margin of the anterior bar, a weakly developed ventral strut, and a low, rounded, anteriorly keeled eminence in the trunk series that becomes more pronounced, tall, robust, and pyramidal caudally; a condition shared with Scutarx deltatylus (Parker, Reference Parker2016b) and Adamanasuchus eisenhardtae (Lucas et al., Reference Lucas, Hunt, Spielmann, Lucas and Spielmann2007a). The dorsal eminence contacts the posterior margin, a condition shared with most taxa except Desmatosuchus (Parker, Reference Parker2005, Reference Parker2008a), Lucasuchus hunti (Long and Murry, Reference Long and Murry1995), Neoaetosauroides engaeus (Desojo and Báez, Reference Desojo and Báez2005), and paratypothoracins (Parker, Reference Parker2007). Additionally, PEFO 46222 and PEFO 49321 preserve asymmetrical osteoderms with dorsal and lateral flanges, flexed at the center of ossification, and articulate with paramedian osteoderms, indicating they are lateral osteoderms from the two outer columns of the dorsal carapace (Parker and Martz, Reference Parker and Martz2010). The lateral osteoderms are broadly flexed, similar to those of non-desmatosuchin and non-typothoracine taxa (Parker, Reference Parker2007). The dorsal eminence is a low keeled triangular boss, unlike desmatosuchins, which exhibit a conical spike (Parker and Martz, Reference Parker and Martz2010), or paratypothoracins, which exhibit a dorsoventrally flattened horn (Parker, Reference Parker2007).

The suite of characters described above supports the referral of PEFO 46222 and PEFO 49321 to the more inclusive clade Calyptosuchini n. clade, which includes Calyptosuchus wellesi, Scutarx deltatylus, and Adamanasuchus eisenhardtae (Parker, Reference Parker2016a). Those three taxa are further differentiated from each other based on a combination of characters and autapomorphies of the trunk paramedian osteoderms. Scutarx deltatylus exhibits a strong dorsal protuberance on the posteromedial dorsal surface, which is an autapomorphic condition of the taxon (Parker, Reference Parker2016b). In Adamanasuchus eisenhardtae (Lucas et al., Reference Lucas, Hunt, Spielmann, Lucas and Spielmann2007a) the dorsal surface exhibits an unornamented triangular area on the dorsal surface of the posteromedial corner, which is an autapomorphy of the taxon (Parker, Reference Parker2016a, Reference Parkerb), as well as a strongly incised ‘cut-off’ posterolateral corner for its contact with the adjacent lateral osteoderm shared with paratypothoracins (e.g., PEFO 3004). In Calyptosuchus wellesi (Parker, Reference Parker2018a) the posteromedial corner is flat and exhibits radial ornamentation instead of a protuberance or unornamented area and lacks a strongly incised (‘cut-off’ corner) posterolateral margin. Thus, the osteoderm anatomy of PEFO 46222 and PEFO 49321 aligns with that of Calyptosuchus wellesi (Long and Ballew, Reference Long, Ballew, Colbert and Johnson1985) following the revision by Parker (Reference Parker2018a).