Phylogenetic analysis

The analysis with the OTU of AMNH FARB 9503 based on Colbert and Cosgriff (Reference Colbert and Cosgriff1974) recovered nine MPTs with a length of 1,081 steps (CI = 0.340; RI = 0.685; HI = 0.664; Fig. 7.1). The strict consensus topology is poorly resolved with respect to Stereospondylomorpha, with many major clades, including Stereospondyli, either not recovered at all or not recovered with all nominal members. AMNH FARB 9503 clusters with lapillopsids in this analysis; this clade is one branch of a large stereospondylomorph polytomy. Examination of individual MPTs revealed that there are three different positions of this clade: five MPTs (1–2, 5–7) recover this clade at the base of Stereospondylomorpha; three MPTs recover this clade at the base of Stereospondyli (3–4, 8); and one MPT (9) recovers this clade as highly nested within Stereospondyli as the sister group to Chinlestegophis jenkinsi + Rileymillerus cosgriffi, which is the collective sister group of Brachyopoidea. Note that these three positions do not correspond precisely to the three different islands identified in the search (MPTs 1–6, 7–8, and 9). Bremer and bootstrap support are low for relevant nodes (e.g., a Stereospondylomorpha that includes AMNH FARB 9503; clustering of this specimen with lapillopsids).

Figure 7.

Strict consensus topologies from the phylogenetic analysis of AMNH FARB 9503. (1) Resultant topology from the analysis using the OTU derived from Colbert and Cosgriff's (Reference Colbert and Cosgriff1974) interpretations; (2) resultant topology from the analysis using the OTU derived from this study's reevaluation of the specimen. Non-stereospondyl clades are visually consolidated; due to space constraints, these are not depicted in part 2, but the relationships are unchanged. Bold numbers above lines are Bremer decay indices; non-bold numbers below lines are bootstrap values. Support values are grayed out when they do not meet standard thresholds of strong support (Bremer > 2; bootstrap ≥ 50%).

The analysis with the revised OTU of AMNH FARB 9503 based on this study's findings recovered three MPTs with a length of 1,074 steps (CI = 0.340, RI = 0.687; HI = 0.663; Fig. 7.2). The strict consensus topology is much more resolved and is largely congruent with that recovered by previous versions of the matrix. Virtually all major clades were recovered, although Capitosauria forms a grade (as with Pardo et al., Reference Pardo, Small and Huttenlocker2017), and the rhytidosteids do not form a clade (as in Schoch, Reference Schoch2013, and Pardo et al., Reference Pardo, Small and Huttenlocker2017). AMNH FARB 9503 forms a polytomy with the two lapillopsids, with the rhytidosteid Nanolania anatopretia as the sister taxon to this clade. In one MPT, AMNH FARB 9503 and Rotaurisaurus contundo are sister taxa. The lapillopsids + AMNH FARB 9503 are collectively the sister group of Chinlestegophis jenkinsi + Rileymillerus cosgriffi, and together are the sister group of Brachyopoidea. Bremer and bootstrap support are relatively unchanged compared to the previous analysis.

The analysis with FMNH PR 5020 recovered five MPTs with a length of 1,073 steps (CI = 0.340, RI = 0.687; HI = 0.663; Fig. 8). The strict consensus topology is nearly identical to that of the previous analysis, with Nanolania anatopretia clustering with lapillopsids, and this clade clusters with Chinlestegophis jenkinsi + Rileymillerus cosgriffi and brachyopoids. FMNH PR 5020 is recovered in a polytomy at the base of Stereospondyli, with branches for the rhinesuchids Broomistega putterilli and Uranocentrodon senekalensis (Van Hoepen, Reference Van Hoepen1911) and one branch for all other stereospondyls (Superstes of Eltink et al., Reference Eltink, Schoch and Langer2019). Examination of the individual MPTs reveals three positions for FMNH PR 5020: nesting with rhinesuchids to form a clade (1–2, 5); diverging after a monophyletic Rhinesuchidae (3); and in a polytomy with a monophyletic Rhinesuchidae and Superstes (4). Bremer and bootstrap support are low for most clades. Compared to the analyses with AMNH FARB 9503, support for Lapillopsidae has increased, while support for Lydekkerinidae has decreased, and Rhinesuchidae was not recovered as a clade (although it was recovered in 88% of bootstrap replicates).

Figure 8.

Strict consensus topology from the phylogenetic analysis of FMNH PR 5020. Non-stereospondyl clades are visually consolidated. Bold numbers above lines are Bremer decay indices; non-bold numbers below lines are bootstrap values. Support values are grayed out when they do not meet standard thresholds of strong support (Bremer > 2; bootstrap ≥ 50%).

We opted to first conduct the analyses without any ordered characters to mirror previous versions of this matrix, but if characters that can be reasonably inferred to occur along a morphocline are ordered, the results are not substantially different. The analyses of the revised OTU of AMNH FARB 9503 and the OTU of FMNH PR 5020 recovered the same number of MPTs and the same strict consensus topology. The analysis of the Colbert and Cosgriff (Reference Colbert and Cosgriff1974) OTU of AMNH FARB 9503 recovered a single MPT (compared to nine MPTs without ordering), but the position of this specimen (sister to lapillopsids) is unchanged, and the relationships of other taxa are as with the strict consensus from the analyses using the revised OTU for this specimen.

We emphasize caution in literal interpretation of the results of any of these analyses, in particular those that sample AMNH FARB 9503, which clustered with lapillopsids (Figs. 7, 8). For one, there remains the question of whether lapillopsids nest within Lydekkerinidae (Eltink et al., Reference Eltink, Schoch and Langer2019), a result that has not been recovered previously in temnospondyl-wide matrices (e.g., McHugh, Reference McHugh2012; Schoch, Reference Schoch2013, and derivates), including this study. If so, then clustering of AMNH FARB 9503 with lapillopsids would not necessarily indicate non-lydekkerinid affinities. Lapillopsids have never been recovered in the position found here—highly nested as part of a sister clade to Brachyopoidea (Figs. 7, 8)—but neither have they been previously sampled with so many small-bodied taxa, and Lapillopsis nana is usually the only representative. The only reasonable conclusion is that the longstanding uncertainty in the position of lapillopsids continues to persist (e.g., Warren and Hutchinson, Reference Warren and Hutchinson1990a; Yates, Reference Yates1999; Yates and Warren, Reference Yates and Warren2000; Ruta et al., Reference Ruta, Pisani, Lloyd and Benton2007; McHugh, Reference McHugh2012; Schoch, Reference Schoch2013; Pardo et al., Reference Pardo, Small and Huttenlocker2017). AMNH FARB 9503 does not actually share any apomorphies with lapillopsids (sensu Yates and Sengupta, Reference Yates and Sengupta2002). Most of the skeletal regions where lapillopsid synapomorphies occur are not even preserved in AMNH FARB 9503 (e.g., palate, mandible). We interpret the recovered position of this specimen and that of lapillopsids as support for our hypothesis that small body size (whether or not due to immaturity) is a homoplastic feature that can lead to clustering of unrelated taxa that are united by ontogenetically variable qualitative features.

Additional support for this hypothesis is found in the position of the rhytidosteid Nanolania anatopretia, which does not cluster with the other sampled rhytidosteids (Laidleria gracilis, Sangaia lavina [Dias-da-Silva, Marsicano, and Schultz, Reference Dias-da-Silva, Marsicano and Schultz2006]) but rather with lapillopsids (Fig. 7) or with Chinlestegophis jenkinsi + Rileymillerus cosgriffi (Fig. 8). Nanolania anatopretia is the smallest rhytidosteid, and therefore may be attracted to other similarly sized taxa, given both the size and morphological disparity from other rhytidosteids (e.g., Dias-da-Silva and Marsicano, Reference Dias-da-Silva and Marsicano2011). The historical recovery of lapillopsids as the sister group to Dissorophoidea in some previous analyses with fewer small-bodied taxa (Schoch, Reference Schoch2013; Pardo et al., Reference Pardo, Small and Huttenlocker2017) also confers support to our hypothesis. Lapillopsis nana was originally considered to be a micropholid amphibamiform (Warren and Hutchinson, Reference Warren and Hutchinson1990a), but exhibits many stereospondyl features, and therefore morphologically converges on small-bodied dissorophoids only due to shared small size (e.g., Yates, Reference Yates1999). The shift in position in this analysis may result from the addition of Rotaurisaurus contundo, N. anatopretia, and the specimens of interest in this study, which are all small in size.

Similar caution should be exercised in interpreting the results of the analysis with FMNH PR 5020. Falling within a polytomy with rhinesuchids and Superstes does not necessarily indicate that it is a rhinesuchid. Its early diverging position may merely reflect the relatively low number of characters for which it can be scored and the generic suite of plesiomorphies that can be identified. In three of the MPTs, FMNH PR 5020 clusters with rhinesuchids, which are the earliest diverging stereospondyls. In one of the other MPTs, it is the earliest diverging stereospondyl, and in the last MPT, it diverges after rhinesuchids but before lydekkerinids. Given our reservations about the phylogenetic results for both specimens, the following sections provide a detailed qualitative anatomical comparison to further evaluate their affinities.

Anatomical interpretations of ‘Cryobatrachus kitchingi’ based on AMNH FARB 9503

Re-examination of AMNH FARB 9503 leads to the conclusion that most of the sutures depicted by Colbert and Cosgriff (Reference Colbert and Cosgriff1974) cannot be validated unless the permanent marker is removed. It is possible that doing so might reveal that most of the sutures still cannot be validated. Most of the sutures may have been tenuous all along based on the prevalence of dashed lines, and we could not identify any traces of sutures in the spaces between these lines. It is certainly possible that most of the original illustrated sutures (Fig. 2) could have been inferred based on either a stereotypical lydekkerinid or stereospondyl configuration (the “typical” configuration). Not all sutures, especially in the postorbital region of the skull, have substantial variation among stereospondyls. For this reason, the sutural interpretations are essentially wholly uninformative for assessing its taxonomy. Two features warrant particular discussion because they have potentially stronger phylogenetic implications.

The first feature is the purported dorsal exposure of the septomaxilla bordering the naris posteriorly, thereby separating the lacrimal from the naris (or separating the prefrontal from the naris in taxa without a lacrimal). Among clades with Mesozoic representatives, this condition is found in some brachyopoids (inclusive of plagiosaurids in this context; Schoch, Reference Schoch2013; Schoch and Milner, Reference Schoch and Milner2014), dvinosaurs, lydekkerinids, rhinesuchids, and trematosaurs (e.g., Shishkin and Welman, Reference Shishkin and Welman1994; Shishkin et al., Reference Shishkin, Rubidge and Kitching1996; Sequeira, Reference Sequeira1998, Warren and Marsicano, Reference Warren and Marsicano1998; Shishkin and Rubidge, Reference Shishkin and Rubidge2000; Damiani and Jeannot, Reference Damiani and Jeannot2002; Damiani and Yates, Reference Damiani and Yates2003; Dias-da-Silva et al., Reference Dias-da-Silva, Marsicano and Schultz2006), and it was one of the more compelling lines of evidence for lydekkerinid affinities of AMNH FARB 9503 (Shishkin et al., Reference Shishkin, Rubidge and Kitching1996). Septomaxillae are rarely preserved in other clades, which suggests that it was a loosely articulated intranarial ossification that was susceptible to taphonomic loss; this can be seen in capitosaurs (e.g., Howie, Reference Howie, Joysey and Kemp1972; Muhkerjee and Sengupta, Reference Mukherjee and Sengupta1998), metoposaurids (e.g., Chakravorti and Sengupta, Reference Chakravorti and Sengupta2018), and amphibamiforms (e.g., Schoch and Rubidge, Reference Schoch and Rubidge2005). Both taphonomic and intraspecific variation in the preservation and configuration of the septomaxilla has been documented in Lydekkerina huxleyi by Jeannot et al. (Reference Jeannot, Damiani and Rubidge2006, fig. 5) and Hewison (Reference Hewison2007, fig. 6). The entire septomaxilla in AMNH FARB 9503 is marked with dashed lines, and we could not identify a suture there, either externally or in the CT data. Posterior to the left naris is a short suture that Colbert and Cosgriff (Reference Colbert and Cosgriff1974) did not identify; this either could be one margin of the septomaxilla or a short maxilla-nasal contact (either would exclude the lacrimal from the naris). The lacrimal can only be identified tentatively by position because it is not fully defined. Because the septomaxilla is more frequently lost or not dorsally exposed (as in the lydekkerinids Chomatobatrachus Cosgriff, Reference Cosgriff1974, Deltacephalus Swinton, Reference Swinton1956, and Luzocephalus Säve-Söderbergh, Reference Säve-Söderbergh1935), the condition in AMNH FARB 9503 should be regarded as indeterminate.

The second feature is the purported anteriorly extensive postfrontal. If this interpretation is correct, it would single-handedly re-validate the taxon because it would be autapomorphic among Temnospondyli. Prefrontal-postfrontal contact is plesiomorphic for temnospondyls, and it is found in the vast majority of stereospondyls. Among stereospondyls, capitosaurs are the one exception in which the frontal enters the orbit in a large number of taxa (e.g., Schoch, Reference Schoch2000, Reference Schoch2008b; Schoch and Milner, Reference Schoch and Milner2000; Damiani, Reference Damiani2001). This condition is also found in the two lapillopsids (of three total species) for which cranial material is known (Yates, Reference Yates1999) and in some of the higher-nested non-stereospondyl stereospondylomorphs (e.g., Gubin, Reference Gubin1991; Golubev, Reference Golubev1995). As far as is known, this condition does not change throughout ontogeny across Temnospondyli (e.g., Bystrow and Efremov, Reference Bystrow and Efremov1940; Welles and Cosgriff, Reference Welles and Cosgriff1965; Warren and Hutchinson, Reference Warren and Hutchinson1988; Yates, Reference Yates1999; Shishkin and Rubidge, Reference Shishkin and Rubidge2000; Steyer, Reference Steyer2003; Schoch and Rubidge, Reference Schoch and Rubidge2005; Witzmann, Reference Witzmann2005; Schoch and Witzmann, Reference Schoch and Witzmann2012), with perhaps the exception of the earliest stage of development in the zatracheid Acanthostomatops vorax (Credner, Reference Credner1883) (Witzmann and Schoch, Reference Witzmann and Schoch2005). Regardless of the arrangement of the circumorbital elements or cranial proportions, the postfrontal almost never extends beyond a point just anterior to the mid-length of the orbit, and it never forms part of the anterior orbital margin as interpreted for ‘C. kitchingi’ by Colbert and Cosgriff (Reference Cosgriff1974; Fig. 2). The only exception to the latter is the Late Triassic Chinlestegophis jenkinsi Pardo, Small, and Huttenlocker, Reference Pardo, Small and Huttenlocker2017, a putative brachyopoid (or close relative of brachyopoids) with a markedly different cranial anatomy; in this taxon, the apparent loss of a distinct lacrimal has resulted in an anterolateral shift in the position of the prefrontal and the postfrontal. A few branchiosaurids and brachyopids have relatively anteriorly extensive postfrontals that extend well past the orbital mid-length, but the postfrontal does not extend onto the anterior orbital margin (e.g., Schoch and Milner, Reference Schoch and Milner2014, figs. 26, 46, for comparisons). There are also some taxa in which the anterior terminus of the postfrontal closely approaches the level of the anterior orbital margin, but in these taxa (e.g., Sclerocephalus haeuseri Goldfuß, Reference Goldfuß1847, Onchiodon labyrinthicus Geinitz, Reference Geinitz1862; Boy, Reference Boy1988, Reference Boy1990) the postfrontal does not follow the contour of the orbit anteriorly such that the anterior terminus is well separated from it. Colbert and Cosgriff's (Reference Colbert and Cosgriff1974) interpretation is predicated on two points: (1) a short oblique suture extending from the anterior orbital margin as the posteromedial edge of the right prefrontal; and (2) an associated oblique suture set at a right angle as the medial edge of the right frontal (Fig. 2). The former appears to be marked by a partial incisure at the orbital margin, but there is clear complex interdigitation posterior to this point (Fig. 5). This position is more “typical” in being around the level of the anteriormost extent of the postfrontal when it meets the prefrontal or with the posteriormost extent of the prefrontal when it does not meet the postfrontal. The inferred frontal suture is dashed and cannot be validated. If indeed it were present, it could essentially be an extension of the posteromedial margin of the prefrontal under our interpretation.

Ontogenetic interpretation of AMNH FARB 9503

In the absence of a robust understanding of the cranial sutures, an interpretation of this specimen's ontogenetic status is provided to facilitate taxonomic interpretations. It is important to note the limitations of such an approach given a paucity of ontogenetic series for most taxa, widespread size variation within Temnospondyli, and heterochrony between and within clades (e.g., Steyer, Reference Steyer, Mazin, Vignaud and de Buffrenil1996, Reference Steyer2000; Boy and Sues, Reference Boy, Sues, Heatwole and Carroll2000; Schoch, Reference Schoch2009, Reference Schoch2010, Reference Schoch2014; Witzmann et al., Reference Witzmann, Scholz and Ruta2009; Pérez-Ben et al., Reference Pérez-Ben, Báez and Schoch2019). Consequently, very few features have been demonstrated to be applicable or useful to even the majority of temnospondyl families because most conserved patterns are gradational features, such as elongation of the snout, negative allometry of the orbits, and narrowing of the postparietals (e.g., Witzmann et al., Reference Witzmann, Scholz and Ruta2009; Pérez-Ben et al., Reference Pérez-Ben, Báez and Schoch2019). These conundrums lie at the root of the uncertainty regarding the taxonomic status of AMNH FARB 9503, which encapsulates the historic difficulties associated with identifying small-bodied individuals, whether as juveniles of large-bodied taxa or as adults of small-bodied taxa.

A few features hint at the relative immaturity of AMNH FARB 9503. The first is the size of the orbits, which scale with negative allometry in most temnospondyls (e.g., Bystrow and Efremov, Reference Bystrow and Efremov1940; Welles and Cosgriff, Reference Welles and Cosgriff1965; Boy, Reference Boy1988, Reference Boy1989, Reference Boy1990, Reference Boy1993; Warren and Schroeder, Reference Warren and Schroeder1995; Boy and Sues, Reference Boy, Sues, Heatwole and Carroll2000; Steyer, Reference Steyer2003; Witzmann and Pfretzschner, Reference Witzmann and Pfretzschner2003; Schoch and Rubidge, Reference Schoch and Rubidge2005; Witzmann, Reference Witzmann2005; Witzmann and Schoch, Reference Witzmann and Schoch2005; Witzmann and Scholtz, Reference Witzmann and Scholz2007; Witzmann et al., Reference Witzmann, Scholz and Ruta2009; Pérez-Ben et al., Reference Pérez-Ben, Báez and Schoch2019; Schoch, Reference Schoch2019), as in vertebrates generally. AMNH FARB 9503 has relatively large orbits, a feature that also characterizes miniaturized taxa, such as amphibamiform dissorophoids and lapillopsids (Table 1). However, those taxa tend to have substantially larger orbits than AMNH FARB 9503 or similarly sized juveniles of large-bodied taxa (e.g., Witzmann et al., Reference Witzmann, Scholz and Ruta2009). The relative orbital size observed in this specimen is more comparable to that seen in the earlier stages of development for the majority of non-miniaturized temnospondyls (e.g., Warren and Schroeder, Reference Warren and Schroeder1995; Steyer, Reference Steyer2003). Orbit position also tends to shift posteriorly over ontogeny in stereospondyls such that the orbits are prominently in the posterior half of the skull (e.g., capitosaurs, rhinesuchids, trematosaurs), although this is not the case for short-snouted taxa such as brachyopoids or some other clades like lydekkerinids.

Table 1.

Comparative measurements for small-bodied Early Triassic temnospondyls. References refer to specific figures rather than specimens because all measurements were taken from published figures. Abbreviations: Io, minimum interorbital width; OL, maximum orbit length, measured along the longest axis; Po, postorbital length; Pr, preorbital length; SL, skull length measured along the midline; SW, maximum skull width. Asterisk indicates that the figure is a reconstruction.

The position of the pineal foramen is another frequently mentioned ontogenetic feature (e.g., Bystrow and Efremov, Reference Bystrow and Efremov1940; Welles and Cosgriff, Reference Welles and Cosgriff1965; Warren and Hutchinson, Reference Warren and Hutchinson1988; Hunt, Reference Hunt1993; Schoch, Reference Schoch and McNamara1995; Boy and Sues, Reference Boy, Sues, Heatwole and Carroll2000). The opening is thought to shift posteriorly throughout ontogeny, although this requires at least two data points to determine a taxon-specific reference landmark. In capitosaurs, the level of the posterior orbital margin is sometimes used (e.g., Welles and Cosgriff, Reference Welles and Cosgriff1965), but this landmark is less reliable for most other clades in which the foramen is always well posterior to the orbit. However, the paucity of taxa in which the pineal foramen is in line with, or even anterior to, the posterior orbital margin suggests that when either of these conditions occur, it is fairly informative. The foramen of AMNH FARB 9503 is in line with the posterior orbital margin, the “juvenile” condition in capitosaurs. This condition is also found in juveniles of the early diverging trematosaur Benthosuchus sushkini (Efremov, Reference Efremov1929) (Bystrow and Efremov, Reference Bystrow and Efremov1940), but not in adults or in specimens of other trematosaurs (e.g., Säve-Söderbergh, Reference Säve-Söderbergh1935; Dutuit, Reference Dutuit1976; Getmanov, Reference Getmanov1989; Steyer, Reference Steyer2003; Schoch, Reference Schoch2008a, Reference Schoch2019) or in lydekkerinids of any size (e.g., Cosgriff, Reference Cosgriff1974; Shishkin, Reference Shishkin1980; Hewison, Reference Hewison1996, Reference Hewison2007; Shishkin et al., Reference Shishkin, Rubidge and Kitching1996; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006). A similar ontogenetic shift from the level of the posterior orbital margin is noted in the Permian stereospondylomorph Sclerocephalus haeuseri (Boy, Reference Boy1988; Schoch and Witzmann, Reference Schoch and Witzmann2009a) and the Permian zatracheid Acanthostomatops vorax (Witzmann and Schoch, Reference Witzmann and Schoch2005), and it might be a hallmark of early ontogeny in other taxa. Only three temnospondyl clades commonly have this condition in presumed “adults”: amphibamiforms, lapillopsids, and plagiosaurids (e.g., Yates, Reference Yates1999; Schoch and Milner, Reference Schoch and Milner2000, Reference Schoch and Milner2014; Warren and Marsicano, Reference Warren and Marsicano2000). The first two are miniaturized clades, and the third is a brachycephalic clade.

A final feature is the preservation of the specimen. The skull roof is sheared off at the level of the tooth row, essentially symmetrically on each side, and without any remnants of palatal, occipital, or neurocranial elements. The lateral margins of the external nares also have been lost, indicating complete detachment of the dentulous portion of the maxilla from the skull roof in this region. Larger temnospondyl skulls can be variably fragmented, but they are rarely observed in this specific fashion where the roof is nearly complete, but without a trace of the palate or neurocranium. This hints at breakage along a plane of weakness, perhaps due to a lack of marked ossification and articulation between different skeletal regions.

Previous authors have not commented on specific features that might indicate relative maturity for AMNH FARB 9503, and no feature observed here indicates as such. If AMNH FARB 9503 was a lydekkerinid, it would be a “sub-adult” at most based on the size range of the most abundantly known taxon, Lydekkerina huxleyi. As mentioned in the description, it is not possible to characterize the evenness of the ornamentation across the skull, and regions where growth zones (indicating either immaturity or marked paedomorphosis) tend to occur (e.g., cheek region) do not have good surface resolution. Features related to the palate, neurocranium, or postcrania cannot be assessed. Other cranial features are of limited utility because they require taxonomic context (e.g., depth of otic notch, development of tabular horn); the “adult” condition of one taxon may only be the “juvenile” condition of another, especially among temnospondyls in which paedomorphosis is common.

Taxonomic affinities of ‘Cryobatrachus kitchingi’ based on AMNH FARB 9503

Previous discussion of the taxonomy of ‘Cryobatrachus kitchingi’ (mostly viz. the holotype) has centered on whether it is a relatively mature lydekkerinid (as argued by Colbert and Cosgriff, Reference Colbert and Cosgriff1974) or a juvenile of a much larger stereospondyl, either a capitosaur or a benthosuchid trematosaur (as suggested by Schoch and Milner, Reference Schoch and Milner2000, who placed it as Stereospondyli incertae sedis). Here we discuss all temnospondyl clades known from the Early Triassic to be as thorough as possible in our assessment. The primary motivation is to confidently exclude (or to state when we cannot) all clades that are presently recognized from the Early Triassic (mostly stereospondyl clades) to avoid preconceived biases and to explicitly state our lines of reasoning to make them more accessible to broader audiences. One important consideration is that size alone should not be used to exclude some clades in the identification of small-bodied individuals. Small, juvenile individuals of many large-bodied taxa are unknown, and the understanding of ontogenetic transformations in most clades is limited to a handful of well-known taxa (e.g., Bystrow and Efremov, Reference Bystrow and Efremov1940; Welles and Cosgriff, Reference Welles and Cosgriff1965; Steyer, Reference Steyer2003). Figure 9 includes 11 similarly sized specimens representing most of the stereospondyl clades for comparison. The following comparisons focus on qualitative aspects of the skull to avoid an overreliance on the uncertain sutures of AMNH FARB 9503.

Figure 9.

Comparison of cranial reconstructions of various small-bodied temnospondyls from the Lower Triassic. (1) Immature stage of the lydekkerinid Lydekkerina huxleyi (after Schoch and Milner, Reference Schoch and Milner2000, fig. 57); (2) mature stage of L. huxleyi (after Schoch and Milner, Reference Schoch and Milner2000, fig. 57); (3) the lydekkerinid Chomatobatrachus halei (after Schoch and Milner, Reference Schoch and Milner2000, fig. 58); (4) slender-headed morph of the amphibamiform Micropholis stowi (after Schoch and Rubidge, Reference Schoch and Rubidge2005, fig. 2A); (5) relatively mature stage of the lapillopsid Lapillopsis nana (after Yates, Reference Yates1999, fig. 5A); (6) the lapillopsid Rotaurisaurus contundo (after Yates, Reference Yates1999, fig. 15A); (7) relatively immature stage of the rhinesuchid Broomistega putterilli (after Shiskin and Rubidge, Reference Shishkin and Rubidge2000, fig. 3A); (8) the rhytidosteid Nanolania anatopretia (after Yates, Reference Yates2000, fig. 4A); (9) immature stage of the trematosaur Benthosuchus sushkini (after Bystrow and Efremov, Reference Bystrow and Efremov1940, fig. 56); (10) immature stage of the capitosaur Watsonisuchus aliciae Warren and Hutchinson, Reference Warren and Hutchinson1988 (after Warren and Hutchinson, Reference Warren and Hutchinson1988, fig. 4A); (11) immature stage of the capitosaur ‘Parotosaurus wadei’ (after Warren and Hutchinson, Reference Warren and Hutchinson1988, fig. 11C); (12) new reconstruction of AMNH FARB 9503. Scale bars = 1 cm. Fine dashed lines represent inferred sutures; coarse dashed lines represent topographic features.

All previous workers have assumed that AMNH FARB 9503 is a stereospondyl, but the uninformative nature of the specimen questions this assumption. Most synapomorphies of Stereospondyli pertain to the palate (e.g., Yates and Warren, Reference Yates and Warren2000; McHugh, Reference McHugh2012; Schoch, Reference Schoch2013; Eltink et al., Reference Eltink, Schoch and Langer2019), which is entirely absent. The recent stereospondylomorph analysis by Eltink et al. (Reference Eltink, Schoch and Langer2019) recovered a few synapomorphies of Stereospondyli that can be identified in AMNH FARB 9503. These include a parabolic preorbital region; a naris at the same level of the dorsal rostral plane; snout margins continually converging towards the tip; and a slit-like otic notch. Only one synapomorphy (intranarial width greater than interorbital width) is in conflict with AMNH FARB 9503. Because the apomorphies of Stereospondyli are continually being refined, perhaps the strongest argument in support of stereospondyl affinities is the evidence against close relatedness to either of the rare non-stereospondyl clades found in the Triassic: Amphibamiformes and Dvinosauria. These are discussed further below.

Brachyopoids and dvinosaurs are readily excluded on the basis of the relatively long and slender profile of AMNH FARB 9503 (e.g., Warren, Reference Warren1998a; Warren and Marsicano, Reference Warren and Marsicano2000; Schoch and Milner, Reference Schoch and Milner2014) and are not shown in Figure 9. All brachyopoids have short and wide (brachycephalic) skulls with parabolic outlines. The handful of dvinosaurs with similarly slender skulls (e.g., Erpetosaurus Moodie, Reference Moodie1909, Neldasaurus Chase, Reference Chase1965; Milner and Sequeira, Reference Milner and Sequeira2011; Schoch, Reference Schoch2018) are restricted to the late Carboniferous and early Permian, and this clade is rare in the Mesozoic, especially in southern Pangea where the only occurrence is Thabanchuia oomie from South Africa. While other dvinosaurs lack the markedly brachycephalic, parabolic skulls of brachyopoids, their wide snouts and anteriorly positioned orbits are in contrast to other highly nested stereospondyls and to AMNH FARB 9503. Instead, they are more similar to the metoposaurid trematosaurs, with which they were often historically allied (e.g., Säve-Söderbergh, Reference Säve-Söderbergh1935; Shishkin, Reference Shishkin1973; Hunt, Reference Hunt1993). Most brachyopoids and dvinosaurs also lack a well-developed otic notch (but see Neldasaurus as a counterpoint). Relatively small specimens, such as the holotypes of T. oomie and ‘Platycepsion wilkinsoni’ Stephens, Reference Stephens1887, are similar in morphology to larger specimens (either conspecifics or different taxa; Cosgriff, Reference Cosgriff1969, Reference Cosgriff1973; Warren, Reference Warren1998a; Warren and Marsicano, Reference Warren and Marsicano1998). This includes the Late Triassic form Chinlestegophis jenkinsi.

Amphibamiforms are a relict clade that, unlike dvinosaurs, is documented from the lower Fremouw Formation in the form of Micropholis stowi (Gee and Sidor, Reference Gee and Sidor2021; Fig. 9.4), a taxon primarily known from two morphs from South Africa (Boy, Reference Boy1985; Schoch and Rubidge, Reference Schoch and Rubidge2005). The broad-headed morph, the only form known from Antarctica, has a proportionately shorter and wider skull than AMNH FARB 9503, and the tabular horns are not nearly as developed. All Antarctic specimens also have a biconcave occipital margin, although this occurs less frequently in the South African material. The slender-headed morph (Fig. 9.4) has a narrower posterior skull table than either the other morph or AMNH FARB 9503. Both morphs have a relatively wide interorbital region with relatively large orbits in the anterior half of the skull. The ornamentation of AMNH FARB 9503 can be sufficiently resolved to determine that it is not pustular, a distinctive feature of M. stowi. The otic notch is well developed in M. stowi, with a large unornamented, vertically oriented supratympanic flange that may have held a tympanum; this is not preserved in AMNH FARB 9503, which seems to have been genuinely flat in occipital profile. There is no evidence for a lateral exposure of the palatine (LEP) found in many dissorophoids like M. stowi; this exposure is often marked by a distinct depression (e.g., Bolt, Reference Bolt1974).

Lapillopsids are an enigmatic clade of diminutive stereospondyls of similar size to amphibamiforms (Fig. 9.5, 9.6). They occur in both Australia and India, and they were only documented subsequent to the description of ‘Cryobatrachus kitchingi’ (Warren and Hutchinson, Reference Warren and Hutchinson1990a; Yates, Reference Yates1999; Yates and Sengupta, Reference Yates and Sengupta2002). Essentially all of the diagnostic features of the clade (sensu Yates, Reference Yates1999) are largely based on skeletal regions that were not preserved in this specimen, but there are numerous differences from the two taxa for which cranial material is known. For example, in Lapillopsis nana, the occipital margin is biconcave, the quadratojugal terminates anterior to the posterior termination of the tabular horn, and there is no dorsal swelling posterior to the naris (Fig. 9.5). In Rotaurisaurus contundo Yates, Reference Yates1999, the tabular horns curve sharply laterally, and the occipital margin is nearly horizontal (Fig. 9.6). AMNH FARB 9503 is larger than any known lapillopsid. Although there is doubt over whether the largest specimens of L. nana were adults, the dorsal protuberances on the skull roof and the ontogenetic shift from a sharply concave to relatively flat occipital margin in L. nana indicate that AMNH FARB 9503 is not simply a large specimen of L. nana. The proportions of R. contundo are very similar to those of AMNH FARB 9503 (Table 1) despite their similar size and underscores the challenges of differentiating small-bodied adults from juveniles on the basis of proportions alone.

Rhytidosteids are a diverse clade that appeared in the late Permian (Marsicano and Warren, Reference Marsicano and Warren1998), but which are mostly known from the Early Triassic (Dias-da-Silva and Marsicano, Reference Dias-da-Silva and Marsicano2011). Rhytidosteids would be predicted in Antarctica based on abundant occurrences in southern Pangea, including the Lystrosaurus declivis Assemblage Zone of South Africa (Cosgriff and Zawiskie, Reference Cosgriff and Zawiskie1979; Dias-da-Silva and Marsicano, Reference Dias-da-Silva and Marsicano2011; Botha and Smith, Reference Botha and Smith2020), the Sakamena Formation of Madagascar (Lehman, Reference Lehman1966), several formations from Australia (Cosgriff, Reference Cosgriff1965, Reference Cosgriff1974; Howie, Reference Howie, Joysey and Kemp1972; Warren and Black, Reference Warren and Black1985; Warren and Hutchinson, Reference Warren and Hutchinson1987; Marsicano and Warren, Reference Marsicano and Warren1998), and the Sanga do Cabral Formation of Brazil (Dias-da-Silva et al., Reference Dias-da-Silva, Marsicano and Schultz2005, Reference Dias-da-Silva, Marsicano and Schultz2006). Indeed, Cosgriff and Hammer (Reference Cosgriff and Hammer1984) referred three ornamented dermal fragments from the lower Fremouw Formation to Rhytidosteidae, but more substantial remains are unknown. Rhytidosteids have a wide range of skull shapes (Dias-da-Silva and Marsicano, Reference Dias-da-Silva and Marsicano2011, fig. 1), and all taxa have a preorbital region subequal in length to the postorbital region. However, most taxa have triangular skulls with prominent expansion of the temporal region and distinctive posterior projection of the quadratojugal (e.g., Deltasaurus kimberlyensis Cosgriff, Reference Cosgriff1965). Some are represented by small specimens, in particular Nanolania anatopretia Yates, Reference Yates2000, from the Arcadia Formation of Australia (~2 cm skull length; Yates, Reference Yates2000; Fig. 9.8), which has the skull profile most similar to that of AMNH FARB 9503. Dias-da-Silva and Marsicano (Reference Dias-da-Silva and Marsicano2011) identified three ambiguous synapomorphies and one unambiguous synapomorphy of Rhytidosteidae in their review. Of these, one is present in AMNH FARB 9503 (orbits located about halfway along the skull length), two are confidently absent (interorbital distance >50% of the skull width; ‘spider-web’ pattern of ornamentation), and one cannot be assessed (pterygoid quadrate ramus orientation). Well-developed otic notches like those of AMNH FARB 9503 are also absent in most rhytidosteids.

Rhinesuchids have only one Mesozoic representative, Broomistega putterilli, from the LAZ of South Africa (Shishkin and Rubidge, Reference Shishkin and Rubidge2000; Fig. 9.7), but the group is well documented in the Permian of Brazil (Barbarena, Reference Barberena1998; Barbarena and Dias, Reference Barberena and Dias1998; Dias and Richter, Reference Dias and Richter2002; Dias and Schultz, Reference Dias and Schultz2003; Ramos and Vega, Reference Ramos and Vega2011; Eltink and Langer, Reference Eltink and Langer2014; Cisneros et al., Reference Cisneros, Marsicano, Angielczyk, Smith, Richter, Fröbisch, Kammerer and Sadleir2015; Eltink et al., Reference Eltink, Dias, Dias-da-Silva, Schultz and Langer2016; Azevedo et al., Reference Azevedo, Vega and Soares2017; Dias et al., Reference Dias, Dias-da-Silva and Schultz2020) and South Africa (Watson, Reference Watson1962; Damiani and Rubidge, Reference Damiani and Rubidge2003; Damiani, Reference Damiani2004; Pawley and Warren, Reference Pawley and Warren2004; Marsicano et al., Reference Marsicano, Latimer, Rubidge and Smith2017). Rhinesuchids have an overall skull profile comparable to that of AMNH FARB 9503 in being long, modestly slender, and with gradual and slight expansion in the temporal region (e.g., Schoch and Milner, Reference Schoch and Milner2000, fig. 55; Marsicano et al., Reference Marsicano, Latimer, Rubidge and Smith2017). The features used by Colbert and Cosgriff (Reference Cosgriff1974, p. 22) to differentiate ‘Cryobatrachus kitchingi’ from rhinesuchids were based on comparisons with large individuals (e.g., markedly longer preorbital region, quadrate well posterior to level of tabular). These features are, in fact, shared with juvenile rhinesuchids (e.g., Schoch and Milner, Reference Schoch and Milner2000, fig. 55D, E), including individuals of B. putterilli (Shishkin and Rubidge, Reference Shishkin and Rubidge2000). Rhinesuchids have been previously noted to be difficult to differentiate from lydekkerinids when comparing disparately sized individuals; B. putterilli originally was described as a species of Lydekkerina. Shishkin and Rubidge (Reference Shishkin and Rubidge2000, p. 665) proposed that paedomorphosis produced a higher degree of similarity of B. putterilli to juvenile capitosaurs compared to adult rhinesuchids, and the juveniles of B. putterilli therefore share many features with adult lydekkerinids (Shishkin and Rubidge, Reference Shishkin and Rubidge2000, p. 654). It is not possible to assess any of the formal diagnostic features of Rhinesuchidae listed by Marsicano et al. (Reference Marsicano, Latimer, Rubidge and Smith2017).

The main features of AMNH FARB 9503 that discount rhinesuchid affinities are the narrow interorbital region, the position of the orbits (they are more posteriorly situated in rhinesuchids, even in similarly sized juveniles), the well-developed tabular horn, which is offset from the main body of the tabular, and the apparent absence of any lateral line grooves on the snout (although such grooves are absent in the smallest specimens of B. putterilli). The nares appear to be entirely dorsally facing, rather than anterolaterally (a rhinesuchid synapomorphy, per Eltink et al., Reference Eltink, Schoch and Langer2019). The prenarial portion of AMNH FARB 9503 is also relatively long compared to juvenile rhinesuchids (e.g., Shishkin and Rubidge, Reference Shishkin and Rubidge2000), although Eltink et al. (Reference Eltink, Schoch and Langer2019) listed a prenarial region as long or longer than the naris as a rhinesuchid synapomorphy.

Trematosaurs are one of the most widely distributed clades, although they have never been reported from Antarctica, which is surprising in light of their occurrences in adjacent regions of southern Pangea (South Africa, Australia, Madagascar, India). This clade is well known for the longirostrine morphology of later diverging forms, which is absent in AMNH FARB 9503, although this conceivably becomes pronounced throughout ontogeny. Many trematosaurs also have relatively small orbits that are widely spaced compared to AMNH FARB 9503. The earliest stages of trematosaur ontogeny are poorly known outside of Benthosuchus sushkini (Bystrow and Efremov, Reference Bystrow and Efremov1940; Fig. 9.9). Its early diverging position accords with its similarity to early diverging capitosaurs, especially with respect to the relatively short snout, and the taxon was sometimes classified as a capitosaur (e.g., Yates and Warren, Reference Yates and Warren2000; Damiani, Reference Damiani2001). The point here is only that it is not necessarily a good representative of trematosaur ontogeny with respect to cranial allometry and, in fact, seems to share more aspects with capitosaurs (e.g., Welles and Cosgriff, Reference Welles and Cosgriff1965). While Schoch and Milner (Reference Schoch and Milner2000) suggested benthosuchid affinities as one possibility for ‘Cryobatrachus kitchingi,’ benthosuchids are only known from Russia. The general skull profile of AMNH FARB 9503 is proportionately wider than that observed in similarly sized individuals of B. sushkini (Bystrow and Efremov, Reference Bystrow and Efremov1940; Fig. 9.9). Other relatively small trematosaur specimens, such as the holotype of Prothoosuchus blomi Getmanov, Reference Getmanov1989, have skulls with concave lateral margins and more laterally situated orbits (Schoch and Milner, Reference Schoch and Milner2000). Small-bodied Late Triassic taxa (e.g., Rileymillerus cosgriffi Bolt and Chatterjee, Reference Bolt and Chatterjee2000, and Almasaurus habbazi), which have sometimes been placed with trematosaurs (e.g., Schoch and Milner, Reference Schoch and Milner2000; Schoch, Reference Schoch2008a; but see Pardo et al., Reference Pardo, Small and Huttenlocker2017), have slender, tall skulls with laterally facing orbits. Trematosaurs from other southern Pangean regions are represented by either much larger material (e.g., Warren, Reference Warren2012) or very fragmentary material (e.g., Damiani et al., Reference Damiani, Neveling, Hancox and Rubidge2000; Damiani and Welman, Reference Damiani and Welman2001), and all of them are markedly morphologically distinct from AMNH FARB 9503 with respect to the cranial proportions.

With the previous clades excluded, only capitosaurs and lydekkerinids (exclusive of lapillopsids in this specific context) remain as candidate groups (Fig. 9.1, 9.2, 9.10, 9.11). Colbert and Cosgriff (Reference Cosgriff1974, p. 22) listed the short preorbital region as the “prime feature” that identifies AMNH FARB 9503 as a lydekkerinid. They derived this from a dated diagnosis of Lydekkerinidae (Watson, Reference Watson1919; Romer, Reference Romer1947), which listed the orbits being at the mid-length of the skull. This characterization is generally true of lydekkerinids (Fig. 9.1, 9.2), although it cannot account for possible ontogenetic changes, either known or presently unknown, and the orbits of the relatively large Eolydekkerina magna Shishkin, Rubidge, and Kitching, Reference Shishkin, Rubidge and Kitching1996, are slightly in the posterior half. However, the orbits of AMNH FARB 9503 are clearly not at the mid-length of the skull (Fig. 5; Table 1); in this, the specimen is similar to juveniles of large-bodied taxa and not to small-bodied forms like lydekkerinids.

Colbert and Cosgriff (Reference Cosgriff1974, p. 21–22) also listed other partially differential features, such as the position of the jaw articulation relative to the neck articulation and the position of the posterior termini of the quadratojugal and the tabular. For one, neither the jaw articulation (viz. the lower jaw or the quadrate) nor the neck articulation (viz. the occiput or an articulated atlas) are actually preserved in AMNH FARB 9503. Both of these features are also not diagnostic, and there is biological variation within Lydekkerinidae and taphonomic variation within Lydekkerina huxleyi (e.g., compare holotypes of ‘Limnoiketes paludinatans’ and Ly. huxleyi; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006). The skull shape is closer to that of some capitosaurs in having a relatively pointed (not blunted) snout and slightly convex lateral margins, but there is enough taphonomic variation within L. huxleyi and enough ontogenetic and interspecific variation within Capitosauria to limit the utility of this comparison. The narrow interorbital region is a more reliable feature that is found in capitosaurs (Fig. 9.10, 9.11), but not in lydekkerinids (Table 1). Also worth noting is the absence of an interorbital depression (‘hollow’) or raised orbital rims—features found in many lydekkerinids, including the South African taxa (Shishkin et al., Reference Shishkin, Rubidge and Kitching1996; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006; Hewison, Reference Hewison2007). A prominent tabular horn like that of AMNH FARB 9503 is only found in some lydekkerinids (Deltacephalus whitei Swinton, Reference Swinton1956, Eolydekkerina magna, L. huxleyi), but it is found in all capitosaurs. Lastly, the nostrils are relatively large, which is more like capitosaurs than lydekkerinids. These comparisons are only further exemplified when comparing AMNH FARB 9503 to the most similarly sized lydekkerinids (Fig. 9.1, 9.2). The presence of a postnarial septomaxilla cannot be confirmed, nor can the exclusion of the frontal from the orbit. While large capitosaurs have a fully-developed lateral line system, it seems to be absent in juveniles of some taxa (e.g., Warren and Hutchinson, Reference Warren and Hutchinson1988, but see Steyer, Reference Steyer2003), whereas even relatively small lydekkerinids have moderately developed canals on the snout (e.g., Hewison, Reference Hewison1996, Reference Hewison2007; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006). Possibly, the organ overlay the skull in AMNH FARB 9503, but was not yet impressed into the roofing elements (see discussion by Witzmann et al., Reference Witzmann, Scholz, Mueller and Kardjilov2010). No diagnostic feature of lydekkerinids (Jeannot et al., Reference Jeannot, Damiani and Rubidge2006; Eltink et al., Reference Eltink, Schoch and Langer2019) is preserved in AMNH FARB 9503, although many of these relate to skeletal regions that are absent in this specimen. The diagnosis of Capitosauria cannot be properly considered here since apomorphies could be absent in juveniles. Most of the diagnostic features listed by previous authors cannot be assessed, whether they are purported apomorphies (e.g., Yates and Warren, Reference Yates and Warren2000; Schoch, Reference Schoch2008b) or part of a unique combination of features (e.g., Schoch and Milner, Reference Schoch and Milner2000; Damiani, Reference Damiani2001; Steyer, Reference Steyer2003).

If AMNH FARB 9503 were a lydekkerinid, it could range from a relatively mature ‘sub-adult’ to a fully mature adult based on the size of other southern Pangean lydekkerinids. Therefore, it is reasonable to expect certain characteristics (such as the interorbital hollow) in a lydekkerinid of this size. Evidence for this is observed in the small holotype of Deltacephalus whitei (<5 cm in skull length; Hewison, Reference Hewison1996) and the holotype of ‘Limnoiketes paludinatans’ (5.5 cm skull length, a relatively small individual of Lydekkerina huxleyi). Conversely, if AMNH FARB 9503 were a capitosaur, it would undoubtedly be a very immature individual, because capitosaurs frequently exceeded half a meter in skull length. Therefore, it is less reasonable to expect certain characteristics (e.g., the prominent Z-shaped lacrimal flexure of the supraorbital canal) in a capitosaur of this size. In this specific example, lateral line grooves appear to have been absent from the skull roof entirely in very immature individuals of some capitosaurs (Warren and Hutchinson, Reference Warren and Hutchinson1988). On the balance of evidence against affinities with other clades, a few qualitative similarities with juvenile capitosaurs, and evidence against capitosaur affinities, we tentatively conclude that AMNH FARB 9503 is the juvenile of an indeterminate capitosaur, which is one of the two possibilities hypothesized by Schoch and Milner (Reference Schoch and Milner2000) and the one briefly alluded to by Sidor et al. (Reference Sidor, Steyer and Damiani2007, p. 232). AMNH FARB 9503 can more confidently be maintained as a nomen dubium (following Schoch and Milner, Reference Schoch and Milner2000), because the specimen is largely uninformative in the absence of verified sutures, and what can be verified is not diagnostic.

Ontogenetic interpretation of FMNH PR 5020

A few features hint at the relative maturity of FMNH PR 5020. The first line of evidence is the evenness of the ornamentation—certain regions of the temnospondyl skull frequently show radiating ornamentation in the form of grooves or ridges in early stages of development, reflecting rapid zones of growth (e.g., Bystrow, Reference Bystrow1935; Bystrow and Efremov, Reference Bystrow and Efremov1940; Getmanov, Reference Getmanov1989; Boy and Sues, Reference Boy, Sues, Heatwole and Carroll2000; Witzmann et al., Reference Witzmann, Scholz, Mueller and Kardjilov2010). Such zones are most common in the preorbital and cheek regions where positive allometry is noted (e.g., Bystrow, Reference Bystrow1935; Witzmann and Scholz, Reference Witzmann and Scholz2007; Witzmann et al., Reference Witzmann, Scholz, Mueller and Kardjilov2010); in the preserved preorbital region of this specimen, the ornamentation is not radiating. The second line of evidence is the small size of the orbits (see previous discussion of AMNH FARB 9503). Even if the skull were to terminate just posterior to the posteriormost preserved region (resulting in a highly irregular shape), the orbits would still be proportionately quite small. A possible third feature is the absence of sutures. Preservation of the specimen is relatively good with many of the small nutrient foramina being identifiable (i.e., the ornamentation is not badly damaged from preparation or weathering), so the sutures could have been indistinguishably fused, at least dorsally (e.g., Kathe, Reference Kathe1999). The last and perhaps most robust line of evidence is the anterior ossification of the sphenethmoid in the interorbital region. The sphenethmoid tends to either complete ossification only at relatively late stages of ontogeny, or not at all, in stereospondylomorphs (e.g., Bystrow and Efremov, Reference Bystrow and Efremov1940; Welles and Cosgriff, Reference Welles and Cosgriff1965; Schoch, Reference Schoch2002; Witzmann, Reference Witzmann2005, Reference Witzmann2006; Schoch and Witzmann, Reference Schoch and Witzmann2009a, Reference Schoch and Witzmannb). This feature is expanded upon below.

The neurocranium of stereospondyls is best known from capitosaurs, with excellent descriptions of Mastodonsaurus giganteus Jaeger, Reference Jaeger1828, by Schoch (Reference Schoch1999, Reference Schoch2002), but these are based mainly on the relatively large specimens of this taxon. Other taxa provide evidence for an ontogenetic transformation of the sphenethmoid. In Wellesaurus peabodyi Welles and Cosgriff, Reference Welles and Cosgriff1965, there is no sphenethmoid in a skull of 26 cm in length, a partially ossified sphenethmoid in a skull of 33 cm in length, and a well-ossified sphenethmoid in a skull of 44 cm in length (the largest known for the taxon; Welles and Cosgriff, Reference Welles and Cosgriff1965). A very short (3 cm) sphenethmoid ossification, incomplete and well posterior to the interorbital region, is known in the ~24 cm long holotype skull of Watsonisuchus rewanensis Warren, Reference Warren1980. A similarly positioned, poorly ossified sphenethmoid is found in the holotype of Watsonisuchus gunganj Warren, Reference Warren1980 (22.7 cm skull length). No sphenethmoid is apparent in the nearly complete and undistorted holotype of Stanocephalosaurus amenasensis Dahoumane et al., Reference Dahoumane, Nedjari, Aït-Ouali, Taquet, Vacant and Steyer2016, which is estimated to be ~23 cm in length and which was tomographically analyzed by Arbez et al. (Reference Arbez, Dahoumane and Steyer2017). Some remnant of the sphenethmoid is found in the specimens of Edingerella madagascariensis (Lehman, Reference Lehman1961), which are all small and which range from 4.6–14.9 cm (Lehman, Reference Lehman1961; Steyer, Reference Steyer2003; Maganuco et al., Reference Maganuco, Steyer, Pasini, Boulay, Lorrain, Bénéteau and Auditore2009). However, the sphenethmoid is posteriorly restricted and poorly ossified in these individuals (evinced by the scant morphological characterization of previous workers), and various features, such as the absence of a complete lateral line system, set FMNH PR 5020 apart from this taxon. The sphenethmoid is unknown for most trematosaurs, in spite of the large size of many specimens and a large sample size for some taxa (e.g., Trematosaurus brauni Burmeister, Reference Burmeister1849; Schoch, Reference Schoch2019), and it remained unossified in the metoposaurid trematosaurs. In Benthosuchus sushkini, the sphenethmoid appears to begin ossifying at a skull length of ~30 cm; this is less than half of the largest estimated size (~80 cm), but is also several times larger than FMNH PR 5020 (Bystrow and Efremov, Reference Bystrow and Efremov1940). A well-ossified sphenethmoid is only known in a handful of relatively large plagiosaurid specimens among the brachyopoids (e.g., Damiani et al., Reference Damiani, Schoch, Hellrung, Werneburg and Gastou2009; Witzmann et al., Reference Witzmann, Schoch, Hilger and Kardjilov2012). There is scant data on the neurocranium of lapillopsids, rhinesuchids, and rhytidosteids. By comparison, a well-ossified sphenethmoid extending to the interorbital region is found in the putative “adults” of small-bodied clades (e.g., lydekkerinids; Shishkin et al., Reference Shishkin, Rubidge and Kitching1996; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006) and non-branchiosaurid amphibamiforms (e.g., Schoch and Rubidge, Reference Schoch and Rubidge2005; Sigurdsen and Bolt, Reference Sigurdsen and Bolt2010; Bourget and Anderson, Reference Bourget and Anderson2011; Maddin et al., Reference Maddin, Fröbisch, Evans and Milner2013). The presence of this ossification evinces maturation at small size in these clades. Collectively, the presence of a well-ossified sphenethmoid that reached at least the interorbital region strongly suggests that FMNH PR 5020 matured at a small size (<15 cm in skull length).

Taxonomic interpretations of FMNH PR 5020

In the absence of sutures, the interpretations of this specimen are based on qualitative features and our ontogenetic assessment. As we noted above, FMNH PR 5020, with a maximum preserved length of 7.3 cm, appears to have been relatively mature at the time of death, thereby restricting the candidate clades to relatively small-bodied clades (adult skull length <15 cm). The preserved portions of the skull also indicate that the specimen does not belong to a brachycephalic clade with a short and broad skull (e.g., plagiosaurids such as Gerrothorax pulcherrimus Fraas, Reference Fraas1913). In the Lower Triassic, the group of candidates thus encompasses amphibamiforms, lapillopsids, lydekkerinids, rhinesuchids, and rhytidosteids, which is a fortuitously narrow taxon bracket for evaluating FMNH PR 5020.

Amphibamiforms are only confidently represented in the Lower Triassic by Micropholis stowi, which was recently documented from the lower Fremouw Formation (Gee and Sidor, Reference Gee and Sidor2021). FMNH PR 5020 is clearly not M. stowi, because it lacks the apomorphic pustulated ornamentation that covers the skull of M. stowi. The skull is also flat, with gently curved lateral surfaces, rather than the taller profile observed in terrestrial dissorophoids. This includes the anterolateral corner of the orbit. In dissorophoids with a large lateral exposure of the palatine (LEP), which occurs in M. stowi, the skull roof is prominently depressed in this area (e.g., Bolt, Reference Bolt1974), but this region is flat in FMNH PR 5020. The suborbital bar is also taller (wider) than in small-bodied dissorophoids such as M. stowi (e.g., Schoch and Rubidge, Reference Schoch and Rubidge2005). Thirdly, the orbits of amphibamiforms remain relatively large throughout ontogeny, which has been ascribed to miniaturization and consequent reduction in size of the circumorbital bones (e.g., Schoch and Rubidge, Reference Schoch and Rubidge2005; Pérez-Ben et al., Reference Pérez-Ben, Schoch and Báez2018). The orbits of FMNH PR 5020 are proportionately much smaller regardless of the complete size of the individual. Finally, M. stowi is known from abundant remains across the Karoo Basin, in which the largest specimen does not exceed 4.5 cm in midline length. Micropholis stowi is already one of the largest amphibamiforms, being surpassed only by some other micropholid amphibamiforms (e.g., Pasawioops mayi Fröbisch and Reisz, Reference Fröbisch and Reisz2008; Maddin et al., Reference Maddin, Fröbisch, Evans and Milner2013), whose largest known size is distinctly less than the preserved length of FMNH PR 5020.

Lapillopsids encompass only small-bodied taxa, <4.5 cm in skull length, and they have proportionately large orbits, in stark contrast to FMNH PR 5020 (Warren and Hutchinson, Reference Warren and Hutchinson1990a; Yates, Reference Yates1999; Yates and Sengupta, Reference Yates and Sengupta2002). The preserved skull profile of FMNH PR 5020 is somewhat comparable to that of Lapillopsis nana, so there is again the question of whether the largest known specimens of this taxon are only “juveniles” or “subadults” (Yates, Reference Yates1999). However, FMNH PR 5020 lacks the dorsal swelling posterior to the naris that diagnoses L. nana; as ornamentation tends to become more pronounced throughout temnospondyl ontogeny, it would not be expected for the feature to retrograde into a flatter skull roof. FMNH PR 5020 also lacks pustular ornamentation, which would be expected to be more developed in a mature individual of L. nana. The holotype of R. contundo is similar to FMNH PR 5020 only in having proportionately smaller orbits. Unfortunately, the diagnostic features of this taxon are on areas not preserved in FMNH PR 5020. The best lines of evidence against affinities with R. contundo are the profile in dorsal view—broader expansion posteriorly in FMNH PR 5020, with obliquely oriented orbits—and the presence of (at least) an infraorbital lateral line groove in FMNH PR 5020 (lateral line grooves are unknown in any lapillopsid).

Rhytidosteids include a number of taxa known from skulls of a similar length to the inferred complete size of FMNH PR 5020, including some from Lower Triassic rocks of Australia (e.g., Cosgriff, Reference Cosgriff1965; Cosgriff and Zawiskie, Reference Cosgriff and Zawiskie1979; Warren and Black, Reference Warren and Black1985; Warren and Hutchinson, Reference Warren and Hutchinson1987; Yates, Reference Yates2000). However, all rhytidosteids can be confidently excluded on the basis of their characteristic wide interorbital distance and the absences of the so-called ‘spider web’ pattern of ornamentation with pitting radiating outward into a groove-ridge system (Dias-da-Silva and Marsicano, Reference Dias-da-Silva and Marsicano2011).

Lydekkerinids and rhinesuchids are the final groups to warrant consideration. Most rhinesuchids are known from much larger specimens (>20 cm skull length), but those of the only Triassic representative, Broomistega putterilli, are similar in size to large lydekkerinids (Shishkin and Rubidge, Reference Shishkin and Rubidge2000). Most lydekkerinids are smaller than FMNH PR 5020, but there are some specimens of both Lydekkerina huxleyi and Eolydekkerina magna in the same range (the disputed long-snouted lydekkerinids Chomatobatrachus halei and Luzocephalus have much longer skulls; Cosgriff, Reference Cosgriff1974; Shishkin, Reference Shishkin1980). One feature probably shared with both groups is a dorsal exposure of the septomaxilla posterior to the naris. The damage in this region on the left side of FMNH PR 5020 is similar to specimens of L. huxleyi in which the septomaxilla has been lost (e.g., Jeannot et al., Reference Jeannot, Damiani and Rubidge2006; Hewison, Reference Hewison2007). The skull shape and the small, obliquely oriented orbits align more closely with lydekkerinids, particularly those from South Africa. Other shared features include the uniform distribution of relatively shallow pitted ornamentation, without any zones of intensive growth (marked by striating grooves or ridges), but with distinct nodes at the intersections, and the poorly developed and discontinuous lateral line system (most similar to Lydekkerina huxleyi). Although the supraorbital canals are not fully exposed, and their relationship to specific cranial elements is unknown, the left canal does have a medial step (flexure) at the presumed position of the lacrimal (posterior to the naris), a feature not found in rhinesuchids, but common in lydekkerinids (Shishkin et al., Reference Shishkin, Rubidge and Kitching1996). Jeannot et al. (Reference Jeannot, Damiani and Rubidge2006) expressly lists this as a lydekkerinid symplesiomorphy while Hewison (Reference Hewison2007) lists it as a synapomorphy found in the majority of lydekkerinids (bearing in mind his more restrictive composition of the clade). One other feature of note is that the interorbital region of FMNH PR 5020 is shallowly depressed, with the left orbital rim being dorsally offset. Shishkin and Rubidge (Reference Shishkin and Rubidge2000, p. 656) noted this as a differentiating feature of lydekkerinids relative to rhinesuchids. The same feature (termed an ‘interorbital hollow’; see Shishkin, Reference Shishkin1980, p. 91, for early usage) was also noted by Piñeiro et al. (Reference Piñeiro, Ramos and Marsicano2012, p. 73) within the same comparative framework. Hewison (Reference Hewison2007) described this feature in L. huxleyi and ‘Limnoiketes paludinatans,’ although he noted it was absent in Chomatobatrachus halei and Luzocephalus blomi Shishkin, Reference Shishkin1980, neither of which he considered to be lydekkerinids. Regardless of the composition of the Lydekkerinidae, this depression is absent in rhinesuchids, but present in some lydekkerinids, and it thus serves as evidence against a possible interpretation as a diminutive rhinesuchid like B. putterilli.

No feature definitively excludes FMNH PR 5020 from the Lydekkerinidae, and we therefore provisionally refer the specimen to Temnospondyli cf. Lydekkerinidae. The absence of sutures, the posterior skull table, and the palate preclude further taxonomic refinement, although we would predict either conspecificity with, or close relatedness to, one of the southern Pangea taxa such as Eolydekkerina or Lydekkerina. Lydekkerina huxleyi is abundant in the Karoo Basin and is also known from Australia (Warren et al., Reference Warren, Damiani and Yates2006), but autapomorphies of previous authors (e.g., Jeannot et al., Reference Jeannot, Damiani and Rubidge2006) are for skeletal regions not exposed or preserved in FMNH PR 5020. Additionally, there is the conundrum of whether taxonomic differentiators are in part related to size (presumed ontogenetic) differences, which is particularly poignant for differentiating Eolydekkerina magna from L. huxleyi (Pawley and Warren, Reference Pawley and Warren2005; Jeannot et al., Reference Jeannot, Damiani and Rubidge2006). FMNH PR 5020 would have been closer in size to the larger E. magna than to the majority of specimens and the holotype of L. huxleyi, which might explain why the prenarial region is proportionately longer than in the latter. Eolydekkerina is a valid taxon according to Jeannot et al. (Reference Jeannot, Damiani and Rubidge2006), who argued that the size distribution of L. huxleyi, for which specimens very rarely approach the size of E. magna, indicates that E. magna is not simply a large individual of L. huxleyi. However, it should be considered that most individuals of L. huxleyi likely did not survive long enough to reach the maximum body size, perhaps due to predation risk as relatively small tetrapods or to climatic harshness. Histological studies have supported this hypothesis through identification of a particularly fast growth rate in early ontogeny and immaturity in specimens that would typically be considered “adult-sized” (Canoville and Chinsamy, Reference Canoville and Chinsamy2015; McHugh, Reference McHugh2015).

Lower Fremouw temnospondyl assemblage

This study continues to refine the poorly understood temnospondyl assemblage of the lower Fremouw Formation that was first characterized in detail by Colbert and Cosgriff (Reference Colbert and Cosgriff1974) and subsequently expanded upon by Cosgriff and Hammer (Reference Cosgriff and Hammer1984). Collectively, those workers identified remains of what they considered to be unequivocal brachyopids, unequivocal lydekkerinids, unequivocal rhytidosteids, and equivocal capitosauroids. Occasional mentions of a tupilakosaurid dvinosaur in the lower Fremouw Formation are based on personal observations of unspecified material by Shishkin (e.g., Shishkin, Reference Shishkin, Ananjeva and Tsinenko2003, Reference Shishkin2007; Shishkin et al., Reference Shishkin, Sennikov, Novikov and Ilyina2006), but such material has never been described or figured in any capacity, and Tupilakosauridae is not considered to be present in Antarctica at present (Schoch and Milner, Reference Schoch and Milner2014).

The extremely fragmentary nature of practically all of the material has greatly hindered the characterization of the temnospondyl record from this interval at all taxonomic scales. The holotype of ‘Austrobrachyops jenseni’ is not even a temnospondyl, and the referred material was only identified to Brachyopoidea (not Brachyopidae) by Warren and Marsicano (Reference Warren and Marsicano2000; but see Schoch and Milner, Reference Schoch and Milner2014, who do place this material to the family level). Similarly, the holotype of ‘Cryobatrachus kitchingi’ is interpreted as a juvenile capitosaur by us here, following Schoch and Milner (Reference Schoch and Milner2000). As we noted in the Materials and Methods, none of the referred material of ‘C. kitchingi’ is properly referable to Lydekkerinidae on the basis of identification of apomorphies. We consider the recognition of Capitosauria in the lower Fremouw Formation viz. AMNH FARB 9503 to be more reliable than the putative capitosauroid based on a single isolated tabular that Colbert and Cosgriff (Reference Colbert and Cosgriff1974) argued was either a trematosauroid or a capitosauroid. Those workers identified it formally as Temnospondyli incertae sedis; Cosgriff and Hammer (Reference Cosgriff and Hammer1984) then referred to it specifically as an indeterminate capitosauroid. Schoch and Milner (Reference Schoch and Milner2000) noted this tabular in their section on indeterminate capitosaur material, but it has otherwise not been further discussed in the literature. Similarly, we consider FMNH PR 5020 to represent the most reliable record of Lydekkerinidae from this horizon, because it is the most substantial and well-preserved of the suite of specimens referred to the clade at any given point. The rhytidosteid fragments reported by Cosgriff and Hammer (Reference Cosgriff and Hammer1984) have rarely been discussed. Shishkin (Reference Shishkin1994, Reference Shishkin, Ananjeva and Tsinenko2003) expressed skepticism over their identity, and other references to the Antarctic record do not indicate an expressed opinion (e.g., Dias-da-Silva et al., Reference Dias-da-Silva, Marsicano and Schultz2006); Schoch and Milner (Reference Schoch and Milner2000) essentially repeat the original justification of Cosgriff and Hammer (Reference Cosgriff and Hammer1984).

This collective convoluted history only underscores the need for continued taxonomic work and the essentiality of revisiting historic specimens as taxonomic frameworks shift over time. The recent report of Micropholis stowi, representing the first occurrence of Amphibamiformes in Antarctica, based on recently collected material (Gee and Sidor, Reference Gee and Sidor2021) is both the most substantial and the most robust temnospondyl record from the entire lower Fremouw Formation. It is hoped that further collection and study of new material may provide a richer context through which to interpret the historic material.