Dermal bones of the skull

The preserved portions of the skull roof of †Parapsephurus willybemisi n. gen. n. sp. include the parietals, frontals, dermopterotic, and dermosphenotics; anteriorly, these bones contact the median rostral bones, which are described in the following. In addition, there are small, irregularly shaped canal bones that are positioned posteriorly on the skull roof dorsal to the parietals that likely represent extrascapular bones, although nothing can be described of their arrangement (Figs. 3–5). There is slight ornamentation of the skull roofing bones, with low radiating ridges from the center of ossification and along the central axis of the parietals (Fig. 5). The parietals are well ossified, extend far posteriorly, and have a fimbriate posterior margin as in most other polyodontids. The left and right parietals contact each other (or nearly so) along much of the midline (the anterior end of these is unclear and may have defined a pineal foramen, as found in Psephurus). Anteriorly, the parietals contact the frontals, which are incompletely preserved in all of our specimens.

Laterally, the skull roof of †Parapsephurus willybemisi n. gen. n. sp. has two dermal bones: posteriorly the dermopterotic and anteriorly the dermosphenotic; in addition, the posttemporal is incorporated into the skull roof and is described in this section (Figs. 3–5). The posttemporal is most robust posterolaterally. Medially, there is a stout projection that contacts the ventral surface of the parietal at about the midpoint of the center of ossification of that bone (Fig. 5). The anterolateral arm of the posttemporal contacts the dermopterotic, forming the posterior margin of the posterior fenestra longitudinalis. The dermopterotic is robust and bears low, roughened ornamentation radiating out from its center of ossification. This bone contacts the parietal posteriorly through a slender posteromedial projection, defining the posterior margin of the middle fenestra longitudinalis. Anteriorly, the dermopterotic contacts the lateral margin of the frontal, with which it encloses the middle fenestra longitudinalis, although its exact relation to the frontal is not clearly preserved. This is different from what is found in most other polyodontids, in which the middle fenestra longitudinalis is defined medially by the dermopterotic and laterally by the dermosphenotic (e.g., see Grande and Bemis, Reference Grande and Bemis1991, figs. 32A, 50B, 67A for conditions in Psephurus, †Crossopholis, and †Paleopsephurus, respectively) and is similar to the condition in †Protopsephurus, in which the frontal forms the median margin of the middle fenestra longitudinalis (Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002, fig. 7B); the condition in †P. willybemisi n. gen. n. sp. is unique, however, in that both the frontal and parietal form the median margin of the middle fenestra longitudinalis. Posteriorly, the dermopterotic contacts the posttemporal, and posterolaterally it contacts the dermosphenotic, with which it defines the posterior margin of the anterior fenestra longitudinalis. The precise shapes and extents of the anterior fenestra longitudinalis, the dermosphenotic, and the frontal are unknown.

The parasphenoid of †Parapsephurus willybemisi n. gen. n. sp. bears elongated posterior rami that extend beyond the pectoral girdle. These rami define an elongate aortic notch (Fig. 6) and are posteriorly pointed, unlike the more truncated or fimbriate ends in Psephurus and Polyodon, respectively. As in most polyodontids, the ascending processes are directed laterally to slightly anteriorly, extending from the main body of the parasphenoid at nearly right angles. This is in contrast to the condition in †Paleopsephurus, in which the ascending processes are posteriorly directed. A pair of elongate pharyngobranchials articulate with the parasphenoid at the base of the ascending processes (Fig. 6). The anterior tip of the parasphenoid contacts the median ventral bones of the rostrum and appears to continue anteriorly as a median anterior process, although the shape and extent of this process are not clearly preserved.

Dermal bones of the rostrum

A few rostral bones of †Parapsephurus willybemisi n. gen. n. sp. are present in FMNH PF17626 (Fig. 7), but the rostrum is most completely known from the holotype (FMNH PF17545; Fig. 8). The rostrum of the holotype is exposed in ventral view. This portion of the specimen was collected separately from the large block that contained the rest of the specimen; therefore, a portion of the bones connecting it to the rest of the skull is missing. In FMNH PF17626, the paired vomers are in contact with the anterior tip of the parasphenoid and are relatively long, smooth elements (Fig. 7). The vomers are not well preserved in the holotype due to the gap between the rostrum and the posterior portion of the skull, but those elements interpreted as the anterior tips of the vomers show them to be paired (Fig. 8). The more anterior bones of the ventral rostral skeleton comprise bones of the p, cv, and v series, which extend the length of the rostrum and, collectively, have lateral margins that are parallel. The p_1–2 and cv_1–2 bones are the longest elements of the rostrum, together extending to almost 70% of the preserved rostral length. The anterior cv₃ and v_2–3 bones are substantially shorter than the more posterior elements; there are additional splint-like bones near the anterior tip of the rostrum that are of unclear series.

There is a dense array of stellate bones present along the entire length of the preserved rostrum (Fig. 8). The stellate bones are well developed and largest medially, where as a series, they contact the median rostral bones. Laterally, the stellate bones are smaller, more irregularly shaped elements.

Opercular bones

The opercular bones of †Parapsephurus willybemisi n. gen. n. sp. include the subopercle (which is the largest and best-preserved element) and a single branchiostegal; a fragment of dermal bone positioned dorsally to the subopercle in FMNH PF17627 may represent a portion of an opercle, as found in basal polyodontids (e.g., Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002) and non-acipenserid Acipenseriformes (Hilton and Grande, Reference Hilton, Grande and Schultze2021), but this requires additional material to confirm. The subopercle is preserved most completely in FMNH PF17627 (Fig. 9) and partially in FAU.DGS.ND.161.4559.T (Fig. 4). The subopercle is a largely flat, superficially ornamented bone with a broadly rounded posterior portion and a long, narrow anterior portion that extends from the midpoint of the main body of the subopercle (Fig. 9). The posterior margin of the subopercle of †Parapsephurus willybemisi n. gen. n. sp. does not bear the splint-like projections found in most other polyodontids. A single slender, elongate bone that is interpreted as a branchiostegal is preserved in FMNH PF17626 (Fig. 7) and FMNH PF17627 (Fig. 9). This bone is similar in shape to that found in Psephurus (Grande and Bemis, Reference Grande and Bemis1991, figs. 34, 36, 37) and †Paleopsephurus (Grande and Bemis, Reference Grande and Bemis1991, fig. 68) and unlike other polyodontids, which have a narrow, needle-like extension similar to the subopercle.

Jaws, suspensorium, and hyoid arch

The upper jaw of †Parapsephurus willybemisi n. gen. n. sp. comprises paired dermopalatines, palatopterygoids, and quadratojugals. The dermopalatine is a relatively narrow element that narrows anteriorly and forms the anterolateral margin of the upper jaw (for consistency with our other recent descriptions of the osteology of Acipenseriformes [Grande and Hilton, Reference Grande and Hilton2006; Hilton et al., Reference Hilton, Grande and Bemis2011, Reference Hilton, Dillman, Zhang, Zhang and Zhuang2016, Reference Hilton and Grande2023], we use the term “dermopalatine” here, with the acknowledgment that this element may represent the maxilla of other actinopterygians [Datovo and Rizzato, Reference Datovo and Rizzato2018]). At its posterior end, a well-developed ectopterygoid process extends posteromedially (Figs. 2, 10, 11.1–11.3) and appears to contact the palatopterygoid in the holotype. The palatopterygoid is a large, thin sheet of bone. There is no trace of an autopalatine. The posterior margin of the upper jaw is defined by the quadratojugal, which bridges the dermopalatine laterally and the palatopterygoid medially. The quadratojugal has elongate dorsal and ventral extensions that flare to broadly contact the other elements of the jaws. This is similar in form to the quadratojugal found in Psephurus (Grande and Bemis, Reference Grande and Bemis1991, fig. 35) and †Paleopsephurus (Grande and Bemis, Reference Grande and Bemis1991, fig. 69).

The longest bone of the lower jaw in †Parapsephurus willybemisi n. gen. n. sp. is the dentary, which is preserved in the holotype (FMNH PF 17545), as well as in FMNH PF17676, PF17627, and FAU.DGS.ND.161.4559.T. The dentary tapers anteriorly to a rounded, narrow tip that is bent medially at about one-fourth its length to meet its antimere at its symphysis. The dentary is deepest posteriorly but tapers anteriorly and ends posteriorly as a point. The mentomeckelian is best seen in FAU.DGS.ND.161.4559.T and is a conical element that sits in a median sulcus at the anterior tip of the dentary (Fig. 11.8). The prearticular is large and is anteriorly closely applied to the medial surface of the dentary. Posteriorly, the prearticular is separated from the dentary at a point of the jaw that presumably was filled with the articular portion of Meckel's cartilage in life (Fig. 11.5–11.7).

The hyomandibula, preserved only in the holotype of †Parapsephurus willybemisi n. gen. n. sp., is exceptionally long and is wider posteriorly than anteriorly (Fig. 3). The anterior head is narrow and is preserved in a position just posterior to the ascending process of the parasphenoid. The hyomandibula extends from this point at about a 45° angle posteroventrally. It is shaped much like the combined ossified and cartilaginous portions of the hyomandibula of Psephurus (Grande and Bemis, Reference Grande and Bemis1991, fig 34). The interhyal is large, robust, and hour-glass shaped (Figs. 3, 7). The anterior ceratohyal is also large, and there is a smaller irregularly shaped element tentatively identified as the posterior ceratohyal; this element has been displaced and separated from the anterior ceratohyal, but it is associated with the proximal tip of the interhyal (Fig. 7). The left hypohyal is present and is associated with the anterior tip of the anterior ceratohyal in FMNH PF17676; it is small, roughly triangular, and has a rounded anterior end (Fig. 7).

Gill arches

In general, the gill arches of †Parapsephurus willybemisi n. gen. n. sp. are poorly preserved. Although portions of the certatobranchial series (Figs. 3, 7, 12) and pharyngobranchial series (Fig. 5) are preserved, not much detail can be determined. The gill rakers are preserved and exposed in the holotype (FMNH PF17545) as well as in FAU.DGS.ND.161.4559.T (Figs. 3, 12). The gill rakers are relatively short and widely spaced, unlike the gill rakers of Polyodon but similar to all other polyodontids for which these structures are known (see Grande and Bemis, Reference Grande and Bemis1991, fig. 26 for comparisons of gill-raker morphology among polyodontids and other acipenseriforms for which these structures are known). The gill rakers appear to be superficially ossified elements that surrounded a soft-tissue core (possibly cartilage) as there is a space in the center of each element. Anteriorly and posteriorly, each gill raker is flared out proximally where they contact the underlying branchial elements (Fig. 12.3).

Vertebral column

Nothing is known of the vertebral elements of †Parapsephurus willybemisi n. gen. n. sp.

Dorsal and anal fins and supports

The dorsal and anal fins and their supports are preserved only on the holotype of †Parapsephurus willybemisi n. gen. n. sp. (Fig. 13). There are approximately 25 dorsal-fin rays, although posteriorly these become difficult to count. There are nine proximal radials and about four or five middle radials preserved supporting the dorsal fin, but these series are incomplete.

There are at least 32 anal-fin rays in †Parapsephurus willybemisi n. gen. n. sp. supported by 14 middle radials (Fig. 13); there are five proximal radials supporting the anal fin, but this series, as for the dorsal fin, is incomplete.

Caudal fin and supports

The caudal fin is preserved only on the holotype of †Parapsephurus willybemisi n. gen. n. sp., which shows a typically heterocercal tail with a dorsal lobe that is longer than the ventral lobe (Fig. 14). Although present, the caudal-fin rays are not sufficiently preserved to provide counts. There are at least 20 dorsal caudal fulcra preserved along the dorsal margin of the caudal fin. These elements are more slender than those in most other polyodontids. The rhombic caudal scales are present but are largely displaced. However, these do appear smaller and slenderer than those of comparably sized specimens of other polyodontids.

Pectoral and pelvic girdles and fins

The preserved portions of the pectoral girdle of †Parapsephurus willybemisi n. gen. n. sp. comprise the posttemporal, supracleithrum, postcleithrum, cleithrum, and fragments of the clavicle. The supracleithrum is preserved on the holotype but displaced from its presumed articulation with the posttemporal. The supracleithrum is an elongate bone that is slightly broader toward its anterior end than its posterior end before tapering to a sharp anterior point (Fig. 3). A smooth crescent-shaped bone that we interpret as a postcleithrum is present between the posterior end of the supracleithrum and cleithrum but is clearly not in its natural position. The cleithrum is well preserved in the holotype and resembles that of Psephurus in its overall form. There is no distinct notch along its posterior margin (e.g., as in †Crossopholis), but it does have a shallow concavity that presumably indicates where the pectoral fin inserts (this concavity is absent in Polyodon). The dorsal tip of the cleithrum appears notched (Fig. 3). In FAU.DGS.ND.161.4559.T, the cleithrum broadens medially to form the dorsal portion of the opercular wall, as found in other Acipenseriformes (Fig. 4). The clavicle is best preserved in FMNH PF17672 (Fig. 7), in which it is seen in ventral view. The left and right clavicles essentially meet along the midline and make broad contact with the cleithra laterally (Fig. 7), as in Polyodon (Grande and Bemis, Reference Grande and Bemis1991, fig. 20). We did not find an interclavicle, but this is possibly due to lack of preservation. The pectoral fin of the holotype has at least 23 fin rays preserved, and FMNH PF17627 has at least 27 fin rays preserved, but it appears to be missing some medially. The leading fin ray is larger than the more posterior ones but does not appear to be spine-like, as in †Protopsephurus (Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002).

No traces of the pelvic girdle or fin are preserved on the holotype. In FMNH PF17626, there are at least 22 right pelvic-fin rays preserved, but this fin appears incomplete; there are at least five left pelvic-fin radials preserved, but again, this series appears incomplete.

Body scales

The body scales of †Parapsephurus willybemisi n. gen. n. sp. are small and scattered along the length of the holotype (FMNH PF17545) and are well preserved just posterior to the pectoral girdle in FAU.DGS.ND.161.4559.T (Fig. 4). These are the denticular trunk scales of Grande and Bemis (Reference Grande and Bemis1991) and consist of a robust base with one to three spines projecting from the base (Fig. 15). We found no round-based scales associated with the opercular wall formed by the cleithrum and clavicle, as found in other acipenseriforms, although this may be due to lack of preservation.

Dermal bones of the skull

The bones of the skull roof of †Pugiopsephurus inundatus n. gen. n. sp. that are best preserved include the dermosphenotic and the dermopterotic (Fig. 17). The left dermopterotic is more complete than its antimere and may also show the anterior portion of the posterior fenestra longitudinalis, although this is not clear. The dermosphenotic and dermopterotic contact one another through a slender bridge that separates the anterior and middle fenestrae longitudinalis. This bridge is more slender than that of †Parapsephurus willybemisi n. gen. n. sp. The dermosphenotic and c3 bone laterally and the dermopterotic medially define the large anterior fenestra longitudinalis. Portions of the left frontal are preserved and exposed in ventral view, but few details are available.

Portions of the parasphenoid are preserved, including the tips of the posterior rami, which reach to the level of the pectoral fins. Anteriorly, the parasphenoid is exposed and contacts the paired vomers. The parasphenoid tapers anteriorly, but a median anterior process is not distinctly preserved.

Dermal bones of the rostrum

Portions of the dorsal and ventral rostral bones are preserved, including the c₃ bones, which are in contact with the anterior tip of the dermosphenotics. Other lateral rostral bones are dorsally and ventrally in contact with the paired vomers, the cv_1–3 bones, and the p_2–4 bones. There is no indication of stellate bones. The c₃ bone does not have a posterior notch, as that of other polyodontids (e.g., see Grande and Bemis, Reference Grande and Bemis1991), but rather contacts the dermosphenotic through a lap suture. Anterior and lateral to the c₃ bone, there is a series of bones that are trough-like and may be interpreted as canal bones, carrying the infraorbital canal anteriorly. The paired vomers are elongate and extend over one-third the length of the rostrum. The median p₁ bone separates the left and right vomers along much of their length; p_2–4 bones are progressively shorter. The cv_1–3 bones are of similar lengths and are positioned lateral to the p series.

Jaws, suspensorium, and hyoid arch

The only elements of the jaws, suspensorium, and hyoid arch that are preserved in the single specimen of †Pugiopsephurus inundatus n. gen. n. sp. are the left and right dermopalatines, the right quadratojugal, and fragments of the right dentary and other unidentified bones of the suspensoria (Fig. 17). The right quadratojugal is incompletely preserved, but it appears to have the anterior flaring found in Psephurus and some other polyodontids, although the extent and shape of these is unknown. Two bones are identified as the left and right dermopalatines because they are positioned closely to the ventral surface of the parasphenoid (i.e., there are no intervening elements, so it is unlikely these are the dentaries). These bones are exposed in ventral view and appear to be more or less complete. Both elements are bent near their midpoints at about a 30° angle, although their anterior margins are smoothly curved and appear to have a distinct medial expansion, similar to though not as extreme as in the dermopalatines of †Protopsephurus and most Acipenseridae. There is no trace of an ectopterygoid process.

Pectoral and pelvic girdles and fins

Portions of the pectoral girdle are represented by fragments of smooth dermal bone scattered posterior to the skull; no details can be provided. There are 31 pectoral-fin rays present on the right fin, which appears complete; 25 fin rays are preserved in the left pectoral fin, although this fin-ray count appears incomplete medially.

Body scales

Denticulate scales are present (Fig. 18) and are most abundant on the sides of the anterior portion of the body and sparser along the midline; they are largest immediately behind the pectoral girdle and decrease in size posteriorly (the posterior portion of the body is not preserved, but the density of scales is less posteriorly on the preserved portion, suggesting an anterior-to-posterior gradation). The scales are roughly triangular in shape, with the widest edge facing anteriorly.