Autapomorphies

(1) external osteoderms in dorsal view smooth except for few conspicuous peaks; (2) external osteoderms thin (thickness about 1 mm); (3) internal osteoderms with dorsal bi- or multilobed articulation surfaces; (4) anterior neural arches of the same sagittal length than that of the osteoderms; (5) anterior trunk ribs with widened distal ends.

Synapomorphic characters shared with some of the dissorophids

(1) narrow parabolic tip of snout laterally convex (also in Conjunctio multidens Carroll, Reference Carroll1964, sensu Schoch and Sues, Reference Schoch and Sues2013; Cacops morrisi Reisz, Schoch, and Anderson, Reference Reisz, Schoch and Anderson2009; Broiliellus texensis Williston, Reference Williston1914; or Aspidosaurus Broili, Reference Broili1904; but in contrast to more pointed snout of Dissorophus multicinctus Cope, Reference Cope1895, and others); (2) relatively flat relief of roofing elements (in contrast to Broiliellus arroyoensis DeMar, Reference DeMar1967; Fig. 4.3) and Broiellus olsoni DeMar, Reference DeMar1967; (3) cranial ornamentation varies greatly in intensity (also in Cacops Williston, Reference Williston1910a); (4) nasal extends beyond anterior margins of external nares (also in Cacops aspidephorus Williston, Reference Williston1910a); (5) postorbital narrowing posteriorly, resulting in a more triangular dorsal contour (also in Broiellus brevis Carroll, Reference Carroll1964, Cacops woehri Fröbisch and Reisz, Reference Fröbisch and Reisz2012, Conjunctio multidens, see Schoch and Sues, Reference Schoch and Sues2013, Dissorophus Cope, Reference Cope1895, Platyhystrix rugosa Berman, Reisz, and Fracasso, Reference Berman, Reisz and Fracasso1981, and Scapanops Schoch and Sues, Reference Schoch and Sues2013); (6) jugal with large eminence bearing knobby ornament (also in Dissorophus, Scapanops, Broiellus reiszi Holmes et al., Reference Holmes, Berman and Anderson2013, Broiellus brevis, and Broiellus texensis); (7) elevated ridge on jugal (only in Cacops morrisi); (8) interorbital region very narrow (IOw/Sl lower than 0.20; in contrast to nearly all other dissorophids; only similar in Cacops morrisi); (9) dorsal exposure of suture between squamosal and supratemporal more than half as long as supratemporal itself (also in Cacops woehri and Conjunctio multidens); (10) supratemporal without ventral projection into squamosal embayment (semilunar flange sensu Bolt, Reference Bolt1974b) (also in Broiellus reiszi); (11) lacrimal as part of orbital margin (in contrast to Cacops morrisi); (12) dorsolateral exposure of ectopterygoid (also in Broiliellus reiszi Holmes et al., Reference Holmes, Berman and Anderson2013, Cacops morrisi, Cacops aspidephorus; see Anderson et al., Reference Anderson, Scott and Reisz2020); (13) basal plate of parasphenoid 1.7 times wider than long (only shared with Broiellus brevis); (14) carotid foramina clearly spaced in wide distance on basal plate (in contrast to cacopines); (15) mid-trunk neural arches with large expansion of spines along sagittal axis (shared with Cacops; see Dilkes and Brown, Reference Dilkes and Brown2007; Gee et al., Reference Gee, Bevitt and Reisz2019b; Diploseira, Dilkes, Reference Dilkes2020), but in contrast to other ones (Schoch, Reference Schoch2012). (16) narrow external osteoderms only 1.4 times wider than long (in Aspidosaurus/syn. Alegeinosaurus and Dissorophus 2.2–6.5 times wider than long); (17) internal osteoderm unfused with neural arch (also in Dissorophinae, see Schoch, Reference Schoch2012); (18) clavicle ornamented with pitting or grooves similar to the skull (also in Dissorophus); (19) clavicular dorsal process with anterior elbow-like postclavicular process (also in Dissorophus and in amphibamiform Eoscopus Daly, Reference Daly1994).

Skull roof

The right half of the skull is nearly complete and allows us to reconstruct the skull in dorsal and ventral views (Fig. 10.1–10.3). The midline skull length is 11.8 cm. The tip of the snout is parabolic, as in Conjunctio multidens (see Schoch and Sues, Reference Schoch and Sues2013) or Cacops morrisi (see Reisz, Schoch, and Anderson, Reference Reisz, Schoch and Anderson2009). The preorbital length of the skull is greater than the postorbital length (by more than 10%), as in Aspidosaurus binasser Berman and Lucas, Reference Berman and Lucas2003, and Cacops morrisi (compare Table 1). The orbital openings are relatively large (O_l/S_l = 0.29), in contrast to A. binasser. The otic notch is very deep. The minimum distance between the otic notch and the posterior orbital margin is distinctly short, only about 11% of the total midline length. The interorbital region is especially narrow (IO_w/S_l = 0.18), in contrast to all other dissorophids (Table 1). The external narial opening is elongated oval in shape. The angle between the length axis of the naris and the midline suture of the skull is close to 45°, as in Dissorophus and Broiliellus. This angle is very variable within dissorophids; it is more than 45° in Cacops morrisi and Conjunctio multidens, but close to 0° in A. binasser and Scapanops neglectus Schoch and Sues, Reference Schoch and Sues2013. The well-preserved anteromedial margin of the right premaxilla is incised to frame a small broad-oval-shaped ‘internarial’ fenestra (Fig. 3.1), which is similar to those known in Cacops morrisi, Conjunctio multidens, A. binasser and S. neglectus, but not in Broiliellus and probably not in Dissorophus (Schoch, Reference Schoch2012; contra Dilkes, Reference Dilkes2020, p. 22). However, this small ‘internarial’ fenestra is contained entirely within the premaxillae as is known from a few trematopids (e.g., Acheloma; Polley and Reisz, Reference Polley and Reisz2011) but not from dissorophids.

Table 1.

Biometric characters in dissorophids; largest and smallest quotients in bold (compare with Fig. 10.5). aS_w = anterior skull width at level of maxilla-premaxilla sutures; H_l = postorbital midline skull length from level of posterior margins of orbits; H_w = postorbital skull width between lateral margins of supratemporals; IN_w = minimum internarial width; IO_w = minimum interorbital width; mS_w = midlength skull width at midlength level of orbits; Occ-Q = length between levels of occipital midline margin and posterior quadrate margin; ON-O = length between levels of anterior otic notch margin and posterior orbital margin; O_l = maximum length of orbit; PO_l = preorbital midline skull length from level of anterior margins of orbits; Po_l = maximum posterior length of postorbital from posteriormost extent of orbit; pS_w = maximum posterior skull width at level of posterolateral margins of cheeks; S_l = midline skull length; Th_l = length of tabular horn between levels of posterior tabular corner and occipital midline margin

The dermal sculpturing of the dorsal skull roof is very characteristic of this new species. It is a reticulated pattern of mostly low, coarse ornament: small-scale elongated ridges surround rounded pits to form polygons, or they surround more elongated pits and valleys. Buxierophus n. gen. also has a peculiar system of cranial ridges probably to stabilize the skull roof (Figs. 2.1, 3.1, 4.1, 4.4, 4.5, 10.1, 10.3); a pronounced sharp ridge runs on the nasal from the posterior tip of the alary process of the premaxilla to the most anterior part of the prefrontal. This sharp ridge merges on the prefrontal in a large strut-like ridge formed by more knobby ornament, then it curves on the posterior tip of the prefrontal. The next pronounced ridge, which runs on the jugal beside the posteroventral margin of the orbit, is a wall-like sharp ridge of more heavy ornament. A similar ridge is also known in Cacops morrisi (see Reisz et al., Reference Reisz, Schoch and Anderson2009). This sharp ridge continues with a gap on the postorbital where it is Y-shaped and swollen; it indeed runs anteroposteriorly from the posterior end of the postorbital (or from the supratemporal) in two directions towards the anterolateral suture with the jugal and towards the anteromedial part of the postorbital. A relatively large strut-like ridge, formed by the knobby ornamentation of the skull roof bones, runs from the tabular horn anteriorly to the posterior half of the supratemporal. These ridges are not preserved on the frontal, postfrontal, or anterior supratemporal. The nearly orthogonal curved ridge system on the nasal and prefrontal is well known in Broiliellus and Dissorophus. It is similar in Broiellus texensis Williston, Reference Williston1914 (Fig. 4.2), D. multicinctus (see Schoch, Reference Schoch2012), and Broiellus arroyoensis (Fig. 4.3). Other dissorophids also have these ridges, but they are always shallower (Schoch, Reference Schoch2012). The ridges on the postorbital are reported in Broiellus brevis, D. multicinctus, and Conjunctio multidens. In Buxierophus n. gen., the dorsal surface of the jugal surprisingly bears a large eminence located on its posterior part and formed by a rounded triangular knobby ornament. This ornamental element is similarly developed in D. multicinctus (see Schoch, Reference Schoch2012), Broiellus brevis (see Carroll, Reference Carroll1964), Broiellus texensis, Broiellus reiszi, and Scapanops.

The skull roof bones of Buxierophus n. gen. are relatively thin, compared with similar-sized skulls of Cacops morrisi (Reisz et al., Reference Reisz, Schoch and Anderson2009; Schoch Reference Schoch2012) or D. multicinctus (Williston, Reference Williston1910b; DeMar, Reference DeMar1968; Milner, Reference Milner2003). The position of the jaw articulation is slightly posterior to the posterior facets of the basisphenoid-exoccipital.

Most of the proportions discussed here are presented in Table 1, where measurements of related dissorophids are compared. Figure 10.5 illustrates the position of the measurements.

The premaxilla is relatively narrow and has a very elongated distinct alary process overlapping the nasal and reaches the middle or the posterior part of the naris. The existence of a small ‘internarial’ fenestra between both premaxillae is discussed above. The maxilla, an elongated and narrow bone, probably terminates posteriorly into the quadratojugal, up to the level of the anterior margin of the otic notch. Its anterodorsal portion is relatively low. The septomaxilla is rarely recorded in dissorophids. The midlateral corner of the right nasal margin accounts for articulation with a small rectangular bony plate, as does the septomaxilla at the posterior rim of the naris and above the lacrimal. The nasal is the most elongated median skull-roof bone. Its anterior third is narrower because of the curved margin of the narial opening. The pronounced sharp ridge runs on the nasal from the posterior tip of the overlapping premaxilla up to the most anterior part of the prefrontal. On the prefrontal, this ridge is swollen by more knobby ornament and curved to the posterior tip of the prefrontal. The prefrontal contacts neither the postfrontal nor the jugal. It is ventrolaterally expanded, stabilizes the anterior orbital rim and is overlapped by the posterodorsal part of the lacrimal. This ventrolateral prefrontal process is in contact with both lateral exposed palatine (LEP) and ectopterygoid (LEE; Fig. 10.3). The laterally visible palatine is small and triangular. The laterally exposed ectopterygoid has a very narrow but elongated lateral expansion. The lacrimal contacts the septomaxilla anteriorly and the orbital margin posteriorly. The nasolacrimal canal is represented by a narrow distinct furrow which runs from the posteromedian edge of the lacrimal to nearly the middle of this bone. In Buxierophus n. gen., the lacerated and lobed posterior margin of the lacrimal is similar, as in Cacops woehri (Fröbisch and Reisz, Reference Fröbisch and Reisz2012), which has yet shorter furrows.

The jugal, relatively narrow and triangular, is reaching anteriorly to the level of the anterior quarter of the orbit. It is in contact anterolaterally with the lateral exposed ectopterygoid and ventrolaterally with the maxilla, but not with the lacrimal. A pronounced sharp ridge runs beside the posteroventral orbital margin, which in dissorophids is known only in Cacops (e.g., Cacops morrisi, see Reisz et al., Reference Reisz, Schoch and Anderson2009). The jugal bears an additional sculpture pattern element on its posterior part: a large rounded triangular eminence with a knobby ornament, which is known in few dissorophoids (see above).

The quadratojugal is very elongated anteriorly, up to the mid-level of the lateral orbital margin. Its lateroventral margin is convex, as in D. multicinctus (Schoch, Reference Schoch2012, fig. 4C). The posterolateral end of the quadratojugal forms a boss-like process and contacts the prominent posterodorsal outgrowth of the quadrate (i.e., quadrate boss). The anterior part of the quadratojugal probably receives the posterior peaks of the maxilla and jugal. The most posterior part of the quadratojugal is strongly concave and forms the posterolateral margin of the otic notch. The dorsal quadrate shows a small crescent-shaped process. The squamosal surrounds nearly completely the otic notch. It shows a dorsally to laterally exposed ornamented area (= supratympanic flange of Bolt, Reference Bolt1974b; = semilunar flange of Schoch and Rubidge, Reference Schoch and Rubidge2005), which continues ventrally into poorly ornamented or smooth portions of the squamosal (compare Schoch, Reference Schoch2012). The curvature along the ventral border of the supratympanic flange is semilunar in shape. The dorsal suture between the squamosal and supratemporal is long, equal to half the length of the supratemporal, like in Conjunctio, and more elongated than in all other dissorophids (Schoch, Reference Schoch2012). The squamosal and tabular are separated by the supratemporal in dorsolateral view (Fig. 10.3), but their contact may be more ventrolaterally located (compare Schoch, Reference Schoch2012, fig. 4A–C). The supratemporal, which is longer than wide, has no ventral projection into the otic notch (semilunar flange sensu Bolt, Reference Bolt1974b) (i.e., it has neither a semilunar flange nor a supratemporal flange, sensu Schoch and Rubidge, Reference Schoch and Rubidge2005). This absence of a semilunar flange is shared with Broiellus reiszi (Holmes et al., Reference Holmes, Berman and Anderson2013) and Broiellus olsoni (Schoch, Reference Schoch2012) but not with the other dissorophids. The tabular is wider than long and presents posterolaterally some knobby tubercles. The tabular horn is rounded and short, without a distinct tabular process. This absence of a tabular process is shared with few other dissorophids (e.g., Aspidosaurus or Scapanops, see Schoch, Reference Schoch2012). The postorbital is broad, but bears a short, rapidly tapering posterior process, which reaches posteriorly the level of the anterior margin of the otic notch. The postorbital has a relatively wide suture with the supratemporal, therefore precluding contact between the squamosal and postfrontal, as in Cacops morrisi (Reisz et al., Reference Reisz, Schoch and Anderson2009). The postparietal, parietal, frontal, and postfrontal are poorly preserved. The frontal enters the orbital margin. It is slightly shorter than the nasal and especially very narrow in its posterior half (IO_w/S_l = 0.18). The frontal is more elongated than the lateral part of the orbit. The parietal is relatively short, and about twice as long as wide.

The sclerotic ring, which lies in the orbital opening, is composed of relatively small rectangular bony plates (Figs. 2, 3.2). Numerous and relatively large palpebral ossicles are situated at the medial margin of this sclerotic ring. Beside these ossicles, a very large palpebral cup is also visible in the orbital opening: it is much longer than the basal plate of the parasphenoid. The palpebral ossicles show a reticulated dorsal ornamentation, as seen on the skull roof bones, but their ornamentation pattern here is thinner and smaller in size. They have a smooth ventral surface. A large palpebral cup is also visible in Broiellus brevis, Broiellus texensis, and Brevidorsum profundum Carroll, Reference Carroll1964.

Palate

The palate of the right half-skull and the endocranium are nearly complete and well exposed in ventral view. The ventral surface of the palatal bones is flat and does not show any strutting pattern of larger ridges, as it is the case in eryopids (Werneburg and Berman, Reference Werneburg and Berman2012). Yet the vomer of Buxierophus n. gen. has a slightly striated posterior margin. The palatal bones are covered by denticle shagreens or patches. The interpterygoid vacuities are relatively large and semi-circular (i.e., with a maximum width at the level of their midlength) as in D. multicinctus and Broiellus brevis. In contrast, Cacops has narrower interpterygoid vacuities (Fröbisch and Reisz, Reference Fröbisch and Reisz2012; Schoch, Reference Schoch2012), and Kamacops and Parioxys have shorter ones (Schoch and Sues, Reference Schoch and Sues2022). Each palatine borders the interpterygoid vacuity along its entire medial margin. These interpterygoid vacuities are partially filled by numerous bony plates of various size, which are also covered by a shagreen of small teeth in ventral view. These bony plates form a series running along the posterior margin of the vomer and the medial palatine, but they covered the complete interpterygoid vacuities in life. This is also the case in Cacops morrisi (Reisz et al., Reference Reisz, Schoch and Anderson2009).

The vomer is relatively wide; therefore, the snout is not markedly pointed. Its posteromedian margin is straight horizontally, as in Broiellus reiszi (Holmes et al., Reference Holmes, Berman and Anderson2013) but not in the other dissorophids (Schoch, Reference Schoch2012, fig. 2). The vomer of Buxierophus n. gen. has probably two dorsal depressed areas in its anteromedian region, namely the paired anterior palatal fossae. Although these fossae are known in various temnospondyls (e.g., eryopids, plagiosaurids, or cochleosaurids), they are here probably unique among dissorophids. The vomer is covered with a dense shagreen of small teeth. It has a pair of relatively small fangs between the choana and anteropalatal fossa. These fangs are similar in size to those of the ectopterygoid, but smaller than those of the palatine. The fangs of the vomer in Cacops morrisi are much larger and as large as those on its palatine (Schoch, Reference Schoch2012). The vomer of Buxierophus n. gen. has no denticles along the medial edge of the choana. That is an additional difference from Cacops morrisi (Reisz et al., Reference Reisz, Schoch and Anderson2009; Schoch, Reference Schoch2012).

The palatine bears a large fang pair and has an elongated suture with the vomer and ectopterygoid. It enters the interpterygoid vacuity. The lateral exposed palatine is discussed above. The palatine and ectopterygoid are much wider than the maxilla. Both are covered by a shagreen of teeth and are relatively short, so that the ectopterygoid terminates anterior to the posterior margin of the orbit. This ectopterygoid bears a small pair of fangs (see above), which is known in other dissorophids (e.g., Cacops woehri, Fröbisch and Reisz, Reference Fröbisch and Reisz2012; Broiellus reiszi, Holmes et al., Reference Holmes, Berman and Anderson2013).

The pterygoid contacts the palatine but not the vomer. A shagreen of small teeth covers most of the pterygoid. The basipterygoid branch of the pterygoid is firmly sutured with the parasphenoid at mid-level of its widened basipterygoid process. Its palatine ramus is broadening abruptly and forms a pronounced transverse flange, giving its lateral margin a rectangular shape. Schoch (Reference Schoch2012) defined this feature as synapomorphic for the dissorophids. The transverse flange is completely covered by a shagreen of teeth in Buxierophus n. gen. in contrast to D. multicinctus (compare Schoch, Reference Schoch2012, figs. 3D, 4H). The quadrate condyle is relative narrow and intercalated between the pterygoid and quadratojugal.

The parasphenoid has a smooth ventral surface lacking denticles, which is characteristic for dissorophids, most branchiosaurids, and Phonerpeton Dilkes, Reference Dilkes1990 (Schoch, Reference Schoch2012). The carotid artery foramen pierces the anteromedian portion of the parasphenoid basal plate, as in most dissorophids. However, in Cacops, this foramen is situated on the posterior part of the cultriform process (Reisz et al., Reference Reisz, Schoch and Anderson2009; Fröbisch and Reisz, Reference Fröbisch and Reisz2012). In Buxierophus n. gen., the parasphenoid basal plate is much shorter than wide, as in Broiellus brevis (Schoch, Reference Schoch2012), but it differs from the narrower basal plate of all other dissorophids. The cultriform process is narrow and relatively thin. It is clearly narrower and slightly swollen in most of its anterior part. This feature is only shared with Kamacops (Schoch, Reference Schoch1999, Reference Schoch2012). The posterolateral wing of the parasphenoid plate is located well beyond the level of the basipterygoid suture. The posterior margin of the basal plate is crenulated with several narrow lobes and one pair of these lobes is more elongated (Fig. 5.5; compare the trematopid Acheloma Cope, 1882, cited in Maddin et al., Reference Maddin, Reisz and Anderson2010, fig. 4C).

Endocranium

The basioccipital and exoccipital are fused in a basioccipital-exoccipital complex. The width of this complex is slightly superior to the distance between the carotid foramina. The posterior part of the basioccipital-exoccipital complex is bilobed, forming the double cotyloform facet of the occipital condyle which contacts the atlas (Figs. 5.5, 10.2). The ventromedian part of the basioccipital-exoccipital complex is composed of two posterolateral facets and a pair of ventrally directed processes or facets (Fig. 5.5). The narrow opisthotic and the poorly preserved prootic-opisthotic complex are visible laterally to the basioccipital-exoccipital complex. These complexes together have the same width as the basal plate of the parasphenoid. The occipital condyle lies slightly anterior to the level of the jaw articulation, in contrast to D. multicinctus (Schoch, Reference Schoch2012, figs. 2, 3D). The configuration of the basioccipital and exoccipital is very variable in dissorophids (Schoch, Reference Schoch2012). The shape of the basioccipital-exoccipital complex in Buxierophus n. gen. is very similar than that of Broiellus brevis (Carroll, Reference Carroll1964). A small part of the sphenethmoid may be preserved dorsal to the cultriform process (Fig. 3.2).

Visceral skeleton

The right stapes is preserved nearly in its anatomical position. The stapedial footplate is widened and clearly pierced by the stapedial foramen which is combined with an elongated furrow. A small expansion, visible on the midshaft of the bone, may be the dorsal process. The stapes of dissorophids is rarely preserved. The stapes of Buxierophus n. gen. has the same shape as that of Broiellus reiszi but the later shows no stapedial foramen (Holmes et al., Reference Holmes, Berman and Anderson2013). Beyond Buxierophus n. gen., a stapedial foramen also has been reported in the stapes of Cacops morrisi (Schoch, Reference Schoch2012), D. cf. D. multicinctus (Gee et al., Reference Gee, Bevitt and Reisz2019b, fig. 14), and Aspidosaurus binasser (Berman and Lucas, Reference Berman and Lucas2003, p. 251).

Interestingly, Buxierophus n. gen. also shows ossified ceratobranchial elements. This is the first time a preserved ossified ceratobranchial skeleton has been reported in dissorophids. The right ceratohyal and one ceratobranchial (probably cb1) can be confidently recognized (Fig. 5). The ceratohyal is distinctly narrow in ventral view and greatly expanded dorsoventrally. It is very similar in shape to that of the eryopid Glaukerpeton avinoffi Romer, Reference Romer1952 (Werneburg and Berman, Reference Werneburg and Berman2012, figs. 9, 11, 12B). The probable ceratobranchial 1 has an only slightly expanded anterior end and a much more widened posterior end. Its anterior part clearly exhibits a median keel in ventral view. The extremities of the possible ceratobranchials 2 and 3 are preserved on the posterior end of the ceratobranchial 1. Its narrower extremity may be the anterior end of the bone, and the much more widened extremity may be the posterior end of the ceratobranchials. Additionally, two widened posterior ends of possible ceratobranchials are situated in the otic notch in dorsal view (Figs. 2.1, 3.1, 5.4). The ceratohyal and the probable original four ceratobranchials are used in the reconstruction of the visceral skeleton (Fig. 10.2).

Figure 5.

Detailed view of the palate, endocranium, and visceral skeleton of Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21). Ceratobranchial with deep groove: (1, 2) photo and interpretative drawing. (3) Ceratohyal in lateral and ventral views, interpretative drawing; compare with Figure 5.5. (4) Photo of ceratobranchials in dorsal view presented in otic notch, compare with Figures 2.1 and 3.1. (5) Interpretative drawing of the palate, visceral skeleton, endocranium, and first vertebrae. atl = atlas; ax = axis; beo = basi-exoccipital; c1–3 = coronoids 1-3; cb = ceratobranchials; chy = ceratohyal; cg = groove for carotid artery; cl = clavicle; op = opisthotic plate; p.c = cultriform process of parasphenoid; pop = prootic-opisthotic plate; ppl = palatal plates; ps = parasphenoid (basal plate); ps.p = basipterygoid process of parasphenoid; pt = pterygoid; sa = surangular; sq = squamosal; stp = stapes.

Mandible and dentition

Both rami of the lower jaw are partly visible in medial view. The right one and the coronoid-surangular process of the left one are used in the reconstruction of Figure 10.4. The mandible of Buxierophus n. gen. is relatively flat in comparison with the high posterior region of the lower jaw in Broiellus reiszi (Holmes et al., Reference Holmes, Berman and Anderson2013). The posterior part of the mandible in D. multicinctus (DeMar, Reference DeMar1968) is similarly flat as that of Buxierophus n. gen.

A coarse dermal sculpturing pattern extends onto the ventromedial surface of the angular and on the ventral side of the postsplenial. The anteriorly curved rim of the Meckelian fenestra may be preserved on the prearticular. This sculptured part of the postsplenial is rarely preserved in other dissorophids (Cacops morrisi, Gee and Reisz, Reference Gee and Reisz2018, fig. 10; Gee et al., Reference Gee, Bevitt and Reisz2019b, fig. 5; Cacops aspidephorus, Anderson et al., Reference Anderson, Scott and Reisz2020). An inflection of the prearticular along the medial rim of the adductor fossa (Dilkes, Reference Dilkes1990; Schoch, Reference Schoch2012) is absent. The posterior coronoid (c3) appears on the dorsal rim of the hemimandible as a process-like, rounded crest (Figs. 3.2, 10.4). Small denticles are preserved on the middle coronoid. A symphyseal depression is visible between the anterior coronoid and the dentary, but no details are preserved. A separated small bone fragment lies on the posterior end of the prearticular and belongs to the surangular. Two other small bony pieces in this region represent the articular.

The dentary teeth are relatively short, about 3 mm in height. One tooth is well preserved in labiolingual view (Fig. 3.1). Its base is rounded, and the tip of the crown has lateral blades. There is no evidence of labyrinthine infolding or pedicellate structure. The preserved teeth of the dentary show little recurvature. The typical, gradual decrease in tooth height in the posterior direction is not clearly visible.

Vertebrae and ribs

Remains of the first nine presacral vertebrae are visible, but without the dorsal portion of their neural spines (Fig. 6). The vertebrae are of rhachitomous type, which is common in dissorophids. The lateral shape of the intercentrum is triangular and that of the pleurocentrum is bean-like. Remains of the neural arches of the atlas and axis, with crest-like ridges, are preserved posteriorly to the basioccipital (Fig. 5.5). The mid-trunk neural arches have a large expansion of the spines along the sagittal axis (Fig. 6), as known from a few dissorophids (e.g., Cacops, see Dilkes and Brown, Reference Dilkes and Brown2007, fig. 2; Gee et al., Reference Gee, Bevitt and Reisz2019b, fig. 2; Diploseira, see Dilkes, Reference Dilkes2020, p. 15), but in contrast to other ones (Schoch, Reference Schoch2012, fig. 7). The postzygapophyses are very elongated posteriorly compared to those of the other dissorophids.

Figure 6.

Vertebral column with ribs of Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21): (1) interpretative drawing and (3) photo; (2) vertebral reconstruction with osteoderms in lateral view. az = anterior zygapophysis; cl = clavicle; cth = cleithrum; ex.os = external osteoderm; ic = intercentrum; in.os = internal osteoderm; na4–8 = neural arch 4–8; pc = pleurocentrum; pz = posterior zygapophysis; r = rib; rf = rib facet; sc.l = left scapulocoracoid.

Figure 7.

Osteoderms of Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21). (1, 2) Reconstruction of external and internal osteoderms in dorsal view and lateral views; (3, 4) interpretative drawing and photo. ex.os = external osteoderm; in.os = internal osteoderm.

Three anterior ribs are visible (Figs. 2, 3, 6.1, 7.4, 9.3). One of the most anterior ribs has a widened proximal and distal head and may be relatively short. There is no apparent uncinate process as well.

Osteoderms

Two series of osteoderms, comprising an internal and an external osteoderm for each vertebra, are recorded in the region of the 9th and 10th vertebrae (Fig. 7). They are covering the vertebral column dorsally and are transversely short. The osteoderms of the internal series are intersegmental in position and each of the external series may be arranged above the neural spine of the vertebrae. A ventrally projecting flange of the internal osteroderm inserts between successive neural spines but is not fused with the adjacent vertebrae (Fig. 6.2). This osteoderm pattern is well known in most representatives of the Dissorophinae (Carroll, Reference Carroll1964; DeMar Reference DeMar1968; Bolt, Reference Bolt1974a; Schoch, Reference Schoch2012), particularly in Dissorophus and most species of Broiliellus.

The osteoderms of the internal series have a step-like facet on the anterior margin and a pair of lobed facets on the posterior margin to articulate dorsally with the external osteoderms. The external osteoderms are very narrow, only 1.4 times wider than long (Fig. 8). They are as wide as the tabular of the skull roof. Such osteoderms are far narrower than those of the typical dissorophine, the external osteoderms of Dissorophus are 6 times wider than long and reach at least the width of two postparietals (Schoch, Reference Schoch2012).

Figure 8.

Comparison of the ornamentation pattern on osteoderms. (1) Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21). (2) “Alegeinosaurus aphthitos” Case, Reference Case1911, AMNH FARB 4756, which may be a junior synonym of Aspidosaurus sensu Gee, Reference Gee2018. (3) Aspidosaurus glascocki Case, Reference Case1910, AMNH 4864. (4–6) Aspidosaurus sp., MCZ 1477. (7) Dissorophus sp., MCZ 4188. ex.os = external osteoderm; in.os = internal osteoderm.

In Buxierophus n. gen., the external osteoderms are thin, about 1 mm thick. The dorsal surface of the external osteoderms is smooth but bears two lateral peaks and one median conspicuous peak. In nearly all other dissorophids, the dorsal surface of the osteoderms shows a characteristic reticulated ornamentation, as is the case on the skull roof and ventral pectoral girdle bones (Fig. 8.2–8.7). The osteoderms of the dissorophid ‘Aspidosaurus apicalis’ (Cope, Reference Cope1881) do not show this typical ornamentation of the skull either—they seemingly appear to be relatively flat but bear few robust tubercles (see Gee, Reference Gee2021, fig. 24). However, these thick osteoderms with sparse ornamentation are not comparable with the thin and smooth osteoderms of Buxierophus n. gen.

Pectoral girdle

The clavicle is completely preserved and shows a relatively short ventral blade with a striated dermal sculpture (Fig. 9). Its postclavicular process at the end of the ventral blade is pronounced anteriorly and similar to that of Dissorophus (DeMar, Reference DeMar1968) and of the amphibamiform Eoscopus (Daly, Reference Daly1994). The cleithrum is recorded with its relatively thick ventrally directed process, its dorsal end is not preserved. The small, preserved part of the interclavicle suggests that it had a short plate without a posterior process, like in Dissorophus (DeMar, Reference DeMar1968). The central parts of the scapulocoracoids indicate relatively narrow bony scapular blades. The dorsal margin of the glenoid foramen is preserved in lateral view. The pectoral girdle of Buxierophus n. gen. in lateral view is very similar to that of Dissorophus (Carroll, Reference Carroll1964; DeMar, Reference DeMar1968).

Figure 9.

Pectoral girdle of Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21). (1, 2) Reconstruction in ventral and lateral views. (3) Photo. cl = clavicle; cth = cleithrum; fsgl = supraglenoid foramen; gl = glenoid fossa; icl = interclavicle; lsr = lateral supraglenoid ridge; pcl = clavicular process; sc.r = right scapulocoracoid.

Figure 10.

Skull of Buxierophus pouilloni n. gen. n. sp., holotype RH-D 20-03-99 (21). (1–3) reconstructions in dorsal, ventral, and lateral views; (4) reconstruction of the mandible in medial view; (5) measurements on dorsal skull. Abbreviations (1–4): a = angular; adf = adductor mandibular fossa; ap = anterior palatal depression; ar = articular; beo = basi-exoccipital; c1–3 = coronoids 1–3; cb1–4 = ceratobranchials 1–4; ch = choana; chy = ceratohyal; ec = ectopterygoid; f = frontal; ipf = interpremaxillar fontanel; ipv = interpterygoid vacuity; j = jugal; j.e = jugal eminence; la = lacrimal; LEE = lateral exposed ectopterygoid; LEP = lateral exposed palatine; m = maxilla; md = mandible; mf = Meckelian fenestra; n = nasal; n.o = narial opening; op = opisthotic plate; ot.n = otic notch; p = parietal; pa = prearticular; p.c = cultriform process of parasphenoid; pl = palatine; pm = premaxilla; po = postorbital; pop = prootic-opisthotic plate; pp = postparietal; ppc = palpebral cup; ppl = palatal plates; prf = prefrontal; prsp = presplenial; ps = parasphenoid; psp = postsplenial; pt = pterygoid; ptf = postfrontal; q = quadrate; qj = quadratojugal; sa = surangular; sm = septomaxilla; sq = squamosal; st = supratemporal; stf = subtemporal fenestra; stp = stapes; sy = symphyseal region; t = tabular; v = vomer. Abbreviations (5): aS_w = anterior skull width at level of maxilla-premaxilla sutures; H_l = postorbital midline skull length from level of posterior margins of orbits; H_w = postorbital skull width between lateral margins of supratemporals; IN_w = minimum internarial width; IO_w = minimum interorbital width; mS_w = midlength skull width at midlength level of orbits; Occ-Q = length between levels of occipital midline margin and posterior quadrate margin; ON-O = length between levels of anterior otic notch margin and posterior orbital margin; O_l = maximum length of orbit; PO_l = preorbital midline skull length from level of anterior margins of orbits; Po_l = maximum posterior length of postorbital from posteriormost extent of orbit; pS_w = maximum posterior skull width at level of posterolateral margins of cheeks; S_l = midline skull length; Th_l = length of tabular horn between levels of posterior tabular corner and occipital midline margin; W_w = maximum transverse width of cheek from lateral margin of supratemporal anterior to otic notch.