Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998

Baron, Matthew G. & Williams, Megan E., 2018, A re-evaluation of the enigmatic dinosauriform Caseosaurus crosbyensis from the Late Triassic of Texas, USA and its implications for early dinosaur evolution, Acta Palaeontologica Polonica 63 (1), pp. 129-145 : 135-141

Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998

Figs. 1 , 2 .

Holotype: UMMP 8870 , a partial right ilium.

Type horizon:?late Carnian, Late Triassic ( Nesbitt et al. 2007).

Type locality: Tecovas Formation, Dockum Group, Crosby County, Texas, USA; Snyder Quarry, Petrified Forest Member, Chinle Formation, Rio Arriba County, New Mexico, USA.

Material. — NMMNH P-35995, a partial right ilium.

Emended diagnosis.— Caseosaurus crosbyensis represents a valid taxon and a most likely represents a member of the clade Herrerasauria . Caseosaurus crosbyensis is unique among herrerasaurids and dinosaurs in possessing a sharp, well developed ridge that connects the midpoint of the supraacetabular crest to the preacetabular process. Furthermore, Caseosaurus crosbyensis can be referred to

50 mm medioventral supraacetabular acetabular wall crest

Herrerasauria based upon the possession of three of the clade’s synapomorphies: a postacetabular process that is 25–35% of the total length of the length of the iliac blade and a supraacetabular crest that extends down part of pubic peduncle as a ridge without reaching the distal end*; possession of extensive rugosities on the pre- and postacetabular processes* (also present in a similar form in silesaurids, and in a slightly different form in Saturnalia tupiniquim and Coelophysis bauri ). In addition to these features, Caseosaurus crosbyensis possesses a number of other features that can be observed in all other herrerasaurs,including: a preacetabular process of the ilium that expands mediolaterally towards its distal end* (also in silesaurids); absence of a brevis fossa. Furthermore, Caseosaurus crosbyensis and NMMNH P-35995 possess an ischiadic peduncle that is less ventrally extensive than the pubic peduncle in medial and lateral aspect, a condition that is also present in Herrerasaurus ischigualastensis and Staurikosaurus pricei . Caseosaurus crosbyensis can be distinguished from Herrerasaurus ischigualastensis and Staurikosaurus pricei by possessing a preacetabular process that is more than twice as long as it is deep*; having a sharp, distinct anterodorsal ridge that runs from the middle of the supraacetabular crest to the preacetabular process*; and the absence of an acetabular antitrochanter* (which also distinguishes Caseosaurus crosbyensis from Chindesaurus bryansmalli ). Caseosaurus crosbyensis can be further distinguished from Staurikosaurus pricei as it possesses a preacetabular iliac process that is much shorter than the postacetabular process of the ilium (also present in Herrerasaurus ischigualastensis , Eoraptor lunensis , and Eodromaeus murphi ). Further to these distinguishing features, Caseosaurus crosbyensis appears to differ from other herrerasaurs in a several other respects. For example, while Caseosaurus crosbyensis possesses rugose areas on the dorsal and lateral portions of the pre- and postacetabular processes, these areas are less extensive than they are in Herrerasaurus ischigualastensis and Staurikosaurus pricei ( Fig. 6 ). Furthermore, the supracetabular crest of Caseosaurus crosbyensis describes a semicircle in lateral view*, differing from the condition in Herrerasaurus ischigualastensis and Staurikosaurus pricei , in which the supraacetabular crest is not semicircular but instead forms a straighter, anteroventrally oriented lip over the main body of the acetabulum ( Fig. 6 ). For the anatomical features listed above, (*) represents those that are present in both the holotype and the referred specimen of Caseosaurus crosbyensis ; the single autapomorphy of the taxon is also observed in both specimens

Description.— The holotype specimen is missing only the middle portions of the dorsal margin, as well as a small portion of the middle section of the iliac body that lies immediately above the supraacetabular rim. Further to this, judging by the shape of the preserved ventral margin of the ilium, around the acetabulum, a portion of a medioventral acetabular wall that would normally partially close the acetabulum is also missing. Towards its dorsal margin, UMMP 8870 bears rugose areas on the lateral and dorsal surfaces of the pre- and postacetabular processes. This is similar to the condition seen in Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Galton 1977; Bittencourt and Kellner 2009; Delcourt et al. 2012). However, these rugosities, while still quite extensive in Caseosaurus crosbyensis , appear to cover relatively smaller portions of the lateral and dorsal faces of the pre- and postacetabular processes than they do in Herrerasaurus ischigulastensis (PVL 2566) , a specimen in which the rugosities form large, bulbous areas on the majority of both the lateral and dorsal surfaces of the pre- and postacetabular processes. In H. ischigualastensis , these rugosities are so pronounced that they form the dominant anatomical feature of the dorsal part of the ilium, partly obscuring other features, such as the brevis shelf ( Fig. 6 ). Some early sauropodomorph taxa such as Saturnalia tupiniquim and Chromogisaurus novasi also possess a type of rugose area on the postacetabular process of the ilium, but in these taxa this rugose area is restricted to a small trapezoidal area near the posterior margin of the postacetabular process ( Ezcurra 2010: fig. 17), rather than being developed across a broad area upon the postacetabular process, with no clear and geometric boundary. The same reduced and limited rugositites can also be observed in the ilium of Coelophysis bauri (e.g., USNM 529376). Given the differences in form and extent of the rugosities among these taxa, the potential homology of these features is difficult to determine. However, the differences in the form of this feature among these taxa is worth noting.

The preacetabular process of UMMP 8870 extends from the main body of the ilium as an anterodorsally oriented projection. The preacetabular process is mediolaterally and dorsoventrally expanded at its dorsal end, similar to the condition seen in Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Galton 1977; Delcourt et al. 2012). This condition is also similar to that in several silesaurid taxa, such as Silesaurus opolensis (ZPAL Ab III/361; Dzik 2003; Peecook et al. 2013) and Ignotosaurus fragilis ( Martinez et al. 2013) . This differs from the condition in other dinosaurian taxa. For example, in more derived sauropodomorphs, such as Efraasia minor , the preacetabular process is not expanded mediolaterally or dorsoventrally, but instead takes the form of a short, relatively flat and unexpanded triangular projection ( Galton 1973; Yates 2003b; Delcourt et al. 2012); in Saturnalia tupiniquim the preacetabular process is slightly expanded mediolaterally and dorsoventrally at its dorsal point, but to a lesser degree than in herrerasaurs and silesaurids. In early ornithischians the preacetabular process is elongate and strap-like and lacks the expansion at its distal end, for example the preacetabular process of Lesothosasrus diagnosticus (NHMUK PV RU B17; Baron et al. 2017b). In theropod taxa, such as Coelophysis bauri , the preacetabular process is anteriorly oriented, dorsoventrally deep and square in lateral view, but relatively unexpanded mediolaterally at its dorsal end (AMNH 7223, AMNH 7224, USNM 529376). The lateral face of the preacetabular process in Caseosaurus crosbyensis also bears an oval-shaped rugose area, similar to that seen in some silesaurids, such as Silesaurus opolensis (ZPAL Ab III/361; Dzik 2003) and Lutungutali sitwensis ( Peecook et al. 2013) ; this rugose area does not extend far onto the dorsal surface in Caseosaurus crosbyensis and silesaurids, contrasting with the condition in Herrerasaurus ischigualastensis (PVL 2566) , in which the rugose area on the preacetabular process is more extensive and covers a larger portion of the dorsal surface. The condition in Staurikosaurus pricei appears to be somewhere between the conditions seen in Caseosaurus crosbyensis and Herrerasaurus ischigualastensis , in that in Staurikosaurus pricei the rugose area appears to extend some of the way on to the dorsal portion of the preacetabular process, but not as far as the same rugose area does in Herrerasaurus ischigualastensis ( Galton 1977; Delcourt et al. 2012). In Coelophysis bauri a small rugose area is present but is restricted to the dorsal surface only, and the surface of this area is far smoother than the areas in silesaurids, herrerasaurids and Caseosaurus crosbyensis .

The postacetabular process of the holotype of Caseosaurus crosbyensis also bears rugosities on its lateral and dorsal surfaces ( Figs. 1 , 2 , 6 ). These rugose areas in Caseosaurus crosbyensis are similar to those that are seen in Herrerasaurus ischigualastensis (PVL 2566) but are less extensive in the former ( Fig. 6 ). Similar rugose areas also appear in some silesaurids ( Fig. 7 ); Silesaurus opolensis (ZPAL Ab III/361; Dzik 2003), Lutungutali sitwensis ( Peecook et al. 2013) and Ignotosaurus fragilis ( Martinez et al. 2013) all have rugose areas on the dorsal and lateral surfaces of postacetabular process. Again, Staurikosaurus pricei has a condition that appears to be somewhere between Caseosaurus crosbyensis and Herrerasaurus ischigualastensis in terms of overall extent ( Delcourt et al. 2012). Such laterally located rugose areas are not seen in early sauropodomorphs (e.g., Panphagia ; PVSJ 874), theropods (e.g., Coelophysis bauri, AMNH 7223, AMNH 7224; Cope 1887; Colbert 1989) or ornithischians (e.g., Lesothosaurus diagnosticus, NHMUK PV RU B 17; Barrett et al. 2014); some dorsally extensive rugosities can be observed in Saturnalia tupiniquim (e.g., MCP 3844-PV) and Coelophysis bauri (USNM 529376). Ventrally, the postacetabular process of the holotype of Caseosaurus crosbyensis bears a small excavation or fossa, and this feature was considered to be the brevis fossa by Hunt et al. (1998). However, Nesbitt et al. (2007) did not consider this fossa to be homologous to the brevis fossa present in a number of other dinosaurian and silesaurid taxa; these authors argued that because the fossa present in the holotype of Caseosaurus crosbyensis is not associated with a distinct ridge it does not meet the criteria given in their definition of a true brevis fossa, nor the definition given by Novas (1992). This study agrees with the observations of Nesbitt et al. (2007) and does not consider the fossa in Caseosaurus crosbyensis to represent a brevis fossa. In our phylogenetic analyses, we do not score Caseosaurus crosbyensis as possessing a brevis fossa that is homologous to those seen in a range of other dinosauriform taxa. The fossa in question is shallow and can be seen in lateral view and lies directly ventral to a slight, rounded, anteroposteriorly extending swelling on the lateral face of the postacetabular process ( Fig. 1 ). The rugose areas are located dorsal to this swelling in Caseosaurus crosbyensis ( Fig. 1 ), which distinguishes them from those present on the postacetabular processes of Saturnalia tupiniquim and Chromogisaurus novasi , which are ventrally located scars associated with muscle origins ( Langer 2003; Ezcurra 2010). The shape of the distal end of the postacetabular process of Caseosaurus crosbyensis is also different from that in sauropodomorph taxa like Saturnalia tupiniquim and Chromogisaurus novasi . In Caseosaurus crosbyensis the postacetabular process is rounded to square in lateral and medial views, with slightly greater posterior extension occurring ventrally than dorsally, whereas in Saturnalia tupiniquim , Chromogisaurus novasi , and Panphagia protos (PVSJ 874), the postacetabular process is more trapezoidal ( Langer 2003; Ezcurra 2010). In herrerasaurids, such as Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Galton 1977; Bittencourt and Kellner 2009; Delcourt et al. 2012), the postacetabular process is also rounded to square in lateral and medial views, which is more similar to the condition in Caseosaurus crosbyensis . In Tawa hallae (GR 155; Nesbitt et al. 2009b) the postacetabular process is also rounded to square, but with even greater posterior extension occurring ventrally; in Coelophysis bauri the postacetabular is squared posteriorly. Also in Tawa hallae (GR 155; Nesbitt et al. 2009b), the lateral face of the postacetabular process is dominated by a distinct anteroposteriorly oriented ridge. In other theropods, such as Coelophyis bauri (AMNH 7223, AMNH 7224; USNM 529376; Cope 1887; Colbert 1989; Fig. 8 ), the greater posterior expansion occurs dorsally rather than ventrally, a reverse of the condition in seen in Caseosaurus crosbyensis , herrerasaurids and Tawa hallae ; in all other respects the morphology of the postacetabular process in early theropods is similarly round to square-ended. On its medial side, the postacetabular process bears a large longitudinal ridge in Caseosaurus crosbyensis , as in most archosaurs ( Nesbitt 2011; Fig. 1 ).

The supraacetabular crest appears to projects laterally from the iliac body in Caseosaurus crosbyensis , although it is broken laterally. In lateral aspect the crest forms a smooth semicircle before continuing as a faint ridge along the ventral portion of the pubic peduncle, terminating dorsal to the ventral most point of the peduncle; in most theropod dinosaurs, such as Tawa hallae and Coelophysis bauri , this crest reaches the ventral most point of the peduncle. The semicircular shape of the supraacetabular crest contrasts with the condition seen in Herrerasaurus ischigualastensis and early sauropodomorphs, such as Saturnalia tupiniquim and Chromogisaurus novasi , in which the supraacetabular crest follows a less curved line along the lateral surface of the ilium and slopes more anteroventrally towards the pubic peduncle ( Langer 2003; Ezcurra 2010). The condition in another herrarasaurid taxon, Staurikosaurus pricei , appears more similar to that of Caseosaurus crosbyesnsis ,.e., a smoother semicircular crest (see Delcourt et al. 2012). Outside of Herrerasauria the shape of the supraacetabular crest has a complex distribution. While some silesaurids possess a smooth semicircular crest, such as Silesaurus opolensis (ZPAL AbIII404/1; Peecook et al. 2013) and Asilisaurus kongwe ( Peecook et al. 2013) , others do not, as for example Lutungutali sitwensis has a straighter, sloped supraacetabular crest ( Peecook et al. 2013). Extending anterodorsally from the midpoint of the supraacetabular crest in Caseosaurus crosbyensis is a distinct, sharp crest, which extends up to the anterodorsal, bulb-like tip of the preacetabular process ( Figs. 1 , 2 ). This sharp crest, or ridge, is delineated on both sides by concave surfaces and distally it merges into the oval-shaped rugose area of the preacetabular process. This is like the condition seen in many silesaurids, such as Silesaurus opolensis , Asilisaurus kongwe , Sacisaurus agudoensis , Lutungutali sitwensis , and Ignotosaurus fragilis ( Dzik 2003; Ferigolo and Langer 2006; Nesbitt et al. 2010; Martinez et al. 2013; Peecook et al. 2013; Fig. 7 ). In Herrerasaurus ischigualastensis and Staurikosaurus pricei there is a lower, more rounded and less distinct anterodorsally oriented swelling that extends from the same point on the supraacetabular crest as the distinct, sharp ridge in Caseosaurus crosbyensis does. In these herrerasaurid taxa, there is less of a distinction between this low crest and the rest of the lateral surface of the ilium. Similar, low crests also appear in some early sauropodomorphs, such as Saturnalia tupiniquim ( Langer 2003) , in ornithischians, such as Lesothosaurus diagnosticus (NHMUK PV RU B17), and in theropods, such as Coelophysis bauri (AMNH 7223, AMNH7224, USNM 529376; Cope 1887; Colbert 1989). Nesbitt (2011) reported the presence of a sharp and narrow ridge in the Triassic sauropodomorph Saturnalia tupiniquim and scored this taxon for having a distinct ridge that could be considered as homologous to the ridges that are see in silesaurids. However, while a small ridge is certainly present between the supraacetabular crest and the preacetabular process in Saturnalia , this ridge does not extend from the dorsal margin of the supraacetabular crest, does not extend fully onto the lateral surface of the preacetabular process and is not bordered posterodorsally and anteroventrally by smooth concave surfaces ( Langer 2003), but instead takes the form of a small, isolated, almost vertically oriented tuberosity in Saturnalia ( Langer 2003) . In this regard, the ridge in Saturnalia differs considerably from the ridges seen in the silesaurid taxa). We therefore do not consider this ridge in Saturnalia to be homologous to those seen in Caseosaurus crosbyensis, NMMNH P-35995, and silesaurids. As discussed above, Saturnalia appears to possess a low and indistinct anterodorsally oriented crest that does connect the supraacetabular crest to the preacetabular process, in addition to the small tuberosity described as a ridge by Nesbitt (2011). We consider the former to be homologous to the crests seen in other dinosaurs and silesaurids, and our scoring for this character (character 305; Baron et al. 2017a) reflects this.

Ventral to the supraacetabular crest, a medioventral acetabular wall extends ventrally and, while incomplete, this feature appears to be well developed and ventrally extensive in Caseosaurus crosbyensis , judging from the extent of the fragmentary remains that appear on the posterior margin of the pubic peduncle ( Fig. 1 ). All silesaurids have extensive acetabular walls that fully close the acetabulum. Similarly, extensive acetabular walls exist in a number of early sauropodomorphs, such as Saturnalia tupiniquim and Chromogisaurus novasi ( Langer 2003; Ezcurra 2010). However, in these early sauropodomorph taxa, the wall is extensive but does not fully close the acetabulum, as it does in silesaurids. The herrerasaurid Herrerasaurus ischigualastensis also has a medioventral acetabular wall (PVL 2566), but this feature is less ventrally extensive than those in silesaurids and early sauropodomorphs. In Herrerasaurus ischigualastensis the acetabulum also appears to be perforate. The exact condition in Staurikosaurus pricei is hard to assess, as the acetabular wall is broken. However, enough remains of the acetabular wall on the posterior margin of the pubic peduncle in Staurikosaurus to see that the wall is ventrally extensive. Based on what remains of the wall in Caseosaurus crosbyensis it is possible to say that a wall is present and probably quite ventrally extensive as well, but whether or not the acetabulum is fully open, partially open or imperforate is difficult to say, as in Staurikosaurus . It seems likely that the two taxa shared morphology similar to that of Herrerasaurus ischigualastensis . Nesbitt et al. (2007) regarded the acetabulum of Caseosaurus crosbyensis as being at least partially perforate, which they used as evidence when arguing that the taxon could be assigned to Dinosauriformes. We agree that, based upon the preserved material of Caseosaurus crosbyensis , a perforate acetabulum seems to be the likely morphology present in this taxon; we score this taxon for the presence of a perforate acetabulum in each of the phylogenetic analyses that we carried out.

The pubic peduncle in Caseosaurus crosbyensis is a long, anteroventrally oriented projection from the main body of the ilium. For its entire length, the posterior margin of the peduncle includes the remnants of the medioventral acetabular wall. Anteroventrally the pubic peduncle flares out anteroposteriorly, as is also the condition in some dinosaurs, including Tawa hallae , and in herrerasaurs, but not in theropods such as Coelophysis bauri (USNM 529376). In Caseosaurus crosbyensis the level of anteroposterior expansion of the anteroventral end is proportionally greater than it is in Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Galton 1977; Delcourt et al. 2012), as well as in silesaurids ( Dzik 2003; Peecook et al. 2013), but less than it is in the early theropod Tawa hallae (GR 155; Nesbitt et al. 2009b). The ischiadic peduncle in Caseosaurus crosbyensis also includes part of the acetabular wall. The ischadic peduncle is smaller than the pubic peduncle and is almost indistinct from the acetabular wall that lies anterior to it. This is very similar to the condition in herrerasaurids, such as Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Galton 1977; Delcourt et al. 2012), and in silesaurids such as Ignotosaurus fragilis ( Martinez et al. 2013) and Silesaurus opolensis ( Dzik, 2003) ( Fig. 7 ). In other dinosaurs, such as the theropods Coelophysis bauri (AMNH 7223, AMNH 7224; Cope 1887; Colbert 1989) and Tawa hallae (GR 155; Nesbitt et al. 2009b), and some sauropodomorphs, such as Efraasia minor ( Galton 1973; Yates 2003b), the ischiadic peduncle is quite distinct from the medioventral acetabular wall. In other dinosaurian taxa, such as the early ornithischian Lesothosaurus diagnosticus (NHMUK PV RU B17; Baron et al. 2017b) and other sauropodomorphs like Saturnalia tupiniquim ( Langer 2003) , the ischiadic peduncle and medioventral acetabular wall do form a continuous surface but the peduncle appears a little more distinct than it does in Herrerasaurus ischigualastensis and Caseosaurus crosbyensis .

NMMNH P-35995 from the Snyder Quarry is less complete than the holotype of Caseosaurus crosbyensis (UMMP 8870); it is missing a greater proportion of its dorsal margin and almost all of its posterior section, including all of the postacetabular process ( Fig. 3 ). However, in all of the areas that are preserved, NMMNH P-35995 bears a striking resemblance to UMMP 8870. Nesbitt et al. (2007) discussed the similarities between NMMNH P-35995 and the holotype of Caseosaurus crosbyensis in some detail, although they did not refer NMMNH P-35995 to Caseosaurus crosbyensis . The similarities that they noted were “a short, pointed anterior process of the ilium, a strong ridge trending anterodorsally from the acetabular rim to the anterior preacetabular process, a wide, open angle between the anterior process and the pubic process, a moderately developed supraacetabular rim and an ischiadic process with a rounded distal end that is dorsal to the distal end of the pubic process” ( Nesbitt et al. 2007: 217). Further to this Nesbitt et al. (2007) stated that none of these features are present in the theropod taxa Coelophysis bauri (AMNH 7223), Coelophysis rhodesiensis Raath, 1969 ), “ Syntarsus ” kayentakatae ( Rowe, 1989), and Dilophosaurus wetherilli ( Welles, 1954) . Overall, we agree with this anatomical assessment by Nesbitt et al. (2007). We would also add that, of these distinguishing characters, a short, pointed anterior process (preacetabular process), a strong ridge trending anterodorsally from the acetabular rim to the anterior preacetabular process and a wide, open angle between the anterior process and the pubic process are also absent in Herrerasaurus ischigualastensis (PVL 2566) . However, an ischiadic process with a rounded distal end that is dorsal to the ventral extent of the pubic process is present in Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei ( Colbert 1970; Galton 1977; Delcourt et al. 2012) and that a wide, open angle between the anterior process and the pubic process, as described in UMMP 8870 and NMMNH P-35995, is also present in Staurikosaurus pricei ( Colbert 1970; Galton 1977; Delcourt et al. 2012). Generally, we also note that the shared features of UMMP 8870 and NMMNH P-35995, as described by Nesbitt et al. (2007), are not present in ornithischian dinosaurs, such as Lesothosaurus diagnosticus (NHMUK RU B17) and Heterodontosaurus tucki (SAM-PK-K1332), in which the preacetabular process is long and “strap-like” and the ischiadic process (= peduncle) is not as reduced. Furthermore, Lesothosaurus diagnosticus and Heterodontosaurus tucki do not possess the wide, open angle between the anterior process and the pubic process that is seen in UMMP 8870, NMMNH P-35995, and Staurikosaurus pricei ( Galton 1977; Delcourt et al. 2012), nor do theropods such as Coelophysis bauri (e.g., USNM 529376). Adding further comparisons to those provided by Nesbitt et al. (2007), we note that there is no strong, sharp ridge trending anterodorsally from the acetabular rim to the anterior preacetabular process or a reduction on the ischiadic process in some basal sauropodomorphs, for example Pantydrao caducus ( Yates 2003a) and Efraasia minor ( Galton 1973; Yates 2003b; Delcourt et al. 2012). In other sauropodomorph taxa, such as Saturnalia tupiniquim and Chromogisaurus novasi , there is a slight reduction in the ischiadic peduncle ( Langer 2003; Ezcurra 2010), but the difference in the ventral extensions of the pubic and ischiadic peduncles is less than it is in UMMP 8870 and NMMNH P-35995, as well as in the herrerasaurids Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei Galton 1977 ; Delcourt et al. 2012). However, we note that in Saturnalia tupiniquim ( Langer 2003) and Chromogisaurus novasi ( Ezcurra 2010) the pubic peduncle is not as long as it is in UMMP 8870, NMMNH P-35995, Herrerasaurus ischigualastensis (PVL 2566) and Staurikosaurus pricei Galton 1977 ; Delcourt et al. 2012), which may explain why the reduction of the ischiadic peduncle in Saturnalia tupiniquim and Chromogisaurus novasi does not appear to be as great as it is in UMMP 8870, NMMNH P-35995, and herrerasaurids. We also note that, for the feature described by Nesbitt et al. (2007) as a strong ridge trending anterodorsally from the acetabular rim to the anterior preacetabular process, it is the form of this feature, rather than its absence or presence, that is the main discriminating factor between UMMP 8870/NMMNH P-35995 and other dinosaurs; there is a crest trending anterodorsally from the acetabular rim to the preacetabular process in many dinosaurian taxa, but it is only in UMMP 8870 and NMMNH P-35995 (and some silesaurids: see above) that this crest appears as a sharp, distinct ridge. In addition to the features that were discussed by Nesbitt et al. (2007), we also make the following observations about the anatomy of the specimen NMMNH P-35995.

NMMNH P-35995, much like UMMP 8870, preserves a complete preacetabular process that takes the form of a thin, anterodorsally extending process that swells dorsally, both mediolaterally and dorsoventrally, into a rounded end. The lateral surface of this process bears a distinct rugose area that also extends partly on to the dorsal surface. The rugose area on the lateral surface of the preacetabular process is connected to the middle of supraacetabular crest by a sharp, distinct ridge. The supraacetabular crest is partly broken in NMMNH P-35995 but is more complete than in UMMP 8870. In lateral view the supraacetabular crest in NMMNH P-35995 takes the form of a smooth semicircle above the acetabulum, like in Tawa hallae and Coelophysis bauri (USNM 529376), but only extends some of the way along the length of the pubic peduncle, as in UMMP 8870, but unlike in Coelophysis bauri . Also like UMMP 8870, NMMNH P-35995 shows evidence of a medioventral acetabular wall that would at least partially close the acetabulum. This wall appears to be extensive in NMMNH P-35995, but it is hard to state with confidence whether or not the acetabulum was completely imperforate. The pubic peduncle in NMMNH P-35995 is longer than the ischiadic peduncle, which is reduced and rounded distally. Unfortunately, the postacetabular process (and much of the dorsal iliac blade) is missing in NMMNH P-35995, so the presence, and possible extent, of rugose areas on the postacetabular process cannot be assessed in this specimen.

Despite the many similarities between the specimens, UMMP 8870 and NMMNH P-35995 can also be distinguished in several minor features. Firstly, the acetabulum of NMMNH P-35995 is deeper relative to the height of the iliac blade above the acetabulum than it is in the holotype of Caseosaurus crosbyensis (based upon the dorsal extent of the preacetabular process). This difference in relative dorsoventral depth of the ilia above and below the acetabulum could be related to size. NMMNH P-35995 is smaller than UMMP 8870. UMMP 8870 has a maximum dorsoventral depth of 86.9 mm whereas NMMNH P-35995 has a maximum depth of 57.8 mm. A second difference between UMMP 8870 and NMMNH P-35995 is in the angle between the preacetabular process and the pubic peduncle. In UMMP 8870 this area takes the form of a rounded, wide gap in which the preacetabular process and the pubic peduncle diverge at an obtuse angle. In NMMNH P-35995 this gap is smaller and the angle between the preacetabular process and the pubic peduncle is acute. Despite this subtle difference, Nesbitt et al. (2007) still considered a wide, open angle between the anterior process and the pubic process to be a feature uniting NMMNH P-35995 and UMMP 8870. An acute angle between the preacetabular process and the pubic peduncle is observed in basal sauropodomorphs, such as Saturnalia tupiniquim and Chromogisaurus novasi ( Langer 2003; Ezcurra 2010; Delcourt et al. 2012), in ornithischians, such as Heterodontosaurus tucki (SAM-PK-K1332) and larger specimens of Lesothosaurus diagnosticus (e.g., NHMUK PV R11000; Baron et al. 2017b), in theropods, such as Coelophysis bauri (AMNH 7223, AMNH 7224; Cope 1887; Colbert 1989) and in Herrerasaurus ischigualastensis (PVL 2566) . Conversely, Staurikosaurus pricei has an obtuse angle between the preacetabular process and the pubic peduncle ( Galton 1977; Delcourt et al. 2012), as do the smaller specimens of the ornithischian Lesothosaurus diagnosticus (NHMUK PV RU B17; BP/1/4731; Baron et al. 2017b). Silesaurids also exhibit something of an open angle between the preacetabular process and the pubic peduncle, with most taxa having an angle close to 90° or slightly acute (e.g., Silesaurus opolensis ; Dzik 2003). Currently, whether or not this feature has any taxonomic significance remains to be seen, and none of the datasets used in this study account for this feature through the inclusion of an anatomical character. The intraspecific variation that is seen in Lesothosaurus diagnosticus might suggest that this feature is size related (see Baron et al. 2017b), and this may explain the observed difference between NMMNH P-35995 and the holotype of Caseosaurus crosbyensis .

Remarks. — NMMNH P-35995 is referable to Caseosaurus crosbyensis on the basis of a shared autapomorphy, the possession of a sharp, well developed ridge connecting the midpoint of the supraacetabular crest to the preacetabular process, as well as a number of features that are unique to UMMP 8870 and NMMNH P-35995 among members of Herrerasauria : the possession of a preacetabular process that is more than twice as long as it is deep (for information on how this measurement was taken, see SOM); possession of a sharp and distinct anterodorsal ridge that runs from the middle of the supraacetabular crest to the preacetabular process of the ilium (also present in members of Silesauridae , but absent in herrerasaurs, and dinosaurs like Tawa hallae and Eodromaeus murphi ); a supraacetabular crest that forms a semicircle in lateral view, rather than a straighter, anteroventrally oriented lip.

Stratigraphic and geographic range.— Tecovas Formation, Dockum Group,Crosby County, Texas, USA; Snyder Quarry, Petrified Forest Member, Chinle Formation, Rio Arriba County, New Mexico, USA.

Figures

Fig. 1. The holotype of the herrerasaurian dinosauriform Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998 (UMMP 8870) from Tecovas Formation, Crosby County, Texas, USA; Carnian, Late Triassic; right ilium in lateral (A) and medial (B) views.

Fig. 2. The holotype of the herrerasaurian dinosauriform Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998 (UMMP 8870) from Tecovas Formation, Crosby County, Texas, USA; Carnian, Late Triassic; right ilium in lateral view; stereopair. Image credit: Adam Rountrey, Museum of Paleontology, University of Michigan, Ann Arbor, USA.

Fig. 6. The herrerasaurian dinosauriform Herrerasaurus ischigualastensis Reig, 1963 (PVL 2566) from Carnian, Late Triassic, Ischigualasto Formation, Hoyada de Ischigualasto, San Juan Province, Argentina; ilium in lateral view.

Fig. 7. Comparison of the ilia of Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998 (A, B) and Silesaurus opolensis Dzik, 2003 (C) showing locations of rugosities and lateral ridge/buttress. A. UMMP 8870, right ilium in lateral view. B. NMMNH P-35995, right ilium in lateral view. C. ZPAL AbIII404/1, left ilium in lateral view (reversed).

Fig. 8. Comparison of the ilia of early dinosaurs. A. Buriolestes schultzi. B. Saturnalia tupiniquim. C. Agnosphitys cromhallensis. D. Tawa hallae. E. Coelophysis bauri. F. Lesothosaurus diagnosticus.

Fig. 3. The herrerasaurian dinosauriform Caseosaurus crosbyensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998 (NMMNH P-35995) from Synder Quarry, Rio Arriba County, New Mexico, USA; Carnian, Late Triassic; right ilium in lateral (A) and medial (B) views.