Osmakasaurus depressus Gilmore, 1909

Mcdonald, Andrew T., 2011, The taxonomy of species assigned to Camptosaurus (Dinosauria: Ornithopoda), Zootaxa 2783, pp. 52-68 : 61-64

Osmakasaurus depressus Gilmore, 1909 , comb. nov.

1909 Camptosaurus depressus Gilmore , p. 292.

2004? Camptosaurus depressus Norman , p. 415.

2008 Planicoxa depressa Carpenter & Wilson , p. 257.

Holotype. USNM 4753, fragmentary postcranium.

Generic etymology. The generic name is derived from ósmaka, which means “canyon” in the language of the Lakota People, in reference to the name of the type locality in western South Dakota, an area inhabited by the Lakota. Saurus comes from the transliterated Greek meaning “lizard”. The gender of the generic name is masculine.

Locality and horizon. Calico Canyon, near Buffalo Gap, Custer County, South Dakota ( Gilmore 1909); Lakota Formation, Barremian–Aptian ( Tschudy et al. 1984).

Generic diagnosis. As for the species by monotypy.

Specific diagnosis. Basal styracosternan distinguished from all other iguanodontians by the following unique combination of characters: cranial end of preacetabular process of ilium modified into horizontal boot, straight dorsal margin of ilium, dorsal margin of ilium thickens mediolaterally towards the M. iliocaudalis platform ( Figs. 8 A, B, E).

Remarks. Camptosaurus depressus was originally distinguished from other species of Camptosaurus chiefly by the “narrowness or depressed nature of the ilia” ( Gilmore 1909:293). This condition is exhibited by only the left ilium of USNM 4753, on which the dorsal margin of the ilium above the acetabulum faces dorsolaterally and the postacetabular process is nearly horizontal ( Fig. 8 A). Carpenter & Wilson (2008) considered the horizontal postacetabular process of USNM 4753 similar to that of DMNH 42504, the holotype ilium of Planicoxa venenica ( DiCroce & Carpenter 2001) ; thus, they referred C. depressus to Planicoxa , as P. depressa . However, the horizontal postacetabular process of USNM 4753 is more likely a product of distortion.

The blade of the left ilium of USNM 4753 is heavily fractured by corresponding cracks on the lateral and medial surfaces ( Figs. 8 A, B). Four roughly vertical cracks divide the blade of the ilium into four segments; these four cracks all merge with a roughly horizontal elongate crack extending craniocaudally dorsal to the acetabulum and peduncles. These cracks mean that the three more cranial segments are suspended in filling substance and that their orientations relative to the rest of the ilium are not certain. Furthermore, there is a point of offset on the lateral surface of the ilium at which one of the blade fragments overhangs the acetabular portion ( Fig. 8 C); this overhang is due to filler between the two pieces and does not reflect the actual morphology of the ilium. Notwithstanding uncertainty regarding the reconstruction of the iliac blade fragments, the postacetabular process does project far dorsolaterally relative to the ischial peduncle ( Fig. 8 D). However, comparing the case of USNM 4753 to other basal iguanodonts, it becomes clear that this is probably a product of plastic deformation.

Among the skeletons of Iguanodon bernissartensis excavated from the Bernissart Mine in Belgium ( Norman 1980), there is variation in the forms of the ilia that can be attributed to deformation. The left ilium of the lectotype, IRSNB 1534, is well preserved with a nearly intact, gently convex dorsal margin along which there extends a thickened, laterally everted rim ( Fig. 9 A). The postacetabular process tapers towards its caudal end. This is taken to be the undistorted form of the ilium. In contrast, the left ilia of IRSNB 1536 and 1561 resemble those of USNM 4753 in their dorsolaterally projecting dorsal margins dorsal to the ischial peduncle ( Figs. 9 B, C), giving the ilia a sinuous appearance in lateral view. That this morphology is likely a result of deformation is evinced in IRSNB 1536 by the sacral neural spines being inclined noticeably to the left side of the animal ( Fig. 10 ), an obvious result of deformation that could have also affected the ilia.

Furthermore, the holotype skeleton of Uteodon aphanoecetes , CM 11337, exhibits a left ilium strikingly similar to that of USNM 4753 in lateral and caudal views, with the dorsal margin of the ilium facing almost laterally and the blade of the ilium projecting dorsolaterally far past the ischial peduncle ( Figs. 6 B, C). That this is a consequence of plastic deformation in CM 11337 is indicated by the condition of the sacral neural spines, which incline sharply towards the left side of the skeleton, as documented above in IRSNB 1536 ( Fig. 6 D; see also Carpenter & Wilson 2008, fig. 13E); the blade of the left ilium likely was subject to the same force that distorted the neural spines. Furthermore, while the blade of the right ilium of CM 11337 is incomplete, enough is preserved to indicate that the iliac blade was nearly vertical and projected dorsally relative to the ischial peduncle, as in ilia of Camptosaurus dispar (e.g., YPM 1877, YPM 8622, USNM 5473; Figs. 4 A–C; Fig. 6 E). The situation in USNM 4753 is akin to that of CM 11337, i.e. one ilium is plastically deformed while the other is not.

The preserved morphology of USNM 4753 is itself evidence against the supposed horizontal postacetabular process. The preserved part of the right ilium of USNM 4753 does not exhibit any hint of a folded, horizontally projecting iliac blade (contra Carpenter & Wilson 2008) ( Fig. 8 E). Indeed, the cross-section of the right ilium in caudal view indicates that the iliac blade was nearly vertical, as in Camptosaurus dispar ( Fig. 8 F). Moreover, comparing the left ilium with the preserved portion of the right reveals that if the morphology of the left ilium were genuine, the right ilium should exhibit the cranial region of the folded iliac blade ( Fig. 11 ). Thus, the morphology used by Gilmore (1909) to demarcate C. depressus and by Carpenter & Wilson (2008) to assign the species to Planicoxa is the product of plastic deformation and fragmentation.

Although crushed and broken, a unique combination of characters can be discerned in the ilia of USNM 4753 (see “Specific diagnosis” above). The dorsal margin of the left ilium faces dorsolaterally due to distortion of the iliac blade ( Figs. 8 A, C), making the ilium appear dorsally convex; however, the dorsal margin itself lacks any curvature and is straight. The dorsal margin of the ilium thickens mediolaterally towards the postacetabular process ( Figs. 8 A, C). C. depressus is more derived than Camptosaurus dispar in exhibiting a horizontal boot at the cranial end of the preacetabular process ( Fig. 8 E), a feature shared with other more derived basal iguanodonts such as Cedrorestes crichtoni (DMNH 47994), Iguanodon bernissartensis (IRSNB 1534), and Eolambia caroljonesa (CEUM 52090). It might seem acceptable to refer C. depressus to Dakotadon because USNM 4753 and the holotype of D. lakotaensis , SDSM 8656, both came from the Lakota Formation of South Dakota. However, the only overlapping elements between the two specimens are poorly preserved caudal vertebrae that display no features that would positively link “ C.” depressus with D. lakotaensis . As the specimen clearly does not pertain to the genus Camptosaurus , cannot be confidently referred to another existing genus, and exhibits a unique combination of characters, a new genus should be created to accommodate “ C.” depressus and achieve taxonomic stability. Hence the novel genus and combination proposed herein, Osmakasaurus depressus .

Figures

FIGURE 4. Ilia of Camptosaurus dispar in lateral view. (A) right ilium of YPM 1877 (holotype) from Quarry 13; (B) right ilium of YPM 8622 from Quarry 13; (C) left ilium of USNM 5473 from Quarry 13; (D) left ilium of YPM PU 14553 from Cleveland-Lloyd Dinosaur Quarry. Abbreviations: mlt, mediolateral thickening; pas, break in slope along dorsal margin of postacetabular process. Images in A, B, and D are copyright YPM. Scale bar equals 10 cm.

FIGURE 6. Left ilium of CM 11337 (holotype of Uteodon aphanoecetes) in (A) ventrolateral view showing the convex dorsal margin, (B) lateral view, (C), caudal view. Sacrum and ilia of CM 11337 in (D) dorsal view. Right ilium of CM 11337 in (E) lateral view. Right ilium of OXFUM J. 3303 (holotype of Cumnoria prestwichii) in (F) lateral view. Abbreviations: dmi, dorsal margin of ilium; mlt, mediolateral thickening; pas, break in slope along dorsal margin of postacetabular process.

FIGURE 8. Left ilium of USNM 4753 (holotype of Osmakasaurus depressus) in (A) lateral, (B) medial, (C) caudolateral, and (D) caudal views. Right ilium of USNM 4753 in (E) lateral and (F) caudal views. Abbreviations: hb, horizontal boot; mlt, mediolateral thickening. Scale bar equals 10 cm.

FIGURE 9. Left ilia of Iguanodon bernissartensis in lateral view. (A) IRSNB 1534 (lectotype), dorsal margin outlined in red; (B) IRSNB 1536, dorsal margin outlined in red; (C) IRSNB 1561.

FIGURE 10. Sacrum and ilia of IRSNB 1536 (Iguanodon bernissartensis) in dorsal view.

FIGURE 11. Left (A) and right (B, reversed) ilia of USNM 4753 (holotype of Osmakasaurus depressus) in lateral view, set to the same scale and with the notch between the preacetabular process and pubic peduncle lined up. Scale bar equals 10 cm.