The postcranial skeleton of Vagaceratops irvinensis (Dinosauria, Ceratopsidae)

Robert B Holmes

doi:10.18435/b5159v

Abstract

The postcranial skeleton of Vagaceratops (= Chasmosaurus) irvinensis (CMN 41357), lacking only the tail, most of the left front and left hind limbs, and portions of the pelvis, is preserved in articulation. It is one of the most complete, best articulated ceratopsid skeletons known. Both the manus and vertebral column exhibit conspicuous pathologies, possibly an indication of advanced age at the time of death. The vertebral column comprises a syncervical, six additional cervical vertebrae, and 12 dorsals. A partial synsacrum is represented by two dorsosacrals, four sacrals, two caudosacrals, and a partial third caudosacral centrum. The ribcage, although crushed, is nearly complete. The sternum is unusually wide compared with other ceratopsids. As in other chasmosaurines, the humerus bears a prominent deltopectoral crest that forms the anterior edge of the broad, rectangular proximal humeral expansion. The medial tuberosity is separated from the dorsal surface of the humerus by a distinct notch. The insertion for the latissimus dorsi is conspicuous. The deltopectoral crest extends a full half of the distance to the distal end of the humerus. Epipodials of the forelimb are relatively short compared to the corresponding propodial. The ulna has a long, distinctly triangular olecranon, broadly rounded anterolateral process, prominent medial process, and a deeply concave trochlear notch. The terminal phalanges on the fourth and fifth manual digits are relatively large, and unlike other ceratopsids have distinct distal ?articular facets. The fourth trochanter of the femur is relatively proximal in position. This study and other recent studies of ceratopsid postcrania suggest that potentially useful taxonomic variation is present in the number of dorsosacrals, size of the groove on the ventral surface of the sacrum, morphology of the last dorsal and dorsosacral ribs, morphology of the scapula and proximal expansion of the humerus, morphology of the ulna, ratio of humerus/epipodium, morphology of the fifth manual digit, and position of the fourth trochanter of the femur.

Highlights

The Ceratopsidae is a successful group of ornithischian dinosaurs that represented an important component of the Late Cretaceous terrestrial fauna of Laramidia, a landmass that formed the western shores of the Western Interior Seaway from what is Alaska in the north to Mexico in the south (Sampson and Loewen 2010)
The same morphology occurs in Styracosaurus (Holmes and Ryan 2013) and Chasmosaurus belli (ROM 843, Fig. 12)
This, and other recent studies (Chinnery 2004; Maidment and Barrett 2011) confirm that variation does exist in the postcranial skeleton of ceratopids

Summary

INTRODUCTION

The Ceratopsidae is a successful group of ornithischian dinosaurs that represented an important component of the Late Cretaceous terrestrial fauna of Laramidia, a landmass that formed the western shores of the Western Interior Seaway from what is Alaska in the north to Mexico in the south (Sampson and Loewen 2010). A thick ridge, presumably associated with the origin of the dorsalis scapulae muscle (Johnson and Ostrom 1995) arises on the proximo-ventral corner of the scapula as the glenoid buttress, passes diagonally across the lateral surface of the blade, and subsides near the dorsal margin of the scapula at approximately the midpoint between the scapulocoracoid suture and the distal end of the blade This resembles the condition in Centrosaurus (Lull 1933), Styracosaurus (Holmes and Ryan 2014), cf Anchiceratops (Mallon and Holmes 2010), and Chasmosaurus The proximal portion of the flattened shaft of this process is in the frontal (horizontal) plane, but is twisted around its long axis so that its anterior expansion has an oblique orientation, with its internal surface facing dorsolaterally (Fig. 20A) This twisting has not been described in other ceratopsids, a similar condition has been illustrated for Centrosaurus As in other ceratopsids (e.g., Dodson et al 2004), the size disparity between the shortest digit (digit I) and the longest is distinctly less than in protoceratopsids, and the pes is more nearly symmetrical around the third digit

Findings

DISCUSSION

SUMMARY