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27 June 2012 The Cervical Vertebrae of the Late Cretaceous Abelisaurid Dinosaur Carnotaurus sastrei
Ariel H. Méndez
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Abstract

The cervical vertebral series of Carnotaurus sastrei from Argentina is described in detail, and compared with Majungasaurus crenatissimus from Madagascar, both Late Cretaceous (Maastrichtian) in age. Notable differences in the morphology of the cervical vertebrae, especially in the shape and development of the epipophysis and the neural spines, are observed between these two genera. These differences show a neck much more robust in Carnotaurus than in Majungasaurus, may be linked to the evolution of the clade in relation to the divergence time since the two genera shared a common ancestor, and functionally may relate to the feeding function associated to the extreme reduction of the forelimbs.

Introduction

The record of Abelisauridae, with over 10 recognized species, represents the best-known carnivorous dinosaur group from Gondwana and includes two of the anatomically bestknown dinosaurs, Carnotaurus sastrei from the La Colonia Formation (Maastrichtian) of Argentina (Bonaparte 1985) and Majungasaurus crenatissimus from the Maevarano Formation (Maastrichtian) of Madagascar (Sampson et al. 1998).

Vertebral anatomy of Gondwanan theropod dinosaurs has received increased attention over the past 25 years. Several workers (e.g., Martinez et al. 1986, 2004; Bonaparte et al. 1990; Carrano et al. 2002, 2011; O'Connor 2007; Méndez 2010) have detailed axial anatomy and provided valuable information about the vertebrae and ribs of Abelisauroidea, documenting a number of features that distinguish members of this group from other theropods (i.e., hypertrophied cervical epipophyses, extremely elongate cervical ribs, increased fusion of elements into the sacral complex, caudal vertebrae with long transverse processes, expanded distal ends of transverse processes in caudal vertebrae). In 1990, Bonaparte, Novas, and Coria fully described the vertebral morphology in Carnotaurus sastrei, documenting for the first time these features in abelisaurids, which were hitherto unknown in other theropods. After this initial description, Carnotaurus has received little detailed attention. O'Connor (2007) added to our knowledge of abelisaurid axial morphology with a detailed description of vertebral remains of Majungasaurus crenatissimus, and data on vertebral morphology within Abelisauridae slowly accumulated following constant discoveries of new, but less complete specimens. Preliminary descriptions of two fairly complete abelisaurids, Aucasaurus garridoi (Coria et al. 2002) and Skorpiovenator bustingorryi (Canale et al. 2009), were also presented, although complete osteological descriptions remain to be completed.

In this work I provide a detailed comparison of the cervical series of Carnotaurus sastrei (MACN-CH 894) with that of Majungasaurus crenatissimus (UA 8678). The goal of the present work is to document different structural patterns exhibited by members of Abelisauridae and to emphasize those features of the cervical vertebral column that would enable a more precise analysis and allocation of new abelisaurid remains that are based on incomplete cervical series.

Institutional abbreviations.—FMNH, Field Museum of Natural History, Chicago, USA; MACN, Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina; UA, Université d'Antananarivo, Antananarivo, Madagascar.

Description and comparison

Carnotaurus and Majungasaurus share the same number of vertebrae in the cervical series (10). Throughout, Carnotaurus sastrei exhibits morphological differences in these vertebrae. Vertebral centra are longer than high from the axis to the 6th cervical vertebra, and then the 7th becomes shorter relative to its height (Table 1). In Majungasaurus, only the last three vertebrae in the neck are less rectangular in outline. When the posterior articular surface of the centrum is oriented perpendicularly, the ventral margin of the anterior articular surface is equal to the mid-height of the posterior surface at the axis, whereas it does not exceed this level in any other vertebrae of the neck. In Majungasaurus, the pattern is different: the ventral edge of the anterior surface of the axis is well below the mid-height of the posterior surface; the next six cervical vertebrae show a much more intense anteroposterior slope, with the ventral edge of the anterior face onehalf or more of the vertebral height above the ventral margin of the posterior face. Only in the last two cervical vertebrae of Majungasaurus is the articular surface offset similarly to Carnotaurus. The ventral concavity observed in lateral view is most pronounced in the 10th cervical vertebra of Carnotaurus, whereas it is more developed from the middle to the end of the series in Majungasaurus (Fig. 1).

Fig. 1.

Comparison of cervical series and skull of two abelisaurid dinoasaurs. A. Carnotaurus sastrei Bonaparte, 1985 (MACN-CH 894), Argentina, La Colonia Formation, Maastrichtian, Upper Cretaceous. B. Majungasaurus crenatissimus Sampson, Witmer, Forster, Krause, O'Connor, Dodson, and Ravoavy, 1998 (UA 8678), Madagascar, Maevarano Formation, Maastrichtian, Upper Cretaceous. Dorsal (A1, B1) and lateral (A2, B2) views. Skull of C. sastrei was modified from Bonaparte et al. 1990, and skull of M. crenatissimus from Sampson et al. 1998. Not to scale (for measurements see Table 2).

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Fig. 2.

Abelisaurid dinoasaur Carnotaurus sastrei Bonaparte, 1985 (MACN-CH 894, Argentina, La Colonia Formation, Maastrichtian, Upper Cretaceous). Atlas in anterior (A), posterior (B), dorsal (C), left lateral (D), and ventral (E), views.

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Atlas.—The atlas of Carnotaurus consists of an intercentrum and a neural arch preserving a slightly deformed left neurapophysis (Fig. 2). The intercentrum exhibits a concave surface for articulation with the occipital condyle. This surface is transversely pronounced and anterodorsally oriented.

The articular surface for the axis is large, transversely oriented, and crescentic in outline. There is no evidence of a prezygapophysis, so we infer the absence of a proatlas. The neurapophysis is directed posterolaterally, rather than dorsally, and is less elongated than the neurapophysis of Majungasaurus. The postzygapophysis is small and triangular in outline. The articular surface is ventromedially directed, similar to the situation of Majungasaurus. The postzygapophysis is projected from the medial part of neurapophysis, forming a channel between the inner side of the latter and the ventral end of the postzygapophysis. Incomplete preservation of the atlantal neurapophysis in Carnotaurus precludes a characterization of its distal end (e.g., it is unclear if it has a sickle-shaped epipophysis like that of Majungasaurus; O'Connor 2007). The ventral surface of the intercentrum has two small lateral processes, separated by a wide groove.

Axis.—This element is complete and very well preserved in Carnotaurus (Fig. 3). The vertebral body is long and low (ratio length/height is 2.4, see Table 1), and increases in height towards the posterior end. A thick ventral rim is present on the posterior articular surface, contributing to the visible height difference between anterior and posterior intervertebral surfaces. There is a clear and firmly conjoined line of fusion between the axial centrum and intercentrum. The latter is broad and almost flat ventrally.

The odontoid process projects forwards beyond the lower end of the axial intercentrum. In Majungasaurus the odontoid reaches but does not pass this edge. Noticeable is the high grade of opisthocoelia, similar to Majungasaurus. In Carnotaurus, the centrum bears two small pneumatic foramina on both sides. On the right side the anterior one is dropshaped (with the apex oriented caudally), while the posterior one is subcircular. On the left side, both are elliptic. Majungasaurus shows only one circular pneumatic foramen on each side. However, a second specimen of Majungasaurus (FMNH PR 2293, MAD95-14