Craniofacial Morphology of Simosuchus Clarki (Crocodyliformes: Notosuchia) from the Late Cretaceous of Madagascar
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Society of Vertebrate Paleontology Memoir 10 Journal of Vertebrate Paleontology Volume 30, Supplement to Number 6: 13–98, November 2010 © 2010 by the Society of Vertebrate Paleontology CRANIOFACIAL MORPHOLOGY OF SIMOSUCHUS CLARKI (CROCODYLIFORMES: NOTOSUCHIA) FROM THE LATE CRETACEOUS OF MADAGASCAR NATHAN J. KLEY,*,1 JOSEPH J. W. SERTICH,1 ALAN H. TURNER,1 DAVID W. KRAUSE,1 PATRICK M. O’CONNOR,2 and JUSTIN A. GEORGI3 1Department of Anatomical Sciences, Stony Brook University, Stony Brook, New York, 11794-8081, U.S.A., [email protected]; [email protected]; [email protected]; [email protected]; 2Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, Ohio 45701, U.S.A., [email protected]; 3Department of Anatomy, Arizona College of Osteopathic Medicine, Midwestern University, Glendale, Arizona 85308, U.S.A., [email protected] ABSTRACT—Simosuchus clarki is a small, pug-nosed notosuchian crocodyliform from the Late Cretaceous of Madagascar. Originally described on the basis of a single specimen including a remarkably complete and well-preserved skull and lower jaw, S. clarki is now known from five additional specimens that preserve portions of the craniofacial skeleton. Collectively, these six specimens represent all elements of the head skeleton except the stapedes, thus making the craniofacial skeleton of S. clarki one of the best and most completely preserved among all known basal mesoeucrocodylians. In this report, we provide a detailed description of the entire head skeleton of S. clarki, including a portion of the hyobranchial apparatus. The two most complete and well-preserved specimens differ substantially in several size and shape variables (e.g., projections, angulations, and areas of ornamentation), suggestive of sexual dimorphism. Assessment of both external and internal morphological fea- tures indicates a habitual head posture in which the preorbital portion of the dermal skull roof was tilted downward at an angle of ∼45◦. Functional and comparative assessment of the feeding apparatus strongly indicates a predominantly if not exclusively herbivorous diet. Other features of the craniofacial skeleton of S. clarki are consistent with the interpretation developed from analysis of the postcranial skeleton of a terrestrial habitus, but the current working hypothesis of a burrowing lifestyle is not supported. The atypical appearance of the skull and lower jaw of S. clarki is underscored by the identification of at least 45 autapomorphic features, many of them related to the greatly foreshortened snout. INTRODUCTION Chimaerasuchus paradoxus Wu, Sues, and Sun, 1995, see also Wu and Sues, 1996; Malawisuchus mwakasyungutiensis Gomani, The Upper Cretaceous (Maastrichtian) Maevarano Formation 1997; Mariliasuchus amarali Carvalho and Bertini, 1999, see also in the Mahajanga Basin of northwestern Madagascar has yielded Zaher et al., 2006; Baurusuchus salgadoensis Carvalho, Campos, the articulated or associated cranial remains of several verte- and Nobre, 2005; Adamantinasuchus navae Nobre and Carvalho, brate taxa, including the frog Beelzebufo ampinga Evans, Jones, 2006; Sphagesaurus montealtensis Andrade and Bertini, 2008; No- and Krause, 2008; the turtle Kinkonychelys rogersi Gaffney, tosuchus terrestris, see Fiorelli and Calvo, 2008; Armadillosuchus Krause, and Zalmout, 2009; the lizard Konkasaurus mahalana arrudai Marinho and Carvalho, 2009; Yacarerani boliviensis No- Krause, Evans, and Gao, 2003; the snake Menarana nosymena vas, Pais, Pol, Carvalho, Scanferla, Mones, and Riglos, 2009; LaDuke, Krause, Scanlon, and Kley, 2010; the crocodyliforms Pakasuchus kapilimai O’Connor, Sertich, Stevens, Roberts, Got- Araripesuchus tsangatsangana Turner, 2006, Mahajangasuchus tfried, Hieronymus, Jinnah, Ridgely, Ngasala, and Temba, 2010) insignis Buckley and Brochu, 1999 (see also Turner and Buckley, over the last 10–15 years have effectively dismantled the concept 2008), and Miadanasuchus oblita (Buffetaut and Taquet, 1979) that crocodyliforms varied little in cranial morphology during Simons and Buckley, 2009; the abelisauroid theropods Majun- their evolutionary history (Langston, 1973). Buckley et al. (2000) gasaurus crenatissimus (Deperet,´ 1896) Lavocat, 1955 (see also described the skull of Simosuchus clarki as being dorsoventrally Sampson et al., 1998; Sampson and Witmer, 2007), and Masi- vaulted with a very abbreviated, squared snout, foliform, multi- akasaurus knopfleri Sampson, Carrano, and Forster, 2001 (see cusped teeth, and an anteriorly shifted jaw joint, and therefore also Carrano et al., 2002, in press); and the titanosaurian sauro- deviating significantly from the ‘typical’ crocodyliform Bauplan pod Rapetosaurus krausei Curry Rogers and Forster, 2001 (see of a dorsoventrally flattened skull with an elongate snout, re- also Curry Rogers and Forster, 2004). None, however, are as curved, conical teeth, and posteriorly positioned jaw articulation. complete and undistorted, or preserve surface detail as well, as The morphology and placement of the jaw articulation and the the head skeleton in the holotype (Universite´ d’Antananarivo shape of the teeth of Simosuchus were interpreted by Buckley et [UA] specimen number 8679) of the blunt-snouted basal mesoeu- al. (2000) as reflective of an herbivorous diet. The lateral position crocodylian Simosuchus clarki. This specimen, which also in- of the orbits and the anterior position of the external nares indi- cludes a large portion of the postcranial skeleton, was rapidly cated to the authors a terrestrial habit. The deep cranium, short, buried in a debris flow over 65.5 million years ago (Rogers shovel-like snout, posteroventrally positioned occipital condyle, 2005; Krause et al., this volume), discovered and collected by and underslung lower jaw were interpreted as evidence that a field crew of the joint Stony Brook University/Universite´ Simosuchus may have been a burrower. Finally, the presence of d’Antananarivo Mahajanga Basin Project in 1998, and described two unambiguous synapomorphies—“internal nares divided by a in a preliminary report by Buckley et al. in 2000. septum and strongly spatulate posterior teeth” (Buckley et al., ThediscoveryofSimosuchus clarki and description of many 2000:943)—linked Simosuchus with the Late Cretaceous (Ceno- other notosuchian taxa with unusual cranial architecture (e.g., manian) South American form Uruguaysuchus as sister taxa. Since the collection and description of the holotype specimen * of Simosuchus clarki, several additional cranial specimens of this Corresponding author. species have been recovered that add relatively little in terms 13 14 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 10 of anatomical detail but do permit a preliminary assessment of Sciences, Chapel Hill, U.S.A.; URC, Museu de Paleontologia e intraspecific variability. The discovery of these additional speci- Estratigrafia Prof. Dr. Paulo Milton Barbosa Landim, Universi- mens, further preparation of the holotype skull, the ability to dis- dade Estadual Paulista, Rio Claro, Brazil. cern anatomical structure in greater detail (e.g., positions of su- Anatomical Abbreviations—See Appendix 1. tures, pathways of neurovascular canals) through the use of com- puted tomography (CT) scanning and image-processing software, and the recent discovery, description, and analysis of numerous MATERIALS AND METHODS other notosuchian taxa from elsewhere allow a more detailed, Specimens of Simosuchus clarki comparative, and comprehensive assessment of the anatomy, life habits, and phylogenetic position of Simosuchus than was possi- Three specimens of Simosuchus clarki are now known in which ble a decade ago when the holotype was first described. the skull and lower jaw are preserved nearly completely: UA This report is focused exclusively on the head skeleton, includ- 8679, FMNH PR 2596, and FMNH PR 2597. (See Krause et ing the dentition, of Simosuchus clarki. Its objectives are to (1) al. [this volume] for provenance information and an accounting provide a detailed description of the skull, lower jaw, and denti- of the postcranial elements associated with each of these speci- tion in a comparative context; (2) evaluate aspects of intraspecific mens.) variability in cranial size, shape, relative proportions, and ontoge- UA 8679—This specimen, which serves as the holotype for netic maturity, and to determine the likelihood of sexual dimor- Simosuchus clarki, includes a fully articulated head skeleton (al- phism; (3) assess attributes of the head skeleton that might pro- though its anterior and posterior palpebrals were removed from vide insight into life habits, particularly feeding and locomotion; both sides of the skull during preparation). This specimen in- and (4) discuss the implications of craniofacial morphology for cludes nearly all elements of the skull (only the stapedes and phylogeny reconstruction. epipterygoids are missing), all elements of the lower jaw, and a Institutional Abbreviations—AMNH, American Museum of single fragmentary piece of the hyobranchial apparatus. In gen- Natural History, New York, U.S.A.; BMNH, British Museum of eral, the entire head skeleton is preserved extraordinarily well; Natural History, London, U.K.; BP, Bernard Price Institute for distortion is minimal, and externally visible breakage is limited Palaeontological Research, University of the Witwatersrand, Jo- primarily to one region of the left quadrate within the adductor hannesburg, South Africa; BSP, Bayerische Staatssammlung fur¨