Craniofacial morphology of Simosuchus clarki (Crocodyliformes: Notosuchia) from the Late Cretaceous of Madagascar

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 features 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.

ACKNOWLEDGMENTS

We thank J. Groenke, V. Heisey, and G. Buckley for their skillful preparation of the head skeletons of UA 8679 and FMNH PR 2597; J. Neville for her expertise in photographing these specimens; L. Betti-Nash for her extreme patience and tireless efforts in executing the illustrations of these specimens; M. Colbert and J. Maisano for μCT-scanning the skull and lower jaw of UA 8679 and digitally processing the resultant HRXCT data set; members of the Radiology Department at the Stony Brook University Medical Center for their assistance in CT-scanning the head skeletons of FMNH PR 2596 and FMNH PR 2597; D. Sammis for locating and providing numerous obscure papers; C. Holiday for helpful correspondences regarding various aspects of the morphology of Simosuchus; and C. Brochu and J. Clark for their detailed reviews of an earlier draft of the manuscript. We also gratefully acknowledge the extensive financial support to D.W.K. from the National Science Foundation (EAR-9706302, DEB-9904045, EAR-0106477, EAR-0116517, EAR-0446488) and the National Geographic Society (6400-99, 6940-00, and 7645-04) that facilitated the discovery, preparation, and study of all specimens of Simosuchus clarki examined throughout the course of this study. The digital reconstruction of the cranial endocast was facilitated by a National Science Foundation grant (EAR-0617561) to P.M.O. Finally, N.J.K. acknowledges a grant from the National Science Foundation (IOS-0749750) that helped to support operations in his laboratory during the preparation of the manuscript.

Notes

¹Maximum measurement.

²Measurement made along an axis parallel to the median sagittal plane and extending perpendicularly between two transverse planes passing through defining landmarks.

³Measurement made along a horizontal axis extending perpendicularly between two parasagittal planes passing through defining landmarks.

⁴Measurement made along a vertical axis extending perpendicularly between two frontal planes passing through defining landmarks.

⁵Measurement made with skull resting on lower jaw in articulated position, with entire lower jaw resting on flat surface.

⁶Measurement made in median sagittal plane.

⁷Straight-line measurement.

⁸Measurement made on both sides of specimen.

⁹Minimum measurement.

¹⁰Measurement made along minor axis of condyle/glenoid.

¹¹Measurement made along major axis of condyle/glenoid.

¹²Maximum measurement that can be made perpendicular to lateral margin of angular.

*Measurement estimated because of slight breakage, erosion, or distortion.

*Measurement estimated because of slight breakage, erosion, or distortion.