Simosuchus clarki is a bizarre, pug-nosed notosuchian crocodyliform known only from the Upper Cretaceous (Maastrichtian) Maevarano Formation in the Mahajanga Basin of northwestern Madagascar. When originally named and described in 2000, S. clarki was based entirely on a single specimen that included a nearly complete skull and lower jaw preserved in articulation with the anterior and mid-trunk portions of the postcranial skeleton, as well as several associated elements from the posterior region. The species is now represented by three additional partial and nearly complete articulated skeletons, as well as numerous isolated elements (mostly teeth), that permit detailed description of its entire bony anatomy, the primary subject of other chapters in this volume. These specimens were discovered as part of the ‘Mahajanga Basin Project,’ initiated in 1993 and conducted jointly by Stony Brook University and the Université d'Antananarivo, in the Berivotra and Masiakakoho study areas. The best-preserved specimens of S. clarki were entombed in massive, poorly sorted, clay-rich debris flow deposits (facics 2 of the Anembalemba Member) that accumulated in channel belts in response to exceptional rainfall events. Simosuchus, along with its contemporaries in the Maevarano assemblage, lived in a strongly seasonal, semi-arid climate some 20 million years after Madagascar separated from the India/Seychelles block and became an island isolated in the Indian Ocean.

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.

Simosuchus clarki Buckley, Brochu, Krause, and Pol, 2000, a small, pug-nosed notosuchian crocodyliform from the Late Cretaceous of Madagascar, was described a decade ago in a preliminary report based on a single specimen that included the skull, lower jaw, and anterior portions of the postcranial skeleton; details of the preserved cervical and anterior dorsal vertebrae and ribs were not described. With the subsequent collection of several additional specimens, which collectively also preserve the posterior dorsal, sacral, and caudal regions of the vertebral column, the postcranial axial skeleton of S. clarki is now one of the most completely known of any basal mesoeucrocodylian. This report includes a detailed description of the vertebral and costal morphology of this bizarre notosuchian and an assessment of its functional and phylogenetic significance. Simosuchus had eight cervical, at least 15 dorsal, two sacral, and probably fewer than 20 caudal vertebrae. Most aspects of the vertebral morphology of Simosuchus are consistent with those generally observed within Crocodylomorpha, or more specifically within Crocodyliformes. However, the low number of caudal vertebrae indicates that the tail of Simosuchus was remarkably short, shorter than in any other known crocodylomorph. This short tail is interpreted to have had little to no functional utility in aquatic propulsion. Assessment of the hypothesis that Simosuchus and some other basal mesoeucrocodylians were fossorial on the basis of cervical morphology requires a more extensive comparative and functional analysis. The postcranial axial skeleton currently contributes little to the assessment of the phylogenetic position of Simosuchus.

The notosuchian crocodyliform Simosuchus clarki from the Late Cretaceous of Madagascar, originally known from a single specimen preserving primarily the skull, lower jaw, and anterior postcranial skeleton, is now known from three additional partial skeletons that collectively preserve nearly the entire appendicular skeleton. Although typically crocodyliform in overall morphology, the appendicular skeleton of Simosuchus preserves a number of derived features of phylogenetic and functional significance. Notable features of the pectoral girdle and forelimb include an anteroposteriorly broad, tripartite scapular blade; a laterally directed scapular prominence; a proximally expanded humerus with a shallow deltopectoral crest; a distinct, semi-ellipsoidal glenohumeral condyle; a tightly articulating radius and ulna; and a relatively foreshortened manus with robust distal unguals. Notable features of the pelvic girdle and hind limb include a distinct, spur-like projection of the anterior process of the ischium; a femur with a distinctive anterior flange; and a relatively foreshortened pes. Generally, characteristics of the appendicular skeleton are consistent with a terrestrial lifestyle and provide insight into the behavior and locomotion of Simosuchus. The robust limb morphology observed in Simosuchus, together with expanded areas for insertion of pectoral and pelvic musculature in the proximal humerus and femur, is consistent with non-cursorial terrestrial locomotion in a relatively stout-bodied crocodyliform. Characteristics of the hind limb indicate a primarily semi-erect posture, in contrast to the erect posture inferred for other notosuchians.

The pug-nosed crocodyliform Simosuchus clarki is known from several specimens that collectively preserve, in addition to the skull and postcranial endoskeletal elements, a complete complement of osteoderms covering every major body region. This comprehensive sample of osteoderms facilitates the reconstruction, from multiple sources, of the entire postcranial dermal skeleton of this crocodyliform. In this contribution, I describe the anatomy, arrangement, and histology of the osteoderms of S. clarki, and provide comments on the phylogenetic and functional importance of these structures. As in other crocodyliforms, the osteoderms of Simosuchus were organized into discrete ‘shields’ covering the dorsal and ventral regions of the trunk and tail. Simosuchus also exhibited extensive osteodermal coverage of the limbs, a feature that characterizes very few crocodyliforms. The osteoderms from different shields possess distinctive morphological features, allowing isolated osteoderms to be assigned accurately to their respective shields. Paleohistological investigations reveal porous, lightly built osteoderms overlying dorsal and appendicular regions, and gastral osteoderms with a flat, diploe-like structure. Details about growth and longevity in Simosuchus are equivocal when assessed using data from osteoderms alone. Simosuchus possessed a tetraserial paravertebral shield, with at least four accessory parasagittal rows of osteoderms on either side. Sagittal segmentation probably enhanced flexibility with respect to ancestral crocodyliforms, but the presence of numerous, tightly sutured accessory rows suggests that lateral undulation, and therefore swimming, was likely restricted. Several new morphological characters are described here, and it is proposed that these will be informative in future studies of crocodyliform phylogeny.

The bizarre crocodyliform Simosuchus clarki shares several derived cranial characteristics with basal notosuchian taxa. In the original phylogenetic analysis assessing placement of Simosuchus among other crocodyliforms, it was recovered as a basal notosuchian. Additional preparation of the holotype skull and postcranium, and discovery of additional specimens, provides new information on the suite of morphologies in this notosuchian. Morphological analysis of all available material of S. clarki yields 60 autapomorphies distributed across the skeleton. To provide an updated test of the phylogeny of Notosuchia and placement of Simosuchus within the clade, we added seven new characters and expanded taxon sampling to include recently described notosuchians. A phylogenetic analysis of 301 characters and 84 crocodyliforms supports the position of Simosuchus as sister to Libycosuchus, more derived than Araripesuchus and Uruguaysuchus, and basal to other ziphosuchian notosuchians. We did not recover an Anatosuchus Simosuchus clade, nor a monophyletic Araripesuchus, but did recover a peirosaurid clade including Peirosaurus, Lomasuchus, Uberabasuchus, and Hamadasuchus (but not Stolokrosaurus). We also found weak support for inclusion of Anatosuchus, Araripesuchus, Peirosauridae, and Mahajangasuchidae within Notosuchia and therefore for the conclusion that all basal mesoeucrocodylians from Gondwana, with the exception of Stolokrosuchus, are notosuchians. The results of this analysis demonstrate that basal mesoeucrocodylian relationships are currently in a state of flux and the need for caution in naming higher-level taxa until a more stable consensus topology is achieved. Similarly, large gaps in the notosuchian record render inconclusive the biogeographic origins of Simosuchus and the clade it represents on Madagascar.