Morphology and diversity of the mandibular symphysis of archosauriforms

Archosauromorphs radiated into numerous trophic niches during the Mesozoic, many of which were accommodated by particular suites of cranial adaptations and feeding behaviours. The mandibular symphysis, the joint linking the mandibles, is a poorly understood craniomandibular joint that may offer significant insight into skull function and feeding ecology. Using comparative data from extant amniotes, we investigated the skeletal anatomy and osteological correlates of relevant soft tissues in a survey of archosauromorph mandibular symphyses. Characters were identified and their evolution mapped using a current phylogeny of archosauriforms with the addition of non-archosauriform archosauromorphs. Extinct taxa with the simple Class I condition (e.g. proterochampsids, ‘rauisuchians’), rugose Class II (aetosaurs, protosuchians, silesaurids) and interdigitating Class III symphyses (e.g. phytosaurs, crocodyliforms) and finally fused Class IV (avians) build the joints in expected ways, although they differ in the contributions of bony elements and Meckel’s cartilage. Optimization of the different classes of symphyses across archosauromorph clades indicates that major iterative transitions from plesiomorphic Class I to derived, rigid Class II–IV symphyses occurred along the lines to phytosaurs, aetosaurs, a subset of poposauroids, crocodyliformes, pterosaurs and birds. These transitions in symphyseal morphology also appear to track changes in dentition and potentially diet. The Triassic radiation of vertebrates marked a great time of experimentation in animal design following the end Permian extinction (Sues & Fraser 2010). Archosauromorphs diversified in the Triassic and were at the forefront of this experimentation in body plans, feeding styles, locomotory modes and evolution of novel features including endothermy (Seymour et al. 2004), beaks (Zanno & Makovicky 2011), feathers (Xu et al. 2009) and flight (Gatesy & Middleton 2007). A subset of this group, the crowngroup archosaurs and their closest relatives (archosauriforms), dominated the terrestrial ecosystem during the Triassic although only a subset of this clade (dinosaurs, pterosaurs and crocodylomorphs) survived into the Jurassic. This fascinating group derived a number of adaptations for prey capture, providing insights into the origin and evolution of feeding strategies. The diversity of archosauriforms during this early radiation illuminates many evolutionary patterns common to other groups, particularly patterns of feeding functional morphology. Archosauriforms radiated into numerous trophic niches during the Triassic Period, including species that convergently evolved forms characteristic of predatory, omnivorous and herbivorous feeding behaviours on land (early crocodylomorphs), in the air (e.g. pterosaurs) and in the water (e.g. Vancleavea campi). Inferences of diet and feeding function remain problematic given the incomplete preservation of most archosauriform taxa, the lack of good modern analogues (Barrett 2000) and insufficient understanding of sauropsid craniodental evolutionary and functional morphology. Regardless, new data on the feeding apparatus of fossil archosauriforms, such as from muscle anatomy (Holliday 2009; Sakamoto 2010), biomechanics (Desojo & Vizcaino 2009; Pierce et al. 2009), and comparative arthrology (Norman & Weishampel 1985; Holliday & Witmer 2008), provide novel, critical insights into craniodental functional morphology and skull evolution. In light of this Triassic diversity, the evolution of cranial joints in archosaurs is particularly informative because today’s two living archosaur clades that originated in the Triassic – crocodylians (i.e. crocodyliform suchians) and birds (i.e. avian dinosaurs) – have incredibly disparate articulations within their skulls. In the former, virtually every joint is sutured into an akinetic skull, whereas the latter evolved a number of loosely articulated cranial elements, resulting in a derived form of cranial kinesis. Changes also occurred in the mandibles, with the loss of teeth, but also in the structure of the mandibular symphysis, the joint between the From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications, 379, http://dx.doi.org/10.1144/SP379.2 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics 10.1144/SP379.2 Geological Society, London, Special Publications published online January 24, 2013 as doi: