Tuesday, December 08, 2015

Azendohsaurus madagaskarensis: A Path not Taken to Create a Sauropod in Triassic Madagascar

Postcranial Osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its Systematic Position Among Stem Archosaur Reptiles


Nesbitt et al


During the Triassic, archosauromorphs became one of the first groups of diapsid reptiles to diversify in terms of body size and morphological disparity in both terrestrial and marine ecosystems across Pangaea. This seemingly rapid divergence, and the numerous unique body plans stemming from it, concomitantly has confounded reconstructions of archosauromorph relationships. Teasing apart homology from homoplasy of anatomical characters in this broad suite of body types remains an enormous challenge with the current sample of taxa. Here, we present the postcranial anatomy of Azendohsaurus madagaskarensis, an early archosauromorph from ?Middle to Upper Triassic strata of Madagascar. Azendohsaurus madagaskarensis is known from nearly the entire skeleton in an ontogenetically variable sample. The holotype locality consists of a monotypic bone bed; preservation ranges from complete but disarticulated bones to articulated sections of the skeleton. Azendohsaurus madagaskarensis embodies an aberrant constellation of archosauromorph features, including an elongated neck, a short, stocky tail, robust limbs, and unexpectedly short digits terminating in large recurved unguals on the manus and pes. Together with the cranium, the postcrania reveal A. madagaskarensis to be another representative of a growing coterie of highly apomorphic and bizarre Triassic archosauromorphs. At the same time, recovery and description of the full anatomy of A. madagaskarensis helps to identify a monophyletic grouping of specialized taxa that includes the North American Late Triassic–aged archosauromorphs Trilophosaurus, Spinosuchus, and Teraterpeton, Indian Pamelaria, and Moroccan Azendohsaurus laaroussii. Moreover, information derived from the skeleton of A. madagaskarensis solidifies the systematic position of these taxa among other archosauromorphs. Using the most comprehensively sampled phylogenetic analysis of early archosauromorphs, we found the clade encompassing the aforementioned taxa as the nearest outgroup of Prolacerta broomi + Archosauriformes. The newly recognized clade containing Azendohsaurus, Trilophosaurus, Spinosuchus, Pamelaria, and Teraterpeton demonstrates high morphological disparity even within a closely related group of archosauromorphs, underscores the polyphyly of protorosaurs ( =  prolacertiforms), and suggests that most major divergences within this group occurred in the Triassic. Furthermore, our results indicate that craniodental character states ascribed to a herbivorous diet were much more pervasive across Triassic Archosauromorpha than previously conjectured.

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