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

Abstract

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 (5 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.

Arctosaurus osborni, a Late Triassic archosauromorph reptile from the Canadian Arctic Archipelago

“…Referral to Archosauromorpha is supported by three features (Ezcurra et al 2014): the (in lateral view) parallelogramshaped centrum, the anterior articular surface of which is set dorsal to the posterior one; the well-developed posterior centrodiapophyseal ridge; and the positioning of the zygapophyses close to each other medially. In addition, NMING:F14878 has two features that Nesbitt et al (2015) hypothesized as shared derived character states for the archosauromorph clade Allokotosauria: the apparently anterodorsally canted neural spine (character state 116-1; Nesbitt et al 2015) and the posteriorly pointed epipophyses on the dorsal surface of the postzygapophyses (character state 119-1; Nesbitt et al 2015). Allokotosauria comprises a variety of Triassic non-archosaurian archosauromorphs, most of which developed dental specializations indicative of omnivory or herbivory, and is currently definitely known from the Late Triassic of North America, Madagascar, Morocco, and possibly India (Nesbitt et al 2015).…”

“…It overhangs a deep arcuate groove in the lateral surface of the centrum (Figs. 2A, 3A), as in the archosauromorph Azendohsaurus madagaskarensis (Nesbitt et al 2015) and in the pseudosuchian Arizonasaurus babbitti (Nesbitt 2005). This groove does not open into the interior of the centrum.…”

“…2A, 3), as in certain non-archosaurian archosauromorphs such as Teraterpeton hrynewichorum (Sues 2003), Mesosuchus browni (Nesbitt 2011), and Azendohsaurus madagaskarensis (Nesbitt et al 2015), and in various pseudosuchians and dinosaurs (Nesbitt 2011). It is posteriorly pointed but does not extend posteriorly beyond the articular facet of the postzygapophysis, as in Azendohsaurus madagaskarensis (Nesbitt et al 2015) and Teraterpeton hrynewichorum (Sues 2003) but unlike in Tanystropheus conspicuus (Wild 1973). The postzygapophyseal facet faces almost directly ventrally with only a slight posterolateral inclination.…”

First direct archosauromorph remains from the Early-Middle Triassic transition of the Iberian Peninsula

“…These features have not been used as characters in phylogenetic analyses focused on basal archosauromorphs (e.g., Dilkes, 1998;Nesbitt, 2011;Nesbitt et al, 2015;Pritchard et al, 2015;Ezcurra, 2016), and they seem to be taxonomically uninformative. In addition, the subtle differences observed in the morphology of the denticles may be attributable to variations within the dental series or intraspecific variability.…”

“…Compressed crowns are present in several crocopodan archosauromorphs (sensu Ezcurra, 2016), namely azendohsaurids (e.g., Azendohsaurus madagaskarensis: UA 10603, 10604, 8-29-97-160), prolacertids (e.g., Prolacerta broomi: BP/1/2675), Teyujagua (Pinheiro et al, 2016), Tasmaniosaurus (Ezcurra, 2014) and archosauriforms (e.g., Proterosuchus fergusi: RC 846; Erythrosuchus afri- (Ezcurra, 2016;Pinheiro et al, 2016). By contrast, other archosauromorphs lack finely serrated crowns (e.g., tanystropheids, prolacertids, trilophosaurids, rhynchosaurs; Benton and Clark, 1988;Juul, 1994;Dilkes, 1998;Nesbitt et al, 2015;Ezcurra, 2016;Pinheiro et al, 2016) and, although azendohsaurids possess denticles in the crown, they are considerably larger and coarser (with a lower density; Flynn et al, 2010;Nesbitt et al, 2015). As a result, recent quantitative phylogenetic analyses found compressed and finely serrated crowns as synapomorphies of lesser inclusive clades within Crocopoda (Ezcurra, 2016;Pinheiro et al, 2016) and support the assignment of the three teeth from the late Early to Middle Triassic of Catalonia to this clade.…”