A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible

Abstract

Here, we describe a new species of Azendohsaurus from the Middle-Late Triassic of Madagascar, extending the geographical range of a taxon known otherwise only by a single species from Morocco. Although Azendohsaurus has consistently been regarded as an early dinosaur (based on various advanced dental and gnathic features resembling those characterizing certain dinosaur subgroups), the relatively complete skeletal material, now available from Madagascar, argues strongly against its dinosaurian affinities. Rather, the retention of numerous primitive cranial and postcranial features indicates a surprisingly early divergence of Azendohsaurus within Archosauromorpha and an unusual mosaic of characters in this taxon. Features considered diagnostic of Sauropodomorpha thus are inferred to occur homoplastically in at least one clade of nondinosaurian archosauromorphs, indicating a complex evolution and distribution of features traditionally thought to be derived within archosaurs. Azendohsaurus has teeth resembling those of both early sauropodomorph and ornithischian dinosaurs, yet also possesses numerous inarguable basal archosauromorph cranial and postcranial attributes. This highlights the risk of uncritically referring isolated, Middle-Late Triassic (or even later), 'leafshaped' teeth with denticles to the Dinosauria. Similarly, the occurrence of such teeth in an early diverging archosauromorph indicates that specializations for herbivory originated more frequently within this clade than conventionally assumed. For example, Azendohsaurus and numerous basal sauropodomorph dinosaur taxa share an array of convergently acquired features associated with herbivory, including tooth denticles, expanded tooth crowns, a downturned dentary and the articular located at the ventral margin of the mandible. Some of these features (denticles, expanded crowns and the ventrally deflected articular) are even more widespread among archosauromorphs, including aetosaurs, silesaurs and ornithischian dinosaurs. A downturned dentary also occurs in Trilophosaurus, a taxon further marked by unique specializations for herbivory, including transversely lophate, tricuspid teeth. An array of features associated with herbivory also occurs in rhynchosaurs and certain crocodilians (e.g. Simosuchus). This distribution suggests that craniodental features associated with herbivory were much more pervasive across the archosauromorph clade than previously recognized, possibly evolving at least six to eight times independently.

The origin and early radiation of dinosaurs

“…Additional material and a careful reevaluation of the specimens demonstrate that the purported 'sauropodomorph' material belongs to a taxon only distantly related to dinosaurs that shares uncanny modifications of the skull with plant-eating, large-bodied dinosaurs (Flynn et al, 2008;Flynn et al, 2010). Similarly, Nesbitt et al (2007) demonstrated that several supposed dinosaurs from the Late Triassic of North America actually represent dinosauromorphs, the closest relatives to dinosaurs rather than bona fide members of the group.…”

The first occurrence of the enigmatic archosauriform Crosbysaurus Heckert 2004 from the Chinle Formation of Southern Utah

“…While some 43 authors have suggested that ornithischians were present in the Late Triassic of North America 44 (Heckert 2005) virtually all authors are in agreement that Crosbysaurus cannot be diagnosed 1 beyond an indeterminate archosauriform and new data suggest that it may be instead an non-2 archosauriform archosauromorph based on similar dental features found in the 3 archosauromorph Azendohsaurus (Flynn et al, 2010 Description -MNA V10666 (Fig. 5-7) is a single, nearly complete shed tooth crown.…”

Supplemental Material for 'Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. (Bulletin of the American Museum of Natural History, no. 398)'

“…This great diversification in morphological disparity by the Middle Triassic is perhaps not unexpected given the rapid, almost simultaneous diversification of archosauriforms (Brusatte et al, 2011;Butler et al, 2011; We have previously noted the convergent similarities of skull morphology and form of the marginal teeth between A. madagaskarensis and early sauropodomorph dinosaurs , many of which have been interpreted as herbivorous adaptations in early sauropodomorphs (Weishampel and Norman, 1989;Barrett, 2000;Sues, 2000;Goswami et al, 2005). For example, both A. madagaskarensis and early sauropodomorphs share convergences in marginal tooth crown structure and hemimandible architecture (see Flynn et al, 2010). As highlighted in the descriptions above, however, homoplasious similarities between A. madagaskarensis and early sauropodomorphs extend beyond craniodental features, occurring throughout much of the skeleton.…”

“…Azendohsaurus Dutuit, 1972 Azendohsaurus madagaskarensis Flynn et al, 2010 HOLOTYPE: (field number 7-20-99-653 Kammerer et al, 2010), rhynchosaurs (Isalorhynchus genovefae Buffetaut, 1983, Whatley, 2005, a kannemeyeriifrom dicynodont , an enigmatic archosaur (Nesbitt et al, unpublished data) and dinosauromorphs (Kammerer et al, unpublished data) occur in the Makay Formation within a 10 km radius of the Azendohsaurus producing locality. The stratigraphic relationships among these localities are not well Fig.…”

“…DEFINITION: The most inclusive clade containing Azendohsaurus madagaskarensis, Flynn et al, 2010, but not Trilophosaurus buettneri, Case, 1928a, Tanystropheus longobardicus (Bassani, 1886), Proterosuchus fergusi, Broom, 1903, Protorosaurus speneri von Meyer, 1830, Rhynchosaurus articeps Owen, 1842, or Passer domesticus Linnaeus, 1758 (fig. 5).…”