ScienceDaily (May 18, 2010) — Azendohsaurusjust shed its dinosaur affiliation. A careful new analysis of A. madagaskarensis -- this time based on the entire skull rather than on just teeth and jaws -- aligns this 230-million-year-old animal with a different and very early branch on the reptile evolutionary tree. Many aspects of Azendohsaurus are far more primitive than previously assumed, which in turn means that its plant-eating adaptations, similar to those found some early dinosaurs, were developed independently.
This is a reconstruction of the skull of the new species of Azendohsaurus. On top is a lateral or side view, and on the bottom, a palatal or roof of the mouth view. Note the teeth covering the palate, a highly unusual feature among reptiles, and the downturned front end of the lower jaw, a feature found independently in many herbivorous archosauromorphs. (Credit: S. Nesbitt)
"Even though this extraordinary ancient reptile looks similar to some plant-eating dinosaurs in some features of the skull and dentition, it is in fact only distantly related to dinosaurs," says John J. Flynn, curator in the Division of Paleontology at the American Museum of Natural History. "With more complete material, we re-assessed features like the down-turned jaw and leaf-shaped teeth found in A. madagaskarensis as convergent with some herbivorous dinosaurs."
The fossil is a member of Archosauromorpha, a group that includes birds and crocodilians but not lizards, snakes, or turtles. The type specimen of the genus Azendohsaurus was a fragmentary set of teeth and jaws found in 1972 near (and named for) a village in Morocco's Atlas Mountains. The fossils on which the current research paper is based was discovered in the late 1990s in southwestern Madagascar. Named A. madagaskarensis, this specimen was uncovered by a team of U.S. and Malagasy paleontologists in a "red bed" that includes multiple individuals that probably perished together. This species was initially published as an early dinosaur in Science over a decade ago, but the completeness of the more recently unearthed and studied fossils has provided the first complete glimpse of what this animal looked like and was related to. A. madagaskarensis was not a dinosaur.
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Volume 53 Issue 3, Pages 669 - 688
Published Online: 14 May 2010
© The Palaeontological Association, 2010
by JOHN J. FLYNN*, STERLING J. NESBITT*†‡, J. MICHAEL PARRISH§, LOVASOA RANIVOHARIMANANA¶ and ANDRÉ R. WYSS**
*Division of Paleontology and Richard Gilder Graduate School, American Museum of Natural History, New York NY 10024, USA; e-mailjflynn@amnh.org
†Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, USA; e-mail nesbitt@ldeo.columbia.edu
‡Present address: Jackson School of Geosciences, The University of Texas at Austin, 1 University Station, C1100 Austin, TX 78712-0254, USA
§College of Science, San Jose State University, One Washington Square, San Jose, CA 95192, USA; e-mailmparrish@science.sjsu.edu
¶Départment de Paléontologie et d'Anthropologie Biologique, Université d'Antananarivo, Antananarivo, Madagascar; e-mailranivolova@moov.mg
**Department of Earth Science, University of California, Santa Barbara, CA 93106, USA; e-mail wyss@geol.ucsb.edu
KEYWORDS
Azendohsaurus • Archosauromorpha • Triassic • Madagascar • Isalo Group • herbivory
ABSTRACT
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), 'leaf-shaped' 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.
Typescript received 12 January 2009; accepted in revised form 26 July 2009
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1475-4983.2010.00954.x About DOI
Typescript received 12 January 2009; accepted in revised form 26 July 2009
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1475-4983.2010.00954.x About DOI
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