[Palaeontology, Vol. 53, Part 3, 2010, pp. 669–688]
A NEW SPECIES OF AZENDOHSAURUS (DIAPSIDA: ARCHOSAUROMORPHA) FROM THE TRIASSIC ISALO GROUP OF SOUTHWESTERN MADAGASCAR: CRANIUM AND MANDIBLE by JOHN J. FLYNN*, STERLING J. NESBITT* à, J. MICHAEL PARRISH§, LOVASOA RANIVOHARIMANANA– and ANDRE´ R. WYSS** *Division of Paleontology and Richard Gilder Graduate School, American Museum of Natural History, New York NY 10024, USA; e-mail
[email protected] Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, USA; e-mail
[email protected] àPresent 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-mail
[email protected] –De´partment de Pale´ontologie et d’Anthropologie Biologique, Universite´ d’Antananarivo, Antananarivo, Madagascar; e-mail
[email protected] **Department of Earth Science, University of California, Santa Barbara, CA 93106, USA; e-mail
[email protected] Typescript received 12 January 2009; accepted in revised form 26 July 2009
Abstract: Here, we describe a new species of Azendohsaurus
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.
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
Key words: Azendohsaurus, Archosauromorpha, Triassic, Madagascar, Isalo Group, herbivory.
Azendohsaurus laaroussii Dutuit, 1972, from the Triassic Argana Formation of Morocco, is characterized by an herbivorously specialized dentition. These include apomorphies present in various herbivorous dinosaurs, which have been shown by our current analysis to have yielded spurious phylogenetic results. Proposed on the basis of gnathic remains, A. laaroussii was originally referred to the Ornithischia (Dutuit 1972). Subsequent
workers, including Thulborn (1973, 1974), Bonaparte (1976) and Dutuit himself (Dutuit and Heyler 1983, p. 629) considered A. laaroussii to be a member of the Prosauropoda, usually in a basal position within the group. Gauffre (1993) also considered A. laaroussii to be a nonyunannosaurid prosauropod. In a recent abstract, Jalil and Knoll (2002) mentioned recovery of fragmentary disarticulated postcranial remains from the holotype
ª The Palaeontological Association
doi: 10.1111/j.1475-4983.2010.00954.x
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locality, attributing them to A. laaroussii. As these postcranial remains do not bear any dinosaurian apomorphies, that suggested a phylogenetic position for A. laaroussii outside of the Dinosauria, although they presciently noted that assignment of these disarticulated postcrania to Azendohsaurus was only a taphonomic inference and that associated postcrania and craniodental remains such as those now known in the Malagasy species would be required to resolve the phylogenetic position of Azendohsaurus. Further, taxa traditionally included within the Prosauropoda appear to form a paraphyletic group of sauropodomorphs leading to a monophyletic Sauropoda (Gauthier 1986; Yates 2003, 2007; Pol 2004; Upchurch et al. 2007). Nevertheless, there is a monophyletic clade of basal sauropodomorphs that contains Plateosaurus, Plateosauravus, Massospondylus, Riojasuchus, Coloradosaurus and a few other taxa usually assigned to the ‘prosauropods’. Herein, we refer to ‘prosauropods’ in quotation marks to indicate the likely paraphyly of the taxa usually included in this group, and use the term basal sauropodomorphs to circumscribe the early diverging members of the Sauropodomorpha and to aid in referring to anatomical conditions that likely pertain across this paraphyletic assemblage of taxa. In the late 1990s, we initiated the first field survey of the Mesozoic Isalo Group of western Madagascar dedicated to the recovery of fossil vertebrates (see Flynn and Wyss 2003). Along the drainage of the Malio River, north-west of the town of Ranohira, we collected abundant terrestrial vertebrate remains from basal levels of a lithostratigraphical unit variously referred to as the ‘Isalo II’ (Besairie 1936, 1972) or the Makay Formation (Razafimbelo 1987). At one geographically tightly circumscribed locality (spanning just tens of centimetres in thickness, along less than 100 metres of outcrop), M-28, we uncovered an almost completely monospecific bone bed containing plentiful remains of a taxon closely resembling A. laaroussii (Flynn et al. 1999). The initial elements recovered from this locality (M28) consisted of maxillary and dentary fragments, making them directly comparable to the published sample of elements of A. laaroussii from Morocco. Building on Gauffre’s (1993) analysis, in our initial description of these elements, we concluded that the Malagasy material represented one (or possibly two) ‘prosauropod’ species comparing closely to A. laaroussii (Flynn et al. 1999). The resemblances between the Malagasy material and A. laaroussii we cited originally included the following: a prominent longitudinal keel on the medial surface of the maxilla, a fossa on the medial surface of the maxilla posterior to the dorsal maxillary process, consistent presence of a neck between the crown and root of the teeth and anteroposterior expansion of the tooth crown bases. In addition, we noted features considered diagnostic of
‘prosauropods’ (Gauffre 1993) including the following: downward curvature of the anterior dentary, a robust dorsal process of the maxilla (the base of which is located in the anterior third of the element) and a series of small nutrient foramina on the medial surface of the maxilla. Although we continue to advocate a close relationship between the taxon represented by the Malagasy material and A. laaroussii, the more complete sample now available from Madagascar indicates that Azendohsaurus represents neither a basal sauropodomorph nor even a dinosaur, ornithodiran or archosauriform. Our results indicate instead that it is an early diverging archosauromorph with an herbivorously specialized dentition, convergent on that occurring in some dinosaurs and other herbivorous archosauromorphs. In addition, based on an abundance of newly recovered articulated craniodental and postcranial material, we argue that these deposits in Madagascar contain only a single species of the taxon that we formerly considered a ‘prosauropod.’ Features cited (Flynn et al. 1999) as possibly distinguishing a second Azendohsaurus-like taxon (based on mandibular fragments) from these deposits included tightly packed teeth with elongate, unexpanded crowns. In light of the additional material now available from this locality, it is now clear that a lower dentition with this conventional ‘prosauropod’ appearance is associated with a strikingly dissimilar upper dentition (see below), a feature characterizing Azendohsaurus as a whole – i.e. the new Malagasy and the Moroccan taxa collectively. As detailed below, improved understanding of the skeletal morphology of the Malagasy taxon also calls into question the advisability of assigning isolated dental and tooth-bearing elements to basal sauropodomorphs, given that features once regarded as diagnostic of the clade (Gauffre 1993) are now known to be highly homoplastic, occurring independently far nearer the base of the archosauromorph tree than previously recognized. Since our initial field season, we have recovered dozens of additional specimens from locality M-28, including associated and articulated material, both cranial and postcranial, from a variety of ontogenetic stages. These findings have revealed a phylogenetically far more perplexing taxon than previously thought, one marked by a pastiche of characters combining dental features typically considered dinosaurian with plesiomorphic archosauromorph characters and attributes suggestive of an aberrant basal ornithodiran. Taken together, the several overlapping portions of articulated partial skeletons now known, permit a nearly complete skeletal reconstruction of the Malagasy species of Azendohsaurus. Herein, we formally recognize a new species of Azendohsaurus from Madagascar, provide a comparative description of its craniodental anatomy, and present a revised diagnosis of Azendohsaurus based on craniodental
FLYNN ET AL.: CRANIAL ANATOMY OF AZENDOHSAURUS
features shared by the two species now assigned to this taxon. The biochronological implications of this new taxon also are discussed. Although no single specimen directly associates all elements of the cranium to each other or to every element of the postcranium, the anatomical overlap between specimens recovered from the quarry (Locality M-28) yielding the type and all referred specimens, makes the association of these elements quite secure. Postcranial material referable to this new taxon will be described in a companion article (Nesbitt et al. in prep.), in which a comprehensive phylogenetic analysis incorporating all the anatomical character data from this new taxon will be undertaken. Institutional abbreviations. BP, Bernard Price Institute for Palaeontological Research, University of Witwatersrand, Johannesburg; BSP, Bayerische Staatssammlung fu¨r Pala¨ontologie and historische Geologie, Munich; FMNH, Field Museum of Natural History, Chicago (PR suffix refers to the Fossil Reptile collection, or ‘Palaeontology, Reptiles’); GR, Ruth Hall Museum of Paleontology, Ghost Ranch, New Mexico; NHM, Natural History Museum, London; RC, Rubidge collection, Wellwood, GraaffReinet; SAM, Iziko South African Museum, Cape Town; SMNS, Staatliches Museum fu¨r Naturkunde, Stuttgart; TMM, Texas Memorial Museum, Austin; UA, University of Antananarivo, Madagascar (specimens to be reposited in the University of Antananarivo Palaeontology collections have been assigned either direct collections numbers [format is UA-#####] or field numbers [format UA-month-date-year-#] – all such specimens are the property of, and will be reposited in, the UA vertebrate paleontology collections, and can be tracked via the pertinent collection or field numbers); UCMP, University of California Museum of Paleontology, Berkeley. Figure abbreviations. a., articulates with; al, alveoli; an, angular; bo, basiocciptial; bpt, basipterygoid processes; bt, basitubera; d, dentary; dt, dentary teeth; ec, ectopterygoid; f, foramen; fm, foramen magnum; fo, fossa; fr, frontal; g, groove; h, hump; ic, internal choana; j, jugal; la, lacrimal; ltf, lower temporal fenestra; mk, meckelian groove; mx, maxilla; n, nasal; na, external nares; o, orbit; oc, occipital condyle; p, parietal; pa, palatine; pf, postfrontal; pm, premaxilla; po, postorbital; pp, paroccipital process of the opisthotic; pra, prearticular; prf, prefrontal; pt, pterygoid; ptf, post-temporal fenestra (Text-fig. 2); ptf, pterygoid flange (Text-fig. 9); qh, quadrate head; qj, quadratojugal; qu, quadrate; rt, replacement tooth; sc, sclerotic ossicle; so, supraoccipital; sq, squamosal; st, stapes; su, surangular; sy, symphysis; t, teeth; utf, upper temporal fenestra; vo, vomer.
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Azendohsaurus, in this area, we have collected abundant remains of other terrestrial vertebrates from within the same lithostratigraphical unit, including new rhynchosaurs (Whatley 2005), well-preserved and nearly complete remains of the rhynchosaur Isalorhynchus genovefae (Buffetaut 1983), a variety of synapsids including at least one probainognathian and multiple traversodontid eucynodonts (Flynn et al. 1999, 2000a, b, in prep.; Kammerer et al. 2005, 2008). As mentioned, the name of the fossiliferous unit is disputed, being most widely known the basal level of Besairie’s ‘Isalo II’ (1936, 1972) and as the Makay Formation (Razafimbelo 1987). Although this lithostratigraphical unit is distributed across many degrees of latitude along the eastern border of the Morondava Basin of southwestern Madagascar, we have encountered fossils over a fairly limited area (
