Historical Biology An International Journal of Paleobiology ISSN: 0891-2963 (Print) 1029-2381 (Online) Journal homepage: https://www.tandfonline.com/loi/ghbi20 Early ornithischian dinosaurs: the Triassic record Randall B. Irmis , William G. Parker , Sterling J. Nesbitt & Jun Liu To cite this article: Randall B. Irmis , William G. Parker , Sterling J. Nesbitt & Jun Liu (2007) Early ornithischian dinosaurs: the Triassic record, Historical Biology, 19:1, 3-22, DOI: 10.1080/08912960600719988 To link to this article: https://doi.org/10.1080/08912960600719988 Published online: 10 Oct 2011. Submit your article to this journal Article views: 291 View related articles Citing articles: 70 View citing articles Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ghbi20 Historical Biology, 2007; 19(1): 3–22 Early ornithischian dinosaurs: the Triassic record RANDALL B. IRMIS1, WILLIAM G. PARKER2, STERLING J. NESBITT3,4, & JUN LIU3,4 1Museum of Paleontology and Department of Integrative Biology, University of California, 1101 Valley Life Sciences Building, Berkeley, CA, 94720-4780, USA, 2Division of Resource Management, Petrified Forest National Park, P.O. Box 2217, Petrified Forest, AZ, 86028, USA, 3Lamont–Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY, 10964, USA, and 4Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY, 10024, USA Abstract Ornithischian dinosaurs are one of the most taxonomically diverse dinosaur clades during the Mesozoic, yet their origin and early diversification remain virtually unknown. In recent years, several new Triassic ornithischian taxa have been proposed, mostly based upon isolated teeth. New discoveries of skeletal material of some of these tooth taxa indicate that these teeth can no longer be assigned to the Ornithischia using unambiguous synapomorphies. The Triassic record of ornithischian dinosaurs now comprises only three probable occurrences: Pisanosaurus and an unnamed heterodontosaurid from Argentina, and an unnamed specimen from the uppermost Triassic of South Africa. This revised Triassic record suggests that ornithischians were not very diverse or abundant through the Triassic, and there are large gaps in the Triassic ornithischian fossil record. Moreover, traditional living analogues for interpreting the feeding ecology of early ornithischians from their tooth morphology are generally inappropriate, and “herbivorous” archosaur teeth such as those found in early ornithischians are not necessarily diagnostic of herbivorous feeding. Keywords: Ornithischia, Triassic, Pisanosaurus, Revueltosaurus, Dinosauria, Archosauria Introduction most ornithischian synapomorphies (Sereno 1991), and sheds little light on the initial stages of Despite their extensive fossil record and exceptional ornithischian evolution. diversity during the later Mesozoic, the origin of the Recently, the record of Triassic ornithischians has Ornithischia is poorly understood. Although all been depleted further with the discovery that at least phylogenetic hypotheses in the past 20 years have some isolated teeth previously assigned to the placed ornithischians as the sister-group to the Ornithischia actually belong to other non-dinosaurian Saurischia (SauropodomorphaCTheropoda), basal archosaurs (Parker et al. 2005). This complicates the forms that actually support this phylogenetic relation- identification of isolated ornithischian material in the ship are scarce. Putative Late Triassic records are rare, Triassic, especially teeth. With this new view of criteria representing a handful of occurrences scattered for recognizing Triassic ornithischians, what can we throughout Laurasia and Gondwana (Figure 1), say about the origin, timing, and initial radiation of the most represented only by teeth (Sereno 1991; Hunt Ornithischia? Here, we review worldwide records of and Lucas 1994). The best-known Triassic putative Triassic ornithischians and discuss the ornithischian, Pisanosaurus mertii from Argentina implications of this revised record. (Casamiquela 1967; Bonaparte 1976), is the only Identifying and assigning isolated and fragmentary specimen that includes appreciable post-cranial remains to particular clades requires the recognition of remains. Even the first well-known basal phylogenetically informative character-states. In the ornithischian, Lesothosaurus diagnosticus from the context of a phylogenetic analysis, some character- Lower Jurassic of South Africa, has already acquired states may provide in-group resolution and be Correspondence: R. B. Irmis, Museum of Paleontology and Department of Integrative Biology, 1101 Valley Life Sciences Building, University of California, Berkeley, CA, 94720-4780, USA. E-mail: [email protected] ISSN 0891-2963 print/ISSN 1029-2381 online q 2007 Taylor & Francis DOI: 10.1080/08912960600719988 Published online 10 Oct 2011 4 R. Irmis et al. Butler 2005a); presence of a separate predentary bone (Sereno 1999; Butler 2005a); and a coronoid process formed by a posterior process of the dentary (Sereno 1999; Butler 2005a). Postcranial character-states include: strap-like preacetabular process of the ilium that extends anteriorly beyond the pubic peduncle (Butler 2005a); posteroventrally rotated pubis with enlarged prepubic process (Butler 2005a); enlarged anterior trochanter of the femur that is anteroposter- iorly wide and separated from the body of the femur by a distinct cleft (Butler 2005a); and posterolateral Figure 1. Global distribution of reported Late Triassic flange of the distal tibia extends posterolaterally ornithischian dinosaur occurrences. Black stars indicate those behind entire distal end of fibula (Butler 2005a). occurrences confirmed in this study. Paleogeographic map modified Institutional abbreviations: AMNH, American from Smith et al. (1994). Abbreviations: AF, Africa; AN, Antarctica; Museum of Natural History, New York;BRSMG, Bristol AS, Asia; AU, Australia; IN, India; NA, North America; SA, South America. City Museum and Art Gallery, Bristol, England; CPBA, synapomorphies of a clade; however, they may have a Ca´tedra de Paleontologı´a de la Facultad de Ciencias wider distribution among distantly related taxa or Exactas de la Universidad de Buenos Aires, Argentina; those not included in the analysis. If these other taxa IRSNB, Institut royal des Sciences naturelles de Belgique, are found in similarly aged strata, the character-states Brussels,Belgium;MNA,MuseumofNorthernArizona, that diagnosed a clade in the phylogenetic analysis Flagstaff, Arizona; NMMNH, New Mexico Museum of cannot be used alone to assign isolated remains to this Natural History and Science, Albuquerque, New Mex- clade. For example, several dental character-states ico; NSM, Nova Scotia Museum, Halifax, Nova Scotia; PVL, Instituto Miguel Lillo, Tucuma´n, Argentina; SAM, may diagnose the Ornithischia in phylogenetic ana- South African Museum, Cape Town, South Africa; lyses of the Dinosauria, but they cannot be used alone UCMP, University of California Museum of Paleontol- to identify isolated Triassic ornithischian teeth, ogy, Berkeley, California; YPM, Yale Peabody Museum, because these character-states are found in other New Haven, Connecticut. Triassic non-dinosaur archosaurs (Parker et al. 2005). To be useful in identification of specimens, these character-states need to be used in association with Records of alleged Triassic ornithischians unambiguous synapomorphies that can be recognized in the specimen of interest. North America Given that this restricts the number of character- Hunt and colleagues (e.g. Hunt 1989; Hunt and states available to diagnose fragmentary early Lucas 1994; Hunt et al. 1998; Heckert 2002, 2004) ornithischian specimens, what unambiguous synapo- identified a variety of Triassic ornithischian dinosaur morphies provide robust evidence for an ornithischian taxa based on isolated teeth collected from Upper affinity? The monophyly of Ornithischia has not been Triassic strata throughout North America, especially questioned by any recent phylogenetic analysis, so the south-western US. The first of these taxa synapomorphies diagnosing the clade Ornithischia published was Revueltosaurus callenderi (Hunt 1989) provide a good starting point provided they are not from the Norian-aged Bull Canyon Formation of New found in other early Mesozoic archosaurs. The Mexico. Hunt (1989) referred this taxon to the following character-states are unambiguous synapo- Ornithischia because of the presence of several dental morphies of the clade Ornithischia, and have been synapomorphies proposed by Sereno (1986) including demonstrated as such by recent phylogenetic analyses low, triangular tooth crowns in lateral view, the (Sereno 1999; Butler 2005a). The status of some of absence of recurvature in maxillary and dentary these character-states, as well as other characters not teeth, and a well-developed neck separating crown included in this list but traditionally used to diagnose from root. Hunt (1989) noted that R. callenderi lacked Ornithischia will be discussed later. This list of “cingula” and differed from other ornithischians in synapomorphies includes character-states not pre- having incisiform premaxillary teeth. served or otherwise known in Pisanosaurus, because Padian (1990) supported the referral of Revuelto- its status is explicitly re-assessed below without saurus to the Ornithischia and referred to the taxon assuming a priori that Pisanosaurus is