(044) Nesbitt et al 2013

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Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013RauisuchiaSTERLING J. NESBITT 1,2 *, STEPHEN L. BRUSATTE 2,3 , JULIA B. DESOJO 4 ,ALEXANDRE LIPARINI 5 , MARCO A. G. DE FRANÇA 6 ,JONATHAN C. WEINBAUM 7 & DAVID J. GOWER 81 Department of Biology, University of Washington, Seattle, WA 98195-1800, USA2 Division of Paleontology, American Museum of Natural History, Central Park West at79th Street, New York, NY 10024, USA3 Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA4 CONICET, Comisión Nacional de Investigación Científica y Técnica, Sección PaleontologíadeVertebrados, Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia, Av. AngelGallardo 470, C1405DRJ, Buenos Aires, Argentina5 Departamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federaldo Rio Grande do Sul, Cx.P. 15001, 91540-970, Porto Alegre, RS, Brazil6 Laboratório de Paleontologia de Ribeirão Preto, FFCLRP, Universidade de São Paulo, Av.Bandeirantes 3900, Ribeirão Preto, SP 14040-901, Brazil7 Biology Department, Southern Connecticut State University, New Haven, CT 06515, USA8 Department of Zoology, The Natural History Museum, London SW7 5BD, UK*Corresponding author (e-mail: sjn2104@gmail.com)Abstract: ‘Rauisuchia’ comprises Triassic pseudosuchians that ranged greatly in body size, locomotorstyles and feeding ecologies. Our concept of what constitutes a rauisuchian is changing as aresult of discoveries over the last 15 years. New evidence has shown that rauisuchians are probablynot a natural (monophyletic) group, but instead are a number of smaller clades (e.g. Rauisuchidae,Ctenosauriscidae, Shuvosauridae) that may not be each other’s closest relatives within Pseudosuchia.Here, we acknowledge that there are still large gaps in the basic understanding in the alphaleveltaxonomy and relationships of these groups, but good progress is being made. As a result ofrenewed interest in rauisuchians, an expanding number of recent studies have focused on thegrowth, locomotor habits, and biomechanics of these animals, and we review these studies here.We are clearly in the midst of a renaissance in our understanding of rauisuchian evolution andthe continuation of detailed descriptions, the development of explicit phylogenetic hypotheses,and explicit palaeobiological studies are essential in advancing our knowledge of these extinctanimals.During the Triassic Period, archosaurs radiated intoa diverse array of body sizes, ecologies and morphologies(Benton & Clark 1988; Sereno 1991; Benton1999, 2004; Nesbitt & Norell 2006; Brusatte et al.2008, 2010; Nesbitt 2011). Basally, Archosauriasplit into two major lineages, the pseudosuchians(crocodylian line) and avemetatarsalians (bird line),early in the Triassic (Benton & Clark 1988; Sereno1991; Benton 1999; Gower & Sennikov 2000;Brusatte et al. 2010, 2011; Nesbitt 2003, 2011;Nesbitt et al. 2011a). Pseudosuchians diversifiedinto some easily recognized and clearly monophyleticgroups during the Triassic, such as the heavilyarmoured aetosaurs and the more fleet-footedcrocodylomorphs. However, there are many otherTriassic pseudosuchians that are not easily placedinto discrete, well-diagnosed clades (e.g. Ticinosuchusferox, Gracilisuchus stipanicicorum).Over the past century, large (2–7 m) hypercarnivorousarchosaurs with recurved serrated teeth thatare not clearly referable to other pseudosuchian ormore inclusive archosauromorph clades (e.g. Aetosauria,Phytosauria, Ornithosuchidae or Crocodylomorpha)were often shoehorned into the poorlydefined group ‘Rauisuchia.’ Many of these taxawere often also assigned to smaller subgroups (subdivisionsof ‘Rauisuchia’), such as Prestosuchidae,Poposauridae, Rauisuchidae or various ranksFrom: Nesbitt, S. J., Desojo, J.B.&Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of EarlyArchosaurs and their Kin. Geological Society, London, Special Publications, 379,http://dx.doi.org/10.1144/SP379.1 # The Geological Society of London 2013. Publishing disclaimer:www.geolsoc.org.uk/pub_ethics
Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013S. J. NESBITT ET AL.associated with Rauisuchidae (e.g. Rauisuchiformes,Parrish 1993). Furthermore, many of thesetaxa, such as Prestosuchus chiniquensis and Rauisuchustiradentes, were represented only by incompletefossils that provided information on onlysmall portions of the skeleton. In addition, severalTriassic pseudosuchians with divergent cranialand/orvertebralanatomies (e.g.Lotosaurusadentus,‘Chatterjeea elegans’) were associated with more‘typical’ rauisuchians by virtue of their similar pelvicand hindlimb morphology. As a result, confusionabout what diagnosed rauisuchians and abouttheir relationships to one another and to other pseudosuchiansdominated the literature of the late 20thcentury, and little consensus had been reached. Atthe most basic level, it was not clear if rauisuchianswere a natural group, a paraphyletic group withrespect to other pseudosuchian clades, or a polyphyleticassemblage spread among more easily recognizedpseudosuchian groups (Gower 2000).Given their generally poor fossil record and theabsence of a clear taxonomy, rauisuchians werelargely bypassed in studies of Triassic vertebratemacroevolution until recently (although see Benton1983; Bonaparte 1984). Nonetheless, rauisuchiansare now attracting an increased level of attentionbecause of a series of recent discoveries of newtaxa and also better material regarding previouslyknown taxa (e.g. Gower 1999; Sen 2005; Sulej2005; Li et al. 2006; Nesbitt & Norell 2006; Jalil& Peyer 2007; Desojo & Arcucci 2009; Françaet al. 2011; Gauthier et al. 2011; Trotteyn et al.2011), advances in understanding character statedistributions among pseudosuchians (Brusatte et al.2010; Nesbitt 2011), redescriptions of importantspecimens (Nesbitt 2007; Weinbaum & Hungerbühler2007; Brusatte et al. 2009; Butler et al. 2011;Lautenschlager & Desojo 2011; Weinbaum 2011),progress in pseudosuchian phylogenetics (Brusatteet al. 2010; Nesbitt 2011) and expansion of ageneral interest in these fascinating creatures. As aresult of this influx of new research and fossil discoveries,the anatomy, systematics and evolutionaryhistory of rauisuchians are becoming increasinglyclear, although many debates remain.Whether a natural group or a collection of moredistantly related groups, rauisuchians are importantfor a number of reasons. They were an importantcomponent of Triassic faunas, especially duringthe Early–Middle Triassic when ecosystems werestabilizing after the Permo-Triassic extinction and‘modern ecosystems’ were being established (Sues& Fraser 2010). Rauisuchians were an integralpart of the initial diversification of Archosauria inthe Early Triassic, but went extinct by the end ofthe Triassic, seemingly quite suddenly and possiblydue to the global end-Triassic mass extinction(Benton 2004). Some rauisuchians, such asthe sail-backed ctenosauriscids, are some of the earliestarchosaurs to appear in the fossil record andthus help to date the origin of Archosauria, whichis an important calibration point in many molecularphylogenetic analyses (Nesbitt 2003, 2011; Butleret al. 2011; Nesbitt et al. 2011a; Parham et al.2012). Furthermore, rauisuchians were part ofnearly all Triassic archosaur assemblages and havebeen found on nearly all portions of Pangaea withLower to Upper Triassic deposits.Rauisuchians were both taxonomically and ecologicallydiverse. They include the first large-bodiedcarnivorous reptiles (up to 7 m in length) in the fossilrecord and were sometimes were much larger thancontemporary herbivores, which is unusual, becausethe opposite relationship between largest predatorand herbivore size usually holds in modern terrestrialecosystems (Sookias et al. 2012). Although mostrauisuchians were massive, quadrupedal predators(e.g. Batrachotomus kuperferzellensis, Postosuchuskirkpatricki, Prestosuchus chiniquensis, Saurosuchusgalilei), some taxa departed from this bodytype. Some may have been partially aquatic (e.g.Qianosuchus mixtus) whereas others were morelightly built and probably bipedal (e.g. Effigia okeeffeae,Poposaurus gracilis, Shuvosaurus inexpectatus).Other taxa, such as ctenosauriscids and thebeaked Lotosaurus adentus, sported elaborate sailson their backs. Intriguingly, several rauisuchianswere long misidentified as theropod dinosaursbecause of some remarkable convergences betweenthese groups (e.g. Colbert 1961; Chatterjee 1985,1993; Benton 1986a; Nesbitt & Norell 2006;Nesbitt 2007; Brusatte et al. 2009; Bates & Schachner2012). It seems likely, therefore, that rauisuchiansoccupied many ecological niches that weresubsequently filled by dinosaurs during the Jurassicand Cretaceous (Brusatte et al. 2008).An understanding of the anatomy, ecology andsystematics of rauisuchians is clearly important forunderstanding the major patterns and processes inTriassic archosaur evolution. At this point in time,however, there are a number of problems thatremain to be resolved. First and foremost, we cannotidentify a rauisuchian by unambiguous synapomorphies,which is key to identifying these animalsin the field or in collections. This point also underscoresa major phylogenetic problem: there are noclear apomorphies that unite all rauisuchianstogether as a clade, and the relationships of severalgenera remain unclear. This naturally prevents aclear understanding of character evolution amongthese animals.In general, taxa classically referred to as rauisuchiansshare a large skull relative to their body,recurved teeth, and a few other character states highlightedby researchers over the past 75 years (Fig. 1).However, these long-assumed ‘key characters’ are
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