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Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013S. J. NESBITT ET AL.some of these taxa into discrete clades, althoughthere is a long way to go.Here, we briefly synthesize the current understandingof rauisuchians. We highlight current consensusin rauisuchian taxonomy and review thediscrete clades (such as Poposauroidea and Rauisuchidae)that can be identified by derived charactersand that have a similar taxonomic compositionin recent phylogenetic analyses. We also reviewadvances in studies of rauisuchian palaeobiologybeyond systematics. Finally, we end with a roadmapof what we consider to be important to futurework on rauisuchians.TerminologyHere, we use Archosauria to refer to the crowngroup defined by the common ancestor of birds andcrocodylians, and all of its descendants (Gauthier &Padian 1985; Gauthier 1986; Sereno 1991). Pseudosuchia(Gauthier & Padian 1985) refers to archosaursmore closely related to crocodylians thanto birds, whereas Avemetatarsalia (Benton 1999)refers to archosaurs more closely related to birdsthan to crocodylians. Note that several authors usethe clade name Crurotarsi (node-based group originally)to refer to what we here call Pseudosuchia(e.g. Sereno & Arcucci 1990; Sereno 1991; Benton1999, 2004; Brusatte et al. 2010).In the late 20th century, Rauisuchia Bonaparte1975 came to encompass an assortment of Triassicgenera that were sometimes (although not consistentlyand with varying definitions and content) partitionedinto Linnean families such as RauisuchidaeHuene 1942, Ctenosauriscidae Kuhn 1964, PrestosuchidaeRomer 1966, Poposauridae (Nopcsa1923) and Chatterjeeidae Long & Murry 1995 (seeGower 2000; Nesbitt 2011 for further discussion).In that ‘Rauisuchia’, as understood here, includesgenera generally referred to these higher taxa, it ishighly likely to be non-monophyletic and is thereforeused in inverted commas. Rauisuchians, inthe sense used here, are best defined as almost allsuchians that are not members of Aetosauria orCrocodylomorpha, with additional exceptions comprisinga few non-rauisuchian, possible suchianssuch as Gracilisuchus stipanicicorum and Turfanosuchusdabanensis. Other than Parrish (1993), whoredefined Rauisuchia to apply to a clade that alsoincluded Crocodylomorpha, most authors since the1990s have understood ‘Rauisuchia’ and rauisuchiansin the sense used here.Phylogenetic definitionsShifting ideas (most not strongly supported) aboutthe relationships among rauisuchians and otherpseudosuchians has led to a number of challengeswhen presenting a natural classification based onexplicit phylogenetic definitions (sensu de Queiroz& Gauthier 1990, 1992). Most workers who haveapplied numerical phylogenetic methods agree that‘Rauisuchia’ is not monophyletic, and most haveconverted pre-cladistic names to clade names (e.g.Rauisuchidae: Parrish 1993). Some authors havetied explicit definitions to these latter two types ofclade names (e.g. Sereno et al. 2005; Weinbaum& Hungerbühler 2007; Brusatte et al. 2010), butothers have not (e.g. Parrish 1993).To add to the confusion, different authors evenin the cladistic age have used various names torefer to clades with a similar composition and/orthe same names for different groups (compare, forexample, the use of Rauisuchia by Gauthier (1986)and Parrish (1993) and the use of Rauisuchidae byParrish (1993) and Brusatte et al. (2010) andNesbitt (2011)). Although the higher-level relationshipsof rauisuchians in the most recent large-scaleanalyses (Brusatte et al. 2010; Butler et al. 2011;Nesbitt 2011) are far from full agreement, severalsmaller clades of rauisuchians are compatible (Fig.2). Therefore, the following list of clade definitionsis split into two sections. The first set of cladenames refers to groups that have been found inrecent analyses using different data sets, and thusappear to be relatively stable. The second set refersto monophyletic groups that have been found insome analyses, but denote a set of either paraphyleticor polyphyletic groups in other analyses. The cladenames in the second set should be re-evaluated ifsubsequent phylogenetic analyses find that theclades, as defined by the phylogenetic definitions,turn out to include other pseudosuchians (e.g. Aetosauria)not intentionally included in the definition.First setRauisuchidae Huene 1942. Stem-based definition –the most inclusive clade containing Rauisuchus tiradentesHuene 1938b but not Aetosaurus ferratusFraas 1877, Prestosuchus chiniquensis Huene1938b, Poposaurus gracilis Mehl 1915 or Crocodylusniloticus (Laurenti 1768). (Sensu Sereno et al.2005.)Poposauroidea Nopcsa 1923. Stem-based definition– the most inclusive clade containing Poposaurusgracilis Mehl 1915 but not Rauisuchustiradentes Huene 1938b, Crocodylus niloticus(Laurenti 1768), Ornithosuchus longidens (Huxley1877) or Aetosaurus ferratus Fraas 1877. (SensuWeinbaum & Hungerbühler 2007.)Shuvosauridae. Node-based definition – theleast inclusive clade containing Shuvosaurusinexpectatus Chatterjee 1993 and Sillosuchus