(044) Nesbitt et al 2013

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Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013RAUISUCHIAXilousuchus sapingensis). These well-constrainedages for these ctenosauriscids also double as theoldest confirmed dates for both Pseudosuchia andArchosauria as a whole (Nesbitt 2011; Nesbittet al. 2011a; Butler et al. 2011; Brusatte et al. 2011).A variety of other rauisuchians are known fromthe Middle Triassic, including the Anisian ‘Mandasuchustanyauchen’ (Charig 1967), Stagonosuchusnyassicus (Huene 1938a) and possibly ‘Pallisteriaangustimentum’ from the Manda beds, and Prestosuchuschiniquensis (Huene 1938b) and Decuriasuchusquartacolonia (França et al. 2011) from theLadinian (Dinodontosaurus Assemblage Zone) partof the Santa Maria Formation. However, the exactages of these rocks are not clear.Youngest recordAnalyses of the end-Triassic extinction (e.g. Benton1986b, 1994; Olsen & Sues 1986) have depictedrauisuchians extending to the end of the Triassic.However, the previously presented data have threenotable limitations: (i) the rauisuchians used inthese studies are not monophyletic; (ii) revisionsin the Triassic timescale (Muttoni et al. 2004; Furinet al. 2006; Mundil et al. 2010) have changed stratigraphicranges; and (iii) the vertebrate fossil recordin the latest Triassic is poor globally (Sues & Fraser2010). Fasolasuchus tenax from the top of the LosColorados Formation was previously thought tobe one of the youngest occurrences from the latestTriassic (Lucas 1998; Arcucci et al. 2004), butnew magnetostratigraphic data (Santi Malnis et al.2011) suggest a mid-Norian date approximatelyequivalent to the age of Postosuchus kirkpatricki,c. 217–215 million years ago (mya) (Irmis et al.2010). The youngest known poposauroid is clearlyEffigia okeeffeae from the late Norian or RhaetianCoelophysis Quarry (Nesbitt 2007; Zeigler & Geissman2011). A specimen referred to Postosuchus sp.(CM 73372) was recorded from the same quarry(Long & Murry 1995; Weinbaum 2002; Novak2004; Peyer et al. 2008), and if it belongs to Postosuchuskirkpatricki it would represent the youngestknown occurrence of Rauisuchidae. Recently,however, Nesbitt (2011) hypothesized a crocodylomorphrelationship for this specimen. Clearly, thediscovery and study of latest Triassic rauisuchiansis a pressing area of future research.The only possible early Jurassic record of a rauisuchianbelongs to a single specimen from the upperElliot Formation of South Africa (Nesbitt andR. Smith unpublished data). The specimen (SAM383) consists of the posterior portion of a maxillawith portions of five teeth from an animal with askull length estimated to be c.1 m. It is unclear ifsome features of the maxilla that it shares with rauisuchians(e.g. Fasolasuchus tenax) are apomorphic(rectangular posterior portion of the maxilla inlateral view, fused interdental plates, shallow antorbitalfossa, large posteriorly opening foramen onposterior portion maxilla in medial view). Alternatively,it is possible that the maxilla could belongto an early crocodylomorph. Additional materialand a better understanding of character evolutionin rauisuchians and early crocodylomorphs areneeded to more confidently identify this intriguingspecimen.Relationships and evolutionRauisuchian taxonomy and evolution have beenpoorly understood because of a number of factors,including poor preservation of specimens, a fragmentaryfossil record, incomplete descriptions, confusionin sorting alpha-level taxonomy and anincomplete understanding of Triassic pseudosuchianrelationships. Details of the tortured taxonomichistory of rauisuchian classification areprovided elsewhere (see Gower 2000; Brusatteet al. 2010; Nesbitt 2011) and will not be repeatedhere. However, the combination of an increasinglymore complete fossil record with advances in phylogeneticmethodologies (e.g. character construction,Sereno 2007; taxon inclusion, Brusatte 2010) instudying early archosaurs has led to a number ofbreakthroughs in understanding rauisuchian systematics.For example, early archosaur phylogenies(e.g. Juul 1994) including rauisuchians often usedcomposite scoring for suprageneric taxa thatassumed the monophyly of groups such as Prestosuchidae.Lately, better-sampled archosaur phylogenies(Brusatte et al. 2010; Butler et al. 2011;Nesbitt 2011) including rauisuchians have usedspecies- or genus-level terminal taxa that do notassume the monophyly of Rauisuchia or majorsubgroups of rauisuchians. Even with those methodologiesin place, however, there is still no consensusabout rauisuchian relationships as a whole,although the framework of one does seem to beemerging. This will, of course, only become clearwith further analyses.The following descriptions of the relationshipsand evolution of rauisuchians follow the recentworks of Brusatte et al. (2010) and Nesbitt (2011)and revised iterations of those matrices in Butleret al. (2011). These two large analyses disagree onthe fundamental relationships of rauisuchians; Brusatteet al. (2010) found a monophyletic Rauisuchia(albeit with minimal support, and overturned by therevised analysis in Butler et al. 2011), whereasNesbitt (2011) found a paraphyletic ‘Rauisuchia’with respect to Crocodylomorpha (Fig. 2). However,although these two analyses differ fundamentally,both works recovered similar relationships
Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013S. J. NESBITT ET AL.
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(044) Nesbitt et al 2013

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