(044) Nesbitt et al 2013

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Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013RAUISUCHIAFossilized gastrointestinal contents of rauisuchiansare rare, but are preserved in the nearly completeand articulated holotype of Ticinosuchusferox and the well-preserved, partially articulatedholotype of Postosuchus alisonae. The preservedgastrointestinal contents of Ticinosuchus ferox(Nesbitt 2011, p. 26) include fish scales, althoughthe rather unspecialized skull and mandible of thisspecies bear no indications that it was a specializedpiscivore. The diverse gastrointestinal contents ofthe Postosuchus alisonae specimen include part ofan aetosaur, a traversodontid cynodont, phalangesof a dicynodont and possibly an amphibian (Peyeret al. 2008).The discovery of multiple (ten) associated individualsof Decuriasuchus quartacolonia (Françaet al. 2011) is rare for pseudosuchians. Françaet al. (2011) interpreted this associated assemblageas a possible indication of social grouping, a behaviourwell known in various ornithodirans, includingTriassic dinosaurs (e.g. Sander 1992). This hypothesisis consistent with the discovery of other rauisuchians,such as Batrachotomus kupferzellensis,Heptasuchus clarki, Postosuchus kirkpatricki,Effigia okeeffeae and Shuvosaurus inexpectatus, infossil assemblages that include specimens of variousontogenetic stages. Furthermore, some rauisuchianassemblages include multiple individuals of differentrauisuchian taxa. For example, other rauisuchians(e.g. Prestosuchus chiniquensis UFRGS-PV-0629-T) were found in the same quarry thatyielded the D. quartacolonia specimens (Langeret al. 2007; Mastrantonio 2010; França 2011).Other multi-taxon rauisuchian assemblages are alsoknown, for example from the Manda beds (Nesbittet al. 2010) of East Africa and the Dockum Groupand Chinle Formation of the western USA (e.g.Long & Murry 1995; Nesbitt 2011). More thanone species of rauisuchian are sometimes foundtogether in bone beds. For example, the holotypesof Postosuchus kirkpatricki and Shuvosaurus inexpectatuswere found associated in the Post Quarryin the Upper Triassic Dockum Group of NorthAmerica. It is clear, therefore, that many rauisuchianfaunas in the Triassic were diverse.Cranial and mandibular mechanicsStudying the feeding habits, skull strength, biteforces and possible cranial kinesis of rauisuchiansis an interesting area of research that is only beginningto be explored with explicit, quantitative methodologies.One subject that has been the focusof considerable research is cranial mechanics inextant reptiles (e.g. Erickson et al. 2003; Metzgeret al. 2005), although this is difficult to study infossil taxa. Even in extant taxa it can be difficult todemonstrate functional and active cranial kinesis,even with access to living animals and knowledgeof soft, as well as hard, tissue anatomy (Smith &Hylander 1985). For extinct vertebrates, inferencesare made by studying the shape of and contactsbetween skull bones (Rayfield 2005; Liparini2008), although care must be taken because apparentlymovable bony joints in fossil material mightnot be involved in kinesis in life (e.g. Bühler et al.1988; Gower 1999).With respect to rauisuchians, Chatterjee (1985),Gower (1999) and Liparini (2008) suggested potential,but probably greatly restricted and passivecranial kinesis for adult Postosuchus kirkpatricki,Batrachotomus kupferzellensis and Prestosuchuschiniquensis, respectively (Fig. 6d). Gower (1999)argued that the poor preservation of the edges ofincompletely preserved cranial elements in B. kupferzellensisprevented firm conclusions regardingcranial mechanics, but he was able to rule out notablekinesis due to the rigid skull roof. However, it isimportant to acknowledge that restricted passiveintracranial mobility of local parts might haveoccurred even if other fused regions of the skull preventedmore extensive intracranial movements. Themandibular symphysis, where known, appears to berelatively simple in rauisuchians, but potential interorintra-mandibular joints are difficult to assessbecause of lack of detailed information about themiddle part of the mandibular ramus (e.g. thejoints between dentary, surangular and angular) inthe vast majority of rauisuchian fossils. Someworkers have highlighted a supposedly moveablepremaxilla–maxilla joint in some rauisuchians(e.g. Benton & Clark 1988; Long & Murry 1995),but detailed biomechanical investigations of thisportion of the skull have not been undertaken andno firm evidence has been forwarded in support ofanything other than small amounts of passive movementin this region.Much additional work on cranial mechanics inrauisuchians is clearly needed. Liparini (2008)suggested that the main areas to look for possiblykinetic joints in rauisuchian skulls include the contactbetween the maxilla and premaxilla, jugal andlacrimal, and between the pterygoid and the basipterygoidprocess of the basisphenoid, quadrate andectopterygoid (Fig. 6). However, detailed descriptionsof the histology and bone texture of theseregions of the skull, and comparisons among manyrauisuchian taxa, have yet to be undertaken. Detailedcraniofacial and mandibular muscular reconstructionswould assist in investigations of kinesis, butthese have also not yet been undertaken. Anotherapproach that is likely to yield useful informationin future is integrated biomechanical modelling.For example, finite element analysis (FEA) hasbeen used to examine differences in cranial mechanicsduring biting with or without intracranial
Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013S. J. NESBITT ET AL.Fig. 6. Examples of palaeobiological studies on rauisuchians: (a) three-dimensional reconstruction of themusculoskeletal system of the hindlimb of the poposauroid Poposaurus gracilis (from Bates & Schachner 2012);(b) muscle reconstructions of the hindlimb of Prestosuchus chiniquensis (from Liparini 2011); (c) example of the‘pillar-erect’ hindlimb posture of Poposaurus gracilis, in posterior view (from Schachner et al. 2011);(d) three-dimensional model of skull and mandible of Prestosuchus chiniquensis illustrating movable joints (labelledi–iii) between the skull bones (modified from Liparini 2008); (e) histological sections through a dorsal osteoderm ofBatrachotomus kupferzellensis showing Sharpey’s fibres (Shf) and parallel-fibred bone tissues (PFB) (scale bars: 1 cm)(from Scheyer & Desojo 2011); (f ) histological section through the femur of Effigia okeeffeae (AMNH FR 30589);(g) histological section through the femur of Postosuchus (UCMP 28353). See appendix for institutionalabbreviations.mobility in, for example, extinct theropod dinosaurs(e.g. Rayfield 2004), and might usefully be extendedto rauisuchians.There is also an ontogenetic dimension to cranialmechanics and diet, and this also requiresfuture research with respect to rauisuchians. Somesmaller, presumably younger, rauisuchian individualsare known from disarticulated skullelements, in contrast to the completely articulatedskulls of larger, presumably older, conspecificspecimens. This phenomenon is observed, forexample, in Luperosuchus fractus (Desojo &Arcucci 2009) and Prestosuchus chiniquensis (seeMastrantonio (2010) and Barberena (1978) forcomparison). This might be related to differencesin the relative degree of mobility of cranial joints
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