(044) Nesbitt et al 2013

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Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013RAUISUCHIAbetween juveniles and adults (as well as size, andtherefore amounts of associated connecting softtissue), which might thus indicate a reduction ofkinesis with age. Walker (1990) suggested a similarontogenetic trajectory for the crocodylomorphSphenosuchus acutus, and thought this mightexplain the presence of potentially moveable jointsin a seemingly rigid adult skull (smaller/youngerspecimens were not available to test thishypothesis).Sexual dimorphism, ontogeny and growthThere is no compelling evidence for sexual dimorphismin any rauisuchian and little information ongrowth rates or ontogenetic trends, but this is unsurprisinggiven the fragmentary nature of much ofthe fossil material and the general lack of palaeobiologicalanalyses for rauisuchians. Very recently,however, some rauisuchians have been studied usinghistological analyses, which offer potential toprovide insights into these questions about growthand ontogeny, as well as other areas of rauisuchianpalaeobiology (Nesbitt 2007; Cerda et al. 2011;Scheyer & Desojo 2011; Scheyer et al. 2011). Histologycan provide data on growth rates, the originand development of bony structures, the osteogenicmechanisms linked to the development of thesestructures (e.g. osteoderm ornamentation), and therelation of bones to soft tissues. Few histologicalstudies of the long bones of rauisuchians have beenconducted, but the handful of published studieshas generated some important data. Ricqlès et al.(2003, 2008) examined long bones referred to Postosuchuskirkpatricki (Fig. 6g), ‘Mandasuchus tanyauchen’and Luperosuchus fractus (for which nolimb material was reported to be found with theholotype skull), and found their histology to besimilar to that of phytosaurs, aetosaurs and extantcrocodilians. Based on these comparisons, theysuggested that rauisuchians had high growth ratesearly in ontogeny and achieved large adult sizesthrough protracted cyclical growth. Nesbitt (2007)reported a similar histological structure in thefemur of Effigia okeeffeae (Fig. 6f), although indicativeof perhaps higher growth rates than in otherpseudosuchians except non-crown-group crocodylomorphs.The most general result of thesestudies is that rauisuchian growth rates do notseem to have approached the rapid rates of dinosaursor pterosaurs (Padian et al. 2001; Erickson2005).In many pseudosuchians, osteoderms constitutethe most consistently well-preserved fossil elements,and thus justify detailed analysis. Recentstudies of archosaur osteoderm histology have generatedimportant data for systematic and functionalstudies (e.g. Scheyer & Sander 2004; Main et al.2005; Hill 2005; Hayashi et al. 2010; Cerda &Desojo 2011). Rauisuchian osteoderms (Fig. 6e)were rather compact bones, usually lacking substantialbone remodelling or large areas of cancellousbone, and thus presenting good growth records. Ofthe rauisuchians examined thus far, only Tikisuchusromeri and the possible rauisuchian Yarasuchusdeccanensis deviate from this trend and have osteodermswith a slightly larger central area of cancellousbone, forming a diploë structure. Preliminarystudies of osteoderms of Prestosuchus chiniquensisand indeterminate rauisuchians indicate, however,that there is some intraspecific variation in termsof bone compactness and degree of remodelling(Cerda et al. 2011). Comparison with extantcrocodylians suggests that this variation might berelated to the relative age, sex and reproductivestatus of the individual animal (Scheyer et al. 2011).However, age estimation based on the count ofgrowth marks in rauisuchian osteoderms will beaccurate only in those specimens that lack internalremodelling (i.e. only for individuals that diedat a young age). Interestingly, nearly all poposauroids(with the exception of Qianosuchus) lackosteoderms.Growth-associated changes have been documentedby comparing the cranial and postcranial skeletonsof presumably younger (smaller and lessfirmly sutured) and older (larger and more firmlysutured) individuals of Prestosuchus chiniquensis(Huene 1938b, 1942; Barberena 1978; Mastrantonioet al. 2008). These ontogenetic changes may also becharacteristic for rauisuchians more widely, becausethey also seem to occur in taxa with less completerepresentation of younger (smaller) individuals,such as both species of Postosuchus (Weinbaum2002, 2011; Peyer et al. 2008), Batrachotomus kupferzellensis(Gower 1999) and Decuriasuchus quartacolonia(França et al. 2011). In all these taxa,presumed younger individuals differ from olderones in lacking fusion between some bones (e.g.neural arches and centra; scapula and coracoid)and having less tightly connected cranial andmandibular elements. In Saurosuchus galilei, forexample, one of the most complete skeletonsknown (PVSJ 32) is skeletally immature (based onthe work of Brochu 1996 and Irmis 2007) becauseit has unfused cervical neural arch–centrum articulations.Furthermore, its skull bones are relativelypoorly ossified (e.g. articular end of the quadrate,poor ossification between exoccipital and basioccipital),and several skull bones (e.g. braincase)were preserved disarticulated (Alcober 2000; Trotteynet al. 2011) (Fig. 4). Gower & Schoch (2009,p. 118) reported less robust limb and pelvic girdlebones and less strongly pronounced muscle attachmentsites in smaller individuals of Batrachotomuskupferzellensis.
Downloaded from http://sp.lyellcollection.org/ at CAPES on May 3, 2013S. J. NESBITT ET AL.LocomotionSchaeffer (1941) pointed out that a terminal proximalfemoral head, contrasting with one offset fromthe long axis of the femur, is capable only of horizontalor slightly oblique movements and is associatedwith a generally sprawling locomotion withlittle vertical component, as observed in extant crocodylians.This femoral configuration is plesiomorphicfor Archosauria, characteristic of allrauisuchians, and is also observed in extant crocodylians.This led some authors, notably Charig (1972),to interpret such groups as functionally and evolutionarilyintermediate forms between ancestral‘sprawlers’, such as non-archosaurian archosauriforms,and derived ‘fully improved’ (upright) locomotors,such as dinosaurs and birds. Beyond thelimits of his typological approach, Charig’s interpretationsare incompatible with current understandingof archosaur phylogeny. In addition,Bonaparte (1984) identified an alternative mode oflocomotion and posture in rauisuchians, characterizedby a largely unmodified femur but upright hindlimbsand a parasagittal gait (Fig. 6c). Bonaparte(1984) drew attention to changes in the pelvicgirdle that permitted such a posture without substantialchanges in femur morphology, includingmore ventrally directed distal ends of sacral ribs,an almost horizontally held ilium with a low dorsalblade, a deep acetabulum bordered by a prominentsupra-acetabular crest, and an elongated pubisand ischium (Fig. 5). Since Bonaparte’s (1984)work, it has become apparent that rauisuchianshave notable variations on this theme. For example,Prestosuchus chiniquensis (Fig. 6b) has a combinationof the plesiomorphic and derived pelvic/hindlimb characters and has been considered tohave a less upright and parasagittal gait than Postosuchuskirkpatricki and Effigia okeeffeae (Liparini2011).Although muscular reconstructions for extinctdinosaurs have been attempted since Dollo (1888),only in the 1990s did researchers begin to reconstructthe soft tissues of extinct archosaurs usingthe extant phylogenetic bracketing methodology,which depends on an explicit phylogenetic context(Bryant & Russell 1992; Witmer 1995). The onlycomprehensive soft tissue reconstruction for a rauisuchianthat has been published thus far is for thepelvic and hindlimb myology of Poposaurus gracilis(Fig. 6a) presented by Schachner et al. (2011),although the unpublished theses of Kischlat (2003)and Liparini (2011) also discuss muscular reconstructionsfor Prestosuchus chiniquensis (Fig. 6b)and other south Brazilian rauisuchians. Notable featuresof Schachner et al.’s (2011) reconstructioninclude elongation and expansion of muscle- attachmentareas in the bone for the muscles that flexand extend the hip and knee articulations. For Prestosuchuschiniquensis, similar, but less accentuatedmodifications have been reconstructed by Liparini(2011). The distinctive rauisuchian ridge abovethe supra-acaetabular crest is inferred to be for theorigin of the M. iliofemoralis, the expanded preacetabularprocess of the ilium for part of the M.puboischiofemoralis internus, and the external surfacesof the extended distal parts of the pubes andischia largely for parts of the M. puboischiofemoralisexternus group. The architecture of the hip jointprobably restricted femoral extension, flexure andabduction relative to that found in parasagittalornithodirans. Despite the derived nature of rauisuchianpelvic osteology, these reconstructions havenot had to argue for any novel myological elements.This suggests that the complement of inferredmuscles in extinct and extant archosaurs was probablyfairly conservative, even though musculararrangements and locomotor function were diverse(Liparini 2011; Schachner et al. 2011).Biomechanical and functional analyses of therauisuchian crurotarsal ankle joint and the metatarsusindicate a predominantly plantigrade posture,where the whole plantar region of the foot participatesin at least part of the stride phase (Bonaparte1984; Parrish 1986; Carrano 1997). The caudally/posteriorly orientated calcaneal tuber of rauisuchians(in contrast to a more obliquely or almostlateral one, as observed, for example, in crocodyliansand phytosaurs) suggests a narrower, moreparasagittal gait (Brinkman 1980). A longer calcanealtuber is better suited to support greater bodyweights and to impress greater (more powerful)foot strokes rather than high speed and amplitudemovements of the feet (Carrano 1997) (Fig. 5).The footprint fossil record (e.g. of Chirotheriumstoretonense) provides some evidence in supportof the interpretation that the hindlimbs of evenquadrupedal rauisuchians were held in a relativelyupright and narrow gait (Kubo & Benton 2009).Rauisuchians studied thus far seem to lacknotable adaptations for supporting extreme bodymass or facilitating extreme cursoriality. Mediumsized(4.5 m) individuals of Prestosuchus chiniquensisare estimated to have weighed up to400 kg (Liparini 2011), much less than the tons ofkilograms of members of several dinosaur lineages(e.g. Christiansen & Fariña 2004; Erickson et al.2004; Sereno et al. 2009; Sander et al. 2011). Traitsof Prestosuchus chiniquensis such as relativelyshort limbs, a digit III that is not elongated, marginaldigits that are not notably reduced, similar proportionsof both hindlimb epipodials, and a metatarsalIII that is approximately half the length of thetibia indicate a subcursorial habit for this species,without obvious adaptations for running (Liparini2011). Accordingly, it seems that this and similar
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