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Daniel J. Chure, Observation on the Morphology and Pathology of the Gastral Basket of Allosaurus, Based

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Daniel J. Chure, Observation on the Morphology and Pathology of the Gastral Basket of Allosaurus, Based ORYCTOS,Vol. 3 : 29 - 37,Décembre 2000 OBSERVATIONSONTHE MORPHOLOGYAND PATHOTOGY OF THE GASTRAL BASKET OF ALLOSAURUS,BASED ON A NE\rySPECIMEN FROM DINOSAUR NATIONAL MONT]MENT Daniel J. CHURE DinosaurNational Monument, Box 128,Jensen UT 84035,USA [email protected] Abstract: A well preservedand complete gastral basket of Allosaurus(DINO 11541)corrects a numberof earlier misinterpretationsof this structurein Allosaurus.The basket consists of eighteenrows of gastralia.The first row has only oneelement per sideand these do not overlapmedially but ratherform a craniallyfacing notch. All otherrows havetwo elementsper side,the medialone being the longer.The right andleft medianelements overlap at the mid- line of the basket.Allosaurus has only four elementsper gastralrow. Previousinterpretations for Allosaurushave been as high asnine elementsper row. The supposedfused, V-shaped mediangastral element is now knownto be a furcula.The supposedfour lateralelements in USNM 4734are actual- ly all part of a singlemedian element injured during life andhealed as a seriesof pseudoarthroses.The presenceof pseudoarthrosesand calluses in othermedian gastral elements of USNM 4734 furrhersupport this interpretation. The presenceof similarpathologies in the gastraliaof anotherspecimen of Allosaurus(USNM 8367)as well asin the dorsalribs of a specimenof Acrocanthosaurusatokensis (SMU 74646)suggests that suchinjuries were not uncommonin allosauroidtheropods. These injuries may be the resultof eitherstruggles with prey or intraspecific behavior,such as kicking or head-flankbutting. Key words:Dinosauria, Theropoda, Osteology, Allosaurus, Gastralia, Jurassic, Morrison, Pathology Observationssur la morphologieet les pathologiesde la cagegastrale d'Allosaurus basées sur un nouveauspécimen daDinosaur National Monurnent,Etats-Unis d'Amérique Résumé: Unecage gastrale complète en bon état de préserv ation d'Allosaurus permet de corrigerd'anciennes inter- prétationsde cettestructure chez ce dinosaure.Le cageen questionconsiste en 18 rangéesde gasfralia.La premiè- re rangéeprésente un seulélément par côté; les deuxéléments de cettepaire ne serecoupent pas médialement mais formentune encoche dirigée cranialement. Les auffesrangées possèdent deux éléments par côté,le médianétant le plus long. Les élémentsmédians gauches et droitesse recoupent sur I'axe de symétrie.Allosaurus ne possèdeque quaffeéléments par rangéegasfrale. Les anciennesinterprétations pour A/Iosaurus ont proposéjusqu'à 9 élémentspar rangée.L élémentgastral suppo- séfusionné en formede V s'estrévélé être une furcula. Les quaffeéléments latéraux supposés de USNM 4734sont enfait lesfragments d'un uniqueélément central fracturé durant la vie de I'animalet cicatrisésous forme d'une série depseudarthroses. La présencede pseudarthroseset de calssur d'autres éléments de la sériegastrale de USNM 4734 soutientcette interprétation. La présencede pathologiessimilaires sur les gasfraliad'un autrespécimen d'Allosaurus (USNM 8367)et sur les côtesd'un spécimend'Acrocanthosaurus atokensis (SMU 74646)indique que de tellesblessures n'étaient pas rares chezles théropodesallosauroides. Ces blessures peuvent être le résultatde luttesavec des proies ou de comporte- mentsintraspécifiques tels quedes ruades ou descoups de têtecontre les flancs. Mots clés: Dinosaurta,Theropoda, Ostéologie, Allosaurus, Gastralia, Jurassique, Morrison, Pathologie 29 ORYCTOS.Vo1.3.2000 INTRODUCTION of a furcula, the compositionand morphology of the wrist, and the gastralbasket. Both the wrist and the Allosaurus is the most abundanttheropod in the furcula have already been described (Chure & Late JurassicMorrison Formation (Foster & Chure, Madsen,1996; Chure, 1999,and in press).The pre- 1998,2000 ; Chureet a1.,2000).As a resultof this sentreport focuseson the gastralbasket. rich fossil record it is one of the best known thero- podsand has been the subjectof severalmonographic INSTITUTIONAL ABBREVIATIONS studies(Gilmore, 1920; Madsen,1976; Chure, 2000). AMNH = AmericanMuseum of NaturalHistory NY. Becauseof this Allosaurus is a critical taxon in many BYU = EarthSciences Museum, Brigham Young University, UT. phylogeneticanalyses of the Theropoda. CMNH = CarnegieMuseum of NaturalHistory PA However, the discovery of a spectacularlycom- DINO = DinosaurNational Monument, UT. plete and articulated specimen of Allosaurus in MOR = Museumof Therockies,MT. Dinosaur National Monument has clarified several SMU = SouthernMethodist University, TX. aspectsof Allosaurusmorphology; namely the presence USNM = UnitedStates National Museum, Washington, D.C. B Figure 1. Gastral basket of Allosaurus (DINO 11541) in dorsal view. A. Position of basket between pectoral and pelvic girdles. B. Close up of median and lateral elements.Scale bar in cm. 30 CHURE - OBSERVATIONSON TIIE MORPHOLOGYANDPATHOLOGY OF TIIE GASTRALBASKET OF ALLOSAURUS 'Weigert, DESCRIPTION (Gradzinski, 1969; 1927), which probably explainshow the skeletonremained articulated in the The specimen(DINO 11541)is a new speciesof high energyriver environment.Part of the right (stra- Allosaurus, currently under description (Chure, tigraphically up) forelimb and hindlimb and the right 2000). It was found in the upper portion of a 60 cm. side of the skull is missing, along with the middle layer of conglomerateat the base of a 1.4 m deep part of the tail. The right side of the gastral basket paleochannelin the Salt Wash Member of the was damagedbefore discovery and many of the late- Morrison Formation of Dinosaur National ral elementsare missing or incomplete.However, the Monument (Hubert & Chure, 1992). The skeleton gastralbasket was in-placebetween the pectoralgird- was in the hyperdorsoflexedattitude strongly sugges- le andpelvis (fig. 1). tive of desiccation of the body before burial l2 .,," :.- 15 Si1'':'", t7 t6,e -:!:-' t t' l8 t7 t5 EI:I:n l3 B t2 & Figure 2. 18 Gastral basket of Allosaurus 18 (DrNO 11541) in ventral view. A. View showing narrowing of basket towards caudal end. B. Closeup showing t7 displacementof contact between median elements. Numbers refer to gastral row, 16 numbered from front of basket. Scale bar in cm. 31 ORYCTOS,Vol.3,2000 right pubis left pubis The basket is rn a large block of sandstone along with the vertebral column, left dorsal ribs, the left scapula, and the left femur. As a result. the cranialhalf of the basketis exposed in dorsal view and the caudalhalf in ven- tral view. The cranial half of the cuirassis broad and flat (possiblydue in part to postmortem dorsoventral flattening), with a gentle dorsal curvature.The cau- dal part is narrower and Figure 3. Medial view of left lateral elements of gastral basket in DINO 11541. Note their position more V-shaped in cross- lateral to distal end of left pubis. Numbers refer to gastral row, numbered from front of basket. section(figs. 1, 2) and on the better preserved left side the caudalpart of the basket can be seen to be lateral to the pubic boot (fie.3). The basket contains eighteenrows of gastralia. The first row is different than the rest of the cuirass in severalways (fig. 4). In the flrst row there is only one elementon each side, ratherthan two. Theseele- ments do not overlap medially. Instead, the medial end of each is a vertical, oval surface and the right and left gastralia meet such that their medial faces form a cra- nially facing right angle notch (fig. 44, C). There are no grooveson the cra- nial margin of these ele- ments for contact with lateral gastralia, further confirming that the first row consists only of the medianelements (fig. aB). 32 CHURE - OBSERVATIONSON THE MORPHOLOGYAND PATHOLOGYOF TFM GASTRAL BASKET OF ALLOSAURUS In gastral rows two through eighteenthere are four gastraliaper row, two median and two lateral (fig. 5). There is no median V-shapedelement like the one reported by Gilmore (1920) in USNM 4734. The median ele- mentsare longer and more robustthan the lateral ones and slightly sinuous in dorsalor ventralview (fig. 1). The medialend is cau- dally curved and expan- ded (fig. 2). The medial ends overlap such that in ventral view the right median gastraliumis ven- tral to the left. The ventral surfaceof the medial end of the left gastralium is convex and this fits into a concavity on the dorsal surfaceof the medial end of the right gastralium (fig. 2B). Laterally the medianelements taper to a point (fig. 5B). A groove on the cranial face of the median gastraliais for the contact with the lateral element(figs. 58, 6). There has been post- mortem separation bet- ween the medianelements of the cuirass(fig. 5). The ventral overlappingof the right medianele- gastralia(figs. 1A, B, 5B). This is typical for thero- mentsis bestseen in mediangastral pm I7. Therehas pods, although the condition is reversed in some been slight disarticulationof this contactin segments maniraptorans (Norell & Makovicky, 1997). Each 18 and 16.In segments13,14, and 15 therehas been lateral elementhas a wide, truncatedlateral edgeand considerablecranial shifting of the left median ele- tapersmedially (fig. 5B). Each lateral elementhas a ments,to the extentthat, for example,the medial end groove and fits in a groove on the craniodorsaledge of left mediangastralium 15 overlapsthe shaftof right the medial element (fig. 6). This overlap between median gastralium 14. This offset continues, to a median and lateral elementswas not strong,as there varying degree,through gastralsegment 2. is some separationalong this contact in all gastral The lateral elementsare shorterthan the median rows in DINO lI54l. 33 ORYCTOS,Vol.3,2000 However,the
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