Pneumaticity and soft−tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods DANIELA SCHWARZ, EBERHARD FREY, and CHRISTIAN A. MEYER Schwarz, D., Frey, E., and Meyer, C.A. 2007. Pneumaticity and soft−tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods. Acta Palaeontologica Polonica 52 (1): 167–188. The axial soft−tissue system in the neck of Dicraeosauridae and Diplodocidae, including pneumatic diverticula, liga− ments, and muscles, is reconstructed on the basis of phylogenetic and functional morphological comparisons with extant crocodylians and birds and compared with other soft−tissue reconstructions for sauropods. Bifurcation of the neural spines separated the paired supraspinal ligament into two sheets. A paired interspinal septum was attached to the cranial and caudal margins of the neural spines. The dorsal and the lateral portions of the cervical musculature must have been strongly segmented, whereas the laterocostal portion was divided with one myoseptum per vertebral segment. The hypaxial cervical muscle was most probably small and only poorly segmented. In Diplodocidae and Dicraeosauridae, the distribution of external pneumatic structures is similar, whereas only Diplodocidae possess intraosseous pneumatic struc− tures. Supravertebral pneumatic diverticula are reconstructed for both groups, which, together with dorsal ligaments filled the gap between the metapophyses of bifurcate neural spines. Comparisons between the vertebrae of juvenile and adult diplodocids strongly indicate that pneumatisation proceeded from the supramedullary diverticula into the neural arch and the neural spine. The regular branching pattern of the pneumatic cavities as well as the vertical I−beam construc− tion of the vertebral corpora is interpreted as a consequence of the biomechanical constraints of the vertebral corpora in diplodocids. These reconstructions form the ground for functional morphological considerations in Diplodocidae and Dicraeosauridae while addressing the possible mechanical consequences of pneumatic structures for the integrity of the support system of the neck. Key words: Diplodocidae; Dicraeosauridae, vertebral pneumaticity; cervical ligaments; cervical musculature; func− tional morphology; ontogeny; tomography. Daniela Schwarz [[email protected]] and Christian A. Meyer [[email protected]], Naturhistorisches Mu− seum Basel, Augustinergasse 2, CH−4001 Basel, Switzerland; Eberhard Frey [[email protected]], Staatliches Museum für Naturkunde Karlsruhe, Abteilung Geologie, Erbprinzenstr. 13, D−76133 Karlsruhe, Germany. Introduction (Britt 1993, 1997; Wedel et al. 2000a, b; Wedel 2003a, b). The distribution and general pattern of pneumatic structures With body lengths exceeding 30 m and masses presumably in sauropod presacral vertebrae ranges from simple external reaching more than 26 tons (Henderson 2004), sauropods pneumatic fossae (e.g., Haplocanthosaurus, Dicraeosaurus, were the largest terrestrial vertebrates that ever existed (Up− Amargasaurus; Janensch 1947; Salgado and Bonaparte 1991; church et al. 2004a). Their presacral vertebrae bear external Britt 1993; Wedel 2003a) to large internal pneumatic cham− laminae and are in most cases hollowed out by a complex pat− bers (e.g., Diplodocus, Camarasaurus, Apatosaurus; Britt tern of cavities. These structures have been explained as the 1993; Wedel et al. 2000a; Wedel 2003a) or a densely spaced osteological traces of a pneumatic system similar to that of meshwork of many small camellae (e.g., Austrosaurus, Sau− extant birds (Seeley 1870; Janensch 1947; Britt 1997; Britt et roposeidon, Saltasaurus; Mamenchisaurus; Longman 1933; al. 1998; Wedel et al. 2000a; O’Connor 2006). Pneumaticity Young and Zhao 1972; Wilson and Sereno 1998; Wedel et al. of the sauropod axial skeleton in sauropods was recognized 2000b). Pneumatic structures in sauropods are highly variable early (Seeley 1870; Cope 1877; Marsh 1877), signalled by within species and individuals (Britt 1993; Curtice 1998; presence of large pneumatic foramina and fossae in the pre− Wedel et al. 2000a; Wedel 2003a), but some of their osteolo− sacral vertebrae. The most comprehensive description of gical traces, such as the laminae (Wilson 1999) or the struc− pneumatic structures and a first detailed outline nomenclature ture and position of pneumatic fossae, can be used for taxon− of laminae in sauropods was published by Janensch (1950). omy (see characters in Upchurch 1995; Salgado et al. 1997; Since non−invasive techniques such as computed tomography Upchurch 1998; Wilson and Sereno 1998; Wilson 2002; became available for analyzing the detailed internal morphol− Upchurch et al. 2004a). ogy of sauropod vertebrae in detail, research on the pneu− Hypotheses on the possible functions of vertebral pneu− maticity of sauropod bones has come under renewed scrutiny maticity in sauropods have included mostly weight reduction, Acta Palaeontol. Pol. 52 (1): 167–188, 2007 http://app.pan.pl/acta52/app52−167.pdf 168 ACTA PALAEONTOLOGICA POLONICA 52 (1), 2007 but also other possibilities such as body cooling (Janensch structions. From a phylogenetic viewpoint, diplodocid and 1947; Coombs 1978; Britt 1993, 1997; Wedel et al. 2000a; dicraeosaurid sauropods comprise well−defined and most Wedel 2005; O’Connor 2006). Vertebral pneumaticity in sau− probably monophyletic sauropod clades (Upchurch 1995, ropods indicates the presence of lung dilatations or air sacs in 1998; Wilson 2002, 2005; Rauhut et al. 2005). Both clades the trunk, which could have had an impact on mechanisms of have a rather unique morphology of their cervical vertebral their respiratory apparatus (Perry and Reuter 1999; Wedel column in terms of the bifurcation of neural spines, vertebral 2003b; O’Connor 2006). A possible biomechanical role of pneumaticity and cervical ribs in comparison with other pneumatic structures in the sauropod axial skeleton as stiffen− sauropods, and they have been studied in terms of their neck ing device was suggested by Akersten and Trost (2000, 2001, mobility (Stevens and Parrish 1999, 2005b, a). Therefore, re− 2004). However, there is general agreement on the fact that the constructions of soft−tissues in the neck of diplodocids and main function of vertebral pneumaticity in sauropods was to dicraeosaurids will provide a basis for investigating possible reduce the density of the skeleton, making the attainment of biological roles of pneumatic structures in the neck, for de− such large body sizes possible at all (Britt 1993, 1997; Wedel vising models of the bracing system of extremely long necks 2003b, a, 2004, 2005; O’Connor 2006). in sauropods, and with some restrictions also for reconstruc− A rationale for the distribution, amount, and possible bio− tions of soft−tissues in other eusauropods. logical roles of pneumatic diverticula in the axial skeletons Institutional abbreviations.—AMNH, American Museum of eusauropods is difficult to explain without detailed recon− of Natural History, New York, USA; CM, Carnegie Mu− structions of the distribution of the pneumatic diverticula, in seum of Natural History, Pittsburgh, USA; MACN, Museo context with the tendinomuscular bracing system. Recon− Argentino de Ciencias Naturales, Buenos Aires, Argentina; structions of the distribution of pneumatic diverticula in the NMB, Naturhistorisches Museum Basel, Switzerland; neck of sauropods are provided by Wedel (2005) and Wedel SMNK, Staatliches Museum für Naturkunde Karlsruhe, et al. (2000a), but overall soft−part reconstructions are re− Germany; SMA, Saurier−Museum Aathal, Switzerland; stricted to isolated muscle portions and ligaments (see e.g., SNM, Naturmuseum Senckenberg, Frankfurt, Germany; Tsuihiji 2004) or muscles and ligaments depicted only by USNM, National Museum of Natural History, Smithsonian their assumed force vectors (Wedel et al. 2000a; Wedel and Institution, Washington, USA; YPM, Yale Peabody Mu− Sanders 2002). An integrative functional morphological ana− seum, New Haven, USA; ZMB, Museum für Naturkunde lysis of the cervical vertebral column of different sauropods, der Humboldt−Universität Berlin, Berlin, Germany. including both soft−tissue reconstructions and evaluations of neck mobility, could be used to understand better how the Other abbreviations.—ASP, Airspace Proportion; CT, com− long sauropod necks were supported. The results of such an puted tomography; m., musculus (muscle); sprl, spinopre− analysis could then be taken as basis for the understanding of zygapophyseal lamina; spol, spinopostzygapophyseal lamina. feeding ranges and locomotor options of sauropods, there− fore for niche partitioning amongst sauropods or their settle− ment in different ecosystems. Materials and methods Because birds are the phylogenetically closest extant rel− atives of sauropods, the reconstruction of sauropod soft−tis− Material and techniques.—Postcranial material of Amarga− sues hitherto was mainly based on the anatomy of ratites like saurus, Apatosaurus, Barosaurus, Dicraeosaurus, Diplodo− Struthio or Rhea (Wedel et al. 2000a; Wedel and Sanders cus,andTornieria was examined (DS) at AMNH, CM, 2002; Tsuihiji 2004). Extant crocodylians form the other MACN, NMB, SMA, SNM, USNM, YPM, and ZMB. Only pole of the extant phylogenetic bracket of sauropods (Wit− taxa with well−preserved cervical vertebrae were used, in par− mer 1995, 1997), but topological similarities of crocodylian ticular the diplodocids Apatosaurus (Gilmore 1936; Upchurch and sauropod cervical vertebrae have been generally ne− et