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Wedel, MJ, & Taylor, M. (2013). Neural Spine Bifurcation in Sauropod Dinosaurs of the Morrison Formation

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Wedel, MJ, & Taylor, M. (2013). Neural Spine Bifurcation in Sauropod Dinosaurs of the Morrison Formation Wedel, M. J. , & Taylor, M. (2013). Neural spine bifurcation in sauropod dinosaurs of the Morrison Formation: ontogenetic and phylogenetic implications. PalArch's Journal of Vertebrate Palaeontology, 10(1), 1-34. Publisher's PDF, also known as Version of record Link to publication record in Explore Bristol Research PDF-document University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) NEURAL SPINE BIFURCATION IN SAUROPOD DINOSAURS OF THE MORRISON FORMATION: ONTOGENETIC AND PHYLOGENETIC IMPLICATIONS Mathew J. Wedel* & Michael P. Taylor# *Corresponding author. College of Osteopathic Medicine of the Pacific and College of Podiatric Medicine, Western University of Health Sciences, 309 E. Second Street, Pomona, California 91766-1854, USA. [email protected] #Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. [email protected] Wedel, Mathew J. & Michael P. Taylor. 2013. Neural Spine Bifurcation in Sauropod Dinosaurs of the Morrison Formation: Ontogenetic and Phylogenetic Implications. – Pal- arch’s Journal of Vertebrate Palaeontology 10(1) (2013), 1-34. ISSN 1567-2158. 34 pages + 25 figures, 2 tables. Keywords: sauropod, vertebra, neural spine, ontogeny, Morrison Formation AbsTrAcT It has recently been argued that neural spine bifurcation increases through ontogeny in several Morrison Formation sauropods, that recognition of ontogenetic transforma- tion in this ‘key character’ will have sweeping implications for sauropod phylogeny, and that Suuwassea and Haplocanthosaurus in particular are likely to be juveniles of known diplodocids. However, we find that serial variation in sauropod vertebrae can mimic on- togenetic change and is therefore a powerful confounding factor, especially when deal- ing with isolated elements whose serial position cannot be determined. When serial po- sition is taken into account, there is no evidence that neural spine bifurcation increased over ontogeny in Morrison Formation diplodocids. Through phylogenetic analysis we show that neural spine bifurcation is not a key character in sauropod phylogeny and that Suuwassea and Haplocanthosaurus are almost certainly not juveniles of known diplodo- cids. Skeletochronology based on the sequence of skeletal fusions during ontogeny can provide relative ontogenetic ages for some sauropods. Although such data are sparsely available to date and often inconsistent among sauropod genera they provide another line of evidence for testing hypotheses of ontogenetic synonymy. Data from skeletal fu- sions suggest that Suuwassea and Haplocanthosaurus are both valid taxa and that neither is an ontogenetic morph of a known diplodocid. © PalArch Foundation 1 Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) Introduction cervical vertebrae in certain large-bodied, long- necked birds (Rhea, Tsuihiji, 2004: figure 2b;Ca- Among tetrapods, sauropod dinosaurs are un- suarius, Schwarz et al., 2007: figure 5b; Dromai- usual in that many taxa have deeply bifid neural us, Osborn 1898: figure 1;Theristicus , Tambussi spines in their presacral vertebrae. Many mam- et al., 2012: 7; also in the recently extinct Drom- mals have shallowly bifid spines in their cervical ornithidae, Gastornithidae, and Phorusracidae, vertebrae, but usually only the neurapophysis Tambussi et al. 2012: 7), the thoracic vertebrae is divided, whereas in sauropods the division is in some bovids (e.g. zebu Bos indicus, Mason & more extensive. In the most extreme cases the Maule, 1960: 20), and the lumbar vertebrae of si- midline cleft extends to the roof of the neural renians (Kaiser, 1974). Cervical neural spines in canal, completely dividing the neural spine into humans and many other mammals have paired bilaterally paired metapophyses (figure 1). Bifid tubercles at their tips (Kapandji, 2008: 190- presacral neural spines evolved several times 191; Cartmill et al., 1987: figure 2-3a; figure 3). independently in sauropods, and are present They are therefore sometimes described as in some mamenchisaurids, all known diplodo- being bifid (e.g. White & Folkens, 2000: 145). cids and dicraeosaurids, the basal macronar- The appearance of bifurcation is caused by the ian Camarasaurus, the basal somphospondyls outgrowth of bone at the spine tip to anchor Euhelopus, Erketu, and Qiaowanlong, and the the large transversospinalis muscles. This is derived titanosaur Opisthocoelicaudia (Wilson a different phenomenon from the non-union & Sereno, 1998; Ksepka & Norell, 2006; You & of the endochondral portions of the vertebral Li, 2009; figure 2). In addition, the tips of the spine, which occurs pathologically in humans proximal caudal neural spines are often weakly (and presumably all other vertebrates) as spina bifid in diplodocids (e.g. Diplodocus carnegii bifida cystica and spina bifida occulta (Barnes, CM 84/94, Hatcher, 1901: plate 9). In contrast, 1994: 46-50 and figures 3.5 and 3.6). non-pathological bifid neural spines are uncom- The developmental underpinnings of bifid mon in extant tetrapods, and are limited to the neural spines in sauropods are not well under- Figure 1. A cervical vertebra of Apatosaurus ajax YPM 1860 showing complete bifurcation of the neural spine into paired metapophyses. In dorsal (top), anterior (left), left lateral (middle), and posterior (right) views. © PalArch Foundation 2 Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) Figure 2. Consensus phylogeny of sauropods based on the strict consensus trees of Taylor (2009), Ksepka & Norell (2010) and Whitlock (2011). The first of these provides the skeleton of the tree including outgroups, basal sauropods and macronarians; the second gives the positions of Erketu and Qiaowanlong; the last provides a detailed phylogeny of Diplodocoidea. Taxa with bifid neural spines are highlighted in blue. Haplocanthosaurus and Suuwassea, whose positions are disputed by Woodruff & Fowler (2012) are shown in bold. © PalArch Foundation 3 Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) above, the hypotheses of Woodruff & Fowler (2012) depend on ontogenetic inferences drawn from Morrison Formation sauropod taxa, and therefore we are confining our discussion to those taxa (e.g. Camarasaurus, Haplocanthosau- rus, and the Morrison diplodocoids). Abbreviations AMNH, American Museum of Natural History, New York City, New York, USA; BYU, Earth Sciences Museum, Brigham Young University, Provo, Utah, USA; CM, Carnegie Museum of Natural History, Pitts- burgh, Pennsylvania, USA; Figure 3. A middle cervical vertebra of a human in cranial FMNH, Field Museum of Natural History, Chi- view showing paired bony processes for the attachment cago, Illinois, USA; of dorsal muscles to the neural spine. Uncatalogued MB.R., Museum für Naturkunde Berlin, Germany; specimen from the anthropology teaching collection at the NSMT, National Science Museum, Tokyo, Japan; University of California, Santa Cruz. OMNH, Oklahoma Museum of Natural History, stood. It is possible that in some vertebrae the Norman, Oklahoma, USA; paired embryonic neural arch elements never SMNS, Staatliches Museum für Naturkunde, fused except to form a roof over the neural ca- Stuttgart, Germany; nal. In contrast, in the genus Camarasaurus it USNM, National Museum of Natural History, is possible that many of the presacral neural Washington, D.C., USA; spines were not bifid in young animals, and UWGM, University of Wyoming Geological that the degree of bifurcation increased over Museum, Laramie, Wyoming, USA; the course of ontogeny (see below). WPL, Western Paleontological Laboratories, In a recently-published paper, Woodruff & Lehi, Utah, USA; Fowler (2012) argued that the degree of bifur- YPM, Yale Peabody Museum, New Haven, Con- cation of sauropod neural spines was ontoge- necticut, USA. netically controlled, with the simple, undivided spines of juveniles gradually separating into Materials and Methods paired metapophyses over the course of post- hatching ontogeny. Based on this inferred on- Neural spine bifurcation in sauropods is a con- togenetic trajectory, Woodruff & Fowler (2012) tinuum from completely unsplit spines to those further argued that currently recognized sauro- that are completely separated down to the roof pod taxa are oversplit, and that when ontoge- of the neural canal. For the sake of convenience, netic transformations were taken into account, in this paper we classify neural spines into four it would be necessary to synonymize several categories based on their degree of bifurcation: taxa. In particular, they argued that the Mor- 1) Spines that entirely lack any midline inden- rison Formation diplodocoid Suuwassea was a tation are described as unsplit; juvenile of a known diplodocid (Ibidem: 6-8), 2) Those with extremely shallow notches in the that Haplocanthosaurus and Barosaurus were dorsal margin, whose depth is less than the likewise suspect (Ibidem: 9), and that rebbachis- minimum width of the spine itself, are de- aurids were possibly paedomorphic dicraeosau- scribed as notched; rids (Ibidem: 8-9). 3) Those that are split
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