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Homology and Architecture of the Caudal Basket of Pachycephalosauria (Dinosauria: Ornithischia): the First Occurrence of Myorhabdoi in Tetrapoda

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Homology and Architecture of the Caudal Basket of Pachycephalosauria (Dinosauria: Ornithischia): the First Occurrence of Myorhabdoi in Tetrapoda Homology and Architecture of the Caudal Basket of Pachycephalosauria (Dinosauria: Ornithischia): The First Occurrence of Myorhabdoi in Tetrapoda Caleb Marshall Brown1*, Anthony P. Russell2 1 Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada, 2 Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada Abstract Background: Associated postcranial skeletons of pachycephalosaurids, most notably those of Stegoceras and Homalocephale, reveal enigmatic osseous structures not present in other tetrapod clades. The homology and functional significance of these structures have remained elusive as they were originally interpreted to be abdominal ribs or gastralia, and more recently have been interpreted as de novo structures in the tail. Principal Findings: Analysis of these structures in nearly all pachycephalosaurid skeletons has facilitated a complete description of their architecture, and the establishment of patterns consistent with those of myorhabdoid ossifications — ossifications of the myoseptal tendons associated with myomeres. The presence and structure of myorhabdoid ossifications are well established for teleost fish, but this marks their first recognition within Tetrapoda. These elements are both structurally and histologically distinct from the deep, paraxial ossified tendon bundles of other ornithischian clades, although they may have performed a similar function in the stiffening of the tail. Conclusions/Significance: These myorhabdoi are not de novo structures, but are instead ossifications (and therefore more amenable to fossilization) of the normally unossified plesiomorphic caudal myosepta of vertebrates. The ubiquitous ossification of these structures in pachycephalosaurids (all specimens preserving the tail also exhibit myorhabdoid ossifications) suggests it is a likely synapomorphic condition for Pachycephalosauria. Citation: Brown CM, Russell AP (2012) Homology and Architecture of the Caudal Basket of Pachycephalosauria (Dinosauria: Ornithischia): The First Occurrence of Myorhabdoi in Tetrapoda. PLoS ONE 7(1): e30212. doi:10.1371/journal.pone.0030212 Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America Received September 27, 2011; Accepted December 15, 2011; Published January 17, 2012 Copyright: ß 2012 Brown, Russell. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was supported by an NSERC (Natural Sciences and Engineering Research Council of Canada) Alexander Graham Bell Canada Scholarship and Doris O. and Samuel P. Welles Fund Travel Grant (UCMP) to CMB and an NSERC Discovery Grant to APR, as well as financial contributions from Montessori High School at University Circle (Cleveland, OH), the Cleveland Museum of Natural History, and Lubrizol Corp. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: Funding for travel to Mongolia was provided in part by Lubrizol Corp. This does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials. * E-mail: [email protected] Introduction Osmo´lska (Figure 2A) (MPC-D 100/1201 – formerly GI SPS 100/ 51) [5,6] led to the establishment of homology between the The dome-headed pachycephalosaurids remain the most ‘‘abdominal ribs’’ of Gilmore [3] and the caudal ‘‘basket-work of enigmatic and poorly understood clade of ornithischian dinosaurs tendons’’ of Maryan´ska and Osmo´lska [5] and led to the rejection [1]. Upon its discovery in 1921, and its description in 1924, of their interpretation as gastralia in Stegoceras [7]. Similar UALVP 002 (Stegoceras validum Lambe) was the first pachycepha- disarticulated structures were also noted for Prenocephale prenes losaurid dinosaur for which significant postcranial skeletal material Maryan´ska and Osmo´lska (Z. Pal. No. MgD-I/104; [5]) and was known, and revealed much about the anatomy of this clade Goyocephale lattimorei Perle, Maryan´ska and Osmo´lska (MPC-D [2–4]. Among the preserved material were dozens of small, 100/1501 – formerly GI SPS 100/1501; [8]). These articulated disarticulated, bony elements that presented morphologies incon- elements were described as an intricate ‘‘basket’’ consisting of six sistent with known anatomical structures of ornithischians parallel rows of fusiform ‘‘tendons’’ with the attenuated extremities (Figure 1). These were tentatively identified as ‘‘abdominal ribs’’ of each making contact with the elements of the adjacent row. (gastralia) or possibly ‘‘ossified tendons’’ by Gilmore [3]. The Sues and Galton [9] attempted to assign the five ‘‘abdominal presence of gastralia, if verifiable, would be unique within the rib’’ types of Gilmore [3] to the serially homologous tendon rows Ornithischia. These elements were categorized by Gilmore into of the caudal ‘‘basket’’ of Homalocephale [5]. They interpreted the five morphotypes, some of which showed what appeared to be Type 1 elements (Figure 1A) as representing the thickened medial bilateral symmetry (Figure 1). portions of the elements of the fourth and fifth row of the caudal Subsequent discovery of an articulated set of ‘‘caudal tendons’’ ‘‘basket’’ (Figure 2A, 2B); Type 2 elements (Figure 1B) were stated in a portion of the tail of Homalocephale calathocercos Maryan´ska and to be similar to the medial portions of the elements from the PLoS ONE | www.plosone.org 1 January 2012 | Volume 7 | Issue 1 | e30212 Caudal Myorhabdoi in Pachycephalosaurid Dinosaurs Figure 1. Elements typifying the five tendon morphotypes of Gilmore [3] in internal and external views. A) Type 1 – ‘Heavy segments’ – ‘‘Large, heavy, sinuous segments with more or less flattened ends that are sometimes grooved with ray-like points’’ ([3]:31). These show right and left symmetry. One element reflected in the vertical plane: left; external view, right; internal view. B) Type 2 – ‘Double pointed segments’ – ‘‘Smaller, subround, sinuous segments having both ends slightly flattened and slenderly pointed’’ ([3]:32). Two elements of each ‘side’ are shown, reflected in the horizontal plane: upper; external view, lower; internal view. C) Type 3 – ‘Attenuated segments’ – ‘‘Of about the same size as the second, subround, sinuous, with one flattened end, the other drawn out into a slender attenuated rod-like process’’ ([3]:32). Two elements reflected in the vertical plane and of uncertain orientation. D) Type 4 – ‘Bifurcated segments - Rare (only two elements known). ‘‘Small, with long bifurcated divergent rounded processes at one end, with the other end unknown’’ ([3]:32). Two elements reflected in the horizontal plane: upper; external view, lower; internal view. E) Type 5 – ‘Median segments’ - Also rare. ‘‘Three small bones that are suggestive of being bilateral median segments, but the fact that both are asymmetrical would rather negative this suggestion’’ ([3]:32). These were incorrectly labeled as Type 4 four in Plate 14 of [3]. Two elements reflected in the horizontal plane and of uncertain orientation. doi:10.1371/journal.pone.0030212.g001 second and third rows (Figure 2A, 2B); and Type 3 elements of the ‘‘caudal basket’’ from a region more distal in the tail than (Figure 1C) were stated to represent the posterior portions of the the region preserved in Homalocephale (MPC-D 100/1201 – elements represented as Type 2 (Figure 1). Type 4 elements formerly GI SPS 100/51). This potentially accounted for the (Figure 1D) were thought to be the products of fusion between the imperfect match between the examples preserved in these two rod-like posterior portions of caudal structures, but could not be taxa. Sues and Galton [9], however, concluded that the ‘‘caudal matched with any elements of the caudal ‘‘basket’’ in Homalocephale basket’’ indicated that the tail of pachycephalosaurs was highly (Figure 2A). Sues and Galton were also unable to confidently specialized, and they thus endorsed the postulation of Maryan´ska match Type 5 elements (Figure 1E) with any known structures. and Osmo´lska [5] that it acted as one component of the ‘‘tripodal This attempt [9] to match the isolated elements found associated prop’’, along with the two hind limbs. with Stegoceras validum with the preserved in situ structures of Goodwin et al. [10] subsequently reported the occurrence of Homalocephale resulted in apparent partial success, but confusion ‘‘ossified tendons’’ in a fourth taxon of pachycephalosaurid, was introduced because of a reversal in labeling. In the figures Stygimoloch spinifer Galton and Sues. UCMP 128383 preserves presented by Sues and Galton [9] labels 1, 2, 3, 4 and 5 actually isolated fragments of Gilmore’s Type 1 and 3 elements (Figure 1A refer to types 4, 5, 3, 2, and 1 respectively, as originally indicated and C), and AMNH 21541 preserves isolated fragments of by Gilmore [3] (Figure 1). unspecified type. Goodwin et al. suggested that the ‘‘ossified Overall, the ‘‘ossified tendons’’ described by Gilmore [3] in tendons’’ in the caudal region may have served a protective Stegoceras were interpreted, by Sues and Galton [9], to be elements function by acting as a cuirass against agonistic behaviors such as PLoS ONE | www.plosone.org 2 January 2012 | Volume 7 | Issue 1 | e30212 Caudal Myorhabdoi in Pachycephalosaurid Dinosaurs flank-butting. The
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