Abstract
BackgroundAssociated 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 FindingsAnalysis 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/SignificanceThese 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.
Highlights
The dome-headed pachycephalosaurids remain the most enigmatic and poorly understood clade of ornithischian dinosaurs [1]
We report on most significantly complete pachycephalosaur postcranial skeletons that preserve portions of the tail and that are housed in accessible institutions
Reconstruction Examination of the articulated and isolated superficial caudal elements of the four pachycephalosaur specimens allows for a) a matching of the isolated elements of UALVP 002 with the articulated elements of MPC-D 100/1201, b) a revision of the types of elements present, and a documentation of their proportional representation, and c) a reconstruction of the three-dimensional architecture of the ossified caudal complex
Summary
The dome-headed pachycephalosaurids remain the most enigmatic and poorly understood clade of ornithischian dinosaurs [1]. Upon its discovery in 1921, and its description in 1924, UALVP 002 (Stegoceras validum Lambe) was the first pachycephalosaurid dinosaur for which significant postcranial skeletal material was known, and revealed much about the anatomy of this clade [2,3,4]. Among the preserved material were dozens of small, disarticulated, bony elements that presented morphologies inconsistent with known anatomical structures of ornithischians (Figure 1). These were tentatively identified as ‘‘abdominal ribs’’ (gastralia) or possibly ‘‘ossified tendons’’ by Gilmore [3]. The presence of gastralia, if verifiable, would be unique within the Ornithischia These elements were categorized by Gilmore into five morphotypes, some of which showed what appeared to be bilateral symmetry (Figure 1). 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
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