AN OSSIFIED TENDON TRELLIS IN CHASMOSAURUS (ORNITHISCHIA: CERATOPSIDAE)

Journal of Paleontology, Mar 2007 by Holmes, Robert, Organ, Christopher

INTRODUCTION

THE ORNITHISCHIA and its sister group, the Saurischia, comprise the Dinosauria. Ornithischians (Weishampel, 2004) are a diverse group of primarily quadrupedal herbivores that include such familiar dinosaurs as the armored stegosaurs and ankylosaurs (Thyreophora), as well as the duckbilled hadrosaurs, pachycephalosaurs, and ceratopsians (Cerapoda). The Ceratopsia (You and Dodson, 2004) includes a number of relatively small basal forms such as psittacosaurs and Neoceratopsia, which comprises protoceratopsids and the larger, more derived horned Ceralopsidae.

Ossified tendons occur throughout the body of many birds (Vanden Berge and Storer, 1995), but reports in other Saurischia are rare (but see Sereno et al., 2004). However, they occur in nearly all ornithischian dinosaurs except stegosaurs (Sereno and Dong, 1992). The presence of ossified tendons above the sacrum is thought to be a diagnostic character of the Ornithischia (Weishampel, 2004). Many taxa also develop them in association with the caudal and/or thoracic vertebrae. Ossified tendons usually occur in bundles along the epaxial region of the vertebral column and the hypaxial region of the tail in basal ornithopods such as Hypsilophodon Galton, 1974. Similar tendon bundles are known in pachycephalosaurs (Sues and Galton, 1987) and ankylosaurs possess tendons associated with the tail "club" (Coombs, 1995). The ornilhopod group Iguanodonloidea (Hadrosauridae and Iguanodonlidae plus Protohadros Head, 1998 and Ouranosaurus Taquel, 1976) developed a rhomboidal trellis of ossified tendons (Fig. 1), often considered to be synapomorphic (e.g., Sereno, 1999), along the dorsal side of their vertebral columns (Dollo, 1886; Brown, 1916; Parks, 1920; Lull and Wright, 1942; Norman, 1980, 1986; Prieto-Márquez, 2001; Hornerel al., 2004). Note that Hadrosauriformes is synonymous with Iguanodontoidea, but the latter is preferred (Norman, 2004) and used here.

The occurrence of ossified epaxial tendons in a wide variety of ornithopods suggests thai a well-developed lendon system should be presenl in neoceralopsians as well. However, tendons are only occasionally preserved in neoceralopsians, and their anatomy is poorly documented. Previous work suggested that they were relatively poorly ossified and less well organized than in hadrosaurs (Hatcher et al., 1907; Brown, 1917; Brown and Schlaikjer, 1937; Sternberg, 1951). However, a recently discovered skeleton of Chasmosaurm irvinemis Holmes et al., 2001 possesses a three-layered rhomboidal tendon-trellis of comparable complexily and similar organizalion to those seen in Iguanodonloidea.

MATERIALS AND METHODS

The specimen described here (NMC 41357) is lhe holotype of Chasmosaurm irvinensis, collected from the Dinosaur Park Formation of southern Alberta by W. Langslon (see Holmes tl al., 2001 for delails). The skeleton is largely complete, missing only portions of the left side of the skull, most of the left fore- and hindlimb elements, and tail. The vertebral column was preserved in articulalion. Although ironstone obscured some delails of the cervical and anterior thoracic column, the remainder was encased in easily removed sandstone. Careful preparation permitted the ossified epaxial tendons to be exposed without damage. A well-developed, uninterrupted series of tendons is present throughout the thoracic vertebral column. Posteriorly, the tendons tie the trunk and sacrum together. Anteriorly, they extend onto the posterior cervical vertebrae, but damage sustained during removal of the ironstone matrix makes it impossible to determine the full anterior extent of the trellis.

EPAXIAL MUSCLES IN EXTANT ARCHOSAURS

Sauropsid epaxial muscles can be divided into three groups: M. transversospinalis (including the intrinsic vertebral muscles), M. longissimus dorsi, and M. iliocostalis (Vallois, 1922; Gasc, 1981; Frey, 1982; Molnar and Frey, 1987; Frey et al., 1989). The intrinsic vertebral muscles of M. transversospinalis occur as thin sheets that connect the neural spines (Mm. interspinales and Mm. interarcuales) and zygapophyses (Mm. interarticulares; Mm. intercristales in birds). The bulk of M. transversospinalis sits lateral to the intrinsic vertebral muscles and is composed of multiple slips. From each slip originates a serially homologous array of tendons that attach to the dorsal tip of the neural spines. From medial to lateral, the slips of M. transversospinalis are M. multifidus, M. spinalis, and M. semispinalis. Crocodilian M. multifidus and M. spinalis are called M. neurospinalis and M. spinoarticularis, respectively, by Frey (1982) and Tsuihiji (2005). The former names are preferred because they are more commonly applied to their homologues in reptiles and mammals (Gasc, 1981).

The crocodilian M. transversospinalis is derived in several ways as a consequence of its relationship to the dermal armor above it (Frey, 1982, 1985). M. semispinalis is divisible into two different sections, M. articulospinalis and M. tendinoarticularis. Tendons of M. articulospinalis and M. spinalis split to insert onto scutes, and M. tendinoarticularis forms anteriorly pointed cones of myosepta. Lateral to M. transversospinalis is M. longissimus dorsi. It contains posteriorly pointed cones of myosepta that insert on transverse processes (Mm. intertransversarii connect transverse processes and are generally considered part of the longissimus). The most lateral epaxial muscle is M. iliocostalis, which originates on the anterior aspect of the ilium and inserts on the tubercles of the ribs and lateral aspects of the transverse processes of the thoracic vertebrae.