The Complete Dinosaur

23. Stegosaurs

483 Stegosaurs Peter M. Galton 23 The Stegosauria, or plated dinosaurs, are medium-sized to large (up to 9 m in total body length), quadrupedal ornithischians, the most diagnostic character of which is the two rows of upright dermal plates and/or spines, one on each side of the midline, extending from the neck region to the end of the tail (Fig. 23.1A–D). Their remains are known definitely from the Middle Jurassic to the Early Cretaceous and on all continents except Antarctica (Olshevsky and Ford 1993; Galton and Upchurch 2004; Maidment et al. 2008; Novas 2009; Maidment 2010; Pereda Suberbiola et al. in press). Othniel Charles Marsh of Yale College erected the Stegosauria in 1877 for a new order of large extinct reptiles from the Upper Jurassic of Morrison near Denver, Colorado, USA. He considered that the back of what he then thought was a new aquatic reptile, Stegosaurus armatus (Greek stege, “roof,” saurus, “reptile”; Latin armatus, “armed”), was covered by large osteoderms, or dermal plates, one of which measured more than a meter in length. These large sheets of bone were thought to be completely embedded in the skin and comparable to those of Protostega, a large aquatic turtle from the Late Cretaceous of Kansas (Cope 1871). Marsh (1877, 513) noted that the bones are “embedded in so hard a matrix that considerable time and labor will be required for a full description.” Fortunately for O. C. Marsh, his workers in the field located quarry sites that yielded better material at Garden Park near Cañon City, Colorado, and Como Bluff in southeastern Wyoming (Ostrom and McIntosh 1999), in much softer rocks of what would become known as the Morrison Formation. Unfortunately for science, most of the original material from Morrison at the Yale Peabody Museum of Natural History is still in rock, and only a small part of it has been prepared (see Carpenter and Galton 2001; Galton 2010). Marsh (1880) illustrated the limbs and plates plus dermal spines of Stegosaurus ungulatus and reduced the Stegosauria to a suborder within the Dinosauria of Richard Owen. He recognized that the tail plates were held erect, as were even the largest asymmetrical ones over the back that were arranged in a row, or perhaps rows, on either side of the midline. Marsh noted that the dermal spine might be associated with the forefoot. In this he followed Owen (1875), who, in his description of the reasonably complete, but headless, skeleton of Omosaurus armatus from the Upper Jurassic of England, described a dermal spine found near the radius and ulna (forearm bones) that was compared with the wrist spine of the ornithopod dinosaur Iguanodon (Owen 1872). Marsh (1880) included Omosaurus in his new family, the Stegosauridae. The discovery of an almost complete skeleton Othniel Charles Marsh and Stegosaurus Peter M. Galton 484 of Stegosaurus in 1887, with the plates and spines preserved in articulation, showed that all plates were held vertically and that the spines belonged to the tail (Marsh 1887). The first skeletal reconstruction of Stegosaurus showed the plates in a single row along the midline plus four pairs of tail spines (Fig. 23.1A; Marsh, 1891, 1896–see below and Czerkas 1987 for more details on restorations of plates subsequent to Marsh). The first subdivision of the Dinosauria of Owen (1842) was made by Thomas Henry Huxley (1870), who referred all forms with dermal armor to the Scelidosauridae . This included Scelidosaurus, a reasonably complete skeleton from the Lower Jurassic of England (Fig. 23.1E; Owen 1861, 1863). When Harry Govier Seeley (1887) split the Dinosauria into the Saurischia and Ornithischia, the Scelidosauridae went to the latter. Marsh (1889, 1896) referred all the quadrupedal ornithischians with dermal armor (along with forms with similar tooth morphology) to the Stegosauria. On the basis of differences in the pelvic girdle, Alfred Sherwood Romer (1927) first made a case for the separation of the suborder Ankylosauria (“fused or joinedtogether reptiles”), erected without definition by Henry Fairfield Osborn (1923), from the suborder Stegosauria. Romer limited Stegosauria to Stegosauridae plus Scelidosauridae (for Scelidosaurus plus a few poorly known forms). This restricted use of the Stegosauria for just the plated dinosaurs was followed by most paleontologists in the United States, Canada, and England. However, other workers followed the original usage of Marsh (1889, 1896) with a more inclusive Stegosauria (e.g., Hennig [1925] and Lapparent and Lavocat [1955] as Stegosauroidea; Nopcsa [1915, 1928] as Thyreophora , the “shield bearers,” in a version that included the Ceratopsia...