Sigmoidal Border Ilium

Marginocephalia

Cerapoda

Figure 8.1 I. Cladogram of Cerapoda, emphasizing the monophyly of Pachycephalosauria. Derived characters include: at I thickened skull roof, frontal excluded from orbital margin, tubercles on posterolateral margin of squamosal, thin, plate-like basal tubera, double ridge-and-groove articulations on dorsal vertebrae, elongate sacral ribs, caudal basket of fusiform ossified tendons, ilium with sigmoidal border; medial process on ilium, pubis nearly excluded from acetabulum, tubercles on squamosal, broad expansion of squamosal onto occiput, free ventral margin of the quadratojugal eliminated by contact between jugal and quadrate.

and Lesothosaurus (see introductory text to Part II: Ornithischia, and Chapter 10).

Members of the pachycephalosaur clade share a host of derived features, most of them cranial. Most important are the thickened skull roof (either table-like or domed), modified cheek region that shrouds the jaw joint, extensive ossification of the orbit (including additional bony elements fused to its upper margin), shortening of the floor of the braincase, expansion of the back of the skull, abundant and strongly developed ornamentation of the external surfaces of the skull, special ridge-and-groove articulations between articular processes on the back and tail vertebrae, elongate sacral ribs, a "basket-work" of ossified tendons that cover the end of the tail, and reduction of the pubic bone to where it does not contribute to the formation of the hip joint. Within Pachycephalosauria, there are a few poorly known taxa at the base of the clade (Figure 8.12). Although it lacks skull material and has been referred to numerous other ornithischian groups, Stenopelix, from the Early Cretaceous of Germany, is probably best placed as the most basal pachycephalosaur. It possesses elongate sacral ribs and a pubis that is nearly excluded from the acetabulum but lacks a medial process on the ilium and a sigmoidal dorsal iliac margin, features otherwise found in all remaining pachycephalosaurs for which they are known.

Yaverlandia - the only other Early Cretaceous pachycephalosaur (this time from England) - was a small animal (length of the skull cap is 45 mm). The top of its head is thickened into two small domes, one on each side of the frontal bones. The upper surface of each dome is pitted. Because Yaverlandia is known only from these frontals, it is difficult to position this form within the cladogram of Pachycephalosauria.

Cladogram Pachycephalosaurus
Figure 8.12. Cladogram of Homalocephaloidea. Derived characters include: at I broad parietal bone, broad medial process on ilium; at 2 nasal and postorbitals incorporated into dome.

However, it seems to be basal within the clade, having either an unresolved relationship with the Wannanosaurus + Goyocephale clade or constituting an unresolved pachycephalosaur at its basal node with Stenopelix.

The remainder of pachycephalosaur taxa has been subject to several cladistic analyses, beginning with P. C. Sereno's 1986 study, followed by research published by T. Maryanslca, Goodwin and co-workers, Williamson and Carr, and Maryanslca, Chapman, and Weishampel, each of which has produced roughly consistent cladograms. Pachycephalosaurs more derived than Stenopelix (and Yaverlandia?) emphasize the growth of the frontal dome, yet share a number of derived features involved with the development of the nerves of the sense of smell that are not seen in Yaverlandia. The basal taxon within this less-inclusive clade is Wannanosaurus. Excluding this flat-headed Chinese form, remaining pachycephalosaurs constitute Goyocephale, a clade diagnosed by reduction of the upper temporal fenestra and a rectangular postacetabular process on the ilium. In the same way, the next less inclusive clade - the most recent common ancestor of Homalocephale and Pachycephalosaurus and all of its descendants - is called Homalocephaloidea, diagnosed by having a broad parietal bone and a broad medial process on the ilium.

Successively more restricted clades are found within Homalocephaloidea. The first, more inclusive, includes Stygimoloch, Stegoceras, Tylocephale, Prenocephale, and Pachycephalosaurus as terminal taxa and is as yet unnamed. This clade is diagnosed by doming of the thickened frontal-parietal part of the skull roof, obliteration of the sutures between these bones, and closure of the upper temporal fenestrae.

Thereafter is an unresolved relationship among the two species of Stegoceras and a clade formed by Tylocephale, Prenocephale, and Pachycephalosaurus, together forming Pachycephalosauridae. In these forms, the nasal

Pachycephalosaurs meet history: a short account of their discovery and postorbitals are incorporated into the domed skull roof. Finally, Tylocephale, Prenocephale, and Pachycephalosaurus form an unresolved clade within Pachycephalosauridae, recognized by having the dome formed by the frontal, parietal, postorbital, and squamosal bones, loss of the squamosal and postorbital platform, and the presence of tubercles on the nasals. According to a new study by Williamson and Carr, the newly discovered Sphaerotholus is also a member of this latter clade.

Pachycephalosaurs are now so thoroughly associated with the moniker "dome-headed dinosaurs," it's hard to imagine that human acquaintance with these animals started out in a sorry state of confusion. The story begins in 1856, with Joseph Leidy's studies of some of the first fossil vertebrate material to come back to the Academy of Natural Sciences in Philadelphia from the earliest government explorations of the wilds of the Western Interior of the USA. One of these specimens, discovered in the Judith River Formation of Montana (Late Cretaceous in age), was a curious-looking, triangular, "cuspy" tooth no larger than half a centimeter in height. Leidy thought that this tooth came from a fossil lizard, calling it Troodon (troo - wound; odon - tooth).

Similar lands of teeth began turning up at the turn of the century through the paleontological explorations of Lambe in Alberta and J. B. Hatcher in Wyoming. Despite the ever-increasing North American sample of these Late Cretaceous teeth, it took Franz Baron Nopcsa, Hungary's best-known dinosaur paleontologist, to suggest that these teeth belonged to a theropod dinosaur instead of a lizard. Nevertheless, that didn't stop a number of dinosaur paleontologists from thinking otherwise. The crux of this difference of opinion was Lambe's 1902 discovery of some rather peculiar, yet fragmentary skull fragments in the same Judith River beds where he had been finding Troodon-like teeth. These specimens consisted of domed skull caps that were exceedingly thick. Moreover, the caps were distinctively ornamented with prominent tubercles. The animal from which these skull fragments came he called Stegoceras (stego - cover; keras - horn).

Nearly a quarter of a century later, C. W. Gilmore described the first complete skull and partial skeleton of a dome-headed dinosaur, which he referred to as Troodon validus. No longer than 2 m, this animal had a 20 cm long skull, with an exceedingly thick, knobby-looking frontal-parietal region. On the back of this thickened skull cap was a short platform (the parietal-squamosal shelf) ornamented with knobs and bumps. On beyond, the skeleton was much like many other bipedal ornithischians, having relatively short forelimbs (the hand remains unknown), long four-toed hindlimbs, stiff vertebral column, and long tail. Since he gave his new dome-headed dinosaur the same generic name as Leidy's dinosaur tooth, Gilmore apparently thought the teeth in the jaws of his new specimen were quite similar to the one Leidy had described 75 years previously, the result of which was that dome-headed dinosaurs began being called troodontids. The troodontids, Gilmore felt, had close relationships with ornithopods, perhaps even being members of the clade.

Despite the importance of Gilmore's study, it took another 20 years for a new dome-headed dinosaur to be described. In 1943, as part of their larger study of the existing material from these dinosaurs, the legendary dinosaur hunters Barnum Brown and Eric Schlaikjer described a new form - Pachycephalosaurus (pachys - thick; kephale - head; saurus - lizard) -which consisted of only a skull. But what a skull: nearly complete, it approached 65 cm in length, with a dome of solid bone 20 cm thick. A skull this size would ride at the end of an estimated 8 m long body. Hailing from the Upper Cretaceous strata of Wyoming, South Dakota, and Montana, Pachycephalosaurus was one of the centerpieces of this detailed study, which also included an important review of what was known of these animals up to the 1940s and the first suggestion that these enigmatic dome-heads had evolutionary affinities with Ceratopsia.

Still, Brown and Schlaikjer continued to think that Troodon was a dome-headed dinosaur. It took further studies to sort out this identity problem. In 1945, Sternberg argued that Leidy's original Troodon tooth came from a small theropod dinosaur (see Chapter 12). More importantly for the dinosaurs at hand, the animal described by Gilmore was correctly renamed Stegoceras, and Pachycephalosaurus became the namesake of the entire group of dome-heads, which was christened Pachycephalosauridae. This view was seconded by L. S. Russell, who studied the original material of Troodon and additional jaw specimens from Upper Cretaceous rocks of Alberta. Pachycephalosaurs finally had come into their own.

Although North American pachycephalosaur specimens continued to be discovered over the ensuing decades, it was not until the 1970s that new lands of these dome-headed dinosaurs were recognized elsewhere. The first and only pachycephalosaur from England was named by Galton in 1971. Called Yaverlandia (from Yaverland Point), this animal is known only from a small skull fragment, collected from the famous Wealden beds of the Isle of Wight, off the southern coast of England.

Thereafter, beginning in the mid-1970s, came a virtual flood of new pachycephalosaurs, this time from Mongolia and China. The first of these was Maryanska and Osmolska's 1974 study of Asian pachycephalosaurs, which unleashed in one fell swoop three new forms, including the first of the flat-headed pachycephalosaurs. Two of these came from the Late Cretaceous Nemegt Formation of Mongolia: Prenocephale (prenes - sloping) and Homalocephale (homalos - even). The former - consisting of a complete skull, but partial postcranial skeleton - is the more spectacular in terms of the beautiful preservation of the skull. Apparently all that needed to be done to clean it up after it had been found was to blow a few sand grains out of its nasal cavity.

In contrast to the dome-headed Prenocephale, Homalocephale is a flat-headed pachycephalosaur. Known from a nearly complete skull, and more importantly a virtually complete, articulated skeleton, Homalocephale is perhaps the best known among all pachycephalosaurs. Still lacking a hand (to this day, none has been found for a pachycephalosaur), the rest of the forelimb is quite small, much shorter than the hindlimb. The backbone appears to have been quite rigid, given that each vertebra is locked with the next via tongue-and-groove articulations. Elsewhere, the rib cage is exceedingly rotund, the hips are broad, and the front section of the tail is wide and lacks chevrons along the undersurface of the vertebrae. The tail ends in a criss-cross of ossified tendons, suggesting that this region was very stiff in life.

The last of Maryanslca and Osmolslca's new pachycephalosaurs is Tylocephale (tyle - swelling). Collected from the Barun Goyot Formation of Mongolia, this full-domed pachycephalosaur is known so far from only a partial frontal-parietal specimen that appears to be somewhat similar to Stegoceras. We have no knowledge yet what the rest of the skeleton of this animal is like.

On the other side of the Mongolian border from where Maryanslca and Osmolslca had been working, Chinese pachycephalosaurs began showing up in the late 1970s. Wannanosaurus (Chinese name for a southern part of Anhui Province), so far known only from a partial skull and associated skeletal fragments, was described in 1977 by L. Hou. It comes from the Xiaoyan Formation (Upper Cretaceous) of Anhui Province, China. This was followed shortly thereafter with the discovery of Micro-pachycephalosaurus (micro - small). Named by the Dean of Chinese dinosaur paleontology, Z.-M. Dong, in 1978, Micropachycephalosaurus is presently known only from a lower jaw and an associated, but fragmentary postcranial skeleton from the Wang Formation (Upper Cretaceous) of Shandong, China. In this same paper, Dong coined the name Homalo-cephalidae for those pachycephalosaurs with a thick, yet flattened, skull roof such as Homalocephale, Wannanosaurus, and possibly Micropachycephalosaurus. The full-domed pachycephalosaurs, of course, require a taxonomic name as well, which Dong suggested is Gilmore's Pachycephalo-sauridae. A year later, another new North American pachycephalosaur reared its unimpeachably ugly head. Gravitholus (gravis - heavy; tholos -dome) was named by Wall and Galton in 1979. Consisting of only part of a dome, this nevertheless distinctive pachycephalosaur was collected from the Judith River Formation of Alberta.

The last four pachycephalosaurs so far to be discovered include Goyocephale (goyo - decorated), Stygimoloch (Stig (Styx) - river of Hades (Hell Creek); moloch - demon), Ornatotholus (ornatus - adorned), and Sphaerotholus (sphaira - ball). Goyocephale - a flat-headed pachycephalosaur - was described in 1982 by A. Perle, Maryanslca and Osmolslca. This animal is known from a fragmentary skull and nearly complete postcranial skeleton that came from Upper Cretaceous strata in Mongolia. Stygimoloch was originally described by Sues and Galton in 1983 and consists of a domed skull cap and postcranial fragments collected by a variety of different individuals from Upper Cretaceous layers in central and eastern Montana and adjacent parts of Wyoming. Ornatotholus was also described by Sues and Galton in 1983; it consists of dome fragments found in the Judith River Formation of both Alberta and Montana. Finally, Williamson and Carr described Sphaerotholus from the Upper Cretaceous Kirtland Formation of northwestern New Mexico - it is presently known from several partial skulls.

Important readings Chapman, R. E„ Galton, P. M„ Sepkosld, J. J. and Wall, W. P. 1981. A mor-

phometric study of the cranium of the pachycephalosaurid dinosaur Stegoceras. Journal of Paleontology, 55,608-616.

Colbert, E. H. 1955. Evolution of the Vertebrates: A History of the Backboned Animals Through Time. Wiley, New York, 479pp.

Davitashvili, L. S. 1961. [The Theory of Sexual Selection], Izdatel'stov Alcademia Nauk SSSR, Moscow (in Russian), 537pp.

Galton, P. M. 1971. A primitive dome-headed dinosaur (Ornithischia: Pachycephalosauridae) from the Lower Cretaceous of England, and the function of the dome in pachycephalosaurids. Journal of Paleontology, 45,40-47.

Giffin, E. B. 1989. Pachycephalosaur paleoneurology (Archosauria: Ornithischia) .Journal of Vertebrate Paleontology, 9, 67-77.

Maryañska, T., Chapman, R. E. and Weishampel, D. B. 2004. Pachycephalosauria. In Weishampel, D. B., Dodson, P. and Osmólska, H. (eds.), The Dinosauria, 2nd edn, University of California Press, Berkeley, pp. 464-477.

Maryañska, T. and Osmólska, H. 1974. Pachycephalosauria, a new suborder of ornithischian dinosaurs. Paleontologica Polonica, 30,45-102.

Sereno, P. C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Society Research, 2.234-256.

Sues, H.-D. 1978. Functional morphology of the dome in pachycephalosaurid dinosaurs. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte, pp. 459-472.

Sues, H.-D. and Galton, P. M. 1987. Anatomy and classification of the North American Pachycephalosauria (Dinosauria: Ornithischia). Palaeontographica A, 198,1-40.

Wall, W. P. and Galton, P. M. 1979. Notes on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America, with comments on their status as ornithopods. Canadian Journal of Earth Sciences, 16, 1176-1186.

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