Pachycephalosauria

The most easily recognizable and well-known trait of pachycephalosaurs is an extremely thick, flat or domed bony skull (pachy = "thick" and cephalia = "skull"). For example, Pachycephalosaurus had a skull that was about 22 cm thick. This remarkable feature was formed by fusion of the frontals and parietals and the addition of

FIGURE 13.5 Skulls of Late Cretaceous pachycephalosaurids. (A) Stegoceras as represented only by its fused frontal and parietals forming a high dome, a typical trait of pachycephalisaurids; College of Eastern Utah Prehistoric Museum, Price, Utah. (B) Pachycephalosaurus, showing deeply inset cheek teeth, domal dorsal surface, and fringing osteoderms on dentary (ventral), nasal (anterior), and squamosals (posterior). Specimen is cast of original, Denver Museum of Science and Nature, Denver, Colorado.

FIGURE 13.5 Skulls of Late Cretaceous pachycephalosaurids. (A) Stegoceras as represented only by its fused frontal and parietals forming a high dome, a typical trait of pachycephalisaurids; College of Eastern Utah Prehistoric Museum, Price, Utah. (B) Pachycephalosaurus, showing deeply inset cheek teeth, domal dorsal surface, and fringing osteoderms on dentary (ventral), nasal (anterior), and squamosals (posterior). Specimen is cast of original, Denver Museum of Science and Nature, Denver, Colorado.

thick deposits of bone to this unit. Despite the impressively large and convex heads this fusion caused in some pachycephalosaurs, they did not have a corresponding increase in cranial capacity. In fact, their EQs are only slightly higher than not-so-brainy dinosaurs such as thyreophorans.

In the early twentieth century, Lawrence Lambe (Chapter 3) was the first to examine pachycephalosaur material, and he named Stegoceras (Fig. 13.5A). Pachycephalosaurus (Fig. 13.5B), the inspiration for the naming of the clades Pachycephalosauria and Pachycephalosauridae, was not discovered until about 40 years later. Soon after, recognition of some already discovered but mislabeled pachycephalosaur remains helped to relate them. Later discoveries of new species of pachycephalosaurs led to the insight that they belonged to two different groups. So far, all but two species have been found in Late Cretaceous strata of North America and Asia; the exceptions are Stenopelix and Yaverlandia, which are both from the Lower Cretaceous of Germany and the UK (Table 13.1).

Homocephaloids (see box) are only defined by two genera, Homocephale and Ornathotholus. They are distinguished by flat ventral surfaces on their pitted skulls and supratemporal fenestrae, whereas pachycephalosaurids have smooth (non-pitted), dome-shaped skulls and lack these fenestrae. Some paleontologists hypothesize that the homocephaloids represent a more primitive status, with traits that evolved into those seen in pachy-cephalosaurids. However, the small amount of fossil material and closeness in geologic ages for the various pachycephalosaurs make resolution of such relationships difficult. The largely complete skulls for some species show some basic characteristics of their group, as well as accouterments such as osteoderms that formed bosses (see Pachycephalosaurus; Fig. 13.5B) or small horns (Stygimoloch) that fringed the skull in various places.

The oldest known pachycephalosaur, and probably the most basal, is Stenopelix valdensis, followed by Yaverlandia bitholus, both from the Early Cretaceous. Despite

Pachycephalosauria can be divided into two clades, Homocephaloidea (nicknamed the "flat-headed" dinosaurs) and Pachycephalosauridae ("dome-headed" dinosaurs), with a few outgroups represented by Wannosaurus and Goyocephale.

its geologic age relative to other pachycephalosaurids, Yaverlandia has some advanced features, such as fused frontals and a moderately domed skull. These observations led to the conclusion that the evolutionary history of pachycephalosaurs may extend back into the Late Jurassic. Unfortunately, the skeletal material for both Stenopelix and Yaverlandia is so fragmentary that they cannot be classified as either homocephaloid or pachycephalosaurid. Wannanosaurus, which comes from the Upper Cretaceous strata of China, is the best-defined primitive pachycephalosaur. Its large supratemporal fenestrae were hypothesized to have diminished within the Pachy-cephalosauridae lineage, but instead it retained these into the Late Cretaceous. The Late Cretaceous Goyocephale of Mongolia is another of the pachycephalosaurs that does not fit easily into either Homocephaloidea or Pachycephalosauridae. Some paleontologists proposed that it may represent an ancestral form of homocephaloid, but it is now considered as an outgroup preceding them.

Homocephale, the namesake of the homocephaloids and one of the most completely known pachycephalosaurs, also lived during the Late Cretaceous of Mongolia. Homocephalids were apparently restricted both geographically and temporally, as they only occur in Upper Cretaceous deposits of Asia. Pachycephalosaurids are similar in that all genera are Late Cretaceous, but with most from North America (e.g., Pachycephalosaurus, Sphaerotholus, Stegoceras, and Stygimoloch). All others are from Asia, with three specifically from China (Micropachycephalosaurus) and Mongolia (Prenocephale and Tylocephale), although the few fragmentary remains of Micropachycephalosaurus make its position within Pachycephalosauridae unverifiable.

Pachycephalosaurs are interpreted as small- to medium-sized obligate bipeds on the basis of the admittedly limited data derived from fore limb/hind limb proportions. (Fig. 13.6). Some forms also are known to have had long tails. The preserved tails have distal caudal vertebrae encased by a meshwork of ossified tendons that

FIGURE 13.6 Pachycephalosaurus, reconstructed as an entire skeleton but with little more to inspire this other than a very bony skull. North Carolina Museum of Natural History, Raleigh, North Carolina.

stiffened that half of the tail. This adaptation was probably related to locomotion and helped to keep the distal part of the tail pointing away from the body as a counterbalance, similar to that of some theropods (Chapter 9). Stegoceras is the most commonly encountered pachycephalosaur genus in the geologic record, although the majority of its material comes from pieces of its skull. Nevertheless, enough of these exist that a few paleontologists have been able to hypothesis sexual dimorphism in one species, Stegoceras validum, discussed in more detail later. The largest pachycephalosaur known is Pachycephalosaurus, which would have been about 8 meters long, including its tail. Some other species, such as the aptly named Micropachycephalosaurus, were less than a meter long.

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Responses

  • ernesta
    Can be divided into two clades, homocephaloidea and pachycephalosauridae.?
    2 months ago

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