Marginocephalia Ceratopsia horns and all the frills

From the time of their discovery in the second half of the 1800s to the present day, there has hardly been a group of dinosaurs that has evoked more fascination than ceratopsians (Figure 6.15). Some of these quadrupedal, horned, frilled dinosaurs roamed the Great Plains of North America in the Late Cretaceous. They were rhino-like, ranging upward of 6 or 7 tonnes. Equally famous, but for other reasons, is a host of smaller, lighter (25 and 200 kg) non-horned Asian ceratopsians from slightly earlier in the Cretaceous (Figure 6.16).

We know at lot about ceratopsians: the fossil record from Asia and North America is one of the most outstanding of any dinosaur group (Figure 6.17). Primitively small bipeds, these animals evolved into powerful quadrupedals early in their history, developing thick hooves on all toes and reaching sizes to rival that of small tanks.

With or without horns, it is easy to recognize the ceratopsian familial stamp: cera topsians all had skulls that were narrow, with a hooked beak in front and a skull that flared deeply in the cheek region (Figure 6.18). And at the tip of the snout in the upper jaw was the uniquely evolved rostral bone (Figure 6.19).

As befits their name, many ceratopsians had horns; however, some did not. Those cerat opsians that had them, though, grew some of the most impressive horns ever seen on any vertebrate (Figure 6.20). Like the horns of many mammals, the skulls only preserve the bony horn cores, covered by keratin sheaths that actually comprised the working end of the horn.

Marginocephalia
Western Interior of North America.

The horn visible on the head of the living animal was therefore significantly larger than the horn core alone (Figure 6.21).

Equally memorable is the ceratopsian frill, the marginocephalian shelf gone amok. Extending from the back of the skull, frills vary considerably in size, ornamentation and shape (see Figure 6.20). The largest reach 2 m in length.

Ceratopsian lives and lifestyles

Dressed and ready to chew. Ceratopsians chewed. A hooked rhamphotheca, blocks of cheek teeth in both upper and lower jaws, a sturdy coronoid process, and evidence for the existence of fleshy cheeks all scream CHEWING.

The business end of the ceratopsian mouth was the narrow, hooked, beak-tipped snout, suggesting the potential for careful selection of the plants for food. Individually the cheek teeth were relatively small, but they grew stacked and overlapping together into a single functional

Psittacosaurus Jaw

slicing block in each jaw, the dental battery (Figure 6.22a). Worn teeth were constantly replaced, so that the active chewing surface of each of the four dental batteries was continually refurbished. Inexplicably and unique in the animal kingdom, the orientation of the grinding surfaces migrated, becoming more and more vertical, until, in the large, highly derived North American forms, they occurred nearly vertically along the sides of the teeth comprising the dental battery (Figure 6.22b).

The force behind this high-angle mastication derived from a great mass of jaw-closing musculature, which in the frilled forms crept through the upper temporal opening and onto the base of the frill. The other end of this muscle attached to a massive, hulking coronoid process on the mandible (Figure 6.23). All in all, the chewing apparatus in ceratopsians

Figure6.17. Global distribution of Ceratopsia.
Einiosaurus Skull

Figure 6.20. Left lateral view of the skull of (a) Psittacosautus, (b) Leptoceratops, (c) Bagacetatops, (d) Centrosaurus, (e) Styracosaurus, (f) Pachyrhinosaurus, (g) Pentaceratops, (h) Arrhinoceratops, (i) Torosaurus, (j) Achelousaurus, and (k) Einiosaurus.

Figure 6.20. Left lateral view of the skull of (a) Psittacosautus, (b) Leptoceratops, (c) Bagacetatops, (d) Centrosaurus, (e) Styracosaurus, (f) Pachyrhinosaurus, (g) Pentaceratops, (h) Arrhinoceratops, (i) Torosaurus, (j) Achelousaurus, and (k) Einiosaurus.

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Ceratopsia

was among the most highly evolved of all ver-tebrates.1

Beyond the mouth. Based upon their narrow girth (by comparison with thyreophorans and pachycephalosaurs), the digestive tract does not appear to have been disproportionately large in ceratopsians, and did not likely rely upon wholesale bacterial fermentation for extracting nutrients from plants. Nevertheless, it must have been big enough to accommodate what must have been an endless parade of foliage that formed the diet of these animals.

Even the largest quadrupedal ceratop-sians never browsed particularly high above the ground. The browse height of the largest was probably less than 2 m. Nevertheless, they may have been able to knock over trees of modest size in order to gain access to choice leaves and fruits.

Which plants were preferred by cer-atopsians remains a mystery. The principal plants whose statures match browsing heights of ceratopsians were a variety of shrubby angiosperms, ferns, and perhaps small conifers (see Chapter 13).

Locomotion. The primitive ceratopsian Psittacosaurus appears to have been fully bipedal and must have walked in typical bipedal ornithischian fashion. But the rest were quadrupeds all, and while the back legs were fully erect, the orientations of the front limbs are somewhat controversial. Some have argued that the front limbs were directly under the body, as they are in mammalian quadrupeds. Others have argued, based on the shape of the bones of the front legs, that a more sprawling posture in the front legs is indicated (Figure 6.24).

These considerations naturally affect how we understand the way ceratopsians ran. The sprawling front legs would likely entail slower speeds, and perhaps a more unhurried lifestyle. The mammal-like reconstructions, with fully erect stances for both front and rear legs, suggest faster speeds, reminiscent of a large, Cretaceous rhinoceros. With such uncertainty about stance, speed tentative estimates for walking range somewhere between 2 and 4 km/h (see Box 12.3), while maximum running speeds range from 30 to 35 km/h.

Bringing up baby. The first dinosaur egg nests ever found, in 1922, were thought to belong to the small Asian ceratopsian Protoceratops, and it was with this dinosaur that, for the next 70 years, the eggs were posed in museum displays all over the world. So it was, uh, informative to learn - after some 70 years- that the embryos inside those "Protoceratops eggs" actually belonged to the theropod Oviraptor (see Chapter 9).

Despite the confusion with Oviraptor, however, juvenile ceratopsians from Asia are now relatively well known. Hatchlings of Psittacosaurus, no more than 23 cm long (with tail!) have been found. And recently, complete skeletons of Protoceratops hatchlings - grown somewhat

1. The primitive ceratopsian Psittacosaurus lacked the highly refined chewing specializations of the North American ceratopsians, but it is known to have harbored a packet of gastroliths lodged in its gizzard, which would have doubly pulverized its meal. Gastroliths are known from no other ceratopsian.

Keratinized Horn
Figure6.21. The skull ofTriceratops showing the horn core covered by the keratinized horn as it would have been in life.
Primitive Ceratopsians

Figure6.22. Cross-section through the upper and lower jaws of Triceratops: (a) high-angle grinding motion of the dental batteries; (b) internal view of the dental battery in the lower jaw of Triceratops.

20 cm 40 cm past the newly hatched stage - were found in nests (Figure 6.25). This means that some parental care at the nest after birth is implied for Protoceratops. Moreover, all of the growth stages from hatchling to adult have been documented in Protoceratops, making the ontogeny - or growth and development - of this dinosaur perhaps the best understood of all dinosaurs. And the ontogeny of ceratopsian dinosaurs turns out to hold key clues about their behavior.

Horns, frills, and ceratopsian behavior. Ceratopsian horns were once thought to have functioned to ward off predators at close quarters. More recent interpretations have not completely ruled this out, but have instead focused on intraspecific behaviors such as display, ritualized combat, defense of territories, and establishment of social ordering.

Juvenile Ceratopsian

Figure6.23. Jaw musculature reconstructed in the skull of a long frilled ceratopsian. The major jaw closing (adductor) muscles are (1) the adductor mandibularis externus superficialis (AMES); (2) the adductor mandibularis externus medialis (AMEM); (3) the psuedotemporalis (PT); (4) the ptery-goideus (PG)

Semi Erect Stance
Figure 6.24. Two potential reconstructions of the front limbs in ceratopsians: (a) fully erect stance; (b) semi-erect stance.

The link between dominance, defense, and horns comes from studies of mammals in their natural habitats. In the case of almost all horned mammals, larger males tend to have a reproductive advantage over smaller males. Dominance in these mammals (and in other tetra-pods) is accentuated by the development of structures that "advertise" the size of the animal; these obviously include horns and antlers, as well as the bony horn-like knobs (ossicones) of giraffes and the nasal horns of rhinoceroses. In short, the variety of horn and antler shapes in mammals are known to reflect (1) species-recognition mechanisms that aid in preventing interspecific matings (that is, matings between species), and (2) intraspecific differences in displays and ritualized fighting behavior.

Turning to ceratopsians, few have doubted that the horns were used for combat; the question has been "At whom were they aimed?" Using modern horned mammals as analogs,

Figure6.25. A nest of hatchling Protoceratopsfrom the Late Cretaceous of Mongolia. inset: Reconstruction of one of the babies as it would have appeared in life.

current thought suggests that the large nasal and brow horns of ceratopsians functioned primarily during territorial defense and in establishing dominance. Similarly, the development of elaborate scallops and spikes along the frill margin in many of the more highly derived cerat opsians separates one species from another. Thought of this way, the remarkable variations in the horns and frills in ceratopsians could be used for interspecific identification as well as the establishment of intraspecific dominance (Figure 6.26).

Striking data that bear upon this have come from Protoceratops. Statistical studies of Protoceratops show two populations of adult frill and facial morphologies - strong evidence of sexual dimorphism (Figure 6.27). Moreover, the frills don't appear too large in juvenile specimens - they only develop when the animals reach 75% of adult body sizes. This suggests that frill growth is coordinated with sexual maturity and therefore that there is a reproductive connection to frill size and shape. Sound like sexual selection?

Sexual selection is now thought to also occur in other ceratopsians, among them Centrosaurus and Chasmosaurus. In many of these forms, the development of scallops and spikes on the frill margin would enhance the dimorphic nature of the frill.

Figure 6.26. "Back off": frill display in Chasmosaurus. The very long frill could have provided a very prominent frontal threat display, not only by inclining the head forward but also by nodding or shaking the head from side to side.
Figure 6.27. Sexual dimorphism in Protoceratops. Note in (a), a presumed female, the frill is less showy and the nasal ridge is less prominent; quite the opposite of (b), a presumed male.

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All of this implies social interactions, and it thus comes as no surprise to learn that ceratopsians lived in large herds. The evidence for this comes from an ever-increasing catalog of ceratopsian bonebeds, mass accumulations of single species of organisms. Bonebeds are known for at least nine separate species, including several bonebeds exceeding 100 individuals. Such gregariousness makes sense when putting frills and horns into their behavioral context: territoriality, ritualized combat and display, and the establishment of dominance are to be expected in animals that come together in highly social circumstances such as herds.

These ideas suggest that we ought to find puncture wounds inflicted on faces, frills, and bodies of competing ceratopsians. In fact, such wounds are preserved in at least five forms. These pathologies, not only on the cheek region but also in the frill, provide strong evidence of the blood-letting that comes from head-on engagements between competing members of the same species.

Thoughts of a ceratopsian. Given the complex repertoire of inferred ceratopsian behaviors, it comes as a bit of a surprise that their brains were not particularly large (see Box 12.4). Despite being near opposites in terms of body size and display-related anatomy, both Protoceratops and Triceratops had brains less than the size expected of a similarly sized crocodilian or lizard. Cerebrally, they were above sauropods, ankylosaurs, and stegosaurs, but commanded proportionally less gray matter than either ornithopods or theropods. Regardless, the variety of exotic morphology in and around the head suggests that ceratopsians, large-brained or no, may have had a relatively complicated behavioral repertoire.

Ceratopsia is a monophyletic taxon, indicated by a rich array of derived features (Figure 6.28). The ceratopsian that is thought to represent the primitive condition for the group is

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