Lifestyles Of The Large And Famous

We have surveyed the animals themselves, their distribution, and the diverting history of their discovery. Few amateurs worry at night about whether Stegoceras is a suitable outgroup for the Ceratopsia or whether the lack of parietal fenestrae in Triceratops is a retained basal character or a character reversal. Children and adults alike really want to know what manner of beasts were the horned dinosaurs. We really yearn to know them as once-living, breathing, behaving, socializing, reproducing animals. To understand them as living animals, the modern analogue that springs to mind immediately is the rhinoceros. Reminding ourselves that no grasses carpeted the Cretaceous savannahs, we still feel that this is a reasonable first approximation. African white rhinos reach 4.0 m in length and nearly 1.9 m in height and weigh up to 2.3 metric tons.20 Published estimates for the weights of horned dinosaurs range from 177 kg for adult Protoceratops, to 190 kg for Leptoceratops, to 3.7 to 3.9 metric tons for Styracosaurus, to 8.5 metric tons for Triceratops." Thus it seems that the Judithian horned dinosaurs Styracosaurus, Mono-

clonius, Centrosaurus, and Chasmosaurus were roughly similar in size, though larger than, the African white rhinoceros. Small, basal ceratop-sians were more the size of certain African antelope. The very largest horned dinosaurs, the giants Triceratops, Pentaceratops, Torosaurus, and maybe Pachyrhinosaurus as well, were easily three times the bulk of the largest living rhinoceros.

Rhinos are large-headed herbivores, but they were nothing compared to ceratopsians. Large head size is characteristic of all ceratopsi-ans beyond the level of Psittacosaurus. Pentaceratops reaches the ultimate condition, with a skull greater in total length than the vertebral column from the first vertebra to the pelvis! In other ceratopsians, the total length of the skull ranges from 60 to 85 percent of the length of the backbone.22 Even disregarding the frill, which is an "add-on," not really part of the true skull, the head is large. The basal length of ceratopsian skulls ranges from 30 to 45 percent of the length of the backbone.

And the brain inside the skull? That is another matter. As much as I would like it to be otherwise, it was not large. Casts of the brain have been studied in Triceratops and Anchiceratops. The volume of the brain cast in Triceratops measures about 300 cm'. James Hopson of the University of Chicago studied comparative brain sizes among dinosaurs and uncovered two major findings about ceratopsids. One is that the brain size of ceratopsids compared to estimated body weight is slightly less than that expected in an alligator of ceratopsid size. The second is that ceratopsids had the largest relative brain size of all four-legged herbivorous dinosaurs. Only two-legged dinosaurs had larger relative brain 23

sizes.

R. S. Lull analyzed ceratopsid brains. He reported that a cast of the brain of Triceratops showed a well-developed olfactory region (for smell), small cerebrum ("which gives evidence of an extremely low grade of intelligence, compared with mammalian standards") and relatively large cerebellum, which coordinates movement (Fig. 1.1).21 All vertebrates have a sense of balance (through the vestibular system) that is related to the sense of hearing, both being innervated by the eighth cranial nerve, the vestibulocochlear nerve. The vestibular system uses a set of three semicircular canals, arranged in three perpendicular planes, that are sensitive to motion in each of the three planes (pitch, roll, yaw). In the skull of Anchiceratops ornatus, described by Brown in 1914, the semicircular canals were beautifully preserved. In 1928, John Tait and Barnum Brown further considered the significance of semicircular canals. They determined that the horizontal canal indicates that the skull 1 6

Triceratops Brain

FIG. 1.1. Brain cast of Triceratops. Many regions of the brain can be identified, for example the olfactory region (ol) for the sense of smell, the cerebrum (c), the cerebellum (cb), the medulla oblongata (m), and the pituitary (p). Some of the twelve pairs of cranial nerves are designated by Roman numerals (V, X, XI, XII), except for cranial nerve II (on), the optic nerve (From Hatcher et al. 1907.)

FIG. 1.1. Brain cast of Triceratops. Many regions of the brain can be identified, for example the olfactory region (ol) for the sense of smell, the cerebrum (c), the cerebellum (cb), the medulla oblongata (m), and the pituitary (p). Some of the twelve pairs of cranial nerves are designated by Roman numerals (V, X, XI, XII), except for cranial nerve II (on), the optic nerve (From Hatcher et al. 1907.)

of Anchiceratops was habitually dipped forward, such that the tips of the postorbital horn cores were about 8 cm higher than the bases. They inferred the same head orientation for Triceratops. The two vertical canals on each side were more or less oriented toward the four legs, making the animal sensitive to tripping or stumbling and thus protecting it from falling. The horizontal canals are larger and thus more sensitive to movement. Tait and Brown concluded that the head was tipped about the long axis and elevated or depressed in a vertical plane more than it was swung sideways in a horizontal plane. These dinosaurs did not groom their flanks. Their visual fields were sideways, not forward. "By turning their head on one side, like a hen viewing a hawk, they looked upwards with the upper eye."25

Rhinos have rather poor eyesight. This perhaps might be viewed as a luxury permitted by the fact that African rhinos weigh nearly ten times more than their largest potential predator, the lion. Adult rhinoceros, like elephants and giraffes, essentially have no natural predators. It is mainly young, sick, or injured animals that fall to predators. The world of the ceratopsians was not so benign. Ceratopsians had to confront predators that essentially were their equals in body size. North American ceratopsids that lived between 76 and 70 Ma had to deal with Albertosaurus and Daspletosaurus, and their young with a guild of smaller predators headed by Troodon, followed closely by Saurornitho-lestes and several other little nasties (Fig. 1.2). The giant Lancian cera-

Extinct Animals Last 000 Years
FlG. 1.2. Anchiceratops confronts Albertosaurus, Late Cretaceous of Alberta, circa 70 Ma. (Robert Walters.)

topsids Triceratops and Torosaurus (68-65 Ma) were faced with the greatest predator ever to walk the earth, the 12-m-long Tyrannosaurus rex. That big bully did not have an easy time of it, however. No wonder there were so few of them. A healthy bull Triceratops was undoubtedly an extremely dangerous animal, and a Tyrannosaurus so unwise or inexperienced as to engage one in frontal combat was likely to have paid with his life (Plate I). I would wager that meat-eaters generally did not enjoy a great taste for fresh ceratopsid drumstick. I suspect that more often the treat was savored as carrion.

The Lilliputian version of the struggle was played out in Asia between Protoceratops and Velociraptor. An adult Protoceratops may have outweighed Velociraptor by a factor of three. Protoceratops had no particular weapons other than a sharp beak and an arched nose, and one might have predicted an easy meal for Velociraptor. However, a famous specimen preserved in Mongolia shows both animals, apparently locked in mortal combat, dead upon the playing field. Evidently Velociraptor, nowhere near as intelligent as portrayed in recent print and cinematic fiction, had seriously underestimated the odds for an easy snack. Some artists have portrayed ceratopsids as taking up a defensive circle, musk-oxen-like, to protect themselves and their young against a marauder. Such a posture is within the license permitted an artist, but we must not fall into the trap of believing that this is a scientific representation of known ceratopsian behavior; it is not.

It is easy to gain the impression that ceratopsians had good eyesight. The width of the bony eye socket routinely ranges from 80 to 100 mm in Centrosaurus and Chasmosaurus to 120 mm in Triceratops to 166 mm in a specimen of Torosaurus as reported by Hatcher! Even little Leptoceratops had an orbit that measured 85 mm in width (but only half that amount in height). It is probable that the actual eyeball inside the bony orbit was significantly smaller, but nevertheless the eyeballs were probably large and the sight was probably good. We can also infer that this is so from the elaborate visual signals found all over the skull. Not only was there a showy frill, but there was also sculpture on the frill to enhance its already striking appearance: scallops, hooks, knobs, and processes. Moreover, there were the horns themselves in variable pattern (Figs. 1.3 and 1.4). Yes, some horns were dangerous weapons. The bony horn cores were extended varying distances by keratin sheaths like those found on cows. But other horns, such as the floppy nose horn of the bizarre Einiosaurus of north-central Montana, were hardly dangerous; instead, they were probably species-specific display structures.

Dinosaur Horn Head Names
FIG. 1.3. Representative frill types in ceratopsids (dorsal views), (a) Torosaurus; (b) Styracosaurus; (c) Triceratops; (d) Chasmosaurus; (e) Centrosaurus; (f) Pachyrhinosaurus. (Robert Walters, after various sources.)
Centrosaurus
FIG. 1.4. Reconstructions of various ceratopsid heads in lateral view, showing horns and frills, (a) Triceratops; (b) Centrosaurus; (c) Chasmosaurus; (d) Styraco-saurus; (e) Pachyrhinosaurus. (Robert Walters.)

Ceratopsids were walking billboards, with their names written all over them. The extravagances on the head sent the message; the acute vision of other members of the species received the message. Taking this line of reasoning one step further, it may also be correct to infer that these same structures indicate that ceratopsians were gregarious animals. Rhinos with their poor vision are not gregarious animals, and their horns, though impressive, are not particularly elaborate as display structures go. As a general rule, the greater the population density and the greater the diversity of a community, the more elaborate the display structures.

Horned dinosaurs probably had an advantage that rhinos do not. Like most mammals, rhinos are color blind. How about dinosaurs? There is a simple way to approach this question. If you want to see colors at the zoo, where do you go? You go to the reptile house or the bird house (yes, you could go to the aquarium instead). Dinosaurs are phylogenetically bracketed by reptiles and birds, and it is therefore most economical to assume that they too had color vision. Inasmuch as they had structures with large surface areas that were meant to be seen, I believe that it is reasonable to infer that the conspicuousness of the structure meant to be seen was enhanced by the use of color. If not necessarily the whole animal, at least the particular display structure— be it the plates of stegosaurs, the cranial crests of duck-bills, or the frills of ceratopsians—was thrown into relief by its coloration. When a Toro-saurus dipped its head, it revealed a huge, shimmering display that could have made a male peacock look drab—think about it!

We all know that the crests of horned dinosaurs are defensive structures, don't we? Maybe so, but it is instructive to consider crests at their origins among the protoceratopsids. Protoceratops had a bony crest that was eggshell thin—literally translucent. What kind of defense could an eggshell offer? Moreover, the showy crest of Protoceratops came in two different models: one taller, wider, and more showy, the other narrower, flatter, and less showy. Furthermore, the two models correlate with distinct patterns of arching of the face over the nose (the "proto-horn"), thickening of the jugal (cheek) horns,2' and other features. These two patterns plausibly represent males and females (or vice versa). Are we to believe that one sex needed better defenses than the other, or is it wiser to consider whether the crest actually originated as a display structure?

Surely horns are prima facie evidence for active, aggressive behavior against mortal enemies? There again, the report is mixed. If the object of the horns is purely defensive—to rebuff, repel, maim, or kill a would-be predator—one might expect more uniformity of weaponry. Not all horn patterns are equally effective, and in general longer, sharper horns may be more effective than shorter, blunter ones. The long-horned Chasmo-saurus kaiseni, for example, would seem to be much better protected than the short-horned Chasmosaurus belli; yet the former seems to have been much less common than the latter. Colbert found the situation puzzling: "Why should some ceratopsians have a single nasal horn, while others have large brow horns in addition to a nasal horn? Why should the horns be straight in some and curved in others? Why should the frill be solid in some, perforated in others, and why should the frill be so variously decorated with spikes and scallops and knobs?"27

If we consider the horn patterns of African antelope today, we notice great variation. Some are short, some are long; some are straight, some are spirally twisted; some bend outward, some bend backward. But how does an antelope defend itself from a lion or a cheetah or a leopard? Not by fighting but by fleeing! It seems that for modern horn-bearing mammals the precise pattern of the horns serves to permit recognition within the species. Members of the same species are able to recognize each other by their horns. Furthermore, the horn pattern may serve as an indicator of the type of struggle between males to achieve breeding rights, as discussed in the next section.

SEX IN THE CRETACEOUS?

Given the emphasis on display and male interactions, did both males and females have the same patterns of horns and frills? In my view, it is almost an a priori expectation, based on our knowledge of living horned and antlered animals, that there should be differences in pattern between males and females among horned dinosaurs. Yet in the past, paleontologists were loathe to adopt this interpretation of ceratopsian morphology. Lull was characteristic of his era in 1933 as he dismissed the possibility of sexual differences: "Little or no sexual distinction is provable among ceratopsians and ... apparently both males and females of a species possessed both the horns and crests in equal degree."28

This statement is made despite the fact that there is so much variability in ceratopsian skulls that cries out for interpretation in sexual terms. The variability is seen not in structures that relate to the biomechanical demands of such vital physiological systems as the respiratory, circulatory, nervous, or masticatory systems, but rather in the frills or ornaments of the skull—the finery, the haberdashery (Fig. 1.5). Did these guys spend so much time in their scholarly work that they never attended a society ball? Did they not notice that visually oriented, color-receptive but black-clad males respond to females in bright colors and plumes with pectoral displays?

Pachyrhinosaurus Dimorphism
FIG. 1.5. Triceratops in full display. (Robert Walters.)

One reason that sexual dimorphism went unrecognized is that it was hidden by taxonomy. If a peahen and a peacock are classified as separate species, then there is no sexual dimorphism in peafowl! In 1975,1 pointed out that in duck-billed dinosaurs not only sexual dimorphism but also growth was disguised by the historical artifact of taxonomy.29 In Triceratops, it would seem that the evidence had already been uncovered in terms of two evolutionary lineages. Were these really animals that lived together at the same time and in the same place, but were of separate evolutionary lineages? This seems an awfully elaborate construct to avoid letting sex raise its ugly head in the Cretaceous. Is this an example of Victorian prudery—cultural baggage, as Stephen Jay Gould would put it—affecting science? In 1976,1 published my interpretation of sexual dimorphism in Protoceratops. This was a relatively easy case in which only a single species had been recognized. I was able to demonstrate dimorphism in showy characters that plausibly represent secondary sexual display characters. The interpretation of males and females from this pattern was no great intellectual leap.30 In 1990, Tom

Lehman of Texas Tech University reviewed the species of chasmo-saurines, and he interpreted the historical taxonomy in thoroughly modem terms of sex differences between male and female.31 His interpretations are not beyond dispute. To my mind, however, the important thing is not whether Lehman is correct in every detail but rather the openness of his interpretation to this very fundamental aspect of cerat-opsian biology.

In summary, is it possible to recognize differences between males and females among the Ceratopsia? Yes! Not only is it possible, I maintain that it is required. In general, females resembled males but differed in the degree to which certain characteristics were expressed. The differences are sometimes subtle, and we may not always be correct in our interpretation of them, but the differences are definitely there.

The Canadian zoologist Valerius Geist in 1966 outlined stages in the evolution of hom-like organs. One stage is represented by animals with small, simple horns that engage in potentially dangerous combat. His example was the Rocky Mountain goat (Oreamnos, a goat-antelope, not a true goat). The small horns of this animal have little display value, and the unbranched horns can have a lethal effect on an opponent. A second stage of horn development is represented by cattle (Bos taurus) or bison (Bison bison), in which cranial display is not elaborate, but the horns can be locked together safely so that combat among males is a test of strength, not a struggle to the death. The third stage of horn development involves the development of elaborate, species-specific display structures on the head, usually among males (except for caribou, all female deer lack antlers; but among horn-bearers, females may or may not imitate males). Accompanying the elaborate display structures are behaviors that allow rivals to assess each other's rack of antlers or curl of horns or other display structure. These ritual displays serve to establish dominance hierarchies and avoid damaging combat. Males that are deficient in display structure may elect not to engage a more dominant male. Combat occurs between males equally convinced of their own prowess.32

Jim Farlow and I applied Geist's model of horn-like organs and behavior to homed dinosaurs in 1975. We noted the correlation between short frills and dominant nasal hom cores in centrosaurines, and between long frills and dominant postorbital hom cores in chasmo-saurines. The shorter frill has less impact as a display structure, and a single nasal hom is a dangerous organ that cannot be readily engaged in a safe manner by a rival male. We therefore postulated that animals such as Centrosaurus, Monoclonius, and Styracosaurus were more solitary animals with less elaborately ritualized behavior. We even went so far as to suggest that they were more solitary animals than their twin-horned contemporaries. (The fossil record of bonebeds that has come to light during the past decade suggests that the hypothesis of solitary habits, especially for Centrosaurus, is almost certainly incorrect.) Chasmo-saurines, by contrast, have long, interesting frills that had marvelous display value when the head was dipped and the vast expanse of the frill was exposed as a peacock-like vertical fan, the head rocking gently back and forth to make sure the message was received. At the same time, the dipped head presented the points of the postorbital horns in an unmistakable attitude for combat."

If a furious rival bull Pentaceratops would not back off but presented in a similar fashion, the opponent's paired horns could be joined. A grunting, twisting, writhing wrestling match could ensue. The straining bodies would heave first one way and then another, back and forth across a clearing, oafishly trampling delicate ferns and weedy flowering plants underfoot until, minutes or even hours later, the weaker male would at last capitulate. He would disengage, flatten his head, meekly extend his neck, and lower his chin toward the ground in his most conciliatory posture, then slink off into the gloom of the dawn redwood forest as his rival strutted, stomped his feet, and snorted loudly in triumph.

Did ceratopsians injure each other? There is some debate on this score. Darren Tanke of the Royal Tyrrell Museum of Palaeontology in Alberta is quite definite in his belief that bone injuries documented in ceratopsid bonebeds from Alberta are quite rare, and that ceratopsids therefore were not pugnacious animals." However, the contrary opinion is very persistent.35 There are a number of chasmosaurines that show unusual openings in the squamosal bone on the side of the frill, often in just one squamosal. These include Chasmosaurus, Pentaceratops, Arrhinoceratops, and Triceratops. The openings are often large, measuring 10 cm or more in length. The bone surrounding the openings rarely looks diseased, and it is not beyond question that the openings represent horn punctures. But if they are not, then what are they, and why do they occur with such frequency in animals that we can otherwise judge to be armed and dangerous? Put another way, we have the weapon, we have the motive, so why doubt the result? Whatever the cause of the lesions, they were not fatal in any of the cases recorded. The victims parried the thrusts and lived to go about their business. Perhaps that is an advantage of being an animal of large size and small brain. One is not very vulnerable. Injuries to the squamosal are uncommon in cen-trosaurines, and I do not know of a hole through a squamosal in this subfamily. However, a lovely skull of Centrosaurus at the University of Alberta shows a dent in the squamosal that may be the result of a horn thrust.

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  • joseph gordon
    Is triceratops larger then styracosaurus?
    6 years ago
  • aamos
    Do triceratops have poor eye site?
    1 year ago

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