Whatever the pace of their metabolism, and their capacity for regulating body temperatures, we can be confident that dinosaurs were highly active creatures. The upright postures, the long legs planted firmly under the body, and whiplike tails all suggest muscle power, dynamic bursts of movement, and—as in the case of dinosaurs like the small theropods— lithe, bounding grace. But did dinosaurs act alone or with others? In some ways this is an even more difficult question to address than those pondered for dinosaur function, physiology, and behavior. The societies of certain living species—the ants, the wasps and bees, flocks of birds, herds and hierarchies of mammals—are some of the most complex and refined products of evolution. Projecting such subtle social relationships back over sixty-five million years to the age of the dinosaurs is fraught with difficulty.
Sometimes the circumstantial evidence allows us to infer something about the "high society" of the dinosaurs, but that evidence allows us to go only so far. For instance, let us consider those stratospheric sauropods once again. In places, sauropods occur in huge heaps of bones, as if the animals lived and died together. Also trackways often indicate many individuals. One may speculate that these animals traveled in great herds, crashing their way through vegetation as they consumed enormous amounts of biomass. There is no direct fossil evidence of this clear cutting of forests by thundering armies of sauropods, but it seems likely that they had great impact on the available resource. The suggestion that at least some sauropod species were highly gregarious has been taken one step further. Dodson, Behrensmayer, Bakker, and Mcintosh noted that sauropods were essen tially homogeneous over great distances—as much as six hundred miles. Such a range, coupled with the recognition that the Morrison represents environments with a rainy season and a dry season, reminds one of the Serengeti today. Martin Lockley's ingenious work on fossil trackways certainly supports the vision of great herds of sauropods and other kinds of dinosaurs stomping through wide expanses of territory.
It has been proposed therefore that these sauropods migrated annually in huge herds, following the opportunities for more food at different locales. There are indeed clues that suggest that these dinosaurs were capable of ranging over wide areas. Very similar sauropods like Brachiosaurus and Barosaurus are found in places as far flung as Tanzania and the Rocky Mountain region. Sauropods, either in herds or in small groupings, must have had impressive capacities for dispersal. Can we then proceed automatically to the theory of annual or some kind of regular migration in these beasts? I'm afraid not. How one can demonstrate sauropod migration, as opposed to a simple tendency to herd, is not at all obvious, at least when one considers the limitations of the fossil record.
Another fascinating but risky probe of dinosaur social behaviors concerns the possibility of a mating hierarchy, a pecking order, within the herd. We see such extreme competition and rank in many living species. Magnificent rams of bighorn sheep battle for a flock of females. African gazelle bucks aggressively keep an impressive herd of does away from other ambitious bucks. In a reversal of roles, the top-ranked female hyena—who is much larger than the males and even develops a penislike structure from the clitoris—dominates her den. But these species hardly measure up to the severe hierarchical ranking in elephant seals, where a snorting, ugly, stinking, three-ton alpha bull may inflict death on challengers, and even crush young pups in the heat of battle, in order to maintain his breeding dominance over a harem of as many as 400 females. These realities of rank and competition extend even into the branches of our own sector of the evolutionary tree. Alpha male gorillas beat their chests in dominating and self-aggrandizing ways, putting would-be challengers in their respectively more submissive places. Gorilla relatives—humans—have systems of myriad diversity and complexity that defy a simple biological or evolutionary description. Nonetheless, in our own species we see all kinds of correlations between attributes and societal rank—attributes that include physique, money, and political power.
One might reasonably expect that dinosaurs—large, active, and in some cases even perhaps congregational creatures that they were—might have exhibited such brutally stratified social organization. Indeed there are dramatic paintings showing dinosaurs of the same species in furious mortal combat over the possession of the herd. But once again we must return to the question—what is the real evidence for such depictions? One thought is that some of the ornamentation of certain dinosaur species— emblems that draw a parallel to plumage, horns, antlers, and other adornments in living mammals—provides a clue to such behavior. In Chapter 2, I briefly described the horns and frills in Protoceratops and other rhinolike ceratopsians. All those variations in horns and frills and spikes are impressive. They certainly serve to differentiate the kinds of ceratopsians. But did they have any social significance?
The question has been addressed by research on rich ceratopsid samples from North America. Some excellent work in this vein has been recently completed by Scott Sampson, who studied marvelous bone beds at Landslide Butte, Montana, that contain huge amounts of concentrated bone of horned ceratopsids. Unlike the Gobi situation, the bones are not articulated in skeletons, but they can be sorted into different batches that represent various taxa. The sorting to a great extent relied on the bizarre array of headgear. But it soon became apparent to Sampson and his coworkers, Michael Ryan and Darren Tanke, that these head ornaments were not always helpful in classifying ceratopsids. It seemed curious that so many grades of horn development existed in certain rich samples. The investigative team recognized an important growth pattern that seemed to account for the bewildering variation in an important subgroup of the cer-atopsids, the Centrosaurinae. All juvenile centrosaurines were alike; they had simple, unadorned heads. Subadults or adolescent centrosaurines attained adult-sized skulls but not yet the fancy horns and frills of adults. Their horn cores, where present, were simple, small, and sometimes backward pointing. Juvenile and subadult forms all had essentially the same frills, thin flanges with a scalloped margin. But adults went to baroque ex-
tremes in developing hooks, horns, and spikes on the frill. In addition, not all adults seemed to show marked development of horns. There was some suggestion that some of the sample of adult skulls had more poorly developed horns because they represented the sex (either male or female) not involved in signal dominance or combat.
Thus the variation in these headpieces, if carefully analyzed, tells us something important about the growth patterns and sexual dimorphism as well as the taxonomy of ceratopsids. Now think of some parallel situations on the African plains. Young gazelles, wildebeest, or kudus are virtually hornless. Horn growth is a function of getting older. Horn size is related to body size and often social dominance over the herd. Moreover, one sex, in this case males, shows a much more pronounced development of horns than the other. The pattern in samples of horned dinosaurs studied by Sampson and others suggests a strikingly similar story for social interactions. Just the same, only the possibility, not the confirmation, of a social hierarchy in ceratopsians can be entertained.
This pattern of head accouterments and their possible bearing on social rank or role is not confined to ceratopsians. The duck-billed hadrosaurs are differentiated mainly by the development (or lack thereof) of their emblematic head crests. Some forms have virtually no crest at all. This may have been a primitive feature of the group, which is found in both early and later hadrosaurs (remember, the fossil record doesn't always perfectly mirror the advancement of steps in evolution). In crested forms,
the name of the game is weird elaboration and variation. Saurolophus, our beast from the Gobi (also known from North America), has a prominent bony ridge on top of the snout and face that ends behind in a small spike. The Gobi species of this group shows some distinctiveness in its somewhat longer and more fan-shaped head spike. But the really bizarre species are some of the North American forms. An extremely broad platelike crest is known in Corythosaurus, and there is a long bony tube that extends backward nearly three and a half feet from the skull of Parasaurolophus. Stranger still is the fact that the nasal passages actually extend for some distance into these hollow crests. Their caliber and design vary, just like the differences one sees in the passageways of trombones, saxophones, and tubas. Not all these crests are hollow; for example, hadrosaurines, which include the Gobi beast Saurolophus, lacked such passageways.
These crests have excited some paleontologists to a considerable degree and there is no shortage of speculation on their function. As with many reconstructions of fossils, none of these explanations can be decisively verified, nor are any of them necessarily false. To complicate matters, the crests may have taken on different functions in different species. Yet it is possible to determine which of these ideas make more sense. The suggested use of the crests for breathing, air storage, or air trapping while underwater seems on the whole rather unlikely. These beasts were adept in water, and they might have retreated to lakes, rivers, and seas to escape a rapacious Tarbosaurus or Tyrannosaurus. Nonetheless, their well-supported bodies and their tooth batteries indicate that hadrosaurs probably spent most of their time on land, eating relatively tough branches and leaves of trees and bushes. Fossilized conifer needles, branches, deciduous foliage, and numerous small seeds and fruits have been claimed to be the "stomach contents" of a hadrosaur Edmontosaurus. If this identity is correct, one can assume that the duckbills did not have an overpowering need to feed underwater for long periods of time.
For lack of a better notion, the correlation between the hadrosaur crest development and a signaling function endures. The idea was refined starting with some thoughts expressed by the paleontologist James Hop-son at the University of Chicago and later elaborated by Peter Dodson at the University of Pennsylvania and Dave Weishampel at Johns Hopkins University. Animals that use such obvious cues today often share a number of qualities. They have a keen sense of eyesight and/or hearing. They are often social and sexually dimorphic (males are larger and more aggressively built and armed than females, or vice versa), with individuals in frequent threatening behavior or combat for competition for mates of the opposite sex. Finally, species living in the same area that rely on such signals, like the antelopes of the Serengeti, often have very distinctive, highly different head ornaments like horns, to cue their own species.
Hadrosaurs in a broad sense fit this picture. They have large eye sock ets and intricate ear bones, indicating acute vision and hearing. Weisham-pel developed some ingenious experiments to suggest that the hollow tubular crests of Parasaurolophus and other hadrosaurs were effective sound resonators. Moreover, Dodson's studies have shown that crests are accentuated in adults and, even in the case of adults, both big-crested and smaller-crested individuals are found in samples representing the same species. As in the case of the ceratopsians, this suggests a difference in the sexes pertaining to social behavior. Either the males or the females were establishing mating hierarchies or involved in rituals of signaling threats and combat. Finally, in a few localities in North America more than six different species are found together and most of these are easily discriminated by their varying head crests.
What about our Gobi creature Saurolophus} One might conjure up a picture of massive animals feeding in the marshes and deeps of a lake. Crests on some individuals may have indicated their position in the mating hierarchy, a signal backed up by the honking sounds of protective males. Is this vision the reality of seventy-five million years before Efremov and his band came upon hadrosaur skeletons in the Nemegt Valley? We'll never know. Some ideas, like the use of crests for visual cues, seem to match some circumstantial evidence. Elaboration of the scene is not so easy however. The myriad published color schemes for dinosaur crests, shields, trunks, and tails are purely imaginary. At any rate it's fun to think about them.
But of all the examples relating dinosaur head structures to social and competitive behavior, there is perhaps none more bizarre and entrancing than that provided by the pachycephalosaurs (pachy-ceph; thickheaded) or "bone-heads." Both the Nemegt and Barun Goyot Formations of the Cretaceous Gobi entomb these "bone-heads," as the Polish-Mongolian team found out. Pachycephalosaurs were originally described around the turn of the century from Cretaceous beds in North America. They vary markedly in size, ranging from five feet to twenty-six feet in length, and are ornithischians with the characteristic pelvic structure. The skeleton shows that these animals were bipedal with strong and long hind limbs and much shorter but well-developed forelimbs. The skeleton also shows several areas of reinforcement, as if these animals were adjusted for
a body-shaking shock. Those ossified tendons interlace the vertebrae of the back and tail. The back vertebrae have distinctive interlocking ridges above the body, or centrum, of the vertebrae, the part which houses the vital spinal cord.
These seem to be the parts of a machine that propelled a battering ram. Indeed, as the name indicates, most pachycephalosaurs had extraordinarily thick skulls. More specifically, the bone of the skull roof is dense, extremely thick, and arched upward, like the great cupola of a cathedral. To reinforce this object, the skull roof is studded with many individual plates of bone. Although not all pachycephalosaurs had these accentuated dome heads, in virtually all species—even the "flat-topped" Homalocephale from the Gobi Cretaceous—the skull roofing bones are thickened and encrusted with bony studs. At the back of the skull are distinct depressions and scars for the attachment of powerful muscles to the neck vertebrae.
This list of features seems indicative of the armament for an aggressive attacker, ready to take on even the most obstinate ankylosaur, matching armor and mace with like weapons. Pachycephalosaurs, however, were not predators. Their teeth were small, flattened utensils with a weak serration along their edges, teeth not meant for slicing meat but merely for shredding plants. Why, then, the thick studded heads and reinforced skeletons? One is forced to the conclusion that these features account for some kind of head-butting. Although there is no direct evidence, it seems that the battering ram was not just used in defense against predators. Two pachycephalosaurs might have indulged in head-to-head combat, much like that seen in rams of bighorn sheep today. It is not hard to imagine the drama of such a contest. Two animals lowered their heads and charged at each other full speed ahead, propelled by their powerful hind limbs. The combatants met head to head with a mighty blow resounding through the Cretaceous forest. How could these hulks survive such a joust? Since at the point of contact the skull was pointed down, its skull roof was oriented forward and in line with the neck and the horizontally held vertebrae that were meant to buttress the head during impact. Moreover, Ned Colbert and later Dr. Hans Dieter-Sues and Peter Galton have described aspects of
the skull roofing bones which seem to suitably disperse the shock waves generated by impact with another solid object.
These features helped absorb the force of the blow in combat. Just the same, it is hard to characterize such behavior as cheerful sport. Unlike battering mountain sheep, there are no horns on pachycephalosaurs to take the first impact; the bony studs on the skull are set low and seem ornamental rather than protective. The blow would be applied directly to the skull roof, in which there are no large air spaces to act as a cushion. The shock waves then passed right through the solid skull roof to the brain. Therein the small brains may have been slightly insulated by air spaces and connective tissue. Some authors have floated the rather ludicrous notion that these small brains were a saving grace, as if the dullness of the beasts, like that of a "punch-drunk" fighter, allowed them to tolerate the violations to the health represented by such skull bashing. Perhaps pachycephalosaurs were not the most intelligent of dinosaurs, but there is no reason to assume that they were dim-witted beasts incapable of feeling pain. Their large eye sockets suggest these animals had keen vision and forward-pointing, perhaps even stereoscopic, eyes. Infillings or endocranial casts of the brain cavity reveal very well developed olfactory lobes, the brain centers for the sense of smell. By these indications, pachycephalosaurs were relatively "sensitive" creatures. Besides, even the most intelligent of species often preoccupy themselves with aggressive and self-destructive behavior.
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