DINOSAURS

A small group of troodonts pauses in the face of an approaching sandstorm (Webster, 1996). These were small, agile, Upper Cretaceous carnivores, recently discovered in the Gobi desert of Mongolia. The various fossil sites appear to represent diverse communities of dinosaurs and other animals associated with oasis-like communities in a regional desert setting.

Within the last 30-40 years there has been a revolutionary transformation in our understanding of dinosaur paleobiology. When you watch a motion picture such as "Jurassic Park" or "Lost World", you are seeing a modern version of these fascinating animals - active, agile, and adaptable, as opposed to huge, awkward, lumbering versions of large reptiles. This page is designed to introduce you to some of the major types of dinosaurs with passing notes as to some of the new insights into how they lived and, ultimately, what kind of animals they were. The illustrations for this page are drawn from three publications which are referenced at the end of the page.

Sauropods

A group of Omeisaurus, medium-sized sauropods, are reconstructed browsing in an Upper Jurassic conifer forest (Norman, 1991). With their long, graceful necks, sauropods are commonly reconstructed as "high browsers", feeding on forest foliage, often using a tripodal stance (standing on their hind legs, braced by their massive tails) to reach higher into the forest canopy. While such a reconstruction seems reasonable, there are some problems:

• Biomechanical problems. Some paleobiologists argue that the mechanics of muscle and tendon attachment would not permit these animals to raise their long necks that far above the horizontal. These workers feel that the long necks represent an adaptation to extend the foraging range without the need for the animals to be constantly moving and would favor a reconstruction where sauropods fed on low-growing vegetation.
• Blood pressure. It is difficult to imagine how sufficient blood could have been provided to the head at extreme elevation to avoid unconsciousness when the head was held up for any extended period.
• Teeth. The most common kind of sauropod dentition, exemplified by animals such as Diplodocus, involves small, peg-like teeth that appear unsuited for feeding on tough material such as conifer shoots. Sauropods with teeth of this type could easily have fed on ferns, which were very abundant in the Jurassic and Cretaceous. Brachiosaurs and camarasaurs had more substantial teeth and could have fed on tougher plant material.
• Body size and herd behavior. Many sauropods are known to have lived in herds, Today, herds of large-bodies animals are characteristic of open vegetation types such as grasslands and tundra - not forests. The Jurassic was a time of extensive, open, fern-dominated communities. Diplodocid sauropods, which are known to have aggregated into herds, may well have grazed on ferns in these open plant communities. Brachiosaurs and camarasaurs appear to have been more solitary animals, which, together with their teeth, would have suited then to somewhat more forested communities. Whether they fed on tree foliage or lower levels in the forest cannot be determined, although the anatomical and physiological limitations noted earlier would favor browsing at a lower level in the forests.

Stegosaurs

A group of Stegosaurs moves along a Jurassic riverbank (Norman, 1991). Stegosaurs were distinctive Jurassic herbivores that had largely disappeared by the Cretaceous. The distinctive plates along the spine and the spikes at the end of their tails have generally been interpreted as defensive structures. While the tail spikes certainly served this function, the role of the dorsal plates is uncertain. There appears to have been a dense network of blood vessels beneath the skin covering the bone plates, which is not consistent with structures that would potentially suffer damage in a carnivore attack. It has been suggested that the plates served a role in heat regulation, acting as fins in a radiator to dissipate excess heat. The M.S.U. Museum has a mounted skeleton of Stegosaurus that you might wish to examine, along with an Allosaurus, a common Jurassic carnivore from western North America.

Ankylosaurs

A typical Ankylosaur (Norman, 1991). Ankylosaurs were relatively small, heavily armored Cretaceous herbivores. Their ecology is poorly understood, but their build suggests that they fed primarily on vegetation at ground level.

Hadrosaurs

A small group of Telmatosaurus, hadrosaurs from the Upper Cretaceous of central Europe. Hadrosaurs were a very diverse group of bipedal herbivores and were the most important herbivore group during the Cretaceous.

A crested hadrosaur (Paralophosaurus) forages along an Upper Cretaceous swampy lakeshore in western North America (Norman, 1991). Several lines of evidence suggest that hadrosaurs fed primarily on conifers. The conifers in these swamps are closely related to the modern bald cypress of the southeastern United States. Many hadrosaurs had elaborate head crests and three functions have been suggested for these structures:

• Sexual display. The crests may have served a role in either mating behaviors or in the social hierarchy within the herd. In such cases one would expect crests to be confined to one sex or the other (probably males, based on bird and mammal behavior).
• Vocalizations. The crests may have served as resonant cavities to enhance vocal behaviors.
• Olfactory function. Since cavities in the crests are connected to the airway and nostrils, the cavities might have been lines with olfactory (smell) receptors that would have served to increase the acuity of the sense of smell, assisting in avoiding predators.

Maiasaura parents caring for nestlings (Norman, 1991). In pioneering work at the "egg mountain: site in Montana, Horner and his coworkers documented the presents of a very large number of nest depressions at a site they consider to have been an island. These hadrosaurs appear to have been attentive parents, caring for and feeding their young until they reached approximately four feet in height. Siting the nests on an island may have provided some protection from predators, since the only defense mechanism available to most hadrosaurs was running or swimming from a source of danger.

Ceratopsians

Given the relatively large size of most dinosaur carnivores, they would have required significant numbers of large prey animals to sustain their populations. Carnivores would thus be much less numerous than herbivores in any Mesozoic ecosystem.

A pair of Tyrannosaurus rex search an Upper Cretaceous landscape for possible prey (Norman, 1991). Tyrannosaurs are perhaps the best-known dinosaurs to the general public and may have been the largest terrestrial carnivores in the history of life. They are generally depicted as very active predators that certainly would have occupied the top position in the food chain of western North American communities in the Upper Cretaceous. A skull of T. rex. is on display at the M.S.U. Museum along with a mounted skeleton of Allosaurus a smaller, Jurassic predator that was the ecological equivalent of later tyrannosaurs. Other paleobiologists suggest that T. rex was primarily a scavenger. Given its food requirements, it is reasonable to suppose that the species would certainly have co-opted the prey of smaller predators if it could drive them from the carcass.

Deinonychus, a large velociraptor, as reconstructed by Robert Bakker (Norman, 1991). Although tyrannosaurs were large and impressive, velociraptors may have had a bigger impact as predators. These are relatively small animals (4-6 feet in height) but they appear to have been very fast and agile and the brain case suggests considerable intelligence. Their primary weapons were their large jaws, with razor-sharp teeth, and a large claw on each hind foot that were probably used as weapons in leaping attacks against larger prey. Evidence suggests that velociraptors may have hunted in packs, increasing their ability to bring down animals much larger than the individual predators.

One of the most recent insights into the biology of theropods (the general group to which dinosaurs such as Tyrannosaurus and the various raptors belong) is the extent to which the smaller species (and possibly the juveniles of larger animals) had feathers evolved as a body covering! These feathered dinosaurs provide insight into thermal regulation in theropod dinosaurs and the question of the evolution of birds.

Oviraptor, a newly discovered, small carnivorous dinosaur from the Gobi desert of Mongolia (Webster, 1996). Nest reconstructions suggest that these animals fed and otherwise cared for their young. Bakker has recently documented similar behaviors in Allosaurus from the Jurassic of western North America, suggesting that parental care of the young may be characteristic of most dinosaurs.

An Ornithomimus from the Cretaceous of western North America (Norman, 1991). These graceful animals were probably very quick and agile and may have preyed largely on the eggs of other dinosaurs.

A pair of 10 meter bipedal carnivores (Afrovenator) single out a young sauropod for attack (Sorvino, 1996) over 90 million years ago (Cretaceous) in what is now the Sahara region of North Africa. Current studies are revealing a wealth of African forms, which appear to have evolved independently since the end of the Jurassic. Afroventator, and several much larger predators, appear to have evolved from Jurassic Allosaurus-like ancestors. An attack, such as that shown here, would have been a very dangerous undertaking for the predators, since studies of track-ways suggest that Sauropod herds traveled in three lines. The outer lines were occupied by adult animals with the young traveling between the lines of adults. It is far more likely that predators would attack solitary animals, either very old of very young, that had become separated from the herd. A reconstruction such as this one hardly serves to give a sense of scale to the scene. For example, the razor-sharp teeth of these predators were up to two-feet in length!

Sorvino, Paul C. 1996. Africa's dinosaur castaways. National Geographic Magazine 189:106-119.

Webster, Donovan. 1996. Dinosaurs of the Gobi. National Geographic Magazine 190:70-89.