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Rebuilding the Lost World

New insights, technologies and fossil finds transform our image of dinosaurs
A full-sized Triceratops sculpture at Honolulu Community College Dinosaur Exhibit
It is the curse of paleontologists that they can never attain a definitive understanding of the creatures that so captivate them. A zoologist interested in, say, the predatory behavior of the tiger can go into the field, watch the action unfold and take notes. But answers to even the most basic questions about dinosaurs lie hidden in the silent bones. Undaunted, paleontologists continue to seek out new fossils and reinterpret old evidence to learn what dinosaurs were really like--often with unexpected results.

Some of the most recent work has yielded provocative insights into how dinosaurs reared their young and what dinosaur calls sounded like. Perhaps most startling of all, a Saharan expedition has turned up the remains of a predatory dinosaur that challenges Tyrannosaurus rex as the biggest carnivore ever to tread the earth.

Fragmentary remains of the giant lizard first turned up in northern Africa in 1927. The creature was given the ungainly name Carcharodontosaurus saharicus ("shark-toothed reptile from the Sahara"), but little information could be gleaned from the scattered fossils. Last fall, however, a University of Chicago team led by Paul C. Sereno made an expedition to the Kem Kem region of Morocco and suddenly realized the true enormousness, and enormity, of Carcharodontosaurus. As reported in the May 17 issue of Science, the creature's fossilized skull is 1.6 meters long, even larger than that of T. rex. (Sereno infers that T. rex probably was the taller of the two, but Carcharodontosaurus took the prize for sheer bulk.)

Even aside from its sheer size, Carcharodontosaurus is remarkable in other ways. An examination of its brain case shows a cranial volume of just 205 cubic centimeters, roughly one eighth that of a human and just half that of T. rex. It seems that Carcharodontosaurus was a fighter, not a thinker. Sereno and his collaborators note that Carcharodontosaurus is also of interest for the information it provides about dinosaur evolution. A closely related dinosaur called Gigantosaurus, which also rivaled T. rex in size, lived in Argentina at about the same time. Sereno concludes that there was a rapid evolutionary "radiation" of giant, carnivorous dinosaurs during the late Cretaceous (90 million to 65 million years ago). Those new species were spreading around the globe even as plate tectonics was starting to carry the continents apart.

While some researchers focus on the greatest dinosaur predators, others are intrigued by a more placid but essential aspect of dinosaur studies: How did they rear their young? For years, dinosaurs were popularly portrayed as stupid, sluggish animals. More recently, scientists have begun to recognize that dinosaurs may have been more active and behaviorally complex that previously thought; some researchers now argue that, based on their kinship with birds, dinosaurs may actually have been attentive parents, protecting their eggs and guarding their young.

Last December a group led by Mark A. Norell of the American Museum of Natural History in New York scored a major coup for the supporters of the devoted dinosaur theory. While fossil hunting at the Ukhaa Tolgod site in central Mongolia Norell's team happened across an unprecedented find: the skeleton of an Oviraptor (a relative of the Velociraptors that starred in Jurassic Park) buried atop a nest of dinosaur eggs. In a paper in the December 21/28,1995 issue of Nature, Norell claimed not only that the Oviraptor was tending to its nest but that it did so "in the same posture taken by many living birds when brooding.

This kinder, gentler vision of dinosaur behavior did not go long unchallenged. In the May 3, 1996 issue of Science, Nicholas R. Geist and Terry D. Jones of Oregon State University struck a counterblow. The two paleontologists examined the pelvic structures of newly-hatched modern birds and crocodilians, and compared those with the pelvises of fossilized dinosaur newborns. Geist and Jones deduced that, among living species, the degree of ossification (in essence, the strength) of the bone indicates whether a hatchling is active and independent or weak and vulnerable. Equivalent fossil studies, the two men reasoned, could reveal whether dinosaur hatchlings truly needed the kind of nurturing inferred by Norell's group.

Geist and Jones answer with an emphatic "no." Pelvic and leg bones in baby dinosaurs look robust, they say, leading the researchers to assert that "the nesting behavior of dinosaurs was likely similar to that of modern crocodilians." Norell, not surprisingly, remains unconvinced, arguing that the discovery of an Oviraptor in a nesting position is more persuasive than inferences from studies of bone strength.

The truth may lie somewhere in between. Some birds (shore birds, for instance) are born in a very capable state, and some modern reptiles (such as pythons) are known to brood their nests. In reality, dinosaurs probably shared some parenting traits with both birds and modern reptiles, and probably had other behaviors unlike either. And given the bewildering diversity of dinosaur species, it is improbable that they all reared their young the same.

The controversy over nesting behavior shows just how hard paleontologists have to work to extract information not directly recorded in the fossil record. For instance, they have long puzzled over a peculiar class of dinosaurs, known as crested duck-bills (more technically, as lambeosaurine dinosaurs), that sported huge bony protrusions from the top of their heads. What function did these crests serve? Scientists can only speculate, but another new fossil discovery may be bringing them closer to an answer.

Last fall a team led by Thomas E. Williamson of the New Mexico Museum of Natural History found an exceptionally well-preserved skull of Parasaurolophus, a lambeosaurine dinosaur whose crest extended in a long backward arc. Williamson was eager to learn more about the internal structure of the crest but did not want to fracture the fossil. So he turned to the world of high-technology: St. Joseph's Hospital in Albuquerque, New Mexico donated the use of a medical computed tomography (CT) scanner, which can create three-dimensional x-ray images.

The CT scans revealed that the Parasaurolophus crest has a previously unseen, very complicated internal structure containing multiple chambers. The exact purpose of those chambers is still unknown, but the overall structure is consistent with the theory that the crest functioned as an acoustic resonator--a kind of biological trumpet that may have aided in mating calls or other kinds of audible communication.

To put this theory to the test, David Weishampel of Johns Hopkins University built a model Parasaurolophus crest out of plastic tubing and tried playing it like a wind instrument; it seemed to work well, producing resonant, musical tones. Expanding on that idea, Diane Farish, an undergraduate at the University of New Mexico, constructed a model of the recently unearthed Parasaurolophus crest. She found that it amplifies the low frequency sounds that, Williamson speculates, may have been effective for long-distance communication through thick jungle growth

Williamson is now collaborating with Carl Diegert of Sandia National Laboratories to construct a detailed computer model of the Parasaurolophus skull. The model can then be modified to add in the soft tissues that do not normally survive the fossilization process. Once he is satisfied that he has achieved a plausible reconstruction of the crest from a living Parasaurolophus, Williamson plans to use the computer model to simulate the sounds that the dinosaur could have created.

Nobody will ever know exactly what sort of noises--if any--Parasaurolophus made. But thanks to the ingenuity of a bunch of hairless, bipedal mammals, dinosaur calls may be heard again, for the first time in 65 million years.

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