Dinosaur Behavior

Article excerpt

Though it is hard to believe, it has been twenty years since my friend Bob Makela and I first stopped in the tiny town of Bynum, Montana, fifty miles southeast of Glacier National Park. We had gone there at the suggestion of Berkeley paleontologist Bill Clemens, to identify some dinosaur fossils that rock-shop proprietor Marion Brandvold and family had collected from a bad-land region along the east flank of the Rocky Mountains. In retrospect it was a visit that would help change the way people think about dinosaurs.

Marion Brandvold had a number of dinosaur fossils, but most interesting to me were two bones she showed us as Bob and I were leaving her shop after identifying all her vertebrate fossils. The two little pieces included the proximal end of a femur and the mid-section of a rib, both of which I immediately recognized as being from a baby dinosaur. A baby duckbilled dinosaur, to be specific, otherwise known as a hadrosaur! Marion took us over to her house, where she and her family had been trying to piece additional bits together, and there on a coffee table were numerous skeletal elements of at least four baby dinosaurs. After we explained to Marion the scientific importance of the fossils, she very graciously donated them to science, to the hands of a fossil preparator from Princeton University and a high school teacher from a small rural town in north-central Montana.

Years of experience, study, and persistence had paid off, not in the field or laboratory, but in a rock shop. We had been looking for the remains of juvenile dinosaurs, but before coming to Bynum, had been disappointed. We left Bynum that day with a coffee can brim-full of tiny vertebrae and leg bones, and jaws with teeth. We would soon return to excavate a structure that we would hypothesize to have been a dirt nest containing the remains of fifteen baby hadrosaurs, cared for by their parents. We also would excavate the skull of a new species of adult hadrosaur, discovered by Laurie Trexler, and determine that the babies and adult were of the same species. Bob and I named the dinosaur Maiasaura peeblesorum, in honor of the Peebles family, owners of the ranch land where the specimens were discovered. A small badland area in western Montana, known by geologists as the Willow Creek Anticline, was the tract of land on the Peebles ranch that had yielded the hypothesized nest of baby duckbills. It would be from the same territory, a one-and-a-half-square-mile area, that many more incredible discoveries shedding light on dinosaur behavior would be made over the next twenty years.

SIGNIFICANCE OF THE NESTING SITES

Sediments of the Willow Creek Anticline were deposited in an upland environment near the young Rocky Mountains about seventy-seven million years ago. They are part of what is called the Two Medicine Formation, a 650-meter-thick package of terrestrial sediment representing streams, rivers, and lakes. The remains of a variety of dinosaur species have been found in the Willow Creek Anticline, and two of them, the plant-eaters Maiasaura peeblesorum and Orodromeus makelai, were new to science. Other dinosaurs include armored nodosaurs, and carnivorous taxa such as Troodon, a dromaeosaurid, and a tryannosaurid. But neither the isolated dinosaur skeletons nor the new species have made the Willow Creek Anticline such a special paleontological location; it is the nests, eggs, and babies, the actual evidence supporting dinosaur social behaviors.

Distinct biostratigraphic zones can be found in the fifty-meter-thick vertical section of the Willow Creek Anticline, including river floodplain deposits that produce the remains of nests, eggshell, and baby bones of the hadrosaur Maiasaura, a stratum containing evidence of a massive accumulation of juvenile and adult skeletons of Maiasaura, and lake deposits with adjacent strata containing remains of eggs and skeletal remains of Troodon and Orodromeus. Each of these three zones yields data from which a number of behaviors have been elucidated. …