by Patrick Getty
On March 17-20, 2012, the Geological Society of America’s Northeast Section held its Annual Meeting in Hartford, Connecticut. (The official dates for the conference were March 18-20, but the associated field trips were held the day before.) It was an exciting four days, especially–from Jurassic Roadshow’s point of view–because there were two field trips related to the region’s dinosaur footprints and a full session on trace fossils. We hope to report on some of these sessions, starting with Patrick Getty’s field trip to The Trustees of Reservations Dinosaur Footprint Reservation site in Holyoke, Massachusetts, about 35 miles north of Hartford. Patrick Getty is a geosciences graduate student at the University of Connecticut.
Most people who live in the city of Holyoke, MA, are familiar with the dinosaur tracks on Route 5 near the Connecticut River. Indeed, this area is now owned and protected by The Trustees of Reservations as Dinosaur Footprint Reservation, free and open to the public from April 1 through November 30 each year. Edward Hitchcock, the professor of Geology and Natural Theology at Amherst College during the mid-19th century whose brief biography appears elsewhere on this blog, first reported dinosaur tracks at this site in 1836.
Since the pioneering days of Hitchcock’s trace fossil research, the footprints have been the subject of many scientific investigations. The most important was conducted by John Ostrom of Yale University, published in 1972. Like Hitchcock, Ostrom noted that the largest dinosaur footprints at the site, called Eubrontes giganteus and made by large carnivores, are mostly oriented to the west. (If you are new to this science, be aware that dinosaur tracks and dinosaur skeletons have entirely different naming systems, since we cannot always tell which kind of dinosaur made a given set of tracks.) Ostrom concluded that the trackways were going in the same direction because the animals were traveling as a herd. The rocks are now tilted so that it looks like the animals were walking uphill, but they were not. The sandy surface was a flat lakebed that later was compacted and turned to stone and still later tilted by tectonic activity.
At the time of Ostrom’s publication, the implications of such an argument were quite novel. He came to his conclusion at a time we now call the Dinosaur Renaissance, when paleontologists were rethinking their ideas of how dinosaurs lived. Before this renaissance, dinosaur were generally thought of as slow, ponderous creatures that lived in isolation, but new ways of looking at the fossil evidence led to the lively, energetic creatures that we see today in modern reconstructions in museums, book illustrations, and the 1993 movie Jurassic Park, for example. Along with these changes came the idea, based on parallel trackways at such sites as the one in Holyoke, that many dinosaurs were in fact social animals living in large herds or flocks.
Many paleontologists had lingering doubts about the evidence Ostrom presented in support of his hypothesis. The most important observation used to cast doubt on Ostrom’s argument was the fact that there are ripple marks present in the rocks. Why is that important? Ripple marks form as waves gently wash back and forth along the edge of a large body of water, such as a lake or the ocean, and the crests of the ripples form approximately parallel to the shoreline. If you look down the next time you go wading at the beach, you might see this phenomenon in the sand right beneath your feet. These ripple marks can be preserved, just as fossils are, and their presence in the rocks at the Holyoke site strongly suggest that the animals were walking near the edge of a large lake. Thus, it is possible that the animals were walking singly at different times, simply following a shoreline. Consider, for example, that if you walk along a beach by yourself, and others do the same thing over the course of a day, you all will leave parallel trackways just out of reach of the waves.
But how do we know which is the correct hypothesis? Were the dinosaurs traveling as a group, or were they simply following a shoreline? A combined University of Connecticut and University of Massachusetts research group set about examining these two possibilities in 2010 by conducting the most thorough survey of the dinosaur tracks at the site that has ever been done. Where Ostrom found only 134 tracks, we identified nearly 800 dinosaur tracks of small and large carnivores, small herbivores, and crocodiles. Significantly, we found 231 Eubrontes giganteus tracks representing 53 individual animals, more than twice as many as were found in any other previous study. Our new map of the site shows that the trackways are not oriented randomly across the surface, but show a bimodal distribution heading either west or east; none go north or south. Also significant is that measurements of ripple crests showed that the shoreline at the site trended from the northeast to southwest, in nearly the same orientation as the dinosaur trackways.
So what does this information tell us? The fact that the dinosaur trackways and the shoreline were nearly parallel strongly supports our contention that the dinosaurs were following a path along the shoreline, not herding. The results of our joint research were presented at two meetings of the Geological Society of America, one in Minneapolis late last year and the other in Hartford just two weeks ago. Additionally, we led a field trip to the site during the Hartford meeting so that other geologists could evaluate our results. All seven participants generally agreed with our interpretations that the big meat eaters liked to follow the shoreline (perhaps to catch their prey?).
One might argue that the evidence from Dinosaur Footprint Reservation is also consistent with a whole herd of carnivores paralleling the shoreline. While this is true, our work has not stopped with the one site. We have examined Eubrontes giganteus trackways at numerous other sites in Connecticut and Massachusetts and found that where there is no sign of a shoreline, the trackways go in different directions and show differences in preservation that suggest individual animals walking across the sites at different times. Thus, we conclude that this large predator was not a herding animal.
So where are we now? Are we back to the old idea that dinosaurs were solitary animals? Is Dinosaur Footprint Reservation just another track site? Not at all! Our work specifically addresses one type of large carnivorous dinosaur from the Connecticut River Valley that was apparently a solitary hunter. Many other types of dinosaurs, including long necked sauropods, armored ankylosaurs, and horned ceratopsians, to name just a few, were gregarious, based on multiple sites elsewhere that show parallel trackways or large numbers of skeletons of the same species.
With so many footprints of different types of animals, Dinosaur Footprint Reservation in Holyoke is a treasure trove of scientific and educational value that deserves continued protection to ensure that people can enjoy visiting the site for years to come.