DROUGHT REVEALS MANY NEW DINOSAUR TRACKSITES FROM THE DAKOTA GROUP AT JOHN MARTIN RESERVOIR, BENT COUNTY, COLORADO

The Cretaceous Dakota Group is well-known for yielding dinosaur footprints from the so-called Dinosaur Freeway which extends from the Colorado Front Range near Denver across the High plains to eastern New Mexico. Here we report some particularly significant discoveries from John Martin Reservoir in southeastern Colorado, which is administered by the US Army Corps of Engineers.

Discoveries at 9 sites have been facilitated by recent drought conditions which, during 2004, reduced the reservoir to almost empty. Most of the tracks and trackways are those of ornithopods. The first-reported site revealed a minimum of 79 tracks. A few larger sites reveal multiple, in situ, trackways of dinosaurs that are still in place. One site reveals least eight trackways of ornithopod dinosaurs (ichnogenus Caririchnium) that run parallel towards the east. The trackways are somewhat regularly spaced, 1-2 meters apart, suggesting a gregarious group or herd. Evidence of herd behavior among ornithopods is common at sites in the Dakota. Another site shows a particularly well-preserved, museum-quality, trackway of an ornithopod (Caririchnium) that was collected and replicated for future use in exhibits.

Some sites are dominated by the tracks of crocodilians, mostly on different surfaces than the dinosaur tracks. Generally these tracks consist of three parallel traces that could be described as scratch marks. In the past they have been controversially interpreted as ornithischian, pterosaur or crocodilian tracks. More recently they have been interpreted as “swim tracks” made by buoyant crocodiles. Although crocodiles have five toes, when swimming, they mostly touched the bottom only with their three longest toes. Sometimes a fourth toe trace is seen. These toe traces are commonly oriented in the same direction suggesting that animals swam in preferred directions, presumably inresponse to currents in channels.

The abundance of crocodile tracks at John Martin is reminiscent of similar reports from the Dakota Group in Kansas, and New Mexico. This suggests that such tracks may be more common in areas that form the southern part of the Dinosaur Freeway.