New Mexico Geological Society Annual Spring Meeting — Abstracts

A dinosaur tracksite associated with the basal part of the Pajarito Formation, at the contact with the underlying Mesa Rica Sandstone, at the Mosquero Creek site, Harding County, New Mexico, is just one of several dozen sites comprising a laterally extensive (80,000 km²), but vertically restricted (on the order of 10 meters) track-rich zone known as the "dinosaur freeway" (Lockley and others, 1992). These track-rich beds have been dated as Late Albian to Early Cenomanian.

It was reported by Lucas and others (1987; 1989) that about 31 trackways were visible including a set of 27 representing small individuals heading northwest, and four representing much larger individuals heading southeast. The tracks, assigned to the ichnogenus Caririchnium represent a quadrupedal ornithopod, that is widely represented throughout the track-bearing beds of the dinosaur freeway complex. At Mosquero Creek the ichnocoenosis appears to be monospecific. This, coupled with the large number of parallel trackways, on a single bedding plane, is good evidence for gregarious behavior.

We recently mapped the Mosquero Creek site for the first time and collected trackway orientation data and footprint measurements. This enables us to demonstrate that the site is one of the three largest in the entire dinosaur freeway complex, and by far the most significant in terms of parallel trackway evidence indicative of gregarious behavior. The record of sets of small and large parallel trackways in separate clusters suggests different age groups traveling, in different directions at different times. Similar but smaller groupings of small and large trackways are known from Dinosaur Ridge, near Denver, Colorado.

Traditionally the Albian age Davenport Ranch tracksite, where 23 parallel sauropod trackways were recorded by Bird (1944), is cited as the best example of footprint evidence for herd behavior among dinosaurs. other examples in North America include sauropod trackways from the Late Jurassic Purgatoire tracksite in Colorado, and ornithopod trackways from the Early Cretaceous Gething beds of western Canada. However at all of these localities the total number of parallel trackways is less than at the Mosquero Creek site. Based on the estimates of growth rates for Cretaceous ornithopods proposed by Horner (1992), we infer that the smaller animals may have been no more than one to two years old, whereas the large animals were probably fully grown (at least 5-7 years old). An alternate explanation is that tracks of different sizes represent males and females (sexual dimorphism).

The large amount .of trackway information now available from the Mosquero Creek site and from the entire dinosaur freeway complex, provides a valuable new data base for testing ideas about dinosaur herding behavior. If proposed growth curves are accepted, the data will also shed light on the relative (approximate) age of trackmakers in such groups.