ANKYLOSAUR TRACKS FROM THE DAKOTA GROUP (CRETACEOUS), BACA COUNTY, COLORADO

The “mid” Cretaceous (late Albian-early Cenomanian) Dakota Group of the Western Interior is now famous for dinosaur tracks found at more than 50 localities between Boulder, Colorado and Tucumcari, New Mexico. This complex of track bearing deposits, associated with the upper part of the Dakota Group has been dubbed the “Dinosaur Freeway.” Recent high-resolution stratigraphy reveals that these strata comprise three progressively landward stepping sequences which collectively record the onset of the globally-widespread Cenomanian transgression. Dinosaur tracksites can now be specifically allocated to this revised and more precise sequence stratigraphy, and track-bearing layers can thus be ascribed to specific transgressive events.

Most tracks have been attributed to ornithopod dinosaurs (ichnogenus Caririchnium) or gracile theropods, probably ornithomimids (ichnogenus Magnoavipes). In addition, crocodile swim tracks have been reported from some sites and one site has yielded bird tracks (ichnogenus Ignotornis). Tracks attributed to ankylosaurs have only been reported from one locality within the middle sequence of these three. We herein report a new locality, near Walsh, Colorado, in eastern Baca county, where a small sample from this middle sequence appears to be dominated by ankylosaur tracks, and associated crocodile swim tracks. The tracks at Walsh are the first ankylosaur tracks reported from these Dakota Group sequences, which at this site represent transgressive estuarine strata. Two isolated ankylosaur tracks have also been reported from the Dakota Group in western Baca County. Their exact stratigraphic context is not known.