Mosquero Canyon's Dinosaur Freeway paralleled a prehistoric inland sea. One of the Dinosaur Freeways.
The oldest Cretaceous dinosaurs in New Mexico are known from 100-million-year-old footprints in Harding and Union counties. These are footprints of ornithopod dinosaurs closely related to duck-billed dinosaurs. A representative slab from Mosquero Creek is on display outside the museum. The footprints are best seen in place at Clayton Lake state Park, where an exhibit explains the more than 500 dinosaur footprints exposed at the dam spillway.
The Dakota Formation (early Cretaceous)
The Dakota Formation (~150ft) is generally thin and highly variable throughout the central Colorado Plateau. Where it is present, it generally consists of a lower unit of sandstone or conglomerate, a middle layer of shale and channel sandstone, and an upper layer of marine shale and sandstone. Several pale soils and numerous types of burrows have been reported from the Dakota Formation. Rare fossils include plants (dominantly ferns and horsetails) and petrified wood.
The Dakota sandstone is interpreted as a shoreline facies of the Cretaceous Western Interior Seaway. While thin in the Colorado Plateau region, correlative formations are found throughout western North America. Numerous dinosaur track sites occur in these deposits, and track ways attributed to birds and small, coeleasaur theropods have been found in the Dakota Group as well. Although theropod and large sauropod tracks are present as in the Morrison, a conspicuous feature of these Cretaceous deposits, both in the Colorado Plateau region and elsewhere, is the presence of numerous large ornithopod (Iguanodontid) track ways (~50cm).
Track ways are found at numerous levels. For instance, 18 track bearing levels have been mapped at one site near Eldorado Springs, Colorado, 11 at Roxborough State Park, Colorado. However, no body fossils have been found at these sites. Often these exposures show parallel, monospecific track ways (up to 55 at Mosquero Creek) suggesting gregarious behavior. Lockley and Hunt write:
"Overall, the Dakota Group, from outcrops in Boulder and Eldorado Springs in the north to Mosquero Creek in the south, proves to be a track-rich zone that extends for hundreds of miles along the trend of the western shoreline of the western interior seaway . . . most of the dinosaur tracks are very similar. They are predominantly those of Iguanodon-like ornithopods. Many are virtually identical to the footprints of an early Cretaceous ornithopod from the Carir basin of Brazil and have therefore been named Caririchnium. Others are slightly or subtly different, resembling so called Igunaodon tracks from early Cretaceous strata in England and other parts of the world. Throughout the Dakota megatracksite we also find the tracks of carnivorous dinosaurs. Most of the well-preserved examples are those of medium-sized, slender toed, gracile theropods of the bird-like type"