Localities of the Cretaceous:

Clayton Lake

Clayton Lake dinosaur tracksite

In the northeastern corner of New Mexico, twelve miles north of the town of Clayton, an earth dam was constructed in the 1970s across Seneca Creek that resulted in the formation of Clayton Lake. The excavation of the spillway, and a flood in 1982 that swept away a layer of silt from the spillway, uncovered an unexpected bonanza of dinosaur tracks, preserved in the Early Cretaceous sandstones of the upper Dakota Group, dated at about 100 million years old. Today, the tracksite, with over five hundred dinosaur footprints preserved, is one of the main attractions at Clayton Lake State Park -- and one of the best-preserved and most extensive dinosaur tracksites in the United States.

The Dakota Group consists of rocks that formed along the western shore of the Western Interior Seaway, a shallow oceanic strait that in Cretaceous time connected the Gulf of Mexico with the Arctic Ocean. These rocks were laid down in lakes, rivers, and river deltas, as well as the ocean. Dakota Group strata extend from central New Mexico more or less northward through Colorado to southern Wyoming (east of the present-day Rocky Mountains and extending westward into the Front Range of Colorado). From northern Colorado to central New Mexico, the upper layers of the Dakota Group (dating to the Albian-Cenomanian) are rich in dinosaur tracks and trackways. Crocodilian tracks, as well as the first Mesozoic bird tracks ever found, are also present in some outcrops of the Dakota Group. These tracks are given added importance by the fact that no dinosaur bones have been found in this part of the Dakota Group; the tracks are the only evidence that dinosaurs once lived in the region. Dinosaur Ridge, just west of Denver, Colorado, is one well-known and publically accessible tracksite in the Dakota Group; Roxborough State Park, south of Denver, has another. Clayton Lake, at the south end of what has been called the "Dinosaur Freeway", is yet another accessible tracksite. (Lockley and Hunt 1995)

Map of the contiguous United States, with the Lower Cretaceous seaways superimposed in blue. The dinosaur tracks indicate Clayton Lake (the slightly larger track) and a few of the other tracksites known from the Dakota Group. Note that the actual shoreline tended to fluctuate over time.

Clayton ornithopod track
The most common tracks at Clayton Lake, and indeed throughout the "Dinosaur Freeway", are broad, three-toed tracks like this one, shown at left. The largest of these tracks is about thirty centimeters in length, from the tip of the middle toe to the rear. Very similar tracks have been found in Brazil and in England. The tracks, which have been placed in the form genus Caririchnium, were made by ornithopod dinosaurs, large herbivores. The best-known ornithopods are the hadrosaurs, or "duckbilled dinosaurs", but the Clayton Lake tracks are a little too old to have been made by hadrosaurs. In all likelihood, tracks like the one shown here were made by iguanodontid dinosaurs, whose best-known representative is Iguanodon. Iguanodontids were close relatives and probable ancestors of the hadrosaurs.

The track shown here represents a hind foot impression. Iguanodontids had large hind limbs and smaller forelimbs, and are often reconstructed as being bipedal. However, this track, and countless others like it, is associated with a smaller impression, visible here to the left of the main track. These are impressions of the forefeet. The ornithopod dinosaurs that left these tracks may have been quadrupedal, walking on all fours. Or they might have been facultative bipeds, able to switch between bipedal and quadrupedal walking.

This image (at right) shows parts of two trackways. On the left is an ornithopod trackway, with its distinctive broad, three-toed hindfoot track shape (another forefoot impression is visible at the upper left of the picture). On the right are some rather unusual tracks, looking rather like kites, but clearly showing three toes at the front. These may be the tracks of small theropod dinosaurs, but could also be tracks of small ornithopods; the "kite shape" may result from the metatarsal part of the foot, which was normally off the ground, contacting the ground when these tracks were made. Definite theropod tracks are known from the Clayton Lake spillway. Another part of the spillway has yielded tracks that were originally thought to belong to a pterosaur. However, they are now known to have been made by crocodiles (Bennett, 1993).

Clayton Lake, along with other Dakota Group tracksites, contains examples of parallel trackways. At Mosquero Creek, a site south of Clayton Lake, at least 55 trackways of small ornithopods have been found, all parallel and all trending towards the north. At the same site, ten parallel trackways have been found that were made by larger ornithopods and that trend towards the south. Evidence like this, which is confirmed by similar findings from elsewhere in the Dakota Group, shows that different dinosaur species, and probably different age groups of the same species, traveled together in groups. It may also suggest that the "Dinosaur Freeway" was exactly that -- a migration route. Herds of dinosaurs, primarily ornithopods, may have migrated north and south for hundreds of miles along what was then the western shoreline of the Western Interior Seaway. This is plausible, but difficult to prove; it is hard to be sure just how far dinosaur populations ranged (Lockley and Hunt, 1995).

Clayton trackways
Clayton mudcracks

Clayton ripples

These images (at left) demonstrate features that can be used to reconstruct the environment in which the tracks were made. The markings that crisscross the rock surface in the upper image are dessication cracks. These can be seen forming today on mud that is drying out; the mud cracks, in a polygonal pattern, as it dries. Visible in the lower image is a small patch of ripple marks. These marks formed, and form today, when steady, relatively gentle water currents flow over soft sediments. Ripple marks are also visible in the upper image, but have been cross-cut by the dessication cracks, showing that the cracks formed later. The picture that emerges from these clues is of a sandy or muddy flat, partly dried out, but still retaining pools and inlets of shallow water. Over this sand flat, sinking a bit in the not-fully-dry muddy sand, herds of dinosaurs once walked.

The Clayton Lake tracksite is part of Clayton Lake State Park, under the auspices of the New Mexico State Parks Division. In addition to dinosaur tracks, the park offers wildlife viewing, fishing, boating, and camping opportunities. You can see more of the "Dinosaur Freeway" at Roxborough State Park, Colorado, and at Dinosaur Ridge, near Morrison, Colorado.

A virtual tour of the Clayton Lake tracks is available at the Learning Family Network website. Also, the Dinosaur Trace Fossils page, maintained by Dr. Tony Martin at Emory University, has a wealth of information about dinosaur tracks and other trace fossils.