Trace fossils at Curecanti fill in what dinosaur bones cannot

A mounted dinosaur skeleton shows only part of the ancient world. At Curecanti National Recreation Area in Colorado, burrows, footprints and skin impressions, all trace fossils, record animals and environments that bones alone leave out. A Park Paleontology News article on the park's trace fossils lays out what they say.

Curecanti National Recreation Area, Colorado. NPS photo.

The park and its rocks

Curecanti occupies a forty-mile stretch along the Gunnison River between Montrose and Gunnison, with outcrops of the Morrison Formation, one of the most prolific dinosaur-bearing units in the United States. The Upper Jurassic outcrops are often hard to reach, but over the last thirty years body fossils (bones and teeth) have been recovered from sauropods, including Camarasaurus, Apatosaurus and other diplodocids, from the theropods Ceratosaurus and Allosaurus, and from stegosaurs, turtles, crocodilians and probable mammals. Trace fossils such as the burrows of termites, bees, crawfish, earthworms and bivalves have also been observed.

Image from the National Park Service's page: the park's geographic extent.

Why traces help

A body fossil is part of the organism; a trace fossil is a fossilized sign that the organism was there. An animal has one set of hard parts but can leave many traces in a lifetime, so a track or burrow shows an animal lived or walked somewhere even if it never died there. Some environments do not preserve bone at all. Wind and especially water can move body fossils, whereas trace fossils are generally found in place.

The catch is working out who made a trace, since body fossils are often found apart from them and different animals can make similar structures through convergent behavior, using the environment in similar ways for protection and food. The reading relies on what modern animals do.

Burrows and the environment

Paleontologists reconstruct a setting from several traces in one bed, because such assemblages recur with specific environments such as river, floodplain, bay, delta and beach.

Tracks and the animals that made them

Comparing foot bones with three-toed tracks reveals the animal as well as its setting:

• A small theropod. Grallator-like tracks show only the front part of the foot. This is typical of theropod tracks in the Morrison Formation and points to a digitigrade posture, with the animal's weight on its toes.
• A stegosaur. Deltapodus tracks have an elongate heel compared with the foot bones, which has been suggested to mean a large fleshy pad at the back of the foot to support weight.
• A crocodilian. Hatcherichnus tracks do not outline the foot. They record a partly buoyant animal swimming with the tips of its digits touching the bottom.

Rarely, soft parts are recorded too. Skin impressions at Curecanti, often beside tracks, are either large polygons with five or six sides, often associated with sauropods, or small rounded features.

What comes next

The article calls the park's invertebrate and vertebrate trace fossils a wealth that will help build a picture of the Jurassic landscape, and says continued surveying and documentation is only beginning to scratch the surface.

Sources

• National Park Service, Park Paleontology News, Vol. 15, No. 1 (Spring 2023), article on trace fossils at Curecanti National Recreation Area, whose summary credits the documentation of the park's Mesozoic vertebrate fossil localities to paleontologist Ryan King of Western Colorado University; the article's figures, modified from published sources, are left out here; rewritten in hubnx's own words.