Abstract

Discovery of dinosaur eggshell fragments in a predominantly mudstone layer 125 ft (38 m) below the top of the Salt Wash Member of the Jurassic Morrison Formation in Delta County, Colorado has led to the identification of a dinosaur nest (containing one crushed but nearly complete egg and thousands of eggshell shards) believed to be the oldest known dinosaur nest in the Northern Hemisphere (153 to 156 m.y.). Laboratory dissection of the nest has revealed the presence of a microcosm of tiny complete and fragmentary plant and animal remains that are the residue of materials used in constructing the nest and in feeding its juvenile inhabitants. The microscopic plant remains include silicified and carbonized wood fragments and tiny stems, charophytes, and a few minute agatized seeds (possibly angiosperm). Microscopic animal remains include various types of eggshell, assorted teeth, fragments of carapaces, a wide variety of bone fragments, some ossified tendons, and a few gastroliths (?), apparently documenting the consumption of embryonic and juvenile turtles, lizards, crocodiles, dinosaurs, pterosaurs, birds, and mammals. Also present are ostracodes, pieces of snail and clam shells, and many worm casts. In addition there are three types of tiny black carbonate bodies of unknown affinities. These minute fossils make this a most important locality for microvertebrates and microflora.

The nest, built on the bank of a small stream, shows evidence of repeated occupancy. Its builders are not known with certainty, but the nature of the organic debris in the nest suggests that it was one of the small carnivorous coelurosaurs (allosaurids), possibly Ornitholestes hermanni, the “egg robber” characterized by small conical piercing teeth.