A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus

An unusual trace containing eggs of the 50 kg-plus theropod dinosaur, Trood on formosus, represents one of the best preserved dinosaur nests. This uniq ue specimen (MOR 963) represents the actual nest structure and the direct p roduct of Troodon behavior. The trace comes from the Campanian, Late Cretac eous Two Medicine Formation of Montana, and consists of a bowl-shaped depre ssion with an internal area of similar to 1m(2) surrounded by a distinct ri m. A clutch of 24 tightly-placed eggs sat in the center and both nest and c lutch show bilateral symmetry about a north-south axis. The trace occurs wi thin a moderately well-developed micritic paleosol. A physically and chemic ally distinct mudstone covered the nest and represents overbank deposition. The nest protected the eggs by creating a suitable micro-environment during the lengthy egg-laying and incubation periods. Clutch and nest size, shape , and symmetry and low organic carbon of the overlying mudstone suggests br ooding rather than incubation with vegetative cover, although the latter ca nnot be ruled out. The nest probably played no role in the post-hatching ca re of precocial Troodon young. Reproductive traits indicated by MOR 963 show that Troodon possessed plesio morphies shared with crocodilians (some burial of eggs and lack of egg rota tion), apomorphies shared with birds (open nests, exposed eggs, and incubat ion by a brooding adult), but also at least one unusual feature (steeply-in clined eggs) not found in either extant archosaur group. Some reproductive features typically associated with living birds first evolved within non-av ian coelurosaurian theropods like Troodon.