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

ABSTRACT

An unusual trace containing eggs of the 50 kg-plus theropod dinosaur, Troodon formosus, represents one of the best preserved dinosaur nests. This unique specimen (MOR 963) represents the actual nest structure and the direct product of Troodon behavior. The trace comes from the Campanian, Late Cretaceous Two Medicine Formation of Montana, and consists of a bowl-shaped depression with an internal area of ~1m² surrounded by a distinct rim. A clutch of 24 tightly-placed eggs sat in the center and both nest and clutch show bilateral symmetry about a north–south axis. The trace occurs within a moderately well-developed micritic paleosol. A physically and chemically 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 brooding rather than incubation with vegetative cover, although the latter cannot be ruled out. The nest probably played no role in the post-hatching care of precocial Troodon young.

Reproductive traits indicated by MOR 963 show that Troodon possessed plesiomorphies shared with crocodilians (some burial of eggs and lack of egg rotation), apomorphies shared with birds (open nests, exposed eggs, and incubation by a brooding adult), but also at least one unusual feature (steeply-inclined eggs) not found in either extant archosaur group. Some reproductive features typically associated with living birds first evolved within non-avian coelurosaurian theropods like Troodon.