Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits

Living archosaurs (crocodilians and birds) share several reproductive features, including hard-shelled eggs1, parental care2,3, assembly-line oviducts4 and luteal morphology5. Nevertheless, crocodilians produce many small eggs that they ovulate, shell and deposit en masse, and incubate within sediments or vegetation mounds2,4,6, whereas birds produce fewer but larger eggs7, usually from a single ovary and oviduct3. Further, birds ovulate, shell and lay one egg at a time and incubate eggs directly with body heat3. New discoveries from the Upper Cretaceous of Montana allow re-evaluation of the transition from basal archosaurian to avian reproductive behaviour in the Coelurosauria8,9, the thero-pod dinosaur clade that includes birds. Egg clutches and nests (Figs 1–3) suggest that the small coelurosaurian Troodon formosus (weight, about 50kg) produced two eggs simultaneously at daily or longer intervals and incubated eggs using a combination of soil and direct body contact. Non-avian coelurosaurians thus possess several primitive features found in crocodilians (two functional ovaries and oviducts, lack of egg rotation and chalazae, partial burial of eggs, precocial young) and several derived features shared with birds (relatively larger and potentially asymmetric eggs, one egg produced per oviduct at a time, loss of egg retention, open nests, brooding) (Fig. 4).