Troodon Nests from Montana

Ben Creisler

Fossil nests found at the famous "Egg Mountain" site in Montana that contained elongated eggs originally attributed to the small plant-eating hypsilophodont Orodromeus have been reidentified as nests belonging to the meat-eating coelurosaur Troodon. Full preparation of an embryo found in a supposed "Orodromeus" egg revealed a baby Troodon instead. Even more importantly, a recently discovered nest included partial remains of an adult Troodon that may have been brooding the eggs, much in the manner of the meat-eating dinosaur Oviraptor, whose nest sites have been found in Mongolia with complete skeletons of adults preserved while resting on clutches of twenty or more eggs. These major discoveries provide key insights into dinosaur reproduction and behavior. Such theropods appear to have had a mode of egg-laying and incubation combining features known from both modern crocodiles and precocial birds. Troodon produced two large eggs at a time, perhaps daily or at longer intervals over a period of weeks, and used and soil and direct body contact to incubate the clutch in an open nest -- there is no evidence of accumulated plant material at the known nest sites. Coelurosaurs such as Troodon apparently resembled crocodiles in having two functional ovaries and oviducts (modern birds only use one). However, they laid one large egg per oviduct at a time in a partial analogy with birds, instead of producing masses of small eggs from both oviducts the way crocodiles do. Troodont nests resembled the nests of ostriches and other basal birds that lay eggs in open clutches -- but like crocodiles, troodonts did not rotate their eggs. The eggs remained fixed upright in the soil to incubate, so it seems likely that such dinosaur eggs lacked chalazae -- chords of albumen that hold the embryo when an egg is moved. Eggs of crocodiles and other reptiles lack such inner attachments -- turning crocodile eggs kills or deforms the developing embryos, a hazard that may have applied to dinosaur eggs as well. The long period of time needed for such dinosaurs to create and then brood a clutch of eggs strongly suggests an extended pair-bonding between mates. The baby troodonts appear to have left the nest shortly after hatching, as both crocodiles and precocial birds do. Remains of adults and juveniles found together (at "Jack's birthday site") may indicate that young troodonts enjoyed some parental protection in a family group. (Base on Varricchio, Jackson, Borkowski and Horner 1997 "Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits." Nature 385:247-250 (Jan. 16, 1997), and other sources.)