Eggs and eggshell fragments from the Upper Cretaceous Two Medicine Formation of Montana

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Abstract
The association of eggshell with embryonic remains and nestbound hatchlings in the Upper Cretaceous (Campanian) Two Medicine Formation of Western Montana allows us to positively correlate eggshell morphology with specific dinosaur taxa in North America. Eggshell material from the nesting sites at Willow Creek Anticline, Egg Mountain, and Egg Island are described; they show the presence of four morphotypes. Two of these morphotypes can be assigned to Orodromeus makelai, a hypsilophodontid, and Maiasaura peeblesorum, a hadrosaurid based on embryonic remains and nestbound hatchlings. The third morphotype probably belongs to a ?Troodon, based on contained embryonic remains. A very small isolated egg of ornithoid morphotype was found without associated fossil material; it is probably avian. In contrast with the complete material from the Willow Creek Anticline, Egg Mountain, and Egg Island sites, the nearby Red Rock Locality, also in the Two Medicine Formation, has yielded only concentrations of eggshell fragments. Correlation of this eggshell with those from the nesting sites shows that, although the Red Rock Locality is of Santonian age rather than Campanian, the structural morphotypes described for Orodromeus, Maiasaura, and ?Troodon also dominate here. In addition, an isolated, gecko-like eggshell was found.