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Montanoolithus is an oogenus of fossil egg found in Montana and Alberta. They were probably laid by a dromaeosaur or a caenagnathid.D. K. Zelenitsky and F. Therrien. (2008) "Unique maniraptoran egg clutch from the Upper Cretaceous Two Medicine Formation of Montana reveals theropod nesting behaviour." Palaeontology 51(6):1253–1259
Montanoolithus's eggshell ranges from 0.70 to 0.85 mm thick, and is composed of two layers. The outer layer, called the columnar layer (or squamatic zone, so named because of the peculiar texture of the layer),Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray. II. Eggshell morphology and structure. UCMP Online Exhibits: Fossil Eggshell is twice as thick as the inner mammillary layer. The two layers are divided by a gradual boundary. The mammillae (the cone-shaped structures in the mammillary layer which make up the base of each eggshell unit) are formed by wedge-shaped crystals.
The mother of the Montanoolithus eggs made a mound-shaped nest out of sand, and laid the eggs in a ring around the top. The nest was made from freshly deposited sand (perhaps near to a river), or in a poorly vegetated area. Even though no parent was found with the eggs, it is likely based on its identification as a maniraptoran that the eggs were incubated, since this behavior has been observed in both troodontids and oviraptorids. The pairing of the eggs suggests that, like other maniraptorans, the egg-layer of Montanoolithus had two functioning oviducts which would each form an egg simultaneously.
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