Partial eggs from the Eocene Willwood Formation of Wyoming contain unidentifiable embryonic remains; the eggs are referable to a theropod on the basis of their structural layering of calcite and to an avian theropod because of their Eocene age. We assign the specimens to the oofamily Medioolithidae as Microolithus wilsoni, oogen. et oosp. nov., on the basis of the following unique combination of characters: 600-µm-thick eggshell composed of three structural layers; abrupt and undulating contact between the mammillary and continuous layers; smooth and glossy outer egg surface; faint or obscure prisms of the continuous layer; non-branching pores; and mammillary-to-total shell thickness ratio of 1:4. The eggshell microstructure resembles that of some extant neognath bird eggs. We refer a single egg from Chadron Formation of eastern Nebraska to incertae sedis as Metoolithus nebraskensis, oogen. et oosp. nov. A combination of characters distinguishes the egg from other fossil and modern avian eggs, namely, thicker eggshell, prominent ornamentation, flared upper portion of the prisms, variable mammillary thickness, and irregular squamatic texture in the continuous layer. These characteristics more closely resemble Mesozoic non-avian theropod eggs and likely reflect mosaic evolution in the non-avian to avian-theropod transition. Finally, an external layer occurs in a wide range of Paleogene and Cretaceous eggs of variable size and taxonomic affinities, indicating that this feature may not represent an apomorphic character for the avian crown group with respect to non-avian theropod eggshell. Therefore, an external layer cannot be used to identify neognath birds in the Mesozoic.
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