Bird-like characteristics of troodontid theropod eggshell

Description of well-preserved fossil eggshell (oospecies Prismatoolithus levis), recently assigned to the theropod dinosaur Troodon formosus, reveals traits that are shared with the eggshell of both fossil and Recent avians. Bird-like characteristics of troodontid eggshell include: fibres associated with eisospherites that are attached to the bases of the mammillae, fine radiating crystals that form the spherulites and grade into blocky wedges of the mammillae, and prisms in the outer layer that exhibit squamatic ultrastructure. The presence of two layers (mammillary and squamatic) within the eggshell is either a theropod apomorphy or it arose within Theropoda; the external eggshell layer of crown-group avians is apomorphic with respect to other theropod eggshell. Despite disagreement on the putative sister taxon of birds, recent theropod phylogenies suggest strongly that prismatic ultrastructure evolved independently in Troodontidae and Neognathae. Our identification of squamatic ultrastructure in troodontid eggshell, which was previously thought to lack this structure, reveals problems with the previous assignment of this eggshell to the dinosauroid-prismatic morphotype and with the concept of eggshell morphotype in the current eggshell parataxonomy. We conclude that the dinosauroid-prismatic morphotype should be abandoned.