Recognition of vertebrate egg abnormalities in the Upper Cretaceous fossil record

In fossil vertebrate eggs, the most commonly reported abnormality consists of an unusually thick shell with one or more superimposed eggshell layers. Although rare, taxonomically referable abnormal specimens provide a more reliable basis for inferences regarding reproductive biology of extinct taxa. Eggshell abnormalities recognized in a Cretaceous turtle and three dinosaurs (hadrosaur, sauropod, theropod) are assigned to these taxa base on: 1) normal eggs containing embryos within the same clutch, 2) identical eggshell microstructure as eggs containing embryonic remains from the same stratum; and 3) inclusion of normal eggs of the same oospecies in a cladistic analysis of egg characters. Distinguishing multiple eggshell layers of biological origin from those produced by taphonomic processes is a critical component in the study of fossil specimens. Features useful for distinguishing a pathological condition in fossil eggs include: 1) multiple eggshell layers separated by permineralized membrane, 2) close conformation of the base of the outer eggshell to the unweathered surface of the underlying egg, and 3) structural relationships of eggshell features (e.g. pore truncation, intercalated membrane and shell fragments, optical continuity of superimposed fragments). Documentation of microstructural features of abnormal eggshell provides important information on the timing and magnitude of stress and facilitates recognition of phylogenetically significant patterns of egg abnormality in the fossil record.