A possible egg of the dromaeosaur Deinonychus antirrhopus: phylogenetic and biological implications

Abstract

A unique find of a partial egg in contact with articulated gastralia from a known specimen of the dromaeosaurid Deinonychus antirrhopus (AMNH 3015) is described. Much of the original taphonomic context of the specimen was lost during the 1931 excavation and preparation, but enough information is preserved to provide strong evidence for a parental association between the adult skeleton and egg. The articulated nature of the gastralia suggests that the adult skeleton was at least partially articulated and had not suffered from either extensive subaerial exposure or postmortem transport, and the egg preservation also indicates in situ burial and postburial lithogenic crushing. Additional support stems from the presence of limey claystone matrix that indicates a low-energy depositional event. Phylogenetic characteristics of the eggshell microstructure are consistent with a theropod origin, and skeletochronological analysis suggests that AMNH 3015 was an adult and thus of breeding age bolstering the interpretation that the egg derives from the skeletal specimen. Physiological parameters of D. antirrhopus, such as estimated mass and pelvic canal diameter, as well as eggshell thickness, are very similar to the similar sized and closely related oviraptorid Citipati osmolskae. Closely related maniraptoran theropods of several species have been found brooding their nests with the gastralia close to or in contact with eggs, and such a scenario is consistent with the preservation of AMNH 3015. Alternative explanations to a parental association, such as random co-occurrence or feeding, are improbable given the taphonomic and biological data of the find. AMNH 3015, therefore, probably represents the first identifiable dromaeosaurid egg yet discovered. It shares derived characters such as two eggshell layers with other theropods. Within theropods, the AMNH 3015 eggshell shares derived characteristics with oviraptorids and differs from troodontid eggshell despite the fact that these taxa are often recovered as sister groups in analyses of skeletal characters, but this signal is weak.