Lay, brood, repeat: nest reuse and site fidelity in ecologic time for two Cretaceous troodontid dinosaurs

—Whereas 'biological site fidelity' refers to the regular reuse of a favored locale (e.g., breeding ground or nest) by an individual animal, 'paleontological site fidelity' typically refers to repeated use of a nesting locality by a herd or species over geologic time scales. Two new Cretaceous specimens from the Two Medicine Formation of Montana, U.S.A., and the Liantoutang Formation of Zhejiang, China, each preserve two closely superimposed clutches of the egg form Prismatoolithus. These eggs belong to the Troodontidae, small theropod dinosaurs sharing a close ancestry with Aves. In both specimens, eggs of a lower clutch are truncated at a level below what would normally preserve in an undisturbed hatched clutch. These traces differ from past examples of dinosaur site fidelity in (1) the close or cross-cutting relationship of the clutches, (2) the precise overlay of clutch atop clutch, and (3) the implication of nest reuse and, thus, site fidelity on an ecologic rather than geologic scale and at approximately the individual rather than species level. Given the likely extended occupation of troodontid nesting sites by attending adults, factors such as nesting success and territoriality may, in addition to favorable substrates, have influenced the behaviors recorded by these specimens. The arrangements of eggs as clutches within the geologic record represent trace fossils. Thus, they record past in situ behavior, providing important insight into dinosaur nesting. In addition, they can serve as independent indicators of substrate conditions and sedimentary history, potentially refining our understanding of paleoenvironments.