Bridging the reptilian-avian reproductive continuum through analysis of maniraptoran dinosaur nesting physiology

Abstract

While it has been the general scientific consensus that the dominant avian practice of thermoregulatory contact incubation of fully exposed eggs evolved from a prior habit of crocodilian-like nest guarding of fully buried eggs, the efficacy of specific intermediate behaviors and the role of associated selective pressures is not well understood. Troodon formosus, a late Cretaceous theropod from North America, has provided fossils illuminating this period of transition from reptilian to avian reproductive strategies. Unfortunately, this species has long been plagued by taxonomic and nomenclatural concerns. We believe these issues would be significantly alleviated by the designation of a neotype. Our biological understanding of T. formosus is further bolstered via investigation of behavior and ecology. Daily growth lines in an embryonic tooth attributed to T. formosus suggest an incubation period of around 74 days, a value between what would be expected for a reptilian (107 days) and avian (44 days) egg of the same mass. Experimental analysis of maniraptoran nesting practices indicates that their partially buried eggs (as evidenced in the fossil record) likely could have benefited more from contact incubation than previously believed. Additionally, experimental results suggest that an attending, endothermic adult might have been able to warm even a fully buried clutch through a barrier of sediment. Egg asymmetry is another feature that appears at this junction, and while it has been well studied in modern birds, most of the suggested benefits are inapplicable to the asymmetric eggs of T. formosus. Actualistic and virtual experimentation indicates that elongate, asymmetric egg-shapes could have reduced the likelihood of displacement and provided increased resistance to vertical compression; two concerns that might have accompanied early contact incubation. Given information gleaned from the studies described herein, it seems possible that the gap between modern avian and crocodilian-style reproduction could have been bridged by stages described as indirect contact incubation (fully buried eggs warmed by an attending adult) and weak contact incubation (partially buried eggs warmed by an attending adult)--changes driven by a selective pressure for increased clutch temperatures.