Abstract

Using tangential thin sections, we examined variation in porosity and water vapor conductance across two eggs of Troodon formosus , a small (∼50 kg) theropod dinosaur from the North American Upper Cretaceous, testing two hypotheses of egg incubation: (1) full burial within sediments or vegetation and (2) partial burial with exposed upper egg portions. We divided and sampled the eggs in five zones, 1 through 5 from blunt top to more pointed bottom. A geometric model composed of a hemisphere, cone, and paraboloid was used to estimate total and zonal volumes and surface areas. The 138 × 67 mm idealized Troodon egg has a volume, surface area, and mass of 296.4 cm 3 , 239.23 cm 2 , and 314.2 g, respectively. Zonal surface areas and volumes highlight the strongly asymmetric and elongate form of the Troodon egg. Geometric modeling provides better estimates of volume and surface area where egg shape diverges markedly from that of a typical bird egg. Porosity varies significantly across both Troodon eggs, with zones 2 and 3 having the largest pores and a majority (70–78%) of total conductance, whereas zone 5 has very low conductance. Total water vapor conductance in the two eggs are 31.85 and 40.62 mg H 2 O day − Torr − , values 76% and 97% of those predicted for an avian egg of similar size. Low total conductance compares favorably to values in extant birds and non-avian reptiles that incubate in open nests, arguing against full burial incubation. Together with nesting site evidence, low conductance values favor partial burial and incubation by a Troodon adult. Asymmetric egg shape concentrates volume, surface area, and conductance near or at the point of subaerial exposure. Among non-avian dinosaurs, the eggs of Troodon and troodontids are most similar to those of modern birds in having an asymmetric shape, low porosity, no ornamentation, and three structural eggshell layers.