An Intermediate Incubation Period and Primitive Brooding in a Theropod Dinosaur

Non-avian dinosaurs such as oviraptorosaurs and troodontids share several important reproductive characters with modern birds, including eggshell microstructure and iterative egg production. Nevertheless, debate exists concerning their incubation strategies. Here we estimate incubation period for the troodontid, Troodon formosus, by examining a near-term embryonic tooth. Synchrotron scanning and histologic thin sections allowed counting of daily (von Ebner) growth lines. The tooth preserves 31 intact lines with an average spacing of 3.3 ± 0.96 μm. Adding 8 more for the missing crown tip gives a total age of 39 days. Modern crocodilians begin to establish their functional dentition at approximately 47% through incubation. Thus, this tooth age suggests a Troodon incubation period of 74 days, falling midway between avian (44.4 days) and reptilian (107.3 days) values predicted by the Troodon egg mass (314 g). An accelerated incubation relative to modern reptiles supports brooding and concurs with a suite of features in oviraptorosaurs and troodontids (sequential laying, large complex clutches, and precocial young) that appear dependent upon both adult body and incubation temperatures elevated over ambient conditions. However, the largely buried condition of Troodon clutches may have prohibited efficient brooding, necessitating longer incubation than that of modern birds with fully exposed eggs.