Fig. S1.

Determining incubation period and tooth replacement rates in embryonic dinosaurs. Embryonic dentitions of Protoceratops andrewsi and Hypacrosaurus stebingeri show different numbers of functional teeth in each tooth family and require different methods to determine incubation period. P. andrewsi (Upper) has a single functional tooth and a single replacement. The formation time for the most-developed functional teeth in the dentition was modeled as representing 58% of the total incubation period. In other words, the teeth destined to be the hatchling functional dental compliment were modeled as having been initiated 42% through development. (Note: The mean replacement rate, which equals the rate at which teeth were typically shed from each tooth family, is not used in the incubation period calculation, but was determined for future use in comparative dental studies.) In H. stebingeri (Lower) there are two functional teeth and a single replacement. The time elapsed to make the hatchling functional dentition similarly requires determining how many days it took to form the oldest tooth. However, the crown of that tooth is worn away from in ovo chewing. As such, a subset of the incremental lines have been effaced in the tooth and its formative time cannot be made using a total line count. However, it is a single-replacement cycle older than the next younger functional tooth. By aging that tooth and adding the tooth-replacement rate, the formative time for the oldest functional tooth is revealed. Coupling this value with consideration that that tooth began 42% through incubation provides an estimate of total incubation period.