Edited by Neil H. Shubin, University of Chicago, Chicago, IL, and approved December 1, 2016 (received for review August 17, 2016)
Daily growth lines in embryonic dinosaur teeth and CT rendering of a P. andrewsi jaw and tooth family. (A) Von Ebner’s growth lines (alternating dark and light bands) in the orthodentine surrounding the pulp cavity (at top of graphic) of an embryonic H. stebingeri tooth (polarized microscopy, transverse view). (B) Von Ebner’s growth lines surrounding the pulp cavity (at top of the graphic) in an embryonic tooth of P. andrewsi (polarized microscopy, transverse view). (C) High-resolution CT rendition of a P. andrewsi tooth family within the jaw used to determine tooth-formation times in embryonic teeth.
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.
P. andrewsi nest and embryonic dinosaur jaws. (A) Nest of P. andrewsi eggs and embryos. The embryos were partially prepared within the eggs. (B) Expanded view of an embryo within an egg showing the thin surrounding eggshell. (C) Embryonic P. andrewsi dentary showing functional and replacement teeth. (D) Section of an embryonic H. stebingeri dentary showing functional and developing replacement teeth.
Dinosaur incubation periods compared with those for extant reptiles and birds. (Neontologic data from ref. 12). The dashed regression line for the two dinosaur species studied here is provided only to allow visual comparison with typical incubation periods for the other clades, Aves and reptiles in particular. Equations: All birds: incubation time = 11.64 × (egg mass)0.233. All reptiles: incubation time = 48.53 × (egg mass)0.138.
