Very little is known of the embryology of dinosaurs. Since birds are directly descended from dinosaurs, it has been assumed that dinosaurs and birds develop similarly. An important aspect of avian development is that birds produce relatively small numbers of large eggs with remarkably short incubation periods (average 11 to 85 days). This short incubation is advantageous to a species because the eggs, and any attending parent, are less exposed to predation and other environmental challenges than if the period were longer. It has been assumed that dinosaurs also had a short incubation period, which birds inherited. A recent study by Gregory Erickson, Darla Zelenitsky, David Kay, and Mark Norell has not only shown that this assumption is incorrect, but the authors present a new way to determine the incubation period of fossilized dinosaurs, which has important implications.
Erickson et al. [Reference Erickson1] examined the teeth of two species of dinosaurs, Protoceratops andrewsi (a horned herbivore) and Hypocerosaurus stebingeri (a duckbilled dinosaur) recovered from 75 million year old sediments in Alberta, Canada, and the Gobi desert of Mongolia, respectively. It is known from studies of embryonic teeth (including those from embryos) in crocodilians and humans that incremental lines of von Ebner reflect diurnal pulses of mineralization during tooth formation. Thus counting these lines gives a reliable estimate of the number of days it took a tooth to form. An extrapolation from the period of tooth formation allows the incubation period to be determined. Erickson et al. showed that (1) lines of von Ebner are present in embryonic dinosaur teeth, (2) counts and data on lines of von Ebner revealed different incubation periods for these two species of dinosaurs whose eggs span nearly the entire size range reported for dinosaurs, and (3) they could test whether these dinosaurs show typical rapid avian incubation times or primitive slow reptilian development. This allowed them to explore ramifications of their results with regard to the origin of modern avian incubation and survivorship through the Cretaceous-Paleogene (K-Pg) extinction event that occurred about 66 million years ago.
Using polarized (transmitted light) petrographic microscopy on sections of teeth to measure lines of von Ebner in transverse and longitudinal planes, Erickson et al. could accurately measure the width and number of lines (Figure 1). They also used incidental light (direct illumination) dissection microscopy on fractured teeth within the jaw to examine longitudinal growth-line expression.
Figure 1 Incremental lines of von Ebner in the dentine of an embryonic Hypacrosaurus stebingeri tooth. Approximately 30 days of dentine deposition is represented in this image. Transverse plane, polarized microscopy. Image width = 310 µm. Credit: Gregory M. Erickson. Ph. D.
In a significant departure from earlier assumptions, Erickson et al. showed that the incubation period of dinosaurs is approximately twice as long as birds with similarly-sized eggs. This makes dinosaur reproduction more vulnerable to interference and may be related to the failure of dinosaurs to compete effectively for vacated ecologic niches that occurred after the K-Pg extinction event, offering another reason why dinosaurs became extinct. This discovery is important and may help explain why birds survived and dinosaurs became extinct after an asteroid struck Earth 66 million years ago. [2]
