Protoceratops andrewsi is one of Mongolia’s most celebrated dinosaurs, famous for its fossil nests. Although eggs originally discovered by the American Museum of Natural History’s 1923 Central Asiatic Expedition and attributed to P. andrewsi have since turned out to belong to another dinosaur (the theropod Oviraptor philoceratops), subsequent discoveries confirm that P. andrewsi was indeed a nest-builder. According to Erickson et al. (2017), a nest of 12 P. andrewsi eggs from the Gobi Desert’s Upper Cretaceous (Campanian) Djadochta Formation is shedding new light on one of the mysteries of dinosaur embryology. The fossil nest, discovered by expeditions of the American Museum of Natural History and Mongolian Academy of Sciences, is the first to be attributed with certainty to a ceratopsian dinosaur. The eggs’ estimated volume (177.98 cc) makes them the smallest “proven” nonavian dinosaur eggs yet discovered. Although partly crushed, each egg contains a well-ossified P. andrewsi embryo with a fully formed dentition.

The authors of the study wanted to answer a seemingly simple question: how long was the incubation period for a nonavian dinosaur egg, compared to living dinosaurs (birds) and other reptiles?

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To find the answer, the scientists looked to growth-line counts in the embryos’ teeth. Like tree rings, structures called incremental lines of von Ebner (reflecting diurnal pulses of mineralization during tooth formation) can be counted to estimate the time elapsed as a tooth formed in ovo. This data can then be used to extrapolate the incubation period. For the study, Erickson et al. examined embryonic teeth from Protoceratops andrewsi (with the smallest known eggs) as well as the hadrosaur Hypacrosaurus stebingeri from Alberta, Canada (with eggs of 3,900 cc, among the largest). The researchers used CT scan images and polarized (transmitted light) petrographic microscopy to measure lines of von Ebner in transverse and longitudinal planes, and were able to accurately count the lines.

The results were surprising: whereas previous researchers hypothesized a rapid incubation period for nonavian dinosaurs (similar to modern birds), the new study suggested a much slower pace, closer to typical reptiles. The authors estimated a minimum incubation period for Protoceratops andrewsi of 83.16 days, twice as slow as birds with comparably sized eggs and slightly slower than modern crocodilians and turtles (although faster than other nonavian reptiles). Results for Hypacrosaurus stebingeri were similar, with an estimated incubation period of 171.47 days (nearly half a year), even slower than typical reptiles.

These findings may help explain why nonavian dinosaurs became extinct. In birds, a rapid incubation period compensates for limitations such as small clutch size and having only a single functioning ovary. The fast time to hatching helps birds avoid risks such as prolonged exposure to predation and environmental dangers, and allows time for migration. A much slower incubation period in nonavian dinosaurs could have impeded seasonal migration, which (combined with slow generation times and a higher exposure to predation) may have placed them at a disadvantage in competing for limited resources during the end of the Cretaceous, a period of rapid climatic change.

References:

• GM Erickson et al. 2017. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. Proceedings of the National Academy of Sciences 114:540–545. https://doi.org/10.1073/pnas.1613716114
• SW Carmichael. 2017. Incubation of dinosaur eggs determined by microscopy. Microscopy Today 25(3):8–11. https://doi.org/10.1017/S1551929517000402

Image

• Protoceratops in Djadokhta by PaleoNeolitic via Wikimedia Commons