Cut a tree trunk in half and you can count the rings to reveal the age. Slower growth in winter (when conditions are poor) means a thin and relatively dark line appears and this marks the end of a season. You might be surprised to know that we can do something similar with the bones of many animals including dinosaurs. Cut a big bone from the thigh in half and there are similar rings to count, laid down for the same reason (growth slowing) and that can also, give or take, mark down the number of years the animal has been alive. This is all well and good, but of little help when the owner is less than a year old, and clearly impossible to apply to embryos. Incredibly however, there is an even more specific and detailed measure for single days that is laid down in the teeth.

These tiny indicators are called Von Ebner lines and they actually reflect daily growth and changes in mineralisation of teeth as they develop. We can see these in modern reptiles like crocodiles but also in dinosaurs. Although very rare, we do have fossil embryos of a number of dinosaurs and a new study by Erickson et al., has cut into the tiny teeth of these specimens and looked at the Von Ebner lines to count the number of days that they were in the egg (coupled with an estimate of when teeth first start growing) with some remarkable findings.



Close-ups of sectioned embryo teeth showing the daily von Ebner lines Photograph: Erickson, G.M. et al.

It seems that dinosaur embryos spend a long time in the egg, in fact a very long time. The two dinosaur species assessed took between three and six months to develop and hatch, comparable to the largest reptiles and far longer than birds (which range from 11 to 85 days in the eggs). Comparing like-for-like based on the sizes of the eggs, dinosaurs took about four times longer to develop than do similarly sized birds, a huge difference. That has some major implications for dinosaur behaviour and ecology. We know adult dinosaurs often cared for their hatchlings, but would parents guard nests for months at a time beforehand? Various dinosaurs are thought to migrate large distances each year to forage in new areas but could they do this if their eggs needed attention for so long?

Critically the authors suggest this might have implications for the extinction of the non-avian dinosaurs (and some of the early birds) at the end of the Cretaceous. With long development times in the nest, dinosaurs would be vulnerable to the kinds of major climate change associated with the mass extinction event. Any bad weather or unusual period of drought, cold, rain etc. over the half a year the animals were developing might be enough to wipe out a whole nest or entire breeding grounds. This would also be true of various other reptiles, but smaller species would at least have had smaller eggs and faster development, but not so the dinosaurs which were typically very large.

This would of course be likely one of just many factors that influenced dinosaur extinction, but it’s certainly a strong possibility that this would have been a major issue. This opens up new ideas about dinosaur growth and development as well as bringing a new factor to the table that must have influenced their behaviour. This is early days for this data but it’s an important next step in our understanding of the life, and death, of the dinosaurs.

References

Erickson, G.M., Zelenitsky, D.A., Kay, D.I., Norell, M.A. 2017. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. PNAS – in press.

