Dinosaur embryos reveal remarkable secrets of life – and extinction

A new study shows many dinosaurs may have taken many months to hatch from their eggs, leaving them vulnerable to sudden environmental changes

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.