Dinosaur Eggs

University of Bristol

Evolution of dinosaur egg laying

Dinosaur eggs and nests are rare before the Cretaceous. This could mean one of two things:
  1. There were fewer eggs before the Cretaceous.

  2. The eggs had a less chance of making it into the fossil record before the Cretaceous.
The first possibility is unlikely. There is no evidence that pre-Cretaceous dinosaurs laid fewer eggs, or that they were overall rarer, than their Cretaceous counterparts. The second possibility is more likely. It has been suggested (Sochava 1969) that pre-Cretaceous eggs were pliable and non-calcareous. Sochava suggested the calcareous dinosaur egg developed in response to aridity in the Cretaceous, became harder and more likely to fossilize.

In the Cretaceous the eggs also appear ornamented. Ornamentation can be broadly divided into two types: longitudinal ridges seen in vertically or sub-vertically arranged eggs or multi-directional or nodular ornamentation seen in randomnly laid eggs (Moratalla 1994). Ornamentation on the eggs allowed them to be buried after laying; the ornamentation allowed the flow of air between the ground and the egg. The ability to bury the eggs may have been an advance in the behaviour of dinosaurs, and it may have contributed to their improved preservation in the Cretaceous.

Brooding, even nest spacing, colonial nesting, parental care, and other apparent behavioural advances are seen in Late Cretaceous nests and eggs, but not before. Is this evidence for advance in complexity of dinosaurian behaviour? Oviraptor arranged its eggs vertically into concentric spirals. Troodon arranged its eggs tightly together almost vertically or into pairs. It could be at this stage of the Late Cretaceous that eggs had evolved albumen chords, or chalazae, which would have allowed the eggs to remain upright during any movement (Terres 1995; Varricchio 1997).

We have absolutely no idea of dinosaur behaviour before the Cretaceous and would be wrong to assume that they simply did not bury their eggs.

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