New Study Suggests Egg Shape Might Hint at Clues to Survival

Eggs come in many different shapes and sizes.  There are large ones, small ones, those that are more rounded, others that can be more ovoid in shape and so on.  However, a new study, conducted by evolutionary biologists at Lincoln University (UK), suggests that egg shape could have been a factor in why some birds survived the Cretaceous extinction event, whilst other types of bird and the Dinosauria did not.

New Research

The research published in the on line journal of the Royal Society, looks at the geometry of eggshells and highlights morphological differences between the eggs of birds and those of their extinct, but very close relatives, the theropod dinosaurs.

Birds, Reptiles and Mammals are linked as all these types of creature are descended from Carboniferous tetrapods that evolved an ability to reproduce from an egg that was contained within a semi-permeable eggshell.  These early terrestrial animals were no longer dependent on the presence of water in order to breed and reproduce successfully.  These types of eggs are called amniotic eggs.

A Diagram Showing the Structure of an Amniotic Egg

The growing embryo is protected by a semi-permeable egg shell.

Picture credit: Everything Dinosaur

Amniote Eggs

Safe from drying out, the embryo inside the egg is further protected by a robust, internal membrane called the amnion.  It is the evolution of the amniote egg that permitted tetrapods to conquer all terrestrial environments.

In this new study, the research team noted that there were notable differences between the eggs of birds that survived the Cretaceous mass extinction event that took place around sixty-five million years ago, and the shape of the eggs of those creatures that become extinct.  Although, the fossil record is far from complete when it comes to preserving evidence of eggs and reproductive strategies, the results suggest that early birds from the Mesozoic laid eggs that had different shapes to those of modern birds.

It is possible that egg morphology indicates different physiologies or different rates of embryonic development and this may have implications when it comes to surviving a mass extinction event, such as that which led to demise of around 70% of all terrestrial life, including all the non-avian dinosaurs.

Could Theropod Egg Shape Have Doomed the Dinosauria?

An Oviraptor (theropod) dinosaur sitting on her nest.

Picture credit: Everything Dinosaur

Egg Shape

One of the authors of this new paper, Dr Charles Deeming (School of Life Sciences, Lincoln University) explained:

“These results indicate that egg shape can be used to distinguish between different types of egg-laying vertebrates.  More importantly they suggest Mesozoic bird eggs differ significantly from modern day bird eggs, but more recently extinct Cenozoic birds do not.  This suggests that the range of egg shapes in modern birds had already been attained in the Cenozoic.”

Comparing Extinct and Extant Egg Shapes

As extant amniotic eggs vary considerably in size and shape and this variety reflects different patterns of egg formation and development, then the variation seen in the fossil record of eggs may also reflect different patterns of egg formation, egg development and even nesting behaviour.

Dr Deeming commented:

“From a biological perspective, it is self-evident that different egg shapes by birds, both past and present, might be associated with different nesting behaviours or incubation methods.  However, hardly any research has been carried out on this topic and fossil data are insufficient to draw firm conclusions.  We hope that future discoveries of associated fossil eggs and skeletons will help refine the general conclusions of this work.”

Other Factors to Consider

Although there might be a link between eggshell shape and the ability to survive the Cretaceous mass extinction, it is likely that a lot of other factors contributed to the survival of one group of vertebrates whilst others died out.  The eggshell shape itself may be a part of the story, but palaeontologists are confident that dinosaurs, including many theropod dinosaurs engaged in complex nesting behaviours, brooded eggs on nests and invested a great deal of time and effort in raising the next generation.

A spokesperson from Everything Dinosaur stated:

“Although the fossil record for eggs and nesting sites is extremely fragmentary, there is evidence to suggest that members of the Dinosauria exhibited altricial and precocial behaviours.  How one group of birds, the Neornithines were able to dominate the Aves remains uncertain, more research in this area is needed.  However, this data adds a fresh perspective and it is certainly intriguing.” 

Dr Deeming advised that this new paper does not provide all the answers, but it hints at the tantalising possibility that eggshell morphology could have been an contributory factor in the extinction of the dinosaurs.  Dr Deeming and this paper’s co-author Dr Marcello Ruta (Lincoln University), are continuing their investigations.  The scientists intend to explore how highly variable amounts of yolk (food for the embryo) and albumen (egg white) could possibly effect egg shape.

For models and replicas of prehistoric animals including dinosaurs: Dinosaur Models and Prehistoric Animal Figures.