Fossilized Eggshells Provide Insight into the Evolution of Nesting

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The nests of most dinosaurs, including duck-billed hadrosaurs, consisted of eggs covered under mounds of vegetation and dirt. The vegetation mound is not represented in this illustration to display the eggs. (Art by Julius T. Csotonyi.)

Very little is known about the types of nests built by dinosaurs because nest structures and nesting materials do not usually fossilize. Yet what we do know about dinosaur nesting style can suggest information about the nesting behaviours among archosaurs (a group that includes crocodilians, birds, and dinosaurs), particularly associated with the origin of birds.

In an attempt to resolve this mystery, Kohei Tanaka and Darla Zelenitsky from the University of Calgary, and François Therrien from the Royal Tyrrell Museum of Palaeontology investigated the eggs of over 120 species of living birds and crocodilians. They discovered that eggshell porosity is strongly correlated with nest types (“nest type” refers to either covered nests, in which eggs are fully covered under mounds of vegetation and dirt, or open nests where eggs are left exposed in the nest and brooded by adults), and can therefore be used as a proxy to infer the nesting style of dinosaurs.

The researchers examined the eggs of various dinosaur groups and show, in a study published in the open-access journal PLoS ONE, that most dinosaurs, including the long-necked sauropods, primitive meat-eating theropods, and possibly the plant-eating ornithischians, buried their eggs in mounds of vegetation, like modern crocodiles do. It was only among advanced theropods, those most closely related to living birds (i.e., the maniraptorans), that the transition to open nests occurred.

Interestingly, early open-nesters, such as oviraptorids, Troodon, and enanthiornithine birds (toothed Cretaceous birds), still partly buried their eggs in the ground; it is not until modern-looking birds (i.e., euornithine birds) evolved that eggs were left fully exposed in open nests. The evolution of open nests, together with brooding behaviour, may have allowed advanced theropods to exploit nesting locations other than the ground, potentially lessening the odds of nesting failure due to predation and flooding, which may have played a role in the evolutionary success of birds.

The article was published in the open-access journal PLoS ONE, which means that it can be downloaded freely at the following link: Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs.

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The nests of advanced meat-eating dinosaurs (i.e., theropods) were open with the eggs partly exposed so a brooding parent could keep them warm. (Art by Julius T. Csotonyi.)

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