Dinosaur Eggs and Reproduction

The fossil record and the close link between dinosaurs and birds allow us to infer that most if not all dinosaurs laid eggs. Eggs have been discovered for the three large dinosaur subgroups (theropods, sauropodomorphs, and ornithiscians).  Dinosaur eggs would have served to protect the embryo and provide calcium, like the eggs other living animals. These eggs were almost always hard-shelled, comprised of one or more layers of calcite. Fossilized calcitic egg fragments from the Jurassic and Cretaceous periods were quite prevalent, but are rare in Triassic sediments. Hence, it is possible that Triassic dinosaurs did not lay hard-shelled eggs. The lack of hard-shelled eggs found in Triassic sediment may also be from a preservational basis.

Dinosaurs laid a variety of different eggs that ranged from spherical to elliptical and could be asymmetrical, like their avian counterparts. Fossil remains indicate that sauropods produced spherical eggs, while theropods produced elliptical eggs that were asymmetrical. The eggs could vary from size, but none have been found to be more than 10 liters in volume. Furthermore, there doesn’t seem to be a relationship between egg size and the eventual size of the adult dinosaur.

File:Dinosaur eggs - Kunming Natural History Museum of Zoology - DSC02392.JPG

Dinosaur Eggs from the Kunming Natural History Museum of Zoology (昆明动物博物馆)

Dinosaur Nests

Usually eggs are found isolated and fragmented, but there have been discoveries of well-preserved sites that have dinosaur nests, which can come in an array of shapes and sizes with varying number of eggs. Theropods seem to have built mostly circular nests and laid eggs in the center of the nest. The eggs were usually partially buried to offer stability and kept the circular nature of the nest. The number of eggs that a theropod nest would contain depended on the species. For instance, an Oviraptor nest had 20-36 eggs and the nest of a Troodon had about 12-24 eggs.

There have been findings that indicate that some nests appear with other nests. These sites have been found stacked vertically as well in several vertical stratigraphic sections that point to hundreds or thousands of years of nesting activity.

Parental Care

Given that birds show some degrees of parental care, it is interesting to consider the nature of parental care of dinosaurs. There has been evidence of well-constructed nests. The nests of troodontids are often surrounded by a rim of sediment to protect against flooding and predation. Many discoveries have also demonstrated that theropods sat on their nests in order to incubate their eggs. There is no fossil evidence that hadrosaurs or sauropods did the same, which would have been difficult due to their enormous sizes. Alternatively, passive incubation methods may have been used by burial in sediment or covering the nest with vegetation. There has not been many findings of fossilized plants associated with dinosaur nests, so it is more likely that passive incubation occurred via burial in sediment. Furthermore, there is evidence of parents caring for their offsprings after they hatch. The fossil record has evidence of adult dinosaurs preserved with juveniles, but active parental care cannot be inferred from this alone.

Works Cited

Brusatte, Stephen. Dinosaur Paleobiology. Chichester, West Sussex: Wiley-Blackwell, 2012. Print.

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