First Evidence That Dinosaurs Nested In Colonies: 15 Nests And 50 Eggs Discovered

Tuesday, September 10, 2019

Artistic reconstruction of colonial nesting site of therizinosaur dinosaurs from Mongolia by Masato Hattori

A new fossil discovery in the Gobi Desert of Mongolia shows that some dinosaurs nested in colonies. 15 nests and 50 eggs, all similar in size and shape and unusually well-preserved, were found coated in the same distinct sediment layer, meaning they could be attributed to a single nesting season.

Paleontologists have suspected colonial nesting behavior in dinosaurs for some time but were unable to find definitive support for the hypothesis until the site’s unique geology provided the evidence they needed. Even when numerous dinosaur nests have been discovered within one rock formation, it could not be concluded that the nests were laid in the same breeding season but only that it could have been a common nesting ground which the dinosaurs visited year after year. A geologic unit, one layer of rock, can represent hundreds to millions of years and can only constrain a fossil’s age within several orders of magnitude of time. Even carbon dating, isotopic dating and the like can have a range of error of thousands to millions of years. Dating techniques that reach into geologic time are often best-estimates, even though much tedious laboratory work and interpretation go into the measurements and conclusions of the results. Because of the inherent uncertainty in these data, it is not uncommon in Paleontology and Geology for fossils and geologic formations to be re-examined, reclassified and reorganized according to new data and findings, where the dates, lineages and sequences can change. This site, with the juxtaposition of the eggs, nests, and a unique sediment layer tying them all together, provides the most definitive evidence to date for colonial nesting in these prehistoric animals.

A dinosaur nest and egg clutch is fossilized in the Javkhlant Formation in Mongolia. The thin dark red sediment bed connects 15 nests.KOHEI TANAKA, TANAKA ET AL. (2019), UNIVERSITY OF TSUKUBA

Scientists from the University of Tsukuba in Tsukuba, Japan published their findings in Geology on July 5, 2019, describing the undisturbed nesting site and the Upper Cretaceous aged Javkhlant Formation. A single marker bed, consisting of red-colored fine-grained sediment, conformably overlies the older tan-colored bed beneath it. The thicker, more continuous tan bed contained the nests and eggs. The red bed, a few inches thick, coated and connected the nests and eggs therein, providing the evidence needed to conclude that the nests and clutches of eggs were laid in the same season. This marker is interpreted to have been the result of a small local flood, which also would then indicate that the nesting site was built near flowing water. The similar morphology of the nests and eggs supports the idea that these were constructed and laid by the same species of dinosaur. The paleontologists hypothesized that the hatching success rate of these non-avian theropod dinosaurs is similar to other species that protect their eggs, like modern crocodiles and many types of birds. The numbers they calculated, hatched versus un-hatched eggs in each nest, indicate the additional complex nesting behavior of protecting the nest. Not only did they nest in colonies, but they also stayed and protected the eggs through hatching. The assumption made here is that the eggs that were found broken or opened were that way because they had hatched and not been victims of predation, which is an alternative explanation.

This is not the only indication that dinosaurs led social and cooperative lives. Other fossils from the same area have revealed three adolescent dinosaurs roosting together at the time of their death. The fossil shows three dinosaurs in a unique position, much like that of a sleeping goose or emu, with their abdomen on the ground atop folded legs with their long necks and heads folded back on their bodies. It seems that the dinosaurs were all sleeping closely huddled together. Bats, crows, and other organisms roost together in groups for both protection and thermoregulation, but this is a new concept in Cretaceous dinosaur paleoecology. The species of dinosaur for either of these discoveries has not yet been identified, but both are suspected to be theropods. 

These exciting revelations support a theory of complex social and reproductive behavior of theropod dinosaurs, which helps reconstruct the paleoecology and lifecycles of organisms living on Earth over 65 million years ago.