A new fossil discovery in the Gobi Desert of Mongolia has revealed that some dinosaurs nested in colonies. 15 nests and 50 eggs, all similar in size and shape and remarkably well-preserved, were found coated in an identical distinct sediment layer indicating that they could be attributed to a single nesting season.
Paleontologists have suspected that dinosaurs had colonial nesting behavior for some time but they were unable to find any definitive support for the hypothesis. This site’s unique geology now provides all the evidence they needed. Even the discovery of numerous dinosaur nests within one rock formation could not completely conclude that the nests were laid in the same breeding season. It could only ascertain that it could have been a common nesting ground which the dinosaurs visited year after year.
A geologic unit of one layer of rock can actually represent hundreds to millions of years and can only constrain a fossil’s age within some orders of magnitude of time. Even carbon dating, isotopic dating and similar techniques can have a range of error of thousands to millions of years. Dating techniques that scope into geologic era are frequently best estimates, even though much tedious laboratory study and interpretation go into the measurements and conclusions of the results. Due to the inherent uncertainty in these data, it is not unusual in Paleontology and Geology for fossils and geologic formations to be re-examined, reorganized and reclassified according to fresh data and findings which might change the dates, lineages, and sequences. The new discovery site, with the juxtaposition of the eggs, nests, and an exclusive sediment layer binding them all together, provides the most definitive evidence to date for colonial nesting in the prehistoric dinosaurs.
In the July 5, 2019 edition of Geology, scientists from the University of Tsukuba in Japan published their findings that describe the undisturbed nesting site and the Upper Cretaceous aged Javkhlant Formation. A single marker bed, comprising of red-colored fine-grained sediment, conformably spreads over the surface of the older tan-colored bed beneath it. The thicker, more continuous tan bed held the nests and eggs. The red bed which was a few inches thick, coated and connected the nests and eggs within, providing clear evidence that the nests and clutches of eggs were laid in the same nesting season. This marker is construed to have been the result of a small local flood, which would, in turn, indicate that the nesting site was built near flowing water. The similar morphology of the nests and eggs strengthens the idea that these were created and laid by the same species of dinosaur.
The paleontologists conjectured that the hatching success rate of these non-avian theropod dinosaurs seemed similar to other species that protect their eggs, similar to modern crocodiles and many types of birds. The numbers they calculated, hatched versus un-hatched eggs in each nest, postulate the additional complex nesting behavior of protecting the nest. Not only did the dinosaurs nest in colonies, but the mothers also stayed and protected the eggs through hatching. This followed the assumption that the eggs that were found broken or opened were in that state because they had hatched and not because they were victims of predation, which is an alternative explanation.
This discovery is not the only indication that dinosaurs led to social and cooperative lives. Other conserved fossils from the same region exposed three adolescent dinosaurs roosting together at the time of their death. The fossil clearly depicts three dinosaurs in a unique position, similar to that of a sleeping goose or emu, with their abdomen on the ground on top of folded legs with their long necks and heads folded back on their bodies. It appears that the dinosaurs were all sleeping closely huddled together. Bats, crows, and similar birds roost together in groups for both protection and thermoregulation, but in Cretaceous dinosaur paleoecology, this is a new concept. The species of dinosaur corresponding to either of these discoveries have not yet been identified, but both are expected to be theropods.
These exciting revelations reinforce the theory that theropod dinosaurs exhibited complex social and reproductive behavior and help reconstruct the paleoecology and lifecycles of organisms living on Earth over 65 million years ago.