A pair of fossilized dinosaur nests offer some of the strongest indications yet that certain dinos brooded their eggs, re-used their nests, and possibly defended their nesting territory much like some modern birds, paleontologists say.
The two nests were found half a world apart, and separated by eons, but they bear some crucial similarities.
Both contain the fossilized remains of multiple egg clutches, apparently laid in close succession. And both belonged to the theropod dinosaurs known as troodontids — small, two-legged, meat-eaters thought by some to be among the most bird-like of extinct dinosaurs.
One of the fossil nests was found in the Liantoutang Formation of eastern China and dates back about 95 million years, while the other, from northwestern Montana, is 75 million years old.
In addition to reinforcing troodontids’ apparent similarity to birds, the new discoveries provide extremely rare insights into the dinosaurs’ actual behavior, including their habits as parents.
Dr. David Varricchio, a paleontologist at Montana State University, explained that the re-use of a site for breeding — known as site fidelity — has been glimpsed in the fossil record before, but it’s been difficult to tell whether different clutches of eggs found in other dinosaur nests were laid by the same individual, or whether they were used by different members of the same species thousands of years apart.
“I think [this discovery] is important, because it really moves away from [specimens] where the timing is very vague and the sites likely simply reflect members of the same species, to a very short time frame, and potentially the same individuals.
“So it’s site fidelity on a scale more similar to what biologists call site fidelity.”
The two nests were discovered by separate teams in 1994 and 2010, but soon began to show some striking similarities.
Based on the eggs’ shape, their microscopic structure, and comparisons with earlier specimens that had been found with fossilized embryos — and in one case, even an adult found still on top its nest — researchers surmised that the Montana eggs were laid by Troodon, and the Chinese specimens were from an unidentified but close troodontid relative. [Learn more about eggs and the bird-dinosaur connection: “‘Unusual’ Fossil Egg Reveals Evolutionary Link Between Dinosaurs and Birds“]
Moreover, both nests revealed two distinct layers of egg clutches: In the case of the Chinese nest, one clutch of 14 eggs appeared directly on top of at least 5 others that remained partially buried within the nest.
In the Montana nest, eight eggs sat closely on top of the shells of 11 previously hatched eggs.
“It is tough to say definitively, but the spacing is very close, and in the Chinese example, the uppers cross-cut the lower,” Varricchio said.
“Likely, exposed eggs would not last too long on the surface, so [the span between when both clutches were laid] needs to be in a relatively short amount of time.”
The Montana specimen is only the second theropod nest found in North America that contains multiple clutches of eggs, Varricchio noted.
The first, uncovered by Montana State’s Dr. Jack Horner in 1982, also displayed multiple egg clutches laid by a Troodon, but the layers were much farther apart in position, and therefore in time.
“Horner had nests separated by a meter of sediment, likely a long time,” he said.
The clutches in the newly found specimen, by contrast, are only 3 to 5 centimeters apart.
“So, these are of a much finer temporal and individual scale,” he said.
Together, Varricchio noted, these “two remarkable troodontid specimens” are evidence of “site-fidelity approaching how biologists use the term — an individual returning to the same location.”
While it may never be possible to get direct observations into the nature of dinosaur behaviors like nesting, discoveries like the two newfound nests are “probably as close as one can get at this time.” [See another striking new dinosaur find: “Dinosaur Blood Cells, Soft Tissue Discovered in 75-Million-Year-Old Fossils“]
“Potentially, they imply Troodon, and other troodontids, were tied to their nesting localities and that these were important,” he added.
And if modern birds are any indication, the fact that these ancient animals brooded and re-used their nests may mean that they also behaved similarly, such as in staking out territories and protecting their eggs. [More insights into dinosaur families: “Baby and Adult Dinosaur Tracks in Alaska Prove Duck-Bills Lived in ‘Social Herds,’ Study Says“]
“Dinosaurs like Troodon were invested in egg laying and care,” he said.
“Consequently, they likely were tied to a specific spot for some time each year. This may imply they were territorial and defensive of this area.”
Varricchio and his colleagues report their finds in the Journal of Vertebrate Paleontology.
Varricchio, D., Jin, X., & Jackson, F. (2015). Lay, brood, repeat: nest reuse and site fidelity in ecologic time for two Cretaceous troodontid dinosaurs Journal of Vertebrate Paleontology, 35 (3) DOI: 10.1080/02724634.2014.932797