Alvarezsaurs are Cretaceous mysteries. These small dinosaurs, a feathered subgroup of coelurosaurs, had long jaws studded with tiny teeth, and their arms were short, stout appendages that some researchers hypothesize were used to tear into anthills or termite mounds. But no one knows for sure. We understand very little about the biology of these dinosaurs, but even as we puzzle over their natural history, more previously unknown genera are being found. The latest is Bonapartenykus ultimus from the Late Cretaceous of Patagonia, and what makes this dinosaur so special is what was found with its bones.
Paleontologists Federico Agnolin, Jaime Powell, Fernando Novas and Martin Kundrát describe the new dinosaur in an in-press Cretaceous Research paper. The alvarezsaur was not in good shape when the researchers found it. While some of the bones, particularly those of the leg, were close to their original articulation, Bonapartenykus is represented by an incomplete set of partially damaged bones, without a skull. In life, the dinosaur is estimated to have been about eight and a half feet long. (Subtle characteristics of the preserved vertebra, shoulder girdle, and hips are what led Agnolin and co-authors to identify this animal as an alvarezsaur despite the paucity of bones.) But there was also something else. Next to the bones were the battered remnants of at least two dinosaur eggs. Could these be fossil evidence of a Bonapartenykus that was protecting its nest?
Determining who laid those eggs is a difficult task. No evidence of embryos has been found inside the egg, so we can’t entirely be sure of what kind of dinosaur was growing inside. The close association between the fossils is the primary line of evidence that the eggs might be attributable to Bonapartenykus. This is the hypothesis favored by Agnolin and co-authors, but they doubt that the small site represents parental care. There is no evidence of a nest. Instead the scientists suggest that the two eggs may still have been inside the dinosaur when it died—a hypothesis based on the previous discovery of an oviraptorosaur from China with a pair of eggs preserved where the dinosaur’s birth canal would have been. When the alvarezsaur perished, the eggs may have fallen out of the body and been preserved with the bones.
Yet I wonder if there might be alternative explanations. Just because fossils are found together does not necessarily mean that the organisms those fossils represent interacted in life. Making connections between organisms found at the same site requires a detailed understanding of taphonomy—what happened to those organisms from the time of death to discovery. In this case, the bones of Bonapartenykus are scattered and poorly preserved, and the eggs were also partially broken. Did the animal simply fall apart, as the authors seem to suggest, or were the bones and eggs brought together through rushing water? Perhaps the body of Bonapartenykus was carried by a water flow to the location of the eggs, fell apart after the water receded and then was buried again. This is a bit of armchair speculation on my part, and the hypothesis proposed by Agnolin and co-authors is a reasonable one, but we need a detailed understanding of how this little fossil pocket formed if we are to understand the relationship between the eggs and the bones. The geological and taphonomic details of the fossil site are important for framing hypothesis about what happened so many millions of years ago. We may have to wait for more intricately preserved fossils to be sure. A Bonapartenykus preserved on a nest, or a female dinosaur with eggs preserved within her hips, would do nicely.
Agnolin, F., Powell, J., Novas, F., & Kundrát, M. (2011). New alvarezsaurid (Dinosauria, Theropoda) from uppermost Cretaceous of north-western Patagonia with associated eggs Cretaceous Research DOI: 10.1016/j.cretres.2011.11.014