Making a Home in a Dinosaur Egg

From Smithsonian.com: Making a Home in a Dinosaur Egg

Dinosaur eggs were wonderful things. For the dinosaurs, reproducing by laying eggs may have played an important role in why many species reached enormous sizes. And for the animals that fed on them, dinosaur eggs were tasty packages of protein. Early last year, for example, researchers announced the discovery of a prehistoric snake that probably crushed sauropod eggs to reach the dinosaur embryos inside. Now paleontologists Jorge Genise and Laura Sarzetti have proposed that wasps may have made the most of dinosaur eggs, too.

The Cretaceous rock of Argentina has yielded many dinosaur eggs. The egg at the center of the new study was part of a clutch found in rock dating between about 77 million and 67 million years ago. There were five spherical eggs altogether, but one was special. Cracked in half, the fossil preserved eight cocoons inside. These were delicate structures—the sort that could not be transported without damaging or destroying the cocoons—and so it seems that the association between the egg and cocoons is real and not attributable to some accident of preservation. Invertebrates had been using this dinosaur egg, but what sort of creatures, and why?

As reconstructed by Genise and Sarzetti, the cocoon-containing egg was probably broken by some kind of force which did not affect the other eggs in the clutch. (If the egg had been crushed during burial in sediment, for example, the other eggs in the clutch would have been similarly broken, yet they were not.) Exactly what cracked the egg is unknown, but as the paleontologists point out, the egg would have filled in with sediment while still decaying. This turned the egg into a food source and place where insect scavengers could burrow into the soil filling the structure.

Exactly what species of insect the cocoons belonged to is unknown, but the structure of the preserved cocoons most closely resembles that of wasp cocoons. This finding helps flesh out the story of what happened to the egg after it was crushed. The location and orientation of the cocoons seems to fit a pattern for parasitoid wasps that track down spiders and crickets in their own burrows, immobilize them, and then lay eggs on them. If correct, this means that the wasps were relatively late arrivals at the rotten dinosaur egg—the wasps were there to take advantage of the other invertebrates that had come to feed on and burrow into the impromptu home. Still, even though they did not directly feed on the dead dinosaur egg, the wasps would have been part of a prehistoric cleaning crew—a temporary ecosystem whose existence we now know of thanks to the chance preservation of a special egg.

References:
GENISE, J., & SARZETTI, L. (2011). Fossil cocoons associated with a dinosaur egg from Patagonia, Argentina Palaeontology, 54 (4), 815-823 DOI: 10.1111/j.1475-4983.2011.01064.x