Luis Chiappe and Lowell Dingus organised two expeditions to the badlands of Auca Mahuevo, Patagonia in 1997 and 1999. Their original aim was to look for fossilised birds, and attempt to fill gaps in the fossil record — their intention was to prove that birds should be reclassified as dinosaurs, according to the rules of cladistics. On the second day of prospecting in the rocky Southern Argentinian desert, the crew of palaeontologists and geologists realised that the unusual fragments of rock on the surface of the ground were fossilised eggshells. They quickly realised they had walked into a fossilised hatchery — indeed, many clusters of complete eggs were visible now they knew what they were looking at. It wasn't until they returned to the hatchery in 1999 that they realised its full size — over two square miles of nests 5ft-10ft apart, each containing 15-30 eggs.

The Cretaceous Environment

They had stumbled across the nesting grounds of a species of Titanosaur. Although not the largest Titanosaurs, these sauropods (large quadraped herbivores, including Apatosaurus and Diplodocus) still reached lengths of about 40ft. The members of the crew specialising in environmental geology and palaeontology recognised that the land on which the eggs were laid had been part of a plain, cut through with rivers and streams. There were more eggs than the team could count, and many more egg finds which, due to time restrictions, simply couldn't be studied. It is uncertain whether the sauropods turned up en masse, or rolled up one by one to lay. Either way, their numbers must have been staggering.

Embryos

Embryos were found inside the eggs, along with embryonic skin, complete with skin patterns, which could be compared with known sauropod skins in an effort to identify precisely which species had laid these eggs. The preservation of the embryos was so good that the palaeontologists could make out wear patterns on their teeth, suggesting that they started flexing their jaw muscles in preparation for a lifetime's worth of eating before they even hatched.

Predators

Another interesting find was the nearly complete fossilised skeleton of Aucasaurus garrodi, a completely new species of Abelisaur, with striking bumps projecting from above the eyes (similar to Carnotaurus). Chiappe and Dingus speculate that one of these 20-foot-long therapods (biped carnivores, including raptors and Allosaurus) could quite happily bring down a young sauropod. They further speculate that a small group or pride of Aucosaurus would be sufficient to bring down an adult sauropod — a chilling thought. Another chilling thought was the discovery of the hipbone of another carnivorous therapod larger than Giganotosaurus¹ (a therapod believed to be larger than Tyrannosaurus rex).

Parental Control

There are surprisingly few broken eggs. The shells described in the introduction of this entry were broken by being weathered out of the rock. The team took this as a sign that after the sauropods had laid, they retired to a suitable distance, or disappeared entirely. Another idea is that most of the sauropods left the site, leaving a few adults to patrol the perimeter. Evidence of adults was left in an unusual fashion. One of the palaeontologists noticed bands of white rock around the site. After some considered investigation, they decided to excavate the overburden² to discover what looked like a series of white plates laid out in a row. These turned out to be the shallow footprints of adult sauropods. The explanation was that the footprints in the soft mud were filled by rainwater, which was then evaporated out, leaving a white mineral coating to the footprint. This process occurred several times, until a mineral 'bowl' was formed.

The End?

The event that brought the demise of the nesting site was devastating, but not terribly spectacular. During rainy seasons, the rivers that cut through the plain carried a lot of silt — during one season, a bank burst, flooding the plain with masses of silt-bearing water (albeit slowly, otherwise the eggs would have washed away or been disturbed), which settled out and covered the eggs. This appears to have happened more than once over time, as there are several layers of rock which contain eggs — the highest layer being over 75 feet above the lowest. This would suggest that the nesting site was in use for a long, long time. Some of the nests were dug out of a sandy substrate — a dried stream or riverbed. This led to some excellent preservation, and unrivalled casts of sauropod nests were taken.

Today the land has been purchased by the government of the province of Neuquen and given a protected status. This was prompted in part by the discovery of modern-day oviraptors (egg thieves). They had found the site after the publication of the story and gone in with heavy gear to collect nests and eggs. In one instance, one of the perpetrators left a receipt with his name on it — it was handed in to the local police, but alas, this Researcher can find out no more about the story. Although most of the eggs are safe and have survived, there is a growing need for protection. 'Corporate art' buyers are fond of fossils, and have been known to buy from suspect sources. For further reading, the field notes and photos of the expedition can be found at the Natural History Museum (USA).

The National Geographic ran an article on the eggs, and there is also a transcript available of an interview with Luis Chiappe.