An intimate view of a Cretaceous hatchery shows that some dinosaurs liked their nesting sites steam-heated — by geothermal vents.
A paper in Nature Communications today1 says that certain dinosaurs regularly returned to geothermal fields to shape nests and deposit eggs more than 100 million years ago.
Palaeontologists found the nesting site in Sanagasta Valley in La Rioja province, northwestern Argentina. It is the first to definitively show that some neosauropods — the group that includes Chubutisaurus insignis — had specific nesting grounds, as many migratory birds do today.
"Before this, we had absolutely no information about the selection and environment of dinosaur nests," says Romain Amiot, a palaeontologist at the University of Lyon in France who was not involved in the research. "It gives us a better understanding of dinosaur biology."
Last summer, authors Gerald Grellet-Tinner, an associate researcher at the Field Museum in Chicago, Illinois, and doctoral student Lucas Fiorelli, of the Regional Research Center for Scientific Investigation and Technology Transfer (CRILAR) in La Rioja, mapped 80 clutches of eggs found at Sanagasta to within 3 metres of geothermal conduits. Geochemical analysis of sedimentary minerals in the eggs allowed the team to estimate an incubation period of 1–2 months at 60–100 °C.
The concentration of elements in the eggshells and associated sediments matched those present during the Gondwanic hydrothermic cycle — a particular period of geothermal venting that began 134 million years ago, and is thought to have ended by 110 million years ago.
The team found that the eggs were large and thick-shelled when freshly laid, an adaptation that may have evolved to allow them to safely incubate in the acidic environment near geyser fountains. The eggs were up to 21 centimetres in diameter, and the shell thickness ranged from 1.29 to 7.54 millimetres, suggesting that they got thinner with time.
"We think the acid in the moisture from the geothermal flow leached into the eggs during incubation, eventually making them thinner for the babies to break out," says Grellet-Tinner.
Most of the nests contained 3–12 eggs, but some held up to 35, spread over up 2 square metres. The eggs were stacked in two rows, with more in the top tier.
The substantial numbers of eggs found in some nests might show that the dinosaurs were boosting chances of species survival, says Grellet-Tinner — just as modern crocodiles do when they heap their eggs in nests of mud and plants incubated by sunlight. However, given that geological shifts could cut off the steam, the sites were also highly vulnerable.
No fossilized dinosaur bones or embryos have been located at the new site. Researchers are still looking for fossils to conclusively identify the species that left the nests.
Today, a type of endangered turkey called the megapode (Megapodius prichardii) nests in similarly volcanically heated burrows on Niuafo'ou island, Tonga.
Dinosaur eggs have been found on almost every continent. Argentina has one of the richest egg records, and some have even been discovered with fossilized embryos inside; the Auca Mahuevo site in Neuquén province south of La Rioja has provided a particularly large number. Auca Mahuevo eggs — which range from 12–14 centimetres in diameter — have been shown to be those of the neosauropod titanosaurus, which could reach 20 metres in length.
The newly discovered Sanagasta site consists of a 300,000-m2 field of geothermal conduits atop granite terraces. The geyser system is similar to that at Yellowstone National Park, Wyoming, the authors say.
"This is an amazing find, revealing an unsuspected and very specific nesting behaviour," says Diego Pol, a palaeontologist at Egidio Feruglio Palaeontological Museum in Trelew, Argentina. "The eggs most likely belong to sauropods, the most common herbivorous animals in the Cretaceous in South America. Titanosaurs were the only sauropods there that survived late into the Cretaceous," he says.
Once the species is pinned down, the many artists who specialize in depictions of dinosaurs will have a field day creating reproductions of 'rookeries' resting on granite slabs heated by spewing geysers.
1. Grellet-Tinner, G. & Fiorelli, L. E. Nat. Commun. doi:10.1038/ncomms1031 (2010)