Many dinos liked their nesting sites steam-heated

Some dinosaurs favoured geothermal fields to make nests and lay their eggs, the discovery of a Cretaceous hatchery has revealed.

Palaeontologists identified the nesting site in Sanagasta Valley in La Rioja province, northwestern Argentina. It is the first to conclusively show that some neosauropods - the group that includes Chubutisaurus insignis - had specific nesting grounds just like today's migratory birds.

The finding is reported in Nature Communications.

"Before this, we had absolutely no information about the selection and environment of dinosaur nests. It gives us a better understanding of dinosaur biology," Nature quoted Romain Amiot, a palaeontologist at the University of Lyon in France, who was not involved in the research, as saying.

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 period of geothermal venting that began 134 million years ago, and is believed to have ended by 110 million years ago.

The researchers 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.

Grellet-Tinner said: "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."
 
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, said Grellet-Tinner. 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.

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, according to the team.

Diego Pol, a palaeontologist at Egidio Feruglio Palaeontological Museum in Trelew, Argentina, said: "This is an amazing find, revealing an unsuspected and very specific nesting behaviour. 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."