Giant dinosaurs may have kept their eggs warm using ‘compost’: what 255 shell fragments have revealed

Titanosaurs, which lived in the south of present-day Argentina at the end of the Cretaceous period, probably did not leave their eggs exposed. An analysis of 255 shell fragments from the Chorriillo Formation has shown that the eggs were most likely completely buried in the moist nesting material.

According to the authors, the most likely source of heat was the warmth generated by decomposing vegetation. In essence, this mechanism resembles a modern compost heap. However, this is merely a reconstruction: no intact eggs or preserved nests have yet been found at the site.

Details

A team led by Kohei Tanaka of the University of Tsukuba studied eggshells collected during expeditions in 2020 and 2024 in southern Patagonia. The deposits date back approximately 69–66 million years, to the late Maastrichtian, that is, the very end of the Age of Dinosaurs.

At the time these deposits were formed, the region was located at approximately 55–60° south latitude. For the Southern Hemisphere, this is one of the most high-latitude, reliable pieces of evidence for dinosaur reproduction.

Based on their microstructure, all the fragments examined have been assigned to the oogenus Fusioolithus. Such eggs are associated with titanosaurs—a group of giant, long-necked herbivorous dinosaurs. However, no direct titanosaur embryo has been found inside these particular eggs; therefore, their attribution is based on the structure of the shell and comparison with related types of fossilised eggs.

The main clue is the shell’s very high porosity. In modern archosaurs, this type of gas exchange is characteristic of eggs that are completely covered by a moist substrate. An exposed egg with such a porous shell would lose water far too quickly.

The authors considered three possible sources of heat for the buried nest: geothermal heat, solar heating and heat from the decomposition of organic matter. No signs of geothermal activity were found in the rocks, whilst solar heating was considered less likely for a cool, high-latitude environment. Therefore, the most plausible explanation was the heat released during the microbial decomposition of plants and other organic matter in the nest.

Why this is important

The discovery broadens our understanding of where titanosaurs may have bred. Previously, most of their known nesting sites in the Southern Hemisphere were significantly closer to the equator. The new data show that these giant animals were able to breed successfully even in cooler latitudes.

The study also shows that the size of an adult animal did not necessarily determine the method of incubation. Instead of sitting on the eggs directly, these giants may have utilised the properties of the nest itself — the moisture and heat from decomposing vegetation.

That said, the conclusion regarding the specific source of heat remains a hypothesis. The authors explicitly note that there are too few preserved Fusioolithus nests to statistically determine the type of heating. Nor are there any intact eggs or nests from Chorrillo; therefore, a future discovery of such a site could either confirm or refute this reconstruction.

Source

Kohei Tanaka, Fernando E. Novas, Makoto Manabe, Chisako Sakata, Jordi A. Garcia Marsà, Darla K. Zelenitsky. Titanosaur eggshells from the latest Cretaceous of Patagonia, Argentina: implications for nesting at high latitude. Royal Society Open Science, 2026. DOI: 10.1098/rsos.260333.

Official press release from the University of Tsukuba and the National Museum of Nature and Science: University of Tsukuba.