Fossil eggshells believed to be from some of the biggest dinosaurs that ever lived have been found in a surprisingly cool climate far from the equator.
But those massive titanosaurs couldn’t keep their volleyball-sized eggs warm by sitting on them.
“They’re too big,” said University of Calgary paleontologist Darla Zelenitsky, who has been studying the problem. “The nest would be one big omelette.”
A new study published on Wednesday by Zelenitsky and collaborators from Japan and Argentina has taken a microscopic look and figured out what they did to keep the eggs warm enough to hatch without sitting on their nests.
By looking at the microscopic pores on the eggshells, the researchers believe titanosaurs used the same incubation technique used by modern crocodiles — burying their eggs in warm piles of rotting vegetation, reports the study led by University of Tsukuba paleontologist Kohei Tanaka, Zelenitsky’s former student, and published in Royal Society Open Science.
The world of the giants
Titanosaurs were a group of gargantuan long-necked, long-tailed, plant-eating dinosaurs that lumbered on four legs and weighed up to 75 tonnes, or about half the mass of a blue whale.
Paleontologists unveil one of largest dinosaurs that ever lived.
Volleyball-sized fossil eggs called Fusioolithus have been linked to titanosaurs, and previously found by the dozens in warmer climates.
Then in 2020 and 2024, fragments of Fusioolithus eggshells were found in the Chorillo Formation at the southern tip of Argentina. The rock formation dates back to the Late Cretaceous, 72 to 66 million years ago, just before the dinosaurs went extinct.
Its exact position has changed a little since then, but at the time, its latitude would have been as far from the equator as Red Deer, Alta., but with a climate similar to that of New York City today, Zelenitsky said.
Fossils found there reveal a lush coastal plain ecosystem where roaming dinosaurs included the titanosaur Nullotitan glaciaris, up to 25 metres long, along with the smaller, two-legged herbivore Isasicursor santacrucensis, tyrannosaur-like megaraptors and small birds. They would have lived among crocodiles, snakes, turtles, frogs, fish, mammals, snails and insects.
Still, Zelenitsky was surprised by the discovery of eggs of titanosaurs so far from the equator: “It’s just a bit more tricky with larger animals because you need more resources to survive … and reproduce.”
3 ways to incubate an egg
The problem is that dinosaurs needed to keep their eggs warm in order for their embryos to develop and hatch – for up to many months, Zelenitsky said.
Smaller, feathered dinosaurs were thought to sit on their eggs like most modern birds and use their body heat. That would have been impossible for huge titanosaurs to do without crushing their relatively tiny eggs.
But there are other options.
Sea turtles bury their eggs on sandy tropical beaches to be warmed by the sun – something that wouldn’t work in cool, southern Argentina.
Modern crocodiles and birds called megapodes bury their eggs in decomposing plant matter. Microbes generate heat as they break it down.
Zelenitsky had previously found evidence that hadrosaurs in Alberta used this technique. So the researchers wondered if titanosaurs in Argentina did, too.
Eggshells have microscopic pores so the embryos can breathe. Those need to be bigger and denser to let enough oxygen through when eggs are buried. Eggs need to be less porous if exposed to open air to prevent the egg from drying out.
The researchers measured the porosity of the titanosaur eggshells from the Chorillo Formation using an electron microscope, and found they were very porous, suggesting they were buried.
Zelenitsky said the research shows just how adaptable dinosaurs were. “Even these absolute largest dinosaurs, you know, figured out a way to … not only reproduce or nest in the lower latitude environments but also in these higher-latitude environments, which are usually more challenging.”
Tsu-Ruei Yang, associate curator of vertebrate paleontology at Taiwan’s National Museum of Natural Science, published a study earlier this year suggesting feathered dinosaurs called oviraptors used both sunlight and body heat to incubate their eggs.
In an email, he said he wasn’t surprised that titanosaurs were nesting so far from the equator, as they were a very successful group of dinosaurs that spread around the world.
He noted that titanosaur eggs in places like Spain have been found with thinner, less porous eggshells, indicating more open nests.
“I would be curious if their nesting strategies were as diverse back [earlier] in the Cretaceous.” He wondered if that kind of adaptation might be behind their ability to spread across the globe during the Late Cretaceous, and suggested a more comprehensive study of titanosaur nesting might provide the answer.
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