The first dinosaurs laid soft eggs and it was only later that some groups evolved eggs with hard shells, according to new research. The finding overturns a long-standing assumption that dinosaurs always laid hard-shelled eggs like modern birds.
Palaeontologists have struggled to find eggs from certain dinosaur groups, says Mark Norell at the American Museum of Natural History in New York. If dinosaurs had always laid hard-shelled eggs, they would all be equally easy to find, as soft-shelled eggs barely fossilise, says Norell.
In Mongolia, Norell’s and his team found a clutch of embryos that they believe belonged to a type of dinosaur called Protoceratops that lived between 83 and 72 million years ago. When they found the fossils, “they were in the foetal position, all curled up”, says Norell. Each was surrounded by a thin film.
The team also examined preserved embryos of Mussaurus, a type of early dinosaur that lived around 200 million years ago. These were also surrounded by a thin outer layer.
Team member Jasmina Wiemann at Yale University found that the films around the embryos contained the degraded remains of egg proteins.
Wiemann then examined 26 kinds of egg from extinct and living animals and found that hard-shelled and soft-shelled eggs had different kinds of proteins that left traces after they were fossilised. When she analysed the Protoceratops and Mussaurus samples, “both of them matched the soft-shelled eggs”, she says.
To find out whether the earliest dinosaur eggs were soft or hard-shelled, team member Matteo Fabbri, also at Yale University, compiled a database of information about eggs from 112 living and extinct reptiles and birds whose evolutionary relationships are known. It turned out that the first members of many groups, including lizards and dinosaurs, laid soft-shelled eggs.
The first dinosaurs eventually gave rise to three major groups: the sauropodomorphs, which included Mussaurus; the ornithischians, which included Protoceratops; and the theropods, which included large meat-eaters like Tyrannosaurus rex and birds. “Each branch of dinosaurs evolved, independently, a hard eggshell,” says Fabbri.
The first dinosaurs were more reptilian in their physiology and in their behaviour. They probably buried their soft-shelled eggs to keep them warm and then later abandoned them. “Dinosaurs were, to a certain extent, less bird-like than we thought,” says Wiemann.
Journal reference: Nature, DOI: 10.1038/s41586-020-2412-8
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