Ancient embryo fossils point at land animal origins
South American paleontologists report they have discovered fossilized embryos of the oldest aquatic reptiles, lagoon-dwelling “mesosaurs” that lived about 280 million years ago.
Long sought, the embryo finds may push back by 90 million years the fossil record of live birth and hatchlings from soft-shelled eggs in reptiles that were ancestral to birds, dinosaurs and mammals. And they point to the role that egg evolution played in the move of animals from the seas to land.
“An amazing discovery, more spectacular because of the quality of preservation,” says paleontologist Graciela Piñeiro of Uruguay’s Facultad de Ciencias, who led the team that found the fossil mesosaurs, reported in the current Historical Biology journal. Mesosaurs were elongated galoots, crocodile-like reptiles almost five feet long that preyed on shrimp-like crustaceans. The fossil finds include an “exquisitely preserved” mesosaur embryo discovered in Uruguay — half-a-foot long but seemingly still coiled in an egg — and what resembles another such embryo carried by a pregnant female mesosaur, found in Brazil.
The finds raise at least two possibilities about how mesosaurs gave birth in their salty lagoons. The egg-like embryo suggests that they crawled on marsh land to lay their eggs shortly before they hatched. On the other hand, the seemingly pregnant mesosaur found in Brazil raises the possibility that the creatures may even have given birth to live young, which “hatched” from thin membranes encircling the eggs inside the females.
“We have to go back to the conquest of land by four-legged animals to explain why this is so very important,” says paleontologist Martin Sander of Germany’s University of Bonn, who was not part of the discovery team. “A final step in this transition was when eggs evolved to be deposited on dry land.”
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