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 currentHistorical 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."

Amphibians and fish have simpler eggs than reptiles, birds and mammals. The latter groups have a more complex egg with a yolk and an amniotic sac (hence the name "amniotes" encompassing all of those groups) that led to tougher, bigger embryos and young among land-dwelling creatures. Eventually these eggs evolved into the hard-shelled ones of birds and some reptiles, as well as the complex embryos of mammals. (Hard-shelled calcified eggs don't appear in fossils until about 210 million years ago, for anyone wondering about the whole chicken vs. egg question.) For that reason, fossil hunters have long sought these early eggs.

"Since we are paleontologists, we like to actually have the fossils," Sander says. "Now we have them," he says despite long-standing fears that the parchment-like outer membranes of the earliest amniote eggs wouldn't survive fossilization.

The search has gone on for awhile. In 1939, the late paleontologist Alfred Sherwood Romer, who founded the Society of Vertebrate Paleontology and wrote some of the field's first textbooks, reported discovery of "The Oldest Vertebrate Egg" (vertebrate means back-boned) from a fossil site in Texas in the American Journal of Science. The claim stood until 1979, when a second look at the fossil egg suggested it actually lacked the features of an amniote egg.