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Which Came First? The Reptile or the Egg?
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A research team headed by Graciela Pineiro of the Facultad de Ciencias in Uruguay has recently published a study that further unravels the intricate story of how reptilian life developed. The team’s study focused on two fossil discoveries, one in Brazil and the other in Uruguay. Both are mesosaurs, alligator-like, water-dwelling reptiles from the Permian Period, approximately 280 million years ago, making them the oldest reptile fossils ever described. Even more exciting is that both fossils are at the same stage of development—just about to be born. The fossil record of eggs and embryos is very sparse, so these specimens are doubly important to the paleontological world. But there is still more—the embryo from Brazil is an advanced-stage mesosaur embryo discovered in what is probably the gestation stage, or in the uterus of its mother. This is now the earliest fossil record of viviparity, or giving birth to live young, which mesosaurs have long been known to do. However, alongside 26 additional adult mesosaurs from Uruguay, many of which were also incubating embryos “in uteri,” is a half-foot long, long-nosed, long-tailed baby that appears to be curled up inside an egg, outside the body of its parent. This is giving many paleontologists an occasion to pause. Perhaps some mesosaurs did lay eggs, but these would have been in water and evidence suggests that they were laid at a late period of development, after a period of growth within the female—a reproductive strategy not before described. The mesosaur egg looks like a precursor to the hardier eggs that modern reptiles now lay on land. Whatever the case, the fossil specimens are intriguing for the study of the evolution of development on several levels.

Read more at sci-news.com.

Read the original study in the journal Historical Biology.

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Learn more about mesosaurs at Reptile Evolution.com.

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