A jumble of bones fills an ancient egg in an illustration of one of the oldest known reptile embryos yet found, according to a new study.
Discovered in Uruguay, the 280-million-year-old eggs belonged to mesosaurs—small aquatic reptiles that pre-date dinosaurs.
Study co-author Graciela Piñeiro first noticed what she thought was a coprolite—a piece of ancient dung—while excavating fossils in northeastern Uruguay's Mangrullo Formation, which dates to the Permian period.
But when she took a closer look in the lab, "I thought, Oh my God, I have here a mesosaur egg with an embryo almost ready to hatch!" Piñeiro, an evolutionary biologist at Uruguay's University of the Republic, said in an email.
(Also see "Pictures: Oldest Dinosaur Nests Found in South Africa.")
The team also found fossils of well-developed embryos inside an adult mesosaur in Brazil, at a site that dates to the same time period as the Uruguayan rock. This suggests the embryos stayed in the mother mesosaur's uterus during most of their development—a hallmark of animals that give birth to live young.
The Brazilian discovery may therefore provide the earliest known evidence of live birth and of parental care in the fossil record. (Read about extinct sea monsters in National Geographic magazine.)
If true, the embryos would push back the beginnings of live birth by 60 million years, according to the study, published recently in the journal Historical Biology.