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 mesosaursâ€”small aquatic reptiles that pre-date dinosaurs.
Study co-author Graciela PiÃ±eiro first noticed what she thought was a coproliteâ€”a piece of ancient dungâ€”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!” PiÃ±eiro, an evolutionary biologist at Uruguay’s University of the Republic, said in an email.
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 developmentâ€”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.