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Photo Credit: PIRO |
The earliest reptiles, birds and mammals may have been born live young, researchers from Nanjing University and University of Bristol have revealed.
Until now, the hard-shelled egg was thought to be the key to the success of the amniotes - a group of vertebrates that undergo embryonic or fetal development within an amnion, a protective membrane inside the egg.
However, a fresh study of 51 fossil species and 29 living species which could be categorized as oviparous (laying hard or soft-shelled eggs) or viviparous (giving birth to live young) suggests otherwise.
The findings, published today in Nature Ecology & Evolution, show that all the great evolutionary branches of Amniota, namely Mammalia, Lepidosauria (lizards and relatives), and Archosauria (dinosaurs, crocodilians, birds) reveal viviparity and extended embryo retention in their ancestors.
Extended embryo retention (EER) is when the young are retained by the mother for a varying amount of time, likely depending on when conditions are best for survival.