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hubie [soylentnews.org] writes:

The earliest reptiles, birds and mammals may have borne live young [bristol.ac.uk]:

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 foetal 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 categorised 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.

While the hard-shelled egg has often been seen as one of the greatest innovations in evolution, this research implies it was EER that gave this particular group of animals the ultimate protection.

[...] "EER is common and variable in lizards and snakes today. Their young can be released, either inside an egg or as little wrigglers, at different developmental stages, and there appears to be ecological advantages of EER, perhaps allowing the mothers to release their young when temperatures are warm enough and food supplies are rich."

Professor Benton concluded: "Our work, and that of many others in recent years, has consigned the classic 'reptile egg' model of the textbooks to the wastebasket.

"The first amniotes had evolved extended embryo retention rather than a hard-shelled egg to protect the developing embryo for a lesser or greater amount of time inside the mother, so birth could be delayed until environments become favourable.

"Whether the first amniote babies were born in parchment eggs or as live, snapping little insect-eaters is unknown, but this adaptive parental protection gave them the advantage over spawning earlier tetrapods."

Journal Reference:
Jiang, B., He, Y., Elsler, A. et al. Extended embryo retention and viviparity in the first amniotes. Nat Ecol Evol 7, 1131–1140 (2023). https://doi.org/10.1038/s41559-023-02074-0 [doi.org]

Original Submission