Earlier thought to be oviparous, ancient amniotes were baked in wombs

The researchers concluded that ancestors of amniotes practiced extended embryo retention.
Sejal Sharma
Representational image of an egg
Representational image of an egg


Who came first: the chicken or the egg? 

It’s a safe bet (and also a scientific fact) to say that the egg came first since they existed before chickens came into the picture. And until now, it was believed that amniotes - birds, reptiles, mammals - grew inside a hard-shelled egg.

Amniotes are a group of vertebrates that undergo embryonic or fetal development within the protective membrane inside the egg. But now a team of researchers from Nanjing University and the University of Bristol claims that the earliest known reptiles, birds, and mammals may have been born ‘live young’, meaning they were developed in a womb.

“When the amniotes came on the scene 320 million years ago, they were able to break away from the water by evolving waterproof skin and other ways to control water loss. But the amniotic egg was the key. It was said to be a ‘private pond’ in which the developing reptile was protected from drying out in the warm climates and enabled the Amniota to move away from the waterside and dominate terrestrial ecosystems,” said Professor Michael Benton from the Bristol’s School of Earth Sciences.

The team studied 51 fossil species and 29 living species, which could be categorized into oviparous (laying hard or soft-shelled eggs) or viviparous (giving birth to live young). The researchers conducted an observational study of the amniotes and concluded that their ancestors were viviparous and practiced extended embryo retention (EER).

EER is when the mother retains the young for a varying amount of time for protection, likely depending on when conditions are best for survival, as per the press release by the University of Bristol on EurekAlert.

Project Leader Professor Baoyu Jiang added, “This standard view has been challenged. Biologists had noticed many lizards and snakes display flexible reproductive strategy across oviparity and viviparity. Sometimes, closely related species show both behaviors, and it turns out that live-bearing lizards can flip back to laying eggs much more easily than had been assumed.”

EER is common and variable in lizards and snakes today, explained Dr. Joseph Keating, one of the study's authors. The young lizards and snakes can be released inside an egg or as little wrigglers at different developmental stages. The ecological advantages of EER include the mother having the option to remove the young when temperatures are warm, and the food supply is rich.

The findings of the study were published in Nature Ecology & Evolution.

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