What came first: the chicken, the egg, or the viviparous reptiles?

Chicken or egg? The discovery of the crocodile-like reptile embryo has overturned the idea of egg-laying development and sparked a wave of debate among biologists. It turns out that the first dinosaurs and the reptiles that preceded them did not necessarily lay eggs with a hard shell, as previously thought. Instead, they may have used an alternative approach to hatching cubs that differed from modern birds. This discovery has important implications for how mammals and birds, as well as reptiles, evolved their approach to procreation.

Hard-shelled egg: the key to reptilian success?

The development of hard-shelled eggs was thought to be the key to reptile success. However, the discovery of the choristodera embryo, a type of crocodile-like reptile, calls this into question. Both names Zootoca vivipara refer to her carrying live cubs, but she may not be the exception among reptiles, as previously assumed.

As Bristol University professor Michael Benton said, the old question, “Which came first, the chicken or the egg?” has long been settled. The chicken came from other egg-laying birds, so eggs easily preceded chickens. However, not all eggs are the same.

The failure to lay eggs in water is considered so significant that mammals, reptiles, birds, and their extinct relatives are known collectively as amniotes after the name of the amnion, the membrane that protects the embryos of all these animals.

Intermediate version: prolonged embryo retention

While the amnion of reptiles forms a sac inside the egg, in humans it surrounds the embryo and the amniotic fluid inside the uterus. There is also an intermediate variant known as Extended Embryo Retention (EER). Today, EER is common in lizards and snakes, but variable. Their cubs can be released either inside the egg or as small wriggling individuals, at different stages of development, and the EER seems to have ecological advantages that perhaps allow mothers to release their cubs when temperatures are warm enough and food supplies are plentiful.

Sometimes closely related species exhibit both behaviors, and it turns out that viviparous lizards can return to laying eggs much more easily than expected. As Professor Baoyu Jiang of Nanjing University said, “This discovery has great implications for how mammals and birds as well as reptiles have evolved their approach to procreation.”

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