Why do reptiles lay hard shelled eggs?

According to evolutionary theory, the birds came from the reptiles, so the reason for both laying hard shelled eggs is the same. There are two reasons why this developed.

1. The hard shell offers more protection. Since the eggs are laid on land, they are subject to knocks, and should they break its game over.

You will notice from chickens eggs that the shells harder at the top and the bottom. This extra robustness is useful for when they are being laid, so they don't break when they hit the ground. The shame, whilst being easier to lay than a round egg also offers more protection, as they do not tend to roll off so easily.

A hard shell poses a problem for the offspring, as hard shelled eggs are difficult to escape from. Most reptiles have an "egg tooth" which they use to break through the shell when they are ready to hatch.

2. Dessication is a major problem, and the hard shell prevents fluid from evaporating. Without the shell, in the heat of the sun they would dry out.

Again, game over.

Amphibians however lay soft eggs. They do not need the same level of protection as their eggs are laid in water, which also sorting out the dessication problem.

Many reptiles eggs have shells similar in consistency with birds, in other words a 'hard' shell. An example would be several turtle species.

Why do they lay eggs with a hard shell, then that is because the shell developed as an evolutionary advantage when vertebrate animals began to live on land. The eggs laid by amphibians do not have a similar shell so the animals have to return to water to breed, when the first animals to have hard shelled eggs evolved, they were at an advantage because they could live and breed away from water, so would have been favoured by selection pressures.

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