A species can’t lay a little eggs and give birth a little live, right?

Answer

Dear Philip,

You cite one of those cases that many people who are not biologists find difficult.

The bottom line is that a gradual transition can be seen from egg-laying (oviparous) to viviparous (viviparous) via the intermediate step egg-laying/viviparous (ovi-viviparous).

For example, mammals are ‘typically’ viviparous, the development of the young takes place completely internally until birth.
You know the exceptions to this, such as the marsupials where the embryonic development continues after the ‘first’ birth in the pouch until the young can, as it were, be born again if it is developed far enough to come out of the pouch.
But the echidnas and the platypus are also exceptional because they actually still lay eggs.
The young are so underdeveloped at hatching that they actually still look embryonic after birth.

The step from oviparous to ovi-viviparous is not (yet) observable within the group of mammals.

But you do see that with reptiles and sharks. In our own country, the slow worm, the adder and the viviparous lizard are ovi-viviparous.

After fertilization, the eggs continue to develop in the mother animal, while in other reptiles the eggs are laid relatively soon after fertilization and the development of the young takes place inside the egg, but outside the body of the mother animal. Most reptiles therefore live in warm areas where it is the sun that provides the necessary heat.

In colder regions, delaying egg laying is an advantage for the species.
And since the eggs do not end up in the outside world and cannot lose moisture through evaporation, investing in a hard shell is not necessary. The shell in viviparous lizards is thus no more than a thin membrane, a shell without lime, a bit like a wind egg. That is also an advantage, because not so much calcium has to go from the food to the eggs.

Some of the young leave the membrane before they emerge from the mother animal, but sometimes there are also a number that only crawl out of the membrane after exit.

So you see that you can still make a nice transition from pure egg-laying to viviparous via egg-laying-viviparous.

If you list animals in the different cases, you get:

Greek tortoise: lays hard-shelled eggs.
Bearded dragon: lays eggs with a leathery shell just after fertilization.
Viviparous lizard: Fertilized eggs do not develop a true shell, just a membrane and remain in the body until the young are ready to be born.

In addition, in stressful situations (danger, cold, food shortage, disturbance,….) some reptiles can keep the fertilized eggs for weeks or even months before they are laid.

Not to mention the sharks of which some egg-laying species (e.g. the dogfish) are most oviparous viviparous and there are even those that are viviparous with placenta and all (e.g. the hammerhead shark).