Cape Talk, Monday night

Like frogsbawn, which turns into tadpoles, but some toads birth live tadpoles. It's a mess. So, how did we end up with such a mixed picture? And which evolved first, eggs or live young? We know that because when we go way back in evolutionary history to our even more distant ancestors, things like amphibians and fish, all of them generally lay eggs. So, in fish,

And amphibians, those eggs are soft and they're wet and they're jelly-like, and they have to be laid in water or at least in very damp places because they have no protection and they just dry out. They're basically little lumps of gelatin. Jelly eggs work perfectly well underwater, but once animals started to venture out of the seas, their eggs had to deal with a new set of challenges. So the big innovation that allowed animals to move onto land thoroughly was developing an egg that was resistant to drying out and also that could hold its

Up against the pull of gravity when it was on the land. These early terrestrial explorers developed a few adaptations that allowed their eggs to survive this brave new world. So, some of those are to do with a special set of membranes that surround the embryo within the egg that help to conserve water, and some of those are to do with having the eggshell itself. And it's not clear which one of those necessarily came first, but together they come up with this very efficient, insulated little package.

It holds itself up against gravity. It largely allows it to stop drying out. It allows the exchange of oxygen and carbon dioxide through the egg. So, when did that happen? So, when we look at the relationships of all of these animals that we think would have laid eggs, it suggests that the origin of, if you like, land egg laying occurred probably quite early, probably about 300 or so million years ago, a long time ago. But actually, for various reasons we don't really understand, the first eggs we get in the fossil record that

Recognizably shelled eggs that could have been laid on land were actually laid by dinosaurs. And those eggs date from about 200 million years ago. So they even post-date the origin of dinosaurs by about 35 million years. So there's a really big gap in the fossil record of eggs. So why do you think then that there is this big gap? There are a couple of different reasons. So when we think of an egg today, we often think of a bird egg. And they have those hard, brittle shells made of a mineral called calcite. They're very strong and very resistant. But when we look more broadly across

Reptiles, they tend not to have that same heavy investment of minerals, and instead their shells tend to be a bit more leathery or parchment-like. And it could be that these early shelled animals that were laying their eggs on land were using these parchment-like eggs, which, because they don't have minerals in them, makes them much much harder to preserve. Or it could simply be what we would call, as geologists, a taphonomic bias. This is that the depositional circumstances where these animals are all living are just not conducive.

To preserving those delicate structures. So it could either just be that the geology didn't allow the evidence to preserve, or it could be actually they had a different composition to start with. So dinosaurs weren't the first animals on land to lay hard-shelled eggs, but those are the first ones we see in the fossil record. Which got me wondering: what does a fossilized egg look like? So I'm going to show you a picture of the first sort of