The Origins Of Vertebrates Part 5: Conquest
So after the descendants of fish walked on bony fins in shallow water, after limbs became the next big thing in vertebrate evolution, after lungs and skin had evolved to the point where living out of the water could be an advantage, our ancestors crawled on four legs towards one hell of a free lunch. Once we could get ourselves onto the land, food practically came to us. There were plenty of insects and plants that had spread throughout the land, untapped by creatures like us and relatively free from predation. Needless to say, when our ancestors came upon this, they took advantage of this huge diversity of ecological niches that needed filling, and by the time those niches were filled, tetrapods were widespread in habitat, shape, size and specialities.
At the point where reptiles split off from their amphibian cousins, the fossil record has a relatively hard time distinguishing the difference. As I pointed out in my last post in this series, the line is blurry, and, in fact, we have two species from around the same time that are almost identical, and yet one of them is classified as an amphibian, and one as a reptile. The only difference we could find was a slight difference in the shape of the skull. However, skull shape is an important defining factor in the divergence of our ancestors, as we will find out when things start to look a little more familiar.
The proliferation of reptiles and the relative decline in amphibians probably dates back to the carboniferous period. At this time the first trees had come around, a shape so successful it’s appeared multiple times, and as the climate changed around us and our new shade-bearing friends, our supercontinent became a vast expanse of wastelands with small island forests, areas where trees could take hold in an otherwise parched environment.
As hydration became more of a commodity, semi-aquatic animals like the amphibians found it harder to cope, whereas reptiles took a foothold and managed to make a home of it, helped in no small part by the way that reptilian eggs work.
Whereas amphibians generally lay their eggs in pools of water , reptiles lay eggs with leathery shells that protects against water loss in the eggs, going so far as to provide their own moisture inside the shells and even their own food and protection for the embryo, so that reptilian young can go through many of the most vulnerable stages of development in a predator-lite environment, and by the time the egg hatches, what you get is essentially a smaller version of the adult; not perfectly safe, but a hell of a lot safer than a tadpole.
Reptiles were also well-equipped for terrestrial life with a lighter, more muscular jaw, an upright posture (allowing for longer strides) and watertight skin in the form of scales. At this point you basically know the history, but I want to take you through a little bit more, because it’s fun.
In the mesozoic era (famous for what comes next) we see the proliferation of the dinosaurs, and along with this vast increase in diversity in a relatively closely-related group of creatures, comes something that we later see in mammals; the conquest of the air and the sea.
Although no dinosaurs took to the sea themselves, in the era in which they conquered the land, Mesosaurs, Plesiosaurs and Icthyosaurs took to the oceans. You’ll know the second for being the most common explanation for the Loch Ness Monster (besides nothing being there at all), and the latter looks superficially like a dolphin and actually gave birth to live young. These animals shared the sea with huge swaths of fish and other creatures, but those that took to the air only had insects for company, and for a while they flourished, growing to enormous sizes. Despite their success however, many of us only know of them by the name Pterodactyl, which isn’t a thing, and I’m sorry for that.
Pterosaurs however, were not the only reptiles to take to the air. Arguable the most iconic group of dinosaurs; the Theropods, went from walking on two legs to having wings on all four, and eventually on two powerful limbs. These dinosaurs were the ancestors of modern birds, and when the meteorite at the end of the Cretaceous period hit, they survived to tell the tale.
Now, I don’t want to completely exclude the mammals, despite the fact that, as a mammal myself, I’m aware that my view of life history is pretty mammal-centric. The mammals belong to a group known as the Synapsids; characterized by an extra opening in their skull behind each eye, and, believe it or not, mammal-like reptiles were much more prolific than the dinosaurs once upon a time. After the mass extinction known as ‘the great dying’ however, 70% of all terrestrial species were wiped out, and it was the dinosaurs that managed to find success in the rubble. Mammal-like reptiles eventually devleioped fur, the ability to eat and breathe at the same time, a strong sense of hearing, and, although much further down the line, the opportunity to give birth to live young.
Perhaps one day I’ll go into the mammals in more detail and explain how the diversity we see today emerged from so few creatures, but for now, I’m going to end it with a parallel. As mammals grew to fill the ecological niches that the wildly successful dinosaurs left behind, we began to move into many areas that they once specialized in. Where ferocious Tyrannosaurs had once been dominant predators, once wolves flourished, the huge birds that had taken their place were out-competed by the packs. Where Plesiosaurs had scoured the oceans, whales evolved from hoofed mammals, and many other mammals took to the sea. Where Pterosaurs dared to explore, and where birds followed, bats evolved flight and found a niche in the darkness, when birds had long since retired for the night.
[image credit: biology-blog]