Amphibians and reptiles: how terrestrial living was transformed by the evolution of better adapted species
Monday 09 February 2009
Today's tour of the aeons before the appearance of the human species on Earth begins with a rare and most important piece of evolutionary evidence provided by the fossils of a creature called
Tiktaalik which were discovered on Ellesmere Island, Canada, in 2004.
They represent what are known as "transitional" fossils, and show key features from two different species: one is the lobe-finned fish; the other, the next-to-evolve tetrapod (meaning "four-limbed creature").
Tiktaalik lived about 375 million years ago, towards the end of the Devonian Period, just as the early plants and mosses were adapting to life near the shores. This creature developed the first genuine arms, with shoulders, elbows and wrists – brilliantly suited to heaving its body on to the land or wading through shallow bogs.
Tiktaalik grew to about 3m in length. It had sharp teeth for hunting and, quite unlike any fish, it was able to turn its head from side to side to look for food or danger, using what was probably the world's first ever neck. Its skull was flat, like that of a modern crocodile, with big eyes bulging out on top of its head, suggesting that it spent much of its time lurking just beneath the water in small streams, lakes and shallow swamps.
Several million years later we come across the almost impossible to pronounce Ichthyostega, which lived between about 357 million and 362 million years ago. This land-lover was about 1.5m long and had seven toes on each foot. It was one of the first genuine tetrapods, and certainly not a fish. Young Ichthyostega could easily leap ashore and move around without having to drag an enormous heavy body with them. Land was also safer than the sea because there was plenty to eat and fewer large predators.
As the generations passed by, these creatures tended to spend longer and longer ashore before returning to the water. Ichthyostega were important because they represented a true link between shallow-water fish and the first highly successful family of animals that could live on land: the amphibians (eg, frogs, toads, salamanders).
Amphibians were well suited to life on land, although they nearly always needed to return to the water to lay their eggs and reproduce. Since their appearance, this family has adapted successfully to life on every continent and in every type of climate – from Arctic ice to sandy deserts – although many of the world's estimated 6,000 species are now on the edge of extinction due to global warming, pollution and the destruction of natural habitats such as wetlands, marshes and woodland.
The transition from fish to amphibian was finally completed about 340 million years ago with the evolution of the first known family of amphibians, the temnospondyls. These could be as large as a fully-grown crocodile or as small as a newt. Eryops is a fine example, and first appeared in the fossil record about 270 million years ago. It had a stout body, wide ribs, grew up to 1.5m long, and had a strong backbone that prevented its body from sagging under its own weight.
For a long time, amphibians ruled supreme on land. They were once the Earth's biggest and fiercest creatures. But they had one significant disadvantage: they had to live near enough to the water to lay their eggs and reproduce. In severe droughts, or if the climate became too dry, this could prove a big problem. As time went by it became an even bigger problem, because the Earth's continental plates were gradually converging into a single, giant supercontinent.
An annual drift of 5 to 10cm sounds very slow, but over a million years that means a continent will move almost 100km. Over 100 million years, that's 10,000km – or a quarter of the way round the world. That explains why, between 250 million and 300 million years ago, life on land changed dramatically.
To begin with, it became a lot hotter and drier, as the land masses converged and distances to the sea became much greater. Amphibians, which had to migrate to the water's edge to breed, were confined to the shores, or to areas with large lakes. If some creature could develop a technology that meant its babies could be born tens, hundreds or even thousands of kilometres away from the water, it would have free rein over all other creatures. As the Earth's crusts converged into a single supercontinent, along came the next leap forward in the tale of life on Earth. It is one that proves once and for all which came first, the chicken or the egg. It was definitely the egg.
Reptiles differ from amphibians because, if necessary, they can live their lives away from the water. They have waterproof skins so that if the weather becomes very hot, the water inside their bodies doesn't evaporate, reducing the likelihood of dehydration. They were also the first creatures to be able to lay eggs on land. These were surrounded with tough, waterproof shells that contained a portable ocean in the form of a liquid membrane to protect the embryo and give it all the nourishment it needs until it is old enough to hatch out and survive on its own in the air.
The first known reptile dates back 315 million years ago. Hylonomus was about 20cm long, and survived on a diet of millipedes and small insects, although it was often attacked by those super-huge dragonflies and other tetrapods on the lookout for a tasty meal. The earliest reptiles had skulls with no holes in them. Most of these are now extinct, although modern turtles, terrapins and tortoises still have this design.
Next to emerge was a group that had two sets of holes in their skulls: one used for seeing (eyes), the others for eating (jaws). One such group were the mammal-like reptiles which would become the dominant force on land for many millions of years. This group, to which we humans are distantly related, emerged a very long time before the first dinosaurs.
One of the most successful genera was Dimetrodon, a reptile which first appeared in the early Permian Period, between 260 million and 280 million years ago. Growing up to 3m long, this lumbering giant walked on four side-sprawling legs and had a long, swaggering tail. It was the largest meat-eater of its time. The success of this bizarre-looking creature was mostly due to the spectacular sail on its back, which it used as a radiator to heat itself up more quickly than other creatures at the beginning of each day.
This was the beginning of warm-bloodedness, a feature of all mammals, including humans. It allows living things to maintain the same internal body temperature regardless of the conditions outside, allowing such creatures to hunt at night or whenever it was safe to go out. Dimetrodon also evolved other mammal-like features, such as different types of teeth. In fact, Dimetrodon means "two-measure teeth".
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