Toward the first tetrapods

Tetrapods have walked on land since the end of the Devonian.(40 kb) A tetrapod is an air-breathing animal that has four legs, or has descended from a four-legged animal. Frogs, penguins, snakes, baboons, swallows, porpoises and even humans are all tetrapods. They live in nearly every ecological niche.

Tetrapods evolved from fish. It was long believed that tetrapods developed because fish succumbed to selective pressures that favoured life in the open air. We now know that the first tetrapods were entirely and exclusively aquatic: they could not linger on dry land despite having lungs and four feet with digits. For example, Acanthostega, found in Upper Devonian layers in Greenland, had ribs that were not sufficiently developed to support the weight of its body out of water. It would have collapsed on the first attempt, rendered unable to breathe. Ichthyostega compensated by developing a stronger ribcage that enabled it to overcome the burden of gravity.

(60 kb)A few million years later, when the first tetrapods worthy of the name began to walk on land, they were still dependent on their aquatic habitat, at least for reproduction. Their eggs, not so different from those of fish, had to remain submerged to survive. This is still the case for amphibians like frogs and salamanders. Their small size allows them to escape the drawbacks of gravity out of water, and they spend most of their time in the water. It was only with the appearance of impermeable membranes like the amnion (a membranous sac that surrounds and protects the embryo), that eggs could be safely laid on dry land by reptiles: a new group of tetrapods that appeared around 315 million years ago (315 Ma). Existing today as snakes, turtles, alligators and lizards, reptiles enjoyed considerable diversity when they included the celebrated dinosaur group between 230 and 65 Ma.

It was 220 Ma ago, in the shadow of the first dinosaurs, when a small group of reptiles, the synapsids, evolved into a new tetrapod form: mammals. It was only much later, once the dinosaurs became extinct and several ecological niches were left vacant, that mammals could express their true evolutionary potential.

Are dinosaurs extinct? Not really... 150 million years ago, some became covered in feathers and developed winged flight, giving rise to birds.

Each of these four great groups of tetrapods – amphibians, reptiles, mammals and birds – knew various evolutionary successes and failures over the course of time. They scattered across all regions of the Earth (some even returning to the oceans), and now interact with each other – and with all other living creatures – in the great ecosystem that we call the biosphere.

The evolutionary success of the tetrapods should not make us forget their ancient aquatic origins at the end of Devonian time. Paddling in mud near the soft, bushy banks of equatorial deltas and estuaries, a group of fish, the elpistostegalians, were poised to give rise to the first vertebrates with toed feet. Among these tetrapod forerunners were Panderichthys, now found fossilized in Latvia, Tiktaalik, in the Canadian Arctic, and Elpistostege, at Miguasha. The gradual conquest of land by vertebrates represented a major event in the history of the animal kingdom and raises an important question: what was the selective advantage of the first tetrapods? In other words, why did they leave the water? The land at that time, invaded by inedible plants and a few rare arthropods, offered little in the way of resources for vertebrates the size of a small crocodile, greedy for protein and with a talent for catching fish. It is plausible that, faced with competition from other predator fish of the same size like Eusthenopteron or Holoptychius, the tetrapod forerunners ventured on land to guard their prey. To seize a placoderm like Bothriolepis, a very common fish, and be able to devour it quietly in a place out of reach of envious competitors was undoubtedly a compelling advantage. It could finally eat in peace in the Devonian world where the calmest place – paradoxically – was outside the “silent world” of water!