Top 100 EDGE Amphibians

If you’re wondering why conserving amphibians is so important, take a moment to appreciate just how ancient they are.

Most of the distinctive characteristics of living amphibians – such as their permeable skin and their jelly-like eggs – are associated with the fact that although they are able to live on land, most species still need to be closely associated with water for much of their lifecycles as well. Indeed, the word ‘amphibian’ reflects this and means ‘both lives’ in ancient Greek. This key amphibian trait has been inherited all the way back from the first vertebrates ever to climb out of the water onto land, almost 370 million years ago during the Devonian Period – about 140 million years before the earliest dinosaurs.

Whereas many people think that the fossil record is full of gaps, we now have an extremely good understanding of the earliest amphibians and how they evolved from fish – nearly all of the vital transitional forms, or ‘missing links’, have been discovered in places like Arctic Canada, Greenland and Latvia, which were then far more tropical than they are today. The closest relatives of the first amphibians were lobe-finned fish related to modern coelacanths and lungfish, in which the fins were supported by bone instead of thin cartilage strips, and which also had a bony girdle to support their forelimbs. These structures did not evolve specifically to allow the animals to walk on land, but instead may have enabled them to drag themselves along the muddy shallows of freshwater swamps like today’s walking catfish, anchor themselves to the bottom in fast-moving currents, or alternately to be ‘sit-and-wait’ predators that could make rapid underwater lunges after smaller prey. One of the most closely related lobe-finned fish, Panderichthys, was over a metre long and could breathe water through a tube in the top of its head, suggesting that it buried itself in the mud as it lay in wait for other fish, and used its powerful forelimbs to spring out for an underwater attack.

Although we still don’t really know why amphibians first invaded the land, the fossils of these unusual transitional animals show that they slowly became more and more associated with terrestrial environments, with later species developing key adaptations such as robust ribcages to support the increased weight of their bodies out of water. However, the earliest true tetrapods – the group of four-legged vertebrates that includes all amphibians, reptiles, mammals and birds – were still completely restricted to living in water. The oldest and most primitive known amphibian, Acanthostega, still had gills and could not use its limbs to support its body weight. Remarkably, Acanthostega and many other early tetrapods also had up to eight digits on their hands and feet – whereas all later tetrapods have had a basic plan of five digits, which has then been modified to form structures as diverse as the flippers of whales and the wings of birds and bats. The geographical distribution of these early tetrapods also suggests that they could live in coastal as well as freshwater environments, which makes them even more different from today’s amphibians.

The early tetrapods were also relatively large predatory animals, and in some senses it might be better to imagine them as being ecologically more like crocodiles than frogs or toads. The largest amphibian ever, the late Permian Prionosuchus, reached an amazing length of 9 metres! Some early amphibians also had remarkable and unusual adaptations, such as Diplocaulus, which had a huge head shaped like a boomerang or a coathanger on top of a tiny body. Most of the giant amphibians died out by the start of the Mesozoic Era, after the evolution of true crocodiles and other predatory aquatic reptiles such as phytosaurs, but some species survived in remote isolated regions. Australia is today home to some ancient lineages such as the platypus, and primitive giant amphibians such as the 5 metre, half-ton Koolasuchus survived here as well until the Cretaceous Period, less than 100 million years ago. Koolasuchus may even have hunted dinosaurs that came down to the water’s edge to drink, sucking them into its huge mouth in the same way that giant salamanders hunt smaller prey today.

The amphibians have already survived several mass extinction events. Almost as soon as tetrapods first evolved, the world lost over 75% of its species during the late Devonian mass extinction around 364 million years ago, for reasons that remain unclear. ‘Modern’ amphibians – the Lissamphibia, the group that contains all living frogs, toads, newts, salamanders and caecilians – evolved from some of the large ancestral amphibians at about the same time as the dinosaurs first appeared, over 200 million years ago, but they survived the end-Cretaceous mass extinction that wiped out the dinosaurs and many other groups. However, amphibians may now be facing the worst threat of their long evolutionary history – and we have to work harder than ever to try and prevent them from finally disappearing from the earth forever.

 

 


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