Welcome to Amphibian August!
Amphibians have existed on Earth for several hundred million years, and have survived through multiple mass extinctions that wiped out many other groups. But even though there are over 7500 known modern species – more than mammals! – between one-third and one-half of them are now also at serious risk of extinction. Factors such as habitat loss, human-induced climate change, pollution, and the worldwide spread of a fatal fungal disease are all causing a dramatic decline in amphibian populations.
So while we spend this month looking at amphibians of the past, please consider helping to protect those in the present by donating to a conservation organization such as Amphibian Ark or the Amphibian Survival Alliance.
Amphibian August #01 – Tiktaalik
We’re using a fairly loose definition of “amphibian” here, especially for the first few days – basically, anything in the evolutionary grade between lobe-finned fish and reptiles. (A more accurate term for these starting entries would probably be “anamniote tetrapodomorphs”, but that’s a lot more unwieldy.)
Don’t worry, we’ll get to temnospondyl hell the proper amphibians soon enough.
Tiktaalik is one of the earliest known examples of transitional forms between fish and tetrapods, living during the Late Devonian period (~375 mya) in Nunavut, Canada, and showing a mosaic mixture of anatomical features. Although it had gills, scales, and fins like a fish, it also had basic tetrapod-like limb bones and joints, a mobile neck, and primitive lungs.
It was also surprisingly large, reaching sizes of around 2.5m long (8′2″). With its vaguely crocodile-like head, flattened body, and sharp teeth, it was probably an ambush predator, capable of “walking” along shallow riverbeds and mudflats to reach areas inaccessible to other aquatic hunters.
Although it represents an incredibly important transitional form for science, Tiktaalik probably wasn’t the direct ancestor of living tetrapods – but it would have been very closely related to a common ancestor, and it gives us an idea of what such an animal may have looked like. Fossilized tetrapod footprints found in Poland date to over ten million years older than Tiktaalik, suggesting that they may have actually evolved much earlier than previously thought.
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