Early tetrapod (“fishapod”) sheds light on transition to land—maybe
|January 17, 2014||Posted by News under Evolution, News|
And if so, only for itself.
Remember Tiktaalik? [tik-TALL-ick] A new partial fossil suggests,
“Previous theories, based on the best available data, propose that a shift occurred from ‘front-wheel drive’ locomotion in fish to more of a ‘four-wheel drive’ in tetrapods,” said Neil Shubin, PhD, Robert R. Bensley Distinguished Service Professor of Anatomy at the University of Chicago and corresponding author of the study, which marks his inaugural article as a member of the National Academy of Sciences. “But it looks like this shift actually began to happen in fish, not in limbed animals.”
Tiktaalik is described by the Phys.org news service as “Tiktaalik roseae represents the best-known transitional species between fish and land-dwelling tetrapods.”
“Best-known” is an interesting phrase in context. Discovered in 2004 in the Canadian Arctic (hence the name) and unveiled in 2006, a 375-million year old fossil of a fish with legs instead of paired fins was hailed as the link between fish and amphibians (frogs, toads, etc.). Evidence suggests that fish had developed legs before walking on land, and speculation abounded as to how or why. In 2009, Nature featured Tiktaalik as No. 2 of 15 Evolutionary Gems. Headlines like “Fossil animals found in Arctic Canada provide a snapshot of fish evolving into land animals,” and “Fossil shows how fish made the leap to land” underlined the find’s importance. Shubin went on record at a trial saying
What evolution enables us to do is to make specific predictions about what we should find in the fossil record. The prediction in this case is clear-cut. That is, if we go to rocks of the right age, and the rocks of the right type, we should find transitions between two great forms of life, between fish and amphibian. … What we see when we look at the fossil record, at rocks of just the right age, is a creature like Tiktaalik.
At last, a clear-cut prediction from the fossil record! “Just the right age.” But in 2010, Tiktaalik failed the fishapod trials. Apparent tetrapod trackways at least 20 million years earlier (about 397 mya) were reported in Poland. Tiktaalik is an interesting fossil, but not the key transitional fossil Shubin insisted it was. For Nature, the trackway discoverers “lob a grenade” into the picture. Or as tetrapod paleontologist Jenny Clack put it, the discovery “blows the whole story out of the water, so to speak.”
So to speak. Does Tiktaalik show that “the evolution of hind legs actually began as enhanced hind fins”, as claimed? The problem is that we don’t know that Tiktaalik’s “four-wheel drive” was even typical, let alone essential or ancestral to anything living today. Or even that the new bone structure turned out to be a good development for Tiktaalik. There’s not enough information.
The Polish trackways establish that Tiktaalik wasn’t anywhere near the first tetrapod, so the most important information about the transition to land doesn’t even include Tiktaalik at present. We need to find the creatures that made those tracks.
Some fish today routinely spend time out of the water, using a variety of mechanisms. But there is no particular reason to believe that they are on their way to becoming full time tetrapods or land dwellers. So we would need to be cautious about assuming that specific mechanisms that might be useful on land are definitive evidence of a definite, permanent move to full-time land dwelling. See, for example, the blenny, here and here.
A lobe-finned fish with a broad flat head and sharp teeth, Tiktaalik looked like a cross between a fish and a crocodile, growing up to a length of 9 feet as it hunted in shallow freshwater environments. It had gills, scales and fins, but also had tetrapod-like features such as a mobile neck, robust ribcage and primitive lungs. In particular, its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground.
It would be interesting to know whether the Polish tetrapods were similar to Tiktaalik in these respects. One must, however, protest the claim that Tiktaalik looked like a cross between a fish and an alligator. As portrayed above, it looks as much like a cross between a fish and a tadpole. Subjective interpretations differ, to be sure, but:
Tiktaalik (Attenborough team’s rendering):
Also, fun, cat vs. alligator:
Note: Re cat vs. alligator: Who did you think would win? Why? Alligators have been an endangered species. When were cats ever an endangered species? Anywhere?