Where fish ends, tetrapod begins. My favourite fish fossil


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It's not a fish that you can buy, and it's not a fish that even exists any more, but for evolutionary scientists, Tiktaalik roseae is one of the most important fish there was.

Evolution is notoriously riddled with holes in its fossil record, although these gaps are filling day by day as new evidence is found. Those that wish to decry evolution as false often like to jump on these omissions and cite them as inexplicable flaws in the theory.

It was out of such a case that the search for Tiktaalik began. Fossil history clearly showed that lobe finned fish – the Sarcopterygii – were abundant 380 million years ago, and that the first land dwelling tetrapod existed 363 million years ago, but what was lacking was a bridge between the two worlds.

By researching the geology of the time, scientists were able to calculate where such a creature should exist, if it had existed at all. Expanding on this, they could approximate where in the modern world such fossil sites might be found, sites that represent the habitat that these creatures would have lived in, which were in this case winding lowland streams in near equatorial regions.

In the event, the site where this evolutionary missing link was found turned out to be Arctic Canada. Million of years of continental drift had moved the fish’s remains over vast distances, but they remained intact.

Tiktaalik filled the void perfectly. Dated at 375 million years old, these creatures were fish, but they shared many features with tetrapods; they were more like ‘fishapods.’ They had scales and gills, and the fins of a fish, but also had less fishy features; ribs, shoulder bones, even a movable head which is not only something that modern fish lack, but would have given the Tiktaalik a predatory advantage over their static necked rivals.

Like many beasts of this time period, Tiktaalik were big. Small fossils have been found, representing fish of around 3ft long, where larger fragments indicate individuals that maybe reached 9ft. Armed with a flotilla of teeth, with flattened head, and eyes mounted firmly atop, they would have looked more akin to crocodiles than any fish we know.

Most remarkable of all, even more so than Tiktaalik’s spiracle breathing holes, or swimbladder derived lungs, is the bone structure within the fins. Tiktaalik had developed not only the joints that constitute a wrist, but also finger bones.

These bones should not be confused with the more powerful and weight-bearing structures of true tetrapods, but still conferred Tiktaalik with an advantage over its rivals. It could prop itself up in waters that likely became excessively hot and oxygen deficient, and it would have been able to hold itself into position in strong currents.

Still classed within the Sarcopterygii, modern relatives of Tiktaalik include the equally ancient coelacanth, as well as more familiar species like lungfish.

So does this mean that we are directly descended from Tiktaalik? Probably not, but the existence of this fishapod shows that intermediate evolutionary stages were not only possible, but actively developing over this time period all of those millions of years ago. For an advocate of evolution like myself, Tiktaalik could quite possibly turn out to be one the most amazing fish I’ll never meet.

And to my fellow evolutionists, I’d invite you to take a moment out, and nurture your inner fish…

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