F Rosa Rubicondior: And This Week's 'Non-Existent' Transitional Form Is...

Thursday 14 April 2016

And This Week's 'Non-Existent' Transitional Form Is...

Cryptotora thamicola.
Tetrapod-like pelvic girdle in a walking cavefish : Scientific Reports

There are enough of these 'non-existent' transitional fossils to fill a large museum and several more are added most months, as a scan of the relevant scientific literature will show. This doesn't stop creationist frauds telling their dupes that there aren't any of course, but then where would they be without false claims and misrepresentations of the scientific facts?

One of the transitions that excites creationists the most is that between fish and the early terrestrial tetrapods (salamander-like limbed vertebrates). Despite the example of Tiktaalik and Eusthenopteron with incontrovertible features mid-way between fish and salamanders, they continue to insist on either denying they exist altogether or arbitrarily designating them as either fish or salamanders. This is intended to avoid the embarrassment that they are quite obviously both because, at that stage in the evolution of terrestrial tetrapods, they had not diverged from fish.

One of the slightly more sophisticated arguments creationists use to dismiss the evidence is the argument that the change couldn't have arisen by mutation because (so creationists dogma dictates) all mutations are harmful. This ignores the fact that many mutations are beneficial of course, but it makes it harder for them to explain this new example. The main problems for creationism here are that not only are mutations beneficial and have obviously been selected for by natural selection, and also that this species is doing exactly what the ancient ancestors of salamanders did, in a great example of convergent evolution.

This new example was described in Scientific Reports last March by a team from the Department of Biological Sciences, New Jersey Institute of Technology, Newark, NJ, USA.

Fishes have adapted a number of different behaviors to move out of the water, but none have been described as being able to walk on land with a tetrapod-like gait. Here we show that the blind cavefish Cryptotora thamicola walks and climbs waterfalls with a salamander-like diagonal-couplets lateral sequence gait and has evolved a robust pelvic girdle that shares morphological features associated with terrestrial vertebrates. In all other fishes, the pelvic bones are suspended in a muscular sling or loosely attached to the pectoral girdle anteriorly. In contrast, the pelvic girdle of Cryptotora is a large, broad puboischiadic plate that is joined to the iliac process of a hypertrophied sacral rib; fusion of these bones in tetrapods creates an acetabulum. The vertebral column in the sacral area has large anterior and posterior zygapophyses, transverse processes, and broad neural spines, all of which are associated with terrestrial organisms. The diagonal-couplet lateral sequence gait was accomplished by rotation of the pectoral and pelvic girdles creating a standing wave of the axial body. These findings are significant because they represent the first example of behavioural and morphological adaptation in an extant fish that converges on the tetrapodal walking behaviour and morphology.

© 2016 Macmillan Publishers Limited. All Rights Reserved. Reprinted under the terms of Creative Commons International License (CC BY 4.0).

This blind cave fish, Cryptotora thamicola, discovered only about twenty years ago in Thailand not only walks up waterfalls using its pectoral and pelvic fins but it does so by using them like a salamander uses its legs, rather than using them to hop like a mud-skipper. Perhaps the most striking thing though is that C. thamicola. has a primitive pelvis and interlocking vertebrae; clear skeletal adaptations to walking and weight-bearing out of water, and paralleling the evolutionary changes that their remote and distant cousins underwent.

It's difficult for creationists to continue to claim that these transitional specimens which can be clearly seen are not really there. It's another thing for them to argue that an evolutionary change we can see occurring today is impossible and so couldn't have happened a long time ago. These awkward facts probably won't make any difference because, as the biochemist and author of Life Ascending: The Ten Great Inventions of Evolution, Nick Lane told blogger Jonny Scaramanga:

When I say many of them [creationists] are clever people, that’s true, but clever in a legal-minded kind of way – very good at picking holes and spotting weaknesses, and realising that it doesn’t matter how many times a specific argument has been refuted, they can keep on repeating it because the large majority of people haven’t heard the refutation.

We really have to stop assuming creationists have a truth agenda like normal people.

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1 comment :

  1. Talking about blind cave fish (in other news, that's a very cute fish in your photo) we have the blind Cave Tetra of Africa in which the juveniles have fully functional eyes. As they grow, though, the eyes atrophy and then get totally covered by scale, skin, and fat. This can be seen happening in the aquarium as well and clearly prove that blind Cave Tetras were originally non cave dwellers which secondarily evolved to a subterranean existence. But you won't find creationists mentioning them either.


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