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Fishyfeet!
Jan. 7th, 2010 12:34 amScientists in changing mind based on new data shocker!
Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, and Per Ahlberg (late of the NHM, now firmly ensconced in Sweden) appear to have thrown a cat among the pigeons. Actually what they've done is thrown a tetrapod among the fishes.
Until last tonight we thought we'd got a handle on tetrapod evolution. First you get things like Eusthenopteron, often cited as the standard Devonian "Lobe-Finned Fish". While clearly a fish it has rather limb-like fins. A little while later (385 million years ago to be precise) a group called the Elpistostegids turn up, the most famous of these is probably Tiktaalik which did the rounds couple of years back. These are much more tetrapod-like. The heads have a long snout, the front fins are even more limb-like, with the beginnings of a shoulder and wrist, but the back legs are relatively small, and while Tiktaalik could probably do push-ups, it certainly couldn't walk anywhere.
Finally at about 370mya tetrapods which actually have proper legs with fingers and everything turn up. Animals like Acanthostega, Ichthyostega, and friends. None of these was ideally suited to life on land- Acanthostega was still dependant on gills, and would have died if you'd taken it out of the water. Ichthyostega would have fared better, but wasn't going to go anywhere terribly quickly. The most recent reconstructions indicate it was rather seal-like. Around about this time the elpistostegids disappear, so we have a nice rough evolutionary sequence, with all the usual (or indeed unusual) branching and diversification that entails- Livoniana's seven rows of teeth are my favourite example.
Or rather "did", now the nice Polish scientists have borked it all up. They've just found the earliest tetrapod footprints and trackways. And they're 397 million years old. Over 10 million years older than the elpistostegids and 18 million older than the "proper" tetrapods. Oh dear. Actually, no. Not "oh dear" "How awesome is that?" They went looking for fossils and trackways where no-one expected to find evidence of tetrapods, and they found it anyway. Adam Rutherford justly celebrates the fact that it was pure research with no applications- and Per notes that not even the guys who's job it is to give money for just those sort of projects would have forked out.
The individual prints are clearly from a tetrapod, and preserve digits. (6 in the case of the photos I've seen. This sounds odd but was expected- Acanthostega had eight fingers, and Ichthyostega seven). The trackways aren't so well preserved, but indicate that the animal would have had a side-to-side gait like that of a salamander. This is decidedly weird- Tiktaalik and Panderichthys shouldn't produce those sorts of trackways. And "walking" seems a bit too advanced for Acanthostega- it couldn't place its feet flat on the ground if it tried. The more I think about it the more important this find gets.
What's going on then? Obviously the biggest problem we have is the spottiness of the fossil record. There's a similar problem with all the lovely Chinese dinobirds. They're mostly from the Early Cretaceous, but the "first bird" is from the Jurassic, and significantly older. My guess is that the changeover is an artefact of the fossil record. Somewhere out there are earlier "fishapods", and as for why Tiktaalik and co hung around for so long after they should have got out of the way for these shiny real tetrapods I shall defer to Grzegorz Niedźwiedzki, who suggests that the elpistostegids were the first to colonise the environments where we've been lucky enough to find their bones. Ichthyostega's great grandmother seems to have been happy to have frolicked around the stromatolite studded tidal flats leaving her footprints, but little else behind.
As always the sciencebloggers have excellent commentary. Ed Yong has talked to Shubin and Daeschler who described Tiktaalik, as well as Jenny Clack, Laelaps, discusses it with reference to work done by Owen in the 19th century, and is very excited by all the new questions that this work raises.
Or you could watch the video, which discusses the environment the fossils were found in, and new ideas for why tetrapods might have started out on land (warning, hot field-trip action depicted)
(There's also really nice shot of the fuzzy Dilophosaurus model Darren Naish blogged about a little while ago in the intro)
Hopefully I'll get a post up in the next couple of days about how the press havereported mangled the reports.
Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg Tetrapod trackways from the early Middle Devonian period of Poland: Nature 463, 43-48 (7 January 2010)
Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, and Per Ahlberg (late of the NHM, now firmly ensconced in Sweden) appear to have thrown a cat among the pigeons. Actually what they've done is thrown a tetrapod among the fishes.
Until last tonight we thought we'd got a handle on tetrapod evolution. First you get things like Eusthenopteron, often cited as the standard Devonian "Lobe-Finned Fish". While clearly a fish it has rather limb-like fins. A little while later (385 million years ago to be precise) a group called the Elpistostegids turn up, the most famous of these is probably Tiktaalik which did the rounds couple of years back. These are much more tetrapod-like. The heads have a long snout, the front fins are even more limb-like, with the beginnings of a shoulder and wrist, but the back legs are relatively small, and while Tiktaalik could probably do push-ups, it certainly couldn't walk anywhere.
Finally at about 370mya tetrapods which actually have proper legs with fingers and everything turn up. Animals like Acanthostega, Ichthyostega, and friends. None of these was ideally suited to life on land- Acanthostega was still dependant on gills, and would have died if you'd taken it out of the water. Ichthyostega would have fared better, but wasn't going to go anywhere terribly quickly. The most recent reconstructions indicate it was rather seal-like. Around about this time the elpistostegids disappear, so we have a nice rough evolutionary sequence, with all the usual (or indeed unusual) branching and diversification that entails- Livoniana's seven rows of teeth are my favourite example.
Or rather "did", now the nice Polish scientists have borked it all up. They've just found the earliest tetrapod footprints and trackways. And they're 397 million years old. Over 10 million years older than the elpistostegids and 18 million older than the "proper" tetrapods. Oh dear. Actually, no. Not "oh dear" "How awesome is that?" They went looking for fossils and trackways where no-one expected to find evidence of tetrapods, and they found it anyway. Adam Rutherford justly celebrates the fact that it was pure research with no applications- and Per notes that not even the guys who's job it is to give money for just those sort of projects would have forked out.
The individual prints are clearly from a tetrapod, and preserve digits. (6 in the case of the photos I've seen. This sounds odd but was expected- Acanthostega had eight fingers, and Ichthyostega seven). The trackways aren't so well preserved, but indicate that the animal would have had a side-to-side gait like that of a salamander. This is decidedly weird- Tiktaalik and Panderichthys shouldn't produce those sorts of trackways. And "walking" seems a bit too advanced for Acanthostega- it couldn't place its feet flat on the ground if it tried. The more I think about it the more important this find gets.
What's going on then? Obviously the biggest problem we have is the spottiness of the fossil record. There's a similar problem with all the lovely Chinese dinobirds. They're mostly from the Early Cretaceous, but the "first bird" is from the Jurassic, and significantly older. My guess is that the changeover is an artefact of the fossil record. Somewhere out there are earlier "fishapods", and as for why Tiktaalik and co hung around for so long after they should have got out of the way for these shiny real tetrapods I shall defer to Grzegorz Niedźwiedzki, who suggests that the elpistostegids were the first to colonise the environments where we've been lucky enough to find their bones. Ichthyostega's great grandmother seems to have been happy to have frolicked around the stromatolite studded tidal flats leaving her footprints, but little else behind.
As always the sciencebloggers have excellent commentary. Ed Yong has talked to Shubin and Daeschler who described Tiktaalik, as well as Jenny Clack, Laelaps, discusses it with reference to work done by Owen in the 19th century, and is very excited by all the new questions that this work raises.
Or you could watch the video, which discusses the environment the fossils were found in, and new ideas for why tetrapods might have started out on land (warning, hot field-trip action depicted)
(There's also really nice shot of the fuzzy Dilophosaurus model Darren Naish blogged about a little while ago in the intro)
Hopefully I'll get a post up in the next couple of days about how the press have
Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg Tetrapod trackways from the early Middle Devonian period of Poland: Nature 463, 43-48 (7 January 2010)
