A CT scan of a fossil of Tiktaalik, one of the first fish to crawl on land, has revealed more features of its body that are intermediate between fish and land-dwelling animals. In particular, they show that its fins were becoming connected to its spine, a feature of limbs in land vertebrates, but not of fins in fish.
鈥淭hat鈥檚 an unexpected result given the way we鈥檝e reconstructed this animal in the past,鈥� says at Pennsylvania State University.
at the University of Chicago and his colleagues found the first fossil of Tiktaalik on Ellesmere Island in Canada in 2004 and revealed the find in 2006. It was clear from the start that it had many features intermediate between fish and land vertebrates. In particular, its fins, while still clearly fins, have primitive wrist and elbow joints.
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However, parts of the fossil remain embedded in rock. In 2016, a CT scan was done to reveal hidden features, but at the time, studies of the images focused on the head region. Last year, while studying a relative of Tiktaalik called Qikiqtania, Stewart looked at the images of Tiktaalik鈥檚 spine and ribs and spotted features previously overlooked.
鈥淭here was a lot more in there than we realised,鈥� he says. Stewart, Shubin and others have now created a more detailed reconstruction of Tiktaalik.
In land vertebrates, the hind legs are connected to the spine via the pelvis. In fish, the equivalent of the pelvis is positioned away from the spine with no direct connection to it. In Tiktaalik, the shape of the vertebrae and ribs suggest that the pelvis was closer to the spine and that there was a soft tissue connection between the pelvis and spine, says Stewart.
Other features, such as the shape of Tiktaalik鈥檚 ribs, are also intermediate, says Stewart.
鈥淚t is a very nice looking piece of research,鈥� says at Imperial College London, who wasn鈥檛 part of the team. 鈥淎 connection between the vertebral column and the pelvis is one of those things that鈥檚 neither explicitly expected or excluded based on where Tiktaalik sits in the evolutionary tree.鈥�
But at Uppsala University in Sweden thinks that the team鈥檚 reconstruction is wrong in the way it connects the spine to the head, shoulder and pelvis. 鈥淭his has big consequences for the interpretation,鈥� he says. 鈥淚 hope to engage in a constructive debate.鈥�
Shubin rejects this claim. The reconstruction draws on evidence from other specimens of Tiktaalik and closely related species as well as this one fossil, he says.
There is also debate about Tiktaalik鈥檚 place in the evolutionary tree. No one is claiming that it is the direct ancestor of land vertebrates, but while Shubin and others think Tiktaalik is a close cousin of that ancestor, Ahlberg calls it a distant cousin.
Fossilised tracks show that the first four-limbed land vertebrates, or tetrapods, evolved at least 5 million years before Tiktaalik, says Ahlberg. 鈥淚t鈥檚 rather like looking at a chimpanzee as a model for a human ancestor 鈥� not completely out-to-lunch, but not nearly as informative as you might imagine.鈥�
鈥淭iktaalik almost certainly did live several million years after the origin of tetrapods, but that doesn鈥檛 really impact Tiktaalik鈥檚 position in the story of the evolution of tetrapods,鈥� says Brazeau. Because some animals evolve fast while others change little for millions of years, the anatomy of the fossils can be more informative rather than their appearance in time, he says.
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