Engineering: Fossil fish sheds light on evolution of limbs

The discovery of the 400 million-year-old fossil of a coelacanth fin has helped scientists to bridge the evolutionary gap between fins and limbs.

Evolutionary biologists at the University of Chicago believe that the pattern in the fossilised fin closely resembles those of living ray-finned fishes.

"This ends intense debate about the primitive pattern for lobed fins, which involves the ancestry of all limbs, including our own," said author Dr Michael Coates, associate professor of biology and anatomy.

According to the researchers, the fossil’s pattern is similar to the branching arrangement still observed in the fins of modern day paddlefish, sturgeons and sharks.

"To understand the developmental evolution of the limbs of tetrapods [four-limbed vertebrates], we shouldn’t be looking at the fins of our nearest living fish relatives – lungfishes and coelacanths – because they’re far too specialised," Dr Coates said.

"Part of the reason why this is an interesting discovery is that people think of coelacanths as archetypal living fossils," explained Matt Friedman, lead author of the paper.

"But it’s a common misconception. If you look deep in the fossil record to the first members of that group, they are really different and very diverse."

'Living fossils' are animals or plants which seem identical to a species otherwise only known from fossils. This means that their anatomy is believed to have remained relatively unchanged for millions of years, unlike many other species which have undergone evolutionary changes more rapidly.

Highly specialised and different species

Although both have been called living fossils, the discovery suggests that the two living groups of close fish relatives of tetrapods – lungfish and coelacanth – are both highly specialised. Both groups evolved many of the same features, but independently of one another.

"They give this perception that maybe those are general characters, but we can show with fossils like this one that they’ve actually developed specializations in tandem," Friedman added.

Until now, many biologists have looked at lungfish as a primitive model of the evolution of tetrapods.

"Our fossil shows that what we’ve been using to define a primitive state is actually very specialised, which means it might give a deceptive view of what evolution was like for these fins skeletons," explained Friedman.

"If you’re going to figure out how limbs evolved, you need to know what the ancestral morphology was.

“With things like this fossil, we’re beginning to hone in on the primitive conditions of fins that gave rise to limbs later on," he commented.

Named Shoshonia arctopteryx after the Shoshoni people and the Shoshone National Forest, the fossil was excavated from Paleozoic sediments at Beartooth Butte in northern Wyoming.

Not only does this fossil bridge the gap between primitive ray-finned fish and limbed animals, the new data forces scientists to reassess the characteristics of the coelacanths, Friedman said.

"Living fossils are a problematic concept," he added. "Oftentimes the fossils look like living animals because they are so poorly studied. Once you start to go in depth with the fossils, you start highlighting differences."
 

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