'Missing link' in shark evolution found in 380m-year-old Australian fossil

29. Mai 2015

Fossilised skeleton found in Kimberley shows sharks once had bone cells within cartilage, suggesting a sophisticated evolutionary path

A 380m-year-old fossil found in Western Australia has been hailed as the “missing link” in shark evolution, revealing the marine predator has a far more sophisticated lineage than previously thought.

The fossilised skeleton, jaws and teeth, found at the Gogo formation in the Kimberley region of WA, shows the ancient shark had a small amount of bone as well as cartilage.

As modern-day sharks have fully cartilage skeletons, the fossil suggests they evolved from an earlier, bonier fish, transforming to a cartilage skeleton to make them lighter, more nimble and quicker through the water.It was previously thought that sharks came from a primitive lineage that didn’t ever develop bone, unlike other fish.

“That idea of shark evolution has been completely turned on its head,” said palaeontologist John Long of Flinders University, who discovered the fossil. “Cartilage is seen as the precursor to bone, so sharks must be primitive because they never developed bone. This fossil suggests sharks went in the opposite direction because modern sharks lost bone.”

Long, who has spent 30 years searching the Gogo formation, found the shark fossil in 2005 but has only just got around to having his work published, in the scientific journal PLOS One.

“The day I found it was truly remarkable, a true eureka moment,” he said. “I hit a rock and could see a fossil that was so clearly different from anything else I’d seen.

“This is a big part of the puzzle when it comes to sharks. They appear 250m years before the last dinosaur and they haven’t changed much in that time, they’ve hit a winning formula. But while there hasn’t been much change in their appearances, there have been lots of subtle changes in their tissues.”

The Gogo, which was once a tropical reef, is seen as one of the most valuable sites in the world for fish fossils. This is because the formation of limestone over dead animals millions of years ago preserved them in three-dimensional form, rather than squashing them flat, as happens in the US and Europe.

Long’s 75cm fish fossil was subjected to various imaging techniques to determine its composition.

Prof Per Ahlberg, a palaeontologist at Uppsala University in Sweden, said: “This is a really interesting discovery.

“Modern sharks have skeletons of a peculiar tissue called prismatic calcified cartilage: cartilage that is mineralised, not as solid sheets, but as a mosaic of tiny mineral prisms.

“This tissue is quite different from the bone that forms our own skeleton, and its origin is not well understood. The new Gogo shark shows what seems to be an early version of prismatic calcified cartilage: unlike the modern kind, the gaps between the prisms contain cells that resemble bone cells.”

Weiterlesen auf: The Guardian