Ancient fishes had abs

Artist’s impression of what the placoderms – Eastmanosteus (grey) and Compagopiscis (white) – looked like when alive. Copyright: Brian Choo

The discovery of ancient fishes with rippling abdominal muscles shows that palaeontology still holds an important role in modern science, according to Flinders University’s Professor John Long.

In a new study published in Science today, Professor Long and a team of palaeontologists reveal that placoderms – 380 million year old armour-plated fishes from the Kimberley – had helically arranged tendons that connected the tail skin to the muscles and helped propel the fish through the water like a modern shark.

It is a significant finding as the preservation of soft tissues within fossils is extremely rare: normally only the fossil skeletons are found.

“We now need to understand how soft tissues evolved in the big evolutionary steps from early fishes to humans that built the vertebrate body plan,” Professor Long said.

Previous researchers had to rely on scars on the bone to restore muscles in fossils but this new discovery proves that these older reconstructions were not accurate.

Lead author Curtin University’s Professor Kate Trinajstic – who was Professor Long’s first PhD student – said, “We were stunned to find that our ancient fossil fishes had abs!”

“Abdominal muscles were thought to be an invention of animals that walked onto the land but this discovery shows that these muscles appeared much earlier in out evolutionary history.

“These muscles were never predicted to be present in such ancient, primitive fishes.”

The team conducted their study on fossils found in rocks of the Gogo Formation of north Western Australia, famous for its 3D preserved fish skeletons revealed by etching the bones out of rock using weak acids.

Previous work by Professor Trinajstic and Professor Long had identified 3D preserved soft tissues including nerve and muscle cells in these fossil fishes: a remarkable discovery because such tissues almost never fossilize.

This time the team decided to go beyond merely identifying soft tissues, to mapping out the musculature of the entire fishes.

In addition to traditional techniques they used microCT and synchrotron scanning to identify muscle fibres still buried in the rock, before any kind of preparation had affected the specimen.

Corresponding author Professor Per Ahlberg from Uppsala University in Sweden is thrilled with the result.

“We have managed to produce something close to a dissection guide for placoderms – nothing like this has ever been possible for such early vertebrates,” he said.

Professor Philippe Janvier of the Natural History Museum in Paris, who was not associated with the work, described it as “a milestone in early vertebrate studies”.

“Rotten fish look like disgusting forensic investigations, but exceptional when on completely extinct Devonian fishes whose relationships are still debated,” he said.

“At last we are very close to the actual anatomy of what is probably the root of the jawed vertebrates!”

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