Australasian Science: Australia's authority on science since 1938

Ancient Fish Had Fab Abs

By Stephen Luntz

The development of jaws and heavy body armour in the Devonian era led to muscles in unexpected places, a remarkable fossil reveals.

Most of what we know about prehistoric muscles comes from the shapes of bones and comparisons with living creatures, since soft tissue seldom fossilises. However, Flinders University palaeontologist Prof John Long has added to his extraordinary series of discoveries from the Kimberley Gogo deposits with the unearthing of two 380 million-year-old fish with preserved muscles. The discovery was announced in Science.

A/Prof Kate Trinajstic of Curtin University, who headed the studies of the fish, says the preservation of the muscles was a result of “quite a complex process”. For one thing, the fish were covered quickly before scavengers could feed. “This time bacteria was our friend, rather than breaking things down it formed a film around the fish, holding them together and forming a sticky substance that attracted pieces of calcium carbonate to precipitate out of the oceans.”

The musculature revealed was quite unexpected. “We were stunned to find that our ancient fossil fishes had abs!” Trinajstic says. “Abdominal muscles were thought to be an invention of animals that walked onto the land, but this study revealed that these muscles appeared much earlier in our evolutionary history.”

Both species of fish were placoderms and carried heavy armour around their heads. “We think the abdominal muscles held the back of the fish stable so there was no friction against the armour,” Trinajstic says.

The fossils also reveal tendons connecting the tail skin to the muscles. These suggest that the placoderms swum in a manner similar to modern sharks, with energy stored in the tendons “allowing a stronger beat for the same amount of force,” Trinajstic says.

“We are finding that these animals have a very complex muscle set,” Trinajstic says. “It is turning our ideas that ‘primitive equals simple’ on their heads.”

Among the muscles the fish developed to support their newly evolved jaws was the cucullaris, which humans still possess on the back the neck.

Trinajstic believes modern fish lost many of these muscles, including those of the abdomen, as speed and manoeuvrability became prime marine attributes. “Once they didn’t have body armour they didn’t need these muscles.”