Primitive fish could nod but not shake its head

Ancient fish fossils with preserved muscle tissue offer a glimpse at how necks evolved in early vertebrate animals. The fossils also offer a puzzle: The fish had specialized abdominal muscles found today in land animals, but not in fish, paleontologists report June 13 in Science.

The 380-million-year-old fossils come from Western Australia’s Gogo Formation and contain three-dimensional details of neck, body and tail muscles. The specimens represent several genera of predatory fish armored in bony plates. Called placoderms, these extinct animals were among the earliest vertebrates with jaws. “A lot of structures in us first appear in these fish, particularly muscles that operate the jaw and the neck,” says coauthor Kate Trinajstic of Curtin University in Perth, Australia.

Placoderms were also some of the first vertebrates to have necks separating their heads and shoulder bones, allowing the fish to move their heads independently of the rest of their bodies. The fossils reveal that the animals had several specialized muscles associated with a hinge joint connecting the head to the body. The fish could pivot their heads up and down, but not side to side. Sharks and other jawed vertebrates later evolved simpler muscles and a more flexible neck that had a greater range of motion, Trinajstic says.

More surprising is that placoderms had abdominal muscles running perpendicular to the body’s midline, says coauthor Per Ahlberg of Sweden’s Uppsala University. Modern fish lack such transverse ab muscles. Land vertebrates, however, need these muscles to hold up their bellies. In placoderms, these muscles might have dampened shear forces between an animal’s swinging tail and stiff body armor, Ahlberg says. The muscles might also have prevented the body from swishing around inside the armor, Trinajstic adds.

The researchers suspect that these muscles were common to all early jawed vertebrates. Later, sharks and bony fish lost the muscles and then early four-limbed vertebrates that moved onto land independently evolved them.

Not everyone agrees. It’s too soon to say whether all early jawed vertebrates had transverse ab muscles, says Matt Friedman, a paleobiologist at the University of Oxford in England. To find out, he says, the researchers first need to confirm the muscles were present in all types of placoderms, not unique to the one placoderm group they studied.