The Flesh of Ancient Fish
It is hard to tell from just bones or a fossilized rock what a creature once looked like with muscles. Flesh does not survive well over the eons. Swedish, Australian and French researchers have presented for the first time miraculously preserved musculature of 380 million year old armored fish discovered in north-west Australia. This research will help scientists to better understand how neck and abdominal muscles evolved during the transition from jawless to jawed vertebrates. The scientific paper describing the discovery is published in the journal Science. The team of scientists who studied the fossilized fish was jointly directed by Prof. Kate Trinajstic, Curtin University, Perth, Australia and Prof. Per Erik Ahlberg of Uppsala University Sweden.
The word fossil naturally conjures up a vision of rattling skeletons. Bones and teeth fossilize far more easily than soft tissues and are usually the only traces of the animal that remain. This makes the rare fossils of soft tissues all the more valuable as windows to the biology of extinct organisms. Such tissues almost never fossilize and scientists usually have to extrapolate skin coverings and musculature from knowledge of modern organisms and from the fossilized skeletons.
The Gogo Formation, a sedimentary rock formation in north-western Australia, has long been famous for yielding exquisitely preserved fossil fish. Among other things it contains placoderms, an extinct group that includes some of the earliest jawed fish.
The Gogo Formation in the Kimberley region of Western Australia exhibits exceptional preservation of a Devonian reef community. The formation is named after Gogo Station, a cattle station where outcrops appear and fossils are often collected from, as is nearby Fossil Downs Station.
The Placoderms were a class of armored prehistoric fish, which lived from the mid Silurian to the end of the Devonian period. Their head and thorax were covered by armored plates; the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish, the Gnathostomata. A 380 million year old fossil of one species is the oldest-known example of live birth.
The Placoderms were hugely successful in the Devonian period, which is sometimes called the Age of Fish.
A few years ago, an Australian research team work led by Prof. Trinajstic made the remarkable discovery that these fossils also contained soft tissues including nerve and muscle cells. Now they have collaborated with the research group of Professor Per Ahlberg, Uppsala University, and with the European Synchrotron (ESRF) in Grenoble, France, to document and reconstruct the musculature of the placoderms. "High contrast X-ray images were produced thanks to a powerful beam and a protocol developed for fossil imaging at the ESRF. This is unique in the world and has enabled us to reconstruct some fossilized muscles and document the muscles of neck and abdomen in these early jawed fish, without damaging or affecting the fossilized remains", says Sophie Sanchez, one of the authors, from the ESRF and Uppsala University.
These early vertebrates prove to have a well-developed neck musculature as well as powerful abdominal muscles — not unlike some human equivalents displayed on the beaches of the world every summer.
"This shows that vertebrates developed a sophisticated musculature much earlier than we had thought" says Per Ahlberg, co-author of the project.
For further information see Ancient Fish.
Image showing preserved bundles of muscles attached to the skull plate of a placoderm . Credit: ESRF/Sophie Sanchez