World's oldest heart is discovered in a 380-million-year-old fossilized fish

The world's oldest heart has just been discovered in a 380-million-year-old fossil of an ancient jawed fish.

Scientists at Curtin University found the "beautifully preserved" heart alongside a separate fossilized stomach, intestine, and liver in an ancient fish, yielding clues about the evolution of jawed vertebrates.

"Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a larger leap between jawless and jawed vertebrates, she said. "These fish literally have their hearts in their mouths and under their gills - just like sharks today." Lead researcher John Curtin Distinguished Professor Kate Trinajstic, from Curtin’s School of Molecular and Life Sciences and the Western Australian Museum, said in a statement.

Published Thursday in Science, the new research found that the position of the organs in the body of arthrodires - an extinct class of armored fishes that swam during the Devonian period from 419.2 million years ago to 358.9 million years ago - is similar to modern shark anatomy.

Primitive jawed fish are not so different from us

Trinajstic said that the discovery was "remarkable" considering that soft tissues of ancient species were rarely preserved, and it was even rarer to find 3D preservation.

"As a paleontologist who has studied fossils for more than 20 years, I was truly amazed to find a 3D and beautifully preserved heart in a 380-million-year-old ancestor," she said.

The research presents the 3D model of a complex s-shaped heart in an arthrodire made up of two chambers, with the smaller chamber sitting on top, for the first time.

According to Trinajstic, such features were advanced in early vertebrates. This offered a unique perspective into how the head and neck region began to change to accommodate jaws, an imperative stage in the evolution of our own bodies.

"For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us," Trinajstic said.

"However, there was one critical difference – the liver was large and enabled the fish to remain buoyant, just like sharks today. Some of today’s bony fish such as lungfish and birchers have lungs that evolved from swim bladders but it was significant that we found no evidence of lungs in any of the extinct armoured fishes we examined, which suggests that they evolved independently in the bony fishes at a later date," she continued.

The discovery explains the evolutionary transition to living vertebrates

Paleontologists discovered the fossil during a 2008 expedition at the GoGo Formation, originally a large reef in the Kimberley region of Western Australia. The sedimentary deposit region is famed for its rich fossil record preserving reef life from the Devonian period of the Paleozoic era, including relics of tissues as delicate as nerves and embryos with umbilical cords, reported CNET.

"Gogo has given us world firsts, from the origins of sex to the oldest vertebrate heart, and is now one of the most significant fossil sites in the world. It’s time the site was seriously considered for world heritage status," Co-author Professor John Long from Flinders University said.

Study co-author professor Per Ahlberg of Sweden's Uppsala University stressed that finding soft tissues preserved in three dimensions in Gogo fishes was truly "exceptional."

"We are also very fortunate in that modern scanning techniques allow us to study these fragile soft tissues without destroying them. A couple of decades ago, the project would have been impossible," he said.

According to the release, this new discovery of mineralized organs makes the Gogo arthrodires the most fully understood of all jawed stem vertebrates and explains the evolutionary transition to living jawed vertebrates, which includes mammals and humans.

Study abstract:

The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft anatomy. Skeletal transformations can be examined directly by studying fossil stem gnathostomes; however, preservation of soft anatomy is rare. We describe the only known example of a three-dimensionally mineralized heart, thick-walled stomach, and bilobed liver from arthrodire placoderms, stem gnathostomes from the Late Devonian Gogo Formation in Western Australia. The application of synchrotron and neutron microtomography to this material shows evidence of a flat S-shaped heart, which is well separated from the liver and other abdominal organs, and the absence of lungs. Arthrodires thus show the earliest phylogenetic evidence for repositioning of the gnathostome heart associated with the evolution of the complex neck region in jawed vertebrates.