Researchers have discovered a 380-million-year-old fossilised heart, along with other internal organs of prehistoric fish. This exciting discovery is not only helping palaeontologists to understand the internal organs of placoderms, these fossils are also providing a fresh perspective on our own evolution.

Fossilised Internal Organs in Ancient Armoured, Jawed Fish

The remarkably well-preserved fossils representing the Devonian placoderm Compagopiscis, come from the famous Gogo Formation, in the Kimberley region of Western Australia. The Gogo Lagerstätte preserves the fauna that once thrived on an ancient, shallow water reef. The fossils are thought to be around 380 million years old (Frasnian faunal stage of the Late Devonian).

Soft Tissues Including Liver and Intestines but No Evidence of Lungs

The international research team led by scientists from Curtin University and the Western Australia Museum have published their findings in the academic journal “Science”. They have identified a number of internal organs preserved in three-dimensions including the liver and intestines. Additionally, the fossils show that the lungs are absent, refuting the hypothesis that lungs are ancestral in jawed vertebrates.

Details Revealed by Computerised Tomography

In collaboration with scientists at the Australian Nuclear Science and Technology Organisation in Sydney and the European Synchrotron Radiation Facility in Grenoble, France, researchers used neutron beams and synchrotron x-rays to scan the specimens, still embedded in limestone concretions. They constructed three-dimensional, computer-generated images of the soft tissues inside them based on the different densities of minerals deposited by the bacteria and the surrounding rock matrix.

Lead researcher and John Curtin Distinguished Professor Kate Trinajstic (Curtin’s School of Molecular
and Life Sciences and the Western Australian Museum), commented that to find three-dimensionally preserved tissues in Devonian fossils was an extremely rare event.

Professor Trinajstic explained:

“As a palaeontologist who has studied fossils for more than 20 years, I was truly amazed to find a 3-D and
beautifully preserved heart in a 380-million-year-old ancestor. 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. These fish literally have their hearts in their mouths and under their gills – just like sharks today.”

The Complex Heart of an Arthrodiran

This is the first time a 3-D representation of the heart of a member of the Arthrodira has been found. The Arthrodira are an extinct Order of jawed, armoured fish within the Class Placodermi. They thrived in the Devonian and some arthrodirans evolved into apex predators such as the huge Dunkleosteus.

The surprising complex heart is s-shaped and consists of two chambers with the smaller chamber sitting on top. Professor Trinajstic stated that these features were advanced in such early vertebrates, offering scientists a unique perspective on how the head and neck region began to change to accommodate jaws, a critical stage in the evolution of animals with backbones.

Learning About the Anatomy of a Primitive Jawed Fish

The discovery and subsequent detailed analysis of these mineralised soft tissues, in combination with earlier studies looking at muscle structure, placoderm embryos and evidence of viviparity makes the Gogo Lagerstätte an exceptionally important fossil resource as researchers strive to improve their knowledge of early vertebrates.

Professor Trinajstic added:

“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. 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 bichirs 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.”

The Stuff of Palaeontologists’ Dreams

Co-author of the scientific paper, Professor John Long (Flinders University, Adelaide, South Australia) commented:

“These new discoveries of soft organs in these ancient fishes are truly the stuff of palaeontologists’ dreams, for without doubt these fossils are the best preserved in the world for this age. They show the value of the Gogo fossils for understanding the big steps in our distant evolution. 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.”

To read an earlier blog post about a fossil from the Upper Devonian Gogo Formation that indicates live birth (viviparity) in placoderms: Evidence of Live Birth (Viviparity) in a 380-million-year-old Fish.

Exceptional Fossils Meet Exceptional Technology

Co-author of the paper Professor Per Ahlberg (Uppsala University, Sweden) explained that access to state-of-the-art, non-destructive scanning technology enabled scientists to make such exceptional discoveries.

Professor Ahlberg explained:

“What’s really exceptional about the Gogo fishes is that their soft tissues are preserved in three dimensions. Most cases of soft-tissue preservation are found in flattened fossils, where the soft anatomy is little more than a stain on the rock. 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.”

A Collaborative Effort

The research was truly a collaborative effort not only involving the Australian Nuclear Science and Technology Organisation and the European Synchrotron Radiation Facility, but also scientists from Flinders University, the Western Australian Museum, Uppsala University, South Australia Museum and Monash University’s Australian Regenerative Medicine Institute.

Everything Dinosaur acknowledges the assistance of a media release from Curtin University in the compilation of this article.

The scientific paper: “Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte” by Kate Trinajstic, John A. Long, Sophie Sanchez, Catherine A. Boisvert, Daniel Snitting, Paul Tafforeau, Vincent Dupret, Alice M. Clement, Peter D. Currie, Brett Roelofs, Joseph J. Bevitt, Michael S. Y. Lee and Per E. Ahlberg published in the journal Science.