It pounded 380 million years ago: Paleontologists have discovered a three-dimensional heart and other organs in fossils of 380 million-year-old armored fish. The study results show that in these early representatives of the vertebrates with jaws, the positions of the organs in the body already corresponded to the anatomy of the sharks. The study thus provides important new information on the evolution of vertebrates, say the scientists.
From worm-like beings to increasingly complex shapes to fish-like creatures with an anatomical concept that still shapes us: How the first vertebrates with jaws developed is an exciting question in evolutionary research. It is believed that the first vertebrates were jawless. But then, step by step, the first structures emerged, which can still be found today in different versions. The earliest known jawmouths (Gnathostomata) are considered to be representatives of armored fish (Placodermata) from the Devonian geological era. The mouth structure of the youngest group of these creatures was already well developed: the Arthrodira snapped for prey in the primeval seas around 420 to 360 million years ago.
X-ray vision of seemingly inconspicuous fossils
A particularly large number of well-preserved fossils of these early jawbones have been discovered in the so-called Gogo Formation in the Kimberley region of Western Australia. Due to obviously favorable fossilization conditions in the former reef, not only bone material has been preserved in some finds: three-dimensional conserved muscle structures have already been described. An international team of researchers is now showing that there are apparently other surprisingly well-preserved remains of former soft structures to be discovered in some specimens. For the study, fossils embedded in limestone were scanned with neutron and synchrotron X-rays at the European Synchrotron Radiation Facility in France.
The scientists report that three-dimensional structures of former organs were recognizable based on the different densities of certain areas in the material. "Most of the other cases where soft tissue is preserved are flat-topped fossils, with the soft anatomy being little more than a speck on the rock," says co-author Per Ahlberg of Uppsala University. "It is therefore really extraordinary that some of the gogo fish have retained three-dimensional soft parts," emphasizes the paleontologist.
A 380 million year old heart is emerging
The scientists were finally able to identify a petrified stomach, the intestines, the liver and also the highlight of the study: a heart, based on the position in comparison to other fossil parts as well as on characteristic fine structures. "As a paleontologist who has been studying fossils for more than 20 years, I was amazed to find a beautifully preserved 3D heart in a 380-million-year-old fossil," says lead author Kate Trinajstic of Australia's Curtin University in Bentley. There were also indications of the structure of the organ. It therefore consisted of two chambers, with the smaller one sitting on top. Using the scan data, the scientists were then able to create a 3D model of the heart of an early representative of the jawbones for the first time.
As they report, the position of the heart and other organs was also an important aspect of the examination results. "For the first time we could see all the organs of a primitive jawed fish together," says Trinajstic. A well-known constellation emerged: “These fish literally had their hearts in their mouths and under their gills – just like sharks do today,” reports the paleontologist. It is an interesting insight into the development of the head and neck in relation to the formation of the jaw system. According to her, these features appear to be very advanced in such early vertebrates. "Like modern-day sharks, Arthrodira's liver was large, allowing the fish to stay afloat," reports Trinajstic.
In conclusion, co-author John Long of Flinders University in Adelaide says: “These new discoveries of soft organs in these ancient fish are truly the stuff of paleontologists' dreams. These fossils are undoubtedly the best preserved examples of this age in the world. They show how valuable the Gogo fossils are for understanding the major steps in our evolutionary history," says the scientist.
Source: Curtin University, professional article: Science, doi: 10.1126/science.abf3289