Bombed 380 million years ago: Paleontologists have discovered a three-dimensional heart and other organs in the fossils of a 380 million-year-old armored fish. The results of the study show that in these first representatives of jawed vertebrates, the positions of the organs in the body already correspond to the anatomy of sharks. The study thus provides important new information about vertebrate evolution, the scientists say.
From worm-like creatures to increasingly complex shapes to fish-like creatures with an anatomical concept that still shapes us: how the first jawed vertebrates evolved is an exciting question in evolutionary research. The first vertebrates are believed to have been jawless. But then, step by step, the first structures appeared, which can still be found today in various versions. The oldest known jaw-lobed (Gnathostomata) is a representative of the armored fish (Placodermata) of the Devonian geological period. The mouth structure of the smallest group of these creatures is already well developed: Arthrodira was captured for prey in the primitive seas about 420-360 million years ago.
X-ray view of seemingly blurred fossils
A particularly large number of well-preserved fossils of early jaw bones have been discovered in the so-called Gogo Formation in the Kimberley region of Western Australia. Due to the clearly favorable fossilization conditions in former corals, not only bone material has been preserved in some finds: preserved 3D muscular structures have already been described. An international team of researchers is now showing that there appear to be other well-preserved remains of formerly soft structures that can be detected in some of the samples. For the study, fossils in limestone were surveyed using neutron and synchrotron X-rays at the European Synchrotron Radiation Facility in France.
The scientists reported that the three-dimensional structures of the former organs can be recognized based on the different densities of specific regions in the material. “Most of the other cases where the 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. “So it’s really unusual that some gogo fish have preserved soft, three-dimensional parts,” the paleontologist asserts.
A 380 million year old heart is emerging
Scientists were finally able to identify the fossilized stomach, intestines, liver and also the most important thing in the study: the heart, based on the location compared to other fossil parts as well as on the distinct micro 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 organ’s structure. So it consisted of two rooms, the smaller of which sat on top. Using the survey data, the scientists were then able to create a 3D model of the heart of an early representative of the jawbone for the first time.
As they reported, the position of the heart and other organs was also an important aspect of the scan results. “For the first time we can see all the organs of a primitive jaw fish together,” Trinajstic says. A well-known constellation appeared: “The hearts of these fish were literally in their mouths and under their gills – just as sharks do today,” says the paleontologist. It is an interesting insight into the evolution of the head and neck in relation to the formation of the jaw system. According to her, these features seem to be quite advanced in such early vertebrates. “Like modern sharks, Arthrodera’s liver was large, which allowed the fish to stay afloat,” Trinajstic says.
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 dreams for paleontologists. These fossils are undoubtedly the best preserved examples in the world at this age. It shows the world how important a fossil is. jojo to understand the major steps in our evolutionary history.”
Source: Curtin University, professional article: Science, doi: 10.1126/science.abf3289
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