Fragile seashores were the “cradle of evolution” for the first fish – not coral reefs as previously thought, according to new research.
The study suggests that fish originated in shallow ocean waters – such as lagoons – some 480 to 360 million years ago before diversifying into deeper waters, rivers and even on to land.
The first vertebrates on Earth were fish appearing around 480 million years ago but fossil records from this time are sparse with only small fragments identified.
But by 420 million years ago, there was a proliferation in the fossil record with a huge variety of fish species present en masse.
It was previously assumed fish originated and diversified in coral reefs, which are dying out all over the world, but new research shows this was not the case.
The study was able to clearly identify shallow, near shore waters where primitive, jawless fish made their first appearance.
Fish which developed more flexible body shapes then moved out from the shore and into deeper waters.
Some fish groups moved into freshwater, while others evolved into early land-dwelling vertebrates.
Professor Lauren Sallan, a palaeobiologist at the University of Pennsylvania in the US, said: “It’s been this ongoing question of, well, where were they?
“Where were they hiding? What were their environmental origins?”
Together with researchers from the universities of Michigan in the US and Manchester and Birmingham in the UK, they surveyed fossil records of primitive fish across the globe.
Their aim was to complete a missing link in our evolutionary story as fossils records before the big proliferation were sparse.
So they took a ‘big data’ approach, surveying more than 2,700 early records and using mathematical modelling to predict the likely habitat type from where these early fish types emerged.
Co-lead author Dr Ivan Sansom, of the University of Birmingham, said: “Most of today’s major groups of fishes, from jawless hagfish and lampreys through to sharks and the bony fish we are familiar with make their first appearance during the Ordovician, around 480 million years ago, or shortly thereafter
“The big surprise for us was that all of these seem to have started their evolutionary journey in these very shallow waters close to the shoreline.”
Prof Sallan said: “In modern conceptions, we see that coral reefs are so important for fish biodiversity, so we assume there’s an ancient link between fishes and reefs going back to the beginning.
“But decades of searching in places like the Cincinnati Arch have come up empty.
“Instead, our work shows that almost every major vertebrate division, from the earliest armoured jawless fish all the way up through sharks and our own ancestors, all started out right near the beach, far inshore of the reef.
“Even as older groups spread out, newer groups were also appearing at the shoreline.”
The findings, published in the journal Science, states that movement of these fish into deeper water was restricted to groups with flexible body shape.
They were able to use their increased mobility to move into deeper waters and become established over the following 20 million years – throughout the Silurian and Devonian periods.
The survey showed that more heavily armoured, rigid fish would have had limited swimming ability, and would not have managed to disperse into deeper waters.
Prof Sallan said: “One reason why these fossils have been hard to study is precisely because of the area in which they are found.
“Wave action in the shallow ocean area likely blasted them into tiny fragments,”
It also helps scientists make sense of the fact that, over evolutionary time, many fish groups moved from ocean water to freshwater with some becoming freshwater fish while others evolved into the earliest tetrapods, land-dwelling vertebrates.
Prof Sallan added: “They often went to freshwater before the reefs, which is almost an independent line of evidence that they would have had to have been close to shore before doing so.”
The team realised that the rocks in which many of the earliest fish were preserved were all formed in extremely similar environments, suggesting there were as yet unknown features in these environments which may be particularly advantageous in driving fish evolution.
Dr Sansom added: “We don’t yet know exactly what was special about those early, shallow marine environments that enabled earliest fish to take those first evolutionary steps, but we do think they provided evolutionary ‘hotspots’, responsible for generating this rich diversity.
“This work highlights how important these increasingly vulnerable near shore areas are for species evolution.
“Modern threats from a combination of climate change, elevated sea levels, over-fishing and pollution could have extremely damaging effects on future species diversification.”
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