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There are more than 400 million years ago, the ancient oceans teeming with many fish that might seem foreign to the seas of today.
At the time, some wore bone armor plates and had no jaws, such as arandaspid, which looked like a clam with a tail. Heterostraceans sometimes looked like underwater armadillos with spikes. There were also galeaspids, some of whom wore sword helmets, and osteostracans, who had a horseshoe-shaped head.
All jawless fish were heavily armored. Thelodonts, for example, had a torpedo-shaped body and bony scales resembling shark skin. Some anaspides had scales and a leaf-shaped body. Next are the first jawfish such as armored placoderms, some of which use their tank-shaped exteriors and razor-sharp teeth to dominate the aquatic world.
Scientists have long wondered where these groups of extinct fish and their living relatives evolved in the sea. Was it the open ocean? Maybe on the coral reefs? Or perhaps in the depths of the abyss?
Finding the answer was difficult. Although there is an abundance of fish fossils dating from about 420 million years ago, ancient fossil records are becoming scarce about 480 million years ago, when the fossil record of fossil fishes began to rise. fish are thought to have appeared for the first time.
Now, a new study suggests that fish first swam in the shallows around the coasts of supercontinents before they diversify and conquer the waters of the world. The results, published Thursday in the journal Science, also give a glimpse of the origins of vertebrates who became the ancestors of our ancestors who first ventured on the mainland.
To better understand the ancient evolutionary history of fish, researchers flipped through the scientific literature and created a database of more than 2,700 fish fossils captured without jaws from all continents and dating back from 480 to 360 million years ago. . The database allowed the team to determine where in the ocean the old groups of fish lived and evolved.
"All kept groups hailing from the shallow waters over the entire 100 million year period, which was completely unexpected," said Lauren Sallan, paleontologist at the University of Pennsylvania and lead author of the University of Pennsylvania. 39; study. "It's a hot spot of unexpected diversity that persists for a long time."
The discovery changes what scientists previously thought about where the first fish evolved, said Michael Coates, a vertebrate palaeontologist from the University of Chicago, who was not involved in the study.
"Previously, we thought the first reef systems would be the cradle of diversification," said Dr. Coates. "But no, it seems that these first armored forms were in environments much closer to the coast. This explains why our first disc is so encrypted. "
The team does not know exactly why the fish evolved near the coast in clear, shallow lagoons and tidal flats whose depths were generally no greater than about 100 feet. They think that this may have to do with waves, sea level changes, runoff, rainfall, and other environmental factors in shallow water habitats.
"We have come to suspect that something is going on with water chemistry and potentially with oxygen levels in these active and dynamic environments," said Ivan Sansom, a paleobiologist from the University of California. University of Birmingham, England.
The team also found that when fish move in shallow waters, the most flexible swimmers, such as thelodonts, eventually leave and invade deeper areas such as coral reefs and the deep sea. They may have encountered molluscs, trilobites and formidable marine scorpions.
Over time, many of the lined armored fish became bottom dwellers and may have stayed in nearshore waters and moved to freshwater rivers and lakes.
Today, evolution has left us with two main groups of fish. Bony fish such as salmon, marlin and some 28,000 other species make up osteichthyes. And chondrichthyans are cartilaginous fish like sharks, rays and rays.
But some weird jawless fish are still hiding in the seas, such as myx and the great lamprey. They may not be armored, but they are reminiscent of the evolutionary footsteps and footsteps that ultimately led to all vertebrates, under water and down here on dry land.
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