Tracing the evolutionary origins of fish in shallow ocean waters

The first vertebrates on Earth were fish, and scientists believe that they first appeared about 480 million years ago. But the fossil record of this period is uneven, with only small fragments identified. There are 420 million years ago, however, the fossil record blossoms, with a wide variety of species of fish present en masse.

"It's the recurring question of, well, where were they?" says Lauren Sallan, a paleobiologist at the University of Pennsylvania. "Where were they hiding, what were their environmental origins?"

Sallan Ivan J. Sansom of the University of Birmingham and his colleagues are the first to present a wealth of evidence to answer this question in a report published this week in the newspaper Science.

And the answer, it seems, is near the shore, these areas often being described as an intertidal zone or shallow lagoons.

"In modern designs, we see that coral reefs are so important to fish biodiversity, so we assume that there is an old connection between fish and early reefs," says Sallan. "But decades of research in places such as the Cincinnati Arc have been empty."

"Instead, our work shows that almost all the major vertebrate divisions, from the oldest armored fish without jaws to sharks to our own ancestors – all started right on the beach, right next to the reef. if older groups have dispersed, new groups have also appeared on the shoreline. "

The findings help explain important features of the fossil record, such as: why so few primitive fish fossils are found intact; The action of the waves in the shallow ocean area probably exploded into tiny fragments. It also helps scientists to understand that, as evolution has evolved, many groups of fish have moved from seawater to freshwater. Some became freshwater fish, while others evolved to become the very first vertebrate tetrapods.

"They often went to freshwater before the reefs, which is an almost independent proof that they should have been near the shore before doing so," says Sallan.

The exact origin and diversification of vertebrates has been the subject of intense debate in paleontology. Some fossil groups from this key Middle Paleozoic period told a story, perhaps an original site of freshwater, while other groups could point to a place of birth at sea. , and still others arose in other types of habitat. To complicate matters, the original history of invertebrate biodiversity seems to be settled: they have diversified around coral reefs, their descendants then moving towards deeper or deeper waters.

Sallan, Sansom and his colleagues decided to investigate the issue of vertebrates using a Big Data approach.

"The good thing about the fossil record is that we often find fish in the context of their place of residence," says Sallan. "The rock that holds them tells us what their environment was like, whether it was a reef, shallow water, deep water, a riverbed or a lake."

Combining this environmental background with what we already knew about relationships between vertebrate trees of the Middle Paleozoic, 480 to 360 million years ago, researchers created a database comprising 2,728 early recordings of jaw and jawless fish.

"It's a really big new data set," says Sallan.

The team was then able to reconstruct the missing information in the fossil record using mathematical modeling, which allowed them to make informed predictions about the type of habitat in which the oldest ancestors of various groups of vertebrates appeared.

"For vertebrates, we find that they are native to this shallow and unexpected area of ​​the oceans, very restricted," Sallan said. "And they stay for a long time in this limited area after their emergence."

As they remained in the lowlands, however, they acquired various adaptations that allowed them to compete with others in a shared habitat. The researchers found that many groups had acquired characteristics that made them well adapted to life, either as inhabitants of the lowlands or for free-swimming ecology in the deep waters of the ocean.

A similar divergence has been observed in modern fish, such as sticklebacks, which have evolved into a bottom form and freestyle form of common ancestors in recent years.

No one, however, has conducted such a comprehensive study of living vertebrate species. "One of the things we want to know is whether these shallow waters are still the biological pump that feeds the reef," Sallan said. "Where is the current site of innovation?"

If this were the case, there could be a small consolation in the face of the massive disappearance of reefs all over the world; perhaps the shallow waters will continue to be the cradle of fish diversification, allowing biodiversity to continue despite the lack of reef habitat.

Explore further:
Fish fossils reveal the evolution of the tail

More information:
L. Sallan of the University of Pennsylvania at Philadelphia, PA et al., "The Coastal Cradle of Diversification of the First Vertebrates" Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aar3689

"Our origins in shallow waters," Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aau8461