The ancient fish have evolved in the shallow seas – the very places that humans are threatening today.

It took about 480 million years for these fish to evolve and adapt to different environments and become the many different species (including ourselves) with boneless ridges, known as vertebrates.

But how did it happen?

The field of paleontology looks at when or and Why animals evolving with time and according to their environment. Today, a new paper by Lauren Sallan of the University of Pennsylvania (along with other American and British colleagues) answers the "or" question about our ancestors more distant fish.

For the first time, the work shows that shallow coastal lagoons were vital in the early days of fish evolution. Today, similar seas ("plateau seas") include places such as the Bass Strait, where water forms less than 200 m deep around a continental shelf.

These places are still important today to maintain the biodiversity of life on Earth – so if we do not take care of our shallow oceans, the long-term consequences for marine life could be disastrous.

Invasion of the ocean

To conduct their research, scientists used a database of 2,827 species of fossil vertebrates. These covered the 120 million years that preceded the appearance of fish (about 480 million years ago) until their conquest of the territory as creatures to four branches known as tetrapods (about 360 million years ago). The database has mapped the habitats of species according to their time ranges and has identified important steps in the formation of new species (known as diversification).

The research points out that littoral coastal environments, including estuarine and lagoon environments, were hotspots for major episodes of early vertebrate diversification – not open oceans where we might think that fish would naturally undergo evolutionary events.

Robust body fish (such as finfish species such as fish-lungs) then searched for habitats closer to the land to diversify, while small-scale thin forms such as jawless thelodontids (such as yellow fish in the picture below) and some sharks to go deeper waters.

Lead researcher Lauren Sallan said, "This research introduces an" invasion of the ocean "just as the invasion of the land did." Our early ancestors moved quickly into freshwater, but then moved to the reefs, then to the ocean and to the new continents., required a lot of time and change. "

Fish with and without jaws

The first diversification of the first jawless fish (resembling the lamprey and hagfish today) led to the appearance of many groups of armored fish at the beginning of the Silurian period (about 420 million years ago). The armored fish had hard plaques on the body and most lived in the shallow inland seas of the continents of the Northern Hemisphere.

The oldest fossil of jawless fish comes from Australia, as represented by simple torpedo-like shapes, such as Arandaspis who lived in the shallow seaway of Larapintine across Australia about 470 million years ago.

The first large ray of jaw fish (called "gnathostomes") occurred in China about 440 million years ago. At the beginning of the Devonian period (419 million years ago), all major groups of jaw – fish had appeared and dispersed in all regions of the world.

Other fish have evolved in distinct regions, such as the East Gondwana Province (Australia and Antarctica) and in the Euramerica supercontinent (a continental mass combining much of North America and North America). from Europe).

The animals known as "Wuttagoonaspis fauna" included many strange forms of placoderm and other primitive fish, and are the oldest local group of vertebrates in Australia. The remains of these distinctive fish that lived about 395 million years ago are found in many sites in an area of ​​two million square kilometers in central Australia.

The historic reefs were different

Reefs are well known for being hot spots of biodiversity – today, several thousand species of fish live around coral reefs. But we now know that this has not always been the case.

Prehistoric reefs, known for about 500 million years, have been built by different organisms over time. Devonian reefs, home to many species of fish, were mainly constructed of algae and sponges, with a smaller contribution from corals.

The Devonian Kimberley reefs were home to more than 50 species of ancient fish living on algal structures lining most of northwestern Australia.

But new research shows that the Silurian and Devonian reefs did not exhibit unusually high diversity, as previously assumed. This suggests that reefs played a much less important role than we thought in the early rise of jawed fish.

Take care of our shallow waters

The message of the new document is that the cradle of first vertebrate diversification has occurred mainly in shallow water habitats close to or across a continent's borders.

The steady rise and fall of sea level over time would have had a significant effect on the habitat size of these marine species, making them particularly vulnerable to extinction when communities lived in basins with low oxygen levels.

These extinctions involved many factors and eliminated some of the dominant fish groups at the end of the Devonian, including placoderms. This has allowed modern fish groups, consisting mainly of bony fish and sharks, to establish themselves.

The research has some implications for today, as one of the authors of the paper, Ivan Sansom of the University of Birmingham, has explained: "This work highlights the problem. importance of these increasingly vulnerable coastal areas for the evolution of species.Modern threats resulting from a combination of sea level rise, overfishing and pollution could have effects extremely damaging to the future diversification of species. "

Explore further:
Tracing the evolutionary origins of fish in shallow ocean waters