Mojarra footprints at Lepe reveal extreme fish migration to deep-sea areas 130 million years ago | Science

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Mojarras in Lepe

About 130 million years ago, marine life experienced one of the most extreme migrations known. A portion of the population that lived in shallower waters ventured into the inhospitable deep-sea world in search of food. A journey into the depths forced them to adapt to freezing temperatures, pressures of 200 kilograms per square centimeter and the absence of light. A study published in the journal PNAS has discovered in Italy the oldest “footprints” or tracks left by fish in the deep sea plains of what was then the Tethys Ocean. And one of the keys to the discovery were the traces left by today’s mojarras in the mud of the estuaries of the Huelva coast of Lepe, described in a previous study. An unexpected connection that allows us to redefine the evolution of marine life millions of years ago.

The oldest fossilized fish remains found in deep sea areas are around 50 million years old. Following their tracks at depth is very complex due to the low fossilization potential of the environment and the loss of all evidence in seafloor subduction, the sinking of one lithospheric plate below the edge of another plate. However, an investigation led by Andrea Baucon, a paleontologist and ichnologist (scientist who studies the traces or signs of activity left by living organisms in sediments or rocks) of the University of Genoa, has found new evidence pointing to the presence of Fishing points from surface waters at depths to more than 80 million years.

“Fish have inhabited the deep-sea floor at least since the Lower Cretaceous,” says Baucon, who classifies the traces found in the Barremian and Hauterivian of the geological soils. The traces found correspond to those left by demersal fish when feeding.

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This progress is in contradiction to the main thesis put forward so far, which attributed the conquest of the seafloor to a more recent oceanic anoxic event (depletion of oxygen). However, the new research argues that the fish were migrating before this event and that it wasn’t this lack of oxygen that was the trigger, but the explosion of resources on the sea floor.

A “party” at 2,000 meters depth

“In the early Cretaceous,” explains Baucon, “there was a dramatic increase in ocean productivity. It was a feast for sediment-eating or sediment-eating animals (e.g., crustaceans, worms) and for the fish that fed on these small creatures.”

According to the researcher, this “party” took place in the deep sea plain of what was then the Tethys Ocean at a depth of around 2,000 or 3,000 meters. The work describes feeding signs of various fish. “Most likely,” Baucon clarifies, “there were three species: a toothless neoteleost, another fish that resembled a modern-day chimera with imposing teeth, and a third species that had a large caudal fin; All of this can be inferred from the morphology of the three types of fossil record discovered at the site.”

Examined Fossil Footprints (A And C) And Actual Track Of A Fish On The Ground (B).

One of the tracks shows gullets that, according to the paleontologist, “require the presence of teeth to scrape and produce the sediment.” “On the other hand, the tracks, which are circular indentations, suggest the absence of teeth to create a jet of water that could be used to create the feeding wells (indentations) studied. Other traces are winding grooves that represent the contact of a long caudal fin with the ground,” explains the researcher.

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Ichnology, the study of the tracks (or records of activities) left by living things in sediments or rocks, requires ancient and contemporary examples to draw parallels that support the conclusions. In this way, the fossil traces (or ichnofossils) found in Italy were compared to those left by modern-day species feeding, such as those produced by modern holocephalic cartilaginous fish (chimeras) at a depth of 1,500 meters in the Pacific Ocean. Or the indentations left in the Ligurian Sea (Mediterranean) by the highly developed sucking apparatus characteristic of the Neoteleos group, which has many similarities with the studied ichnofossils.

Current Footprint Of A Mojarra Left In A Lepe Estuary.

But the fundamental key that allowed us to guide the research from the beginning was a previous study of the tracks left by the Sparids of the genus Diplodus (Mojarras) in the shallow intertidal zones of the Río Piedras estuary in Lepe (Spain) .

Between 2010 and 2015, Fernando Muñiz and Zain Belaústegui, ichnologists and professors at the Universities of Seville and Barcelona, ​​led the study of structures created by Mojarras in the side channels of the Nueva Umbría outflow in the Piedras River. “At low tide, a variety of species can be observed interacting with the bottom mud, including fish, worms, crustaceans, snails and mussels. Once the tide goes out and the surface is exposed, you can see the structures left by these species. “Among the numerous footprints observed, some in particular were very similar to those found in Italy by the paleontologist Baucon. They are 130 million years old and are believed to have been left in deep marine environments,” the experts comment.

The Lepera-Lepera connection

“The Mojarra makes two furrows with its incisors when it eats the microorganisms in the mud. In other cases, the nose will bounce against the walls of the ducts, leaving a different type of structure. Until our research, when these structures fossilized, they were attributed to arthropods and not fish, since these types of invertebrates also leave very similar tracks. It is therefore a clear example of one of the principles of ichnology: Different organisms can leave similar marks or marks,” explains Muñiz. “But the tracks of the mojarras give us the clue to think about other species from deeper areas, especially demersal fish, which, after an adaptation process, interacted with the bottom to conquer spaces with more pressure, lower temperature and almost no light,” they explain. Researchers.

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This adaptation process was not easy, forcing the species 130 million years ago to develop similar abilities as the fish that live in deep areas today. “Modern fish suffering from similar conditions show altered eyes, low-density tissues and slow metabolism. In the depths of the sea, the pressure is so high that it destabilizes the proteins; To counteract the destabilizing effects of pressure, deep-sea fish, for example, have high concentrations of trimethylamine oxide (an organic breakdown product) in their tissues. That’s why they usually smell so strong. “Deep-sea fish have evolved a mechanism for secreting oxygen molecules that eliminates the need to breathe air at the surface to inflate the swim bladder,” explains Baucon.

“These adaptations are as surprising as those that allowed vertebrates to colonize air and land, that is, the appearance of wings for flying and limbs for walking,” the paleontologist concludes.

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