Current specimen of Bathysaurus, a bony fish that lives in the depths. NOAA Oceans Explorer Program
About 130 million years ago, marine fauna experienced one of the most extreme migrations known. A portion of the population that lived in more superficial waters ventured into the inhospitable deep sea world in search of food, a journey into the depths that forced them to adapt to freezing temperatures, a pressure of 200 kilograms per square centimeter and the absence of Adjust light. Research published in the journal PNAS has discovered the oldest “footprints” or tracks left by fish in the deep sea plains of what was then the Tethys Ocean in Italy. And one of the keys to the discovery was 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 re-describe the evolution of marine fauna millions of years ago.
The fossilized remains of the oldest fish in deep-sea areas are around 50 million years old. Tracing their footprints or tracks at depth is very complex due to the low fossilization potential of the environment and the loss of all evidence during seafloor subduction, the sinking of a lithospheric plate below the edge of another plate. However, research 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) from the University of Genoa, has found new evidence pointing to the presence of fish , which have been coming from surface water to depths for more than 80 million years.
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“Fish have inhabited the deep seabed since at least the Early Cretaceous,” says Baucon, who places the traces found in the Barremian and Hauterivian ages of the geological soils. The traces found correspond to those left by demersal fish when they feed.
This progress contradicts the main thesis previously held, which attributed the conquest of the sea floor to a recent oceanic anoxic event (decrease in oxygen). However, the new research argues that the fish were migrating before this event and that it was not this lack of oxygen that was the trigger, but rather the explosion of resources on the sea floor.
A “party” at a depth of 2,000 meters
“In the early Cretaceous,” explains Baucon, “there was a dramatic increase in ocean productivity. “It was a feast for sediment-feeding or sediment-feeding animals (e.g. crustaceans, worms) and for fish that fed on these small creatures.”
According to the researcher, this “party” took place in the deep sea level 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. “It is most likely,” Baucon clarifies, “that there were three species: a toothless neoteleost, another fish that resembled a modern chimera with impressive 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 trace fossils discovered at the site.”
Examined fossilized footprints (a and c) and current trace of a fish on the ground (b).Andrea Baucon
One of the tracks shows feeding grooves, which, according to the paleontologist, “require the presence of teeth to scrape and produce the sediment.” “On the other hand, the tracks, which are circular depressions, suggest the absence of teeth to create a jet of water that could create the feeding wells (depressions) studied.” Other tracks are sinuous grooves that ensure contact a long tail fin with the ground,” explains the researcher.
Ichnology, the study of the traces (or records of activity) left by living things in sediments or rocks, requires ancient and current examples to establish parallels that support conclusions. In this way, the fossil tracks (or ichnofossils) found in Italy were compared with those left by modern species when they fed, such as those produced by modern holocephalic cartilaginous fish (chimeras) at a depth of 1,500 meters in the Pacific Ocean. Or the depressions left in the Ligurian Sea (Mediterranean) by the sophisticated suction devices characteristic of the Neoteleost group, which bear many similarities to the studied ichnofossils.
Current footprint of a Mojarra left in a Lepe estuary. Fernando Muñiz and Zain Belaústegui
However, the fundamental key that was able to guide the research from the beginning was a previous study of the tracks left by sparids of the genus Diplodus (Mojarras) in the shallow intertidal zones of the Piedras river mouth 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 branch channels of the Nueva Umbría arrow in the Piedras River. “At low tide, a variety of species can be seen interacting with the mud at the bottom, including fish, worms, crustaceans, snails and mussels. Once the time of low tide is reached and the surface is exposed, the structures left by these species can be observed. Among the numerous footprints observed, some were particularly very similar to those found by the paleontologist Baucon in Italy, which were dated to 130 million years ago and interpreted as traces left in the deep sea,” comment the experts.
The connection with the leprosy mojarra
“The Mojarra leaves two grooves with its incisors as it eats the microorganisms in the mud. In other cases, they bump their noses against the walls of the canals, leaving a different type of structure behind. Until our research, when these structures fossilized, they were attributed to arthropods and not fish, as these types of invertebrates also leave very similar traces. It is therefore a clear example of one of the principles of ichnology: different organisms can leave similar footprints or traces,” explains Muñiz. “But the traces of the Mojarras give the clue to think of other species from deeper areas, especially demersal fishes, which, after a process of adaptation, interacted with the bottom to conquer spaces with more pressure, less temperature and almost no light,” they explain . Researchers.
This adaptation process was not easy and forced species 130 million years ago to develop skills similar to those of fish that live in deep areas today. “Modern fish suffering from similar conditions exhibit altered eyes, low-density tissues and slow metabolism. In the deep sea, the pressure is so high that it destabilizes proteins; To counteract the destabilizing effects of pressure, deep-sea fish, for example, have high concentrations of trimethylamine oxide [compuesto orgánico producto de la descomposición] in their tissues. That’s why they tend to smell so bad. Deep-sea fish have evolved a mechanism for secreting oxygen molecules that eliminates the need to suck in air from the surface to inflate the swim bladder,” explains Baucon.
“These adaptations are as surprising as those that allowed vertebrates to colonize the air and land, i.e. the appearance of wings for flying and limbs for walking,” concludes the paleontologist.
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