The strange life-forms lurking in Earth's deepest seas may seem primordial, as if they are as old as the oceans themselves. But the truth is that these fish and other animals took hundreds of millions of years to adapt to the pressure, cold and darkness of the depths, and a recent study helps pinpoint vertebrates' entry into this inky realm. The rarity of deep-sea fossils had previously left paleontologists with only a patchy understanding of life in the ancient deep. Now fossilized feeding traces created by hungry fish, described in the Proceedings of the National Academy of Sciences USA, indicate they were swimming in the deep seas by 130 million years ago.
The telltale fossils were first discovered a decade ago in Italy's Palombini Shale Formation, which once lay roughly 6,500 feet below the surface of the Cretaceous seas. These fossils didn't preserve the ancient fish themselves but rather recorded their behavior: the fish left behind a variety of pits, scrapes and sinuous trails in the sediments on the ocean floor.
No one knew what to make of these markings at first. “It took more than 10 years to realize the true nature of the trace fossils,” says Andrea Baucon, a paleontologist at the University of Genoa. But after he observed modern Mediterranean fish creating similar structures, he says, “a light went on in my head.” By studying the behavior of living fish, Baucon and his colleagues identified what probably created the ancient marks. The pits, for example, appear to have been made by fish that could jet water at the sediment to reveal hiding prey. The distinctive two-lobed scrapes were created by fish mouthing at the bottom to catch worms, and the S-shaped trails were created by a fish flicking its tail back and forth as it swam just above the seafloor. The results suggest that several fish species were living in deep waters and hunting for squishy prey by the Early Cretaceous period—37 million years earlier than indicated by previous fossil evidence of deep-sea vertebrates.
“The study is a superb example of how trace fossils can fill huge gaps in our understanding,” says Emory University paleontologist Anthony Martin, who was not involved in the work.
This updated time line also hints at why fish adapted to such ocean extremes—they may have been following their wormlike invertebrate prey, Baucon says. These invertebrates spread because of changes in the nutrients available in the ocean's depths. Deep-sea fish are carnivores, explains marine biologist Elizabeth Miller of the University of Oklahoma, who was not involved in the work. So “without something to eat [besides] each other, it's difficult to imagine fishes making a living in the deep sea.”