Vertebrates first appeared in shallow marine habitats around 500 million years ago. They then transitioned to land nearly 370 million years ago and took to the skies around 200 million years ago. For the deep seas, the first fossil evidence found dated back 50 million years, leaving scientists to wonder why it took so much longer to occupy than land and air, that seem more distant and diverse than vertebrates’ original habitat.
The authors of a new study1, led by Andrea Baucon at the University of Genoa, found fossil traces in three sites in Central Italy, two in the Northern Apennines and one on the Tyrrhenian coast near Livorno, that were probably of fish inhabiting the depths of the sea as early as 130 million years ago.
“Trace fossils are resilient to processes that obliterate body fossils, and do not decompose,” Baucon explains. Trace fossils provide information about the environment where they were deposited. “Finding a fish body fossil in a deep-sea deposit would not necessarily prove that it was living at abyssal depths, because when a fish dies its remains could be transported far from its original habitat.”
Researchers found bowl-shaped traces that resemble those left by modern fish, called chimaera and living in Pacific Ocean trenches, when they plunge their mouths into the sediment to find food. They also found swimming traces in the form of sinusoidal incisions in the rocks, that could be attributed to smaller fishes looking for food in the sediment that bigger animals had just stirred up. The third type of trace fossils were parallel tracks, which researchers interpreted as produced by fishes scratching the seabed with pairs of teeth separated by a space.
The authors used a technique called biostratigraphy to date the fossils. “The trace-bearing rocks preserve the fossil remains of microscopic organisms that were deposited when the rocks were soft mud,” says Baucon.
The findings push back the occupation of the abyssal plain by 80 million years, which means it would still have occurred after colonization of land and air, but with a shorter gap. Baucon says that there are at least two reasons why the deep sea may have been the last habitat to be colonized by vertebrates.
First, the absence of light, high pressure, and very low temperature, could have required more drastic adaptations than those needed to live in the land and sky. Second, around 130 million years ago, the deep seas may have become richer in food, thanks to a new input of organic matter fueled by the high and rapid diversification of flowering plants on land. The authors also found vertical burrows in the rocks, which were possibly excavated by worms living in the seabed and feeding on this organic matter.