Fish started to live in the deep sea at least 130 million years ago 鈥� 80 million years earlier than we thought 鈥� fossils found in Italy suggest.
鈥淭he discovery was like finding astronaut footprints on the moon,鈥� says study author from the University of Genoa in Italy. 鈥淲e discovered when and where vertebrates colonised the deep sea.鈥�
The deep sea is an unpromising habitat for fish: It is frigid, pitch-black, periodically swept by powerful currents, and characterised by high-pressure conditions that can crush even hard shells. In the Early Cretaceous Period 鈥� from 145 to 100 million years ago 鈥� only certain species of mushy worms, molluscs and were thought to have adapted to thrive in such forbidding environments.
Advertisement
Yet, in the rocks of the Italian Apennine mountains, which formed in the Early Cretaceous at the bottom of a prehistoric ocean known as Tethys, stumbled upon ancient marks 鈥� or 鈥渢race fossils鈥� 鈥� left behind by something bonier. Preserved in what was once the seafloor, they found scrapings where bowl-like mouths had dug through the mud. There were also parallel raking marks left by the teeth of foraging fish, and sine-wave-shaped trails showing where tail fins had dragged through the sediment as the fish swam near the seafloor.
鈥淲hile the fossil skeletons tell us what the fish was like,鈥� says Baucon. 鈥淭race fossils tell us how ancient fishes behaved. We can act like psychologists.鈥�
The researchers compared the ancient prints to those left by fish currently living in the depths of the ocean that feed by either raking or suctioning the seafloor. The traces look identical.
鈥淭hey were probably a lot like modern ghost sharks, ratfish, chimaeras and rabbitfish. They鈥檙e weird and interesting fish,鈥� says at the University of Utah, who was not involved in the research. 鈥淚t鈥檚 also possible that bony fish like modern sea breams and porgies made some of the traces, and these probably looked like 鈥榥ormal鈥�, perch-like fish that you鈥檇 find at any seafood market.鈥�
Given the variety of fossilised markings, Baucon鈥檚 team thinks they must have been made by at least three different ancient deep-sea fish species, suggesting that what he calls the 鈥減ioneers of the abyssal plains鈥� represented a complex community.
Crucially, the findings suggest fish 鈥� and therefore vertebrates 鈥� started moving out of the shallows and into the unwelcoming ocean abyss 80 million years earlier than scientists previously thought.
The traces were also found in the same areas as deep-sea worm burrows. Since these invertebrates would have served as delicious snacks for the fish, Baucon thinks a sudden increase in the availability of prey may have encouraged the fish to shift from the shallow coast to the deep abyss.
Evolutionary trees constructed on the basis of molecular differences between species suggest that deep-sea fish lineages emerged in the Late Jurassic or Early Cretaceous, says at the University of Oklahoma, who was not involved in the study. 鈥淗owever, prior to this study, there was no direct evidence from the fossil record of deep-sea fishes until the Late Cretaceous. This study provides that [missing earlier] evidence.鈥�
The fossils bridge a gap in scientific knowledge, says at the University of Zurich, Switzerland, who was not involved in the study. It鈥檚 really hard to find fossils from the bottom of the ocean from such an early phase in prehistory, because the geological crust beneath the oceans is constantly created and destroyed 鈥� which means most ancient deep-sea fossils have been destroyed, too.
鈥淭his is an excellent example of how we can learn from seemingly insignificant or not particularly beautiful fossils,鈥� says Klug. 鈥淪uch discoveries represent super-rare insights into the ecology of ocean floors in the past.鈥�
PNAS
