Four-limbed animals from all corners of the planet have several things in common: one of them is that we have five digits (in the case of birds, we would speak in the past tense) at the end of each of our limbs. But, when did these digits appear? as was? At what point did the fish emerge from the depths and begin to feed in shallower and shallower waters and finally crawl to dry land? The answer to these questions has been a great mystery in evolutionary biology until now.
A team of paleontologists has discovered the earliest evidence for this anatomical feature in a fish’s fin, Elpistostege watsoni from the Devonian newspaper, about 380 million years ago. These fossils feature rudimentary fingers that mark one of the most important transitions in the evolution of vertebrates on the planet.
The closest fish to tetrapods
To understand the evolution from a fish fin to a tetrapod limb, paleontologists study the fossils of lobe-finned fish and tetrapods from the Middle and Upper Devonian (393-359 million years ago) known as Elpistostegalians . These include the well-known Tiktaalik roseae from Canadian Arctic, Late Devonian, of which only incomplete but excellently preserved specimens are available (discovered in Canada in 2004).
The remarkably complete new specimen of Elpistostege, which was 1.57 meters long, includes the first full-arm skeleton seen in this order of fish . The researchers, using high-energy CT scans, were able to discover the characteristics of the tetrapod-like arm in the pectoral fin of the fossil. Among the structures observed in the study, they found a humerus (arm), radius and ulna (forearm), carpal row (wrist) and phalanges organized in digits (fingers).
” This is the first time we have unequivocally discovered fingers caught in a fin in any known fish ,” says John Long, professor of paleontology at Flinders University in Australia. “The jointed fingers on the fin are like the finger bones found in the hands of most animals.”
Vertebrates were able to leave the water and conquer the land
The ‘fingers’ of Elpistostege watsoni – which looked like a shark with powerful sharp fangs, a flat head, long snout and small round eyes – allowed our primitive sea-dwelling ‘relative’ to transition from water to land, experts say.
The bones are not exactly real fingers, as they are tucked inside the fin like a mitten and cannot move freely, but it does confirm the animal as an intermediary between fish and tetrapods.
“This finding pushes back the origin of digits in vertebrates to the level of fish , and tells us that the pattern for the vertebrate hand was first developed in deep evolution, just before the fish left the water “, clarify the authors in the journal Nature that collects the study.
“The origin of the digits is related to the development of the fish’s ability to support its weight in shallow water or for short trips on land. A greater number of small bones in the fin allows more planes of flexibility to distribute their weight across the fin, ”they continue.
However, Elpistostege is not necessarily our ancestor , but it is the closest to a true transitional fossil, an intermediary between fish and tetrapods. It was the largest predator living in the shallow marshes of Quebec at the time and would have fed on extinct lobe-finned fish found fossilized in the same Canadian deposits.
Referencia: Elpistostege and the origin of the vertebrate hand, Nature (2020). DOI: 10.1038/s41586-020-2100-8 , https://nature.com/articles/s41586-020-2100-8
