Fossilised fish from around 375 million years ago may have been able to use their fins to prop themselves up at the bottom of lakes or even venture out of the water.
An American research team used CT scanning and imaging software to observe the shape and structure of fins while still encased in surrounding rock.
Structural changes to the fins, including a lack of symmetry that enabled a palm-like ‘flexing’, were instrumental during vertebrate evolution.
This process turned water-bound fish into today’s four-legged animals — which experts refer to as tetrapods.
Researchers found several trends leading to the formation of digits as creatures began to develop limbs fit for walking on land.
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A fossil cast of a fin from a juvenile Sauripterus taylori, a late Devonian fish with primitive features of tetrapods
Artist’s reconstruction of the tetrapodomorphs: From top to bottom Sauripterus taylori, Eusthenopteron foordi and Tiktaalik roseae
EVOLUTIONARY HISTORY OF FISH FIN
Pectoral fins are located on each side and are equivalent to the forelimbs of tetrapods, a group of vertebrates that includes today’s amphibians, reptiles, birds and mammals.
Pelvic or ventral fins are typically located below and behind the pectoral fins and are equivalent to the hindlimbs of tetrapods.
Dorsal fins are located on the back of fish and were lost in the evolution of tetrapods.
‘We’re trying to understand the general trends and evolution of the dermal skeleton before all those other changes happened and fully-fledged limbs evolved,’ said Thomas Stewart, a biologist at the University of Chicago and leader of the study.
‘Animals went from swimming freely and using their fins to control the flow of water around them, to becoming adapted to pushing off against the surface at the bottom of the water.’
Dr Stewart and his team used CT scanning to study fin rays — the skeletal rods that support the fin web, the fleshy part of a fish’s fin — of three adult and juvenile fish specimens: Sauripterus taylori, Eusthenopteron foordi and Tiktaalik roseae.
The first two species were believed to have been fully aquatic and used their pectoral fins — the set of fins just behind a fish’s head — for swimming.
They may also have been able to use their fins to prop themselves up on the bottom of lakes and streams.
Tiktaalik, meanwhile, may have been able to support most of its weight with its fins and perhaps could have even used them to venture out of the water for short trips across mudflats.
The researchers studied the pectoral fin of the fish, which are homologous, or related to, the forelimbs of tetrapods – the four-legged animals of today
‘By seeing the entire fin of Tiktaalik we gain a clearer picture of how it propped itself up and moved about,’ said Neil Shubin, senior author of the study.
‘The fin had a kind of palm that could lie flush against the muddy bottoms of rivers and streams.’
Each fish fin has a series of skeletal rods, which form two halves.
In the Eusthenopteron and Tiktaalik fossils, researchers observed skeletal rays in the top half the fin that were slightly larger and longer than those on the bottom.
Tiktaalik roseae, a 375-million-year-old ‘fishapod’ has features of both fish and four-legged tetrapods
This increasing asymmetry suggests that the fish developed muscles that extended on the underside of its fins — akin to the fleshy base of the human palm — to flex their fins and help support its weight.
‘Asymmetry was an important precursor to the architecture of limbs, foreshadowing the anatomical and functional differences between the flexors and extensors of digits in terrestrial tetrapods,’ the researchers write in Proceedings of the National Academy of Scientists.
Skeletal rays of the pectoral fin of the Sauripterus taylori in 3D scan (a) and photograph of the fossil showing full extent of the fin web (b)
The three specimens are thought to be early relatives of tetrapods — today’s class of four-limbed animals.
The fossils date from the Devonian geologic time period that occurred between 416 million to 358 million years ago.
Paleontological research into the transformation from fin to limb evolution has focused on the evolution of the endoskeleton — the large, distinct bones and pieces of cartilage that correspond to those of our upper arm, forearm, wrist and digits.
The delicate rays and spines of a fish’s fin ray structure form a second, no less important ‘dermal’ skeleton.
Combined with previous findings on the fish endoskeleton, this new study helps fill a knowledge gap in the development of limb architecture in tetrapods.