When tetrapods (four-limbed vertebrates) began moving from water to land about 390 million years ago, they sparked the rise of lizards, birds, mammals, and all land animals that exist today, including humans and some aquatic animals like whales and dolphins.
The earliest tetrapods come from their fish ancestors in the Devonian period and are more than twice as old as the oldest dinosaur fossils. Resembling a cross between a giant salamander and a crocodile, they were about 1 to 2 meters long with gills, webbed feet and caudal fins, and were still strongly attached to water. Their short arms and legs had up to eight digits on each hand and foot, and they were likely ambush predators lurking in the shallow water, waiting for prey to approach.
Scientists know how fish's fins turned into the limbs of tetrapods, but controversy continues over where and how the earliest tetrapods used their limbs. And while many hypotheses have been proposed, very few studies have rigorously tested them using the fossil record.
In an article published on January 22nd in Advances in science An international team of researchers studied three-dimensional digital models of the bones, joints and muscles of the fins and limbs of two extinct early tetrapods and a closely related fossil fish to find out how the function of the foreleg changed as the fins evolved into limbs. The research, led by Julia Molnar, Assistant Professor at the New York Institute of Technology College of Osteopathic Medicine, and Stephanie Pierce, Thomas D. Cabot, Associate Professor of Organismic and Evolutionary Biology at Harvard University, discovered three distinct functional stages in the transition from fins to limbs and that these early tetrapods had a very distinct pattern of muscle strength that did not look like a fish fin or modern tetrapod limbs.
To reconstruct the functioning of the limbs of the earliest known tetrapods, Molnar, Pierce, and co-authors John Hutchinson (Royal Veterinary College), Rui Diogo (Howard University), and Jennifer Clack (University of Cambridge) first had to figure out what muscles were present in fossil animals. A challenging task since muscles are not preserved in fossils and the muscles of modern fish fins are completely different from those of the tetrapod limbs. The team spent several years answering the question of how exactly the few simple muscles of a fin became dozens of muscles that perform all sorts of functions in a tetrapod limb.
"Determining what muscles were in a 360-million-year-old fossil took many years of work to get to the point where we could begin building very complicated models of the musculoskeletal system," Pierce said. "We needed to know how many muscles there were in the fossil animals and where they attached to the bones so we could test how they worked."
They built three-dimensional musculoskeletal models of the pectoral fin in Eusthenopteron (a fish closely related to tetrapods that lived in the late Devonian about 385 million years ago) and the forelegs of two early tetrapods, Acanthostega (365 million years old, living towards the end of the Late Devonian Period) and Pederpes (348-347 million years old, living in the Early Carboniferous Period). For comparison, they also built similar models of the pectoral fins of live fish (coelacanth, lungfish) and the front legs of live tetrapods (salamanders, lizards).
To determine how the fins and limbs function, the researchers used computer software originally designed to study human locomotion. This technique has recently been used to study locomotion in ancestors of humans and also dinosaurs such as T-rexbut never in something as old as an early tetrapod.
By manipulating the models in the software, the team was able to measure two functional characteristics: the maximum range of motion of the joint and the ability of the muscles to move the fin or limb joints. The two measurements would reveal compromises in the musculoskeletal system and enable researchers to test functional hypotheses in extinct animals.
The team found that the forelegs of all terrestrial tetrapods went through three different functional stages: a "benthic fish" stage that resembled modern lung fish, an "early tetrapod" stage that does not resemble an extinct animal, and a "crown tetrapod" stage with characteristics of both lizards and salamanders.
"The fin out Eusthenopteron had a pattern reminiscent of the lungfish, which is one of the closest living relatives of tetrapods, "said Pierce." But the early limbs of the tetrapod showed more similarities to each other than either fish or modern tetrapods. "
"That was perhaps the most surprising," said Molnar. "I thought Pederpes, and possibly Acanthostegawould fall fairly well within the realm of modern tetrapods. But they formed a group of their own that didn't look like a modern tetrapod limb or a fish fin. They weren't in the middle but had their own collection of traits that likely reflected their unique surroundings and behavior. "
The results indicated that early tetrapod limbs were more suitable for propulsion than weight loading. In the water, the animals move forwards or backwards with their limbs so that the water can support their body weight. However, in order to move on land, the animal must go against gravity and push down with its limbs to support the body mass.
This does not mean that early tetrapods were unable to move on land, but that they did not move like a modern living tetrapod. Their means of locomotion were likely unique to these animals, who were still tightly attached to the water but also ventured on land, where there was plenty of opportunity for vertebrate animals but little competition or fear of predators.
"These results are exciting because they independently support a study I published last year using completely different fossils and methods," said Pierce. "This study, which focused on the humerus, showed that early tetrapods had some land movement ability, but they may not have been very good at it."
Researchers are closer to reconstructing the evolution of terrestrial locomotion, but more work is needed. You plan to next model the hind limb to examine how all four limbs work together. It has been suggested that early tetrapods used their front legs for propulsion, but modern tetrapods derive most of their propulsive power from the hind limb.
"We plan to look for evidence of a shift from forward leg locomotion to hind leg locomotion as seen in modern tetrapods," Molnar said. A joint look at the fore and hind limbs could tell more about the transition from water to land and how tetrapods eventually ruled the earthly realm.