In Science Advances, an international team of researchers examined 3D digital models of the bones, joints and muscles of the fins and limbs of two extinct early tetrapods and closely related fossil fish to figure out how the forelimb functioned, and what changed when the fins evolved into limbs.
When quadrupeds vertebrates began to move from water to land about 390 million years ago, this led to the emergence of lizards, birds, mammals and all land animals that exist today, including humans and some aquatic vertebrates such as whales and dolphins.
The earliest tetrapods descended from their fish ancestors in the Devonian period and were more than twice the age of the oldest dinosaur fossils. They resembled a cross between a giant salamander and a crocodile, were about 1 to 2 meters long, had gills, webbed feet and tail fins, and were still strongly attached to water. Their short arms and legs had up to eight fingers, and they were likely predators that hid in shallow water, waiting for their prey to approach.
Scientists know how the fins of fish evolved into the limbs of tetrapods, but controversy remains about where and how the first tetrapods used their limbs. Although there have been many hypotheses, very few studies have thoroughly tested them using fossils.
The study identified three different functional stages in the transition from fins to limbs, and that these early tetrapods had a very distinct pattern of muscle levers that did not resemble fish fins or modern quadrupeds.
To reconstruct how the limbs of the earliest known tetrapods functioned, one first had to figure out which muscles were present in the fossil animals. A difficult task, since they were not preserved in the fossils, and the muscles of the fins of modern fish are completely different from the muscles of the limbs of tetrapods. The team spent several years trying to answer the question: how exactly did a few simple fin muscles become dozens of muscles that perform all kinds of functions in the limbs of tetrapods?
“Determining which muscles were present in the 360 million-year-old fossil took many years of work to get to the point where we could start building very sophisticated models of the musculoskeletal system. We needed to know how many muscles the fossil animals had and where they were attached to the bones so we could test how they functioned.”
Stephanie Pearce, Associate Professor, Department of Organic and Evolutionary Biology, Harvard University
They built a three-dimensional musculoskeletal model of the pectoral fin of the Eusthenopteron (fish closely related to the tetrapods that lived in the Late Devonian about 385 million years ago) and the forelimbs of two early tetrapods Acanthostega (365 million years old, living in the late Late Devonian) and peders ( 348-347 million years old, primitive four-legged “amphibious” animal of the early Carboniferous era). For comparison, they also built analogous models of the pectoral fins of living fish (coelacanth and lungfish) and the forelimbs of modern tetrapods (salamanders and lizards).
To determine how fins and limbs work, the researchers used computer software originally developed to study human movement. This method has recently been used to study movement in the ancestors of humans as well as dinosaurs, but has never been used in ancient animals such as the early tetrapods.
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 fins and limb joints.
The team found that the forelimbs of all terrestrial tetrapods went through three different functional stages: the benthic fish stage resembling modern lungfish, the early tetrapod stage, and the crown tetrapod stage with characteristics of salamanders and lizards.
The results showed that the limbs of the early tetrapods were more adapted to movement than to exercise. In water, animals use their limbs to move forward or backward, allowing the water to support their body weight. However, walking on land requires the animal to act against gravity and push down with its limbs in order to maintain its body mass.
This does not mean that the first tetrapods were unable to move on land; rather, they did not move like modern tetrapods. Their vehicles were probably unique to these animals, which were still very attached to water but also made landfalls where there was plenty of opportunity for vertebrates but little competition or fear from predators.