An article published in the journal “Science Advances” reports a study on the evolution of the forelimbs’ skeleton and muscles in the transition from fish to tetrapods. A team of researchers led by Julie Molnar of the New York Institute of Technology and Stephanie Pierce of Harvard examined three-dimensional models of the bones, joints, and muscles of the fins and limbs of two now-extinct early tetrapods and a fish closely related to them. The conclusion is that those early tetrapods had anatomical features more suited to propulsion than to support their weight.
Tetrapods emerged about 390 million years ago, in the Devonian period, beginning the colonization of the mainland which led to the evolution of reptiles, dinosaurs (and therefore birds), and mammals. The evolution from fish fins to tetrapod limbs has already been studied on various occasions and is now quite well known. However, there are still discussions about the use of those primitive limbs by early tetrapods.
In recent years, the application of modern technologies to paleontology made it possible to study fossils in ways that were unthinkable until a few decades ago. The adaptation of tests such as the CT-scan to the study of fossils now allows to discover all the details of a fossil without damaging it and to create three-dimensional reproductions of them. This also allows to analyze how extinct animals moved.
For this study, the researchers examined fossils of two animals that lived in the Devonian period, a fish of the species Eusthenopteron foordi and an early tetrapod of the species Acanthostega gunnari, and of another early tetrapod of the species Pederpes finneyae (the reconstruction in the image, courtesy Julie Molnar, shows the bones of the foreleg) that lived in the Carboniferous period, which followed the Devonian. The anatomy of these extinct animals has been compared to that of extant species: fish from the Dipnoi and coelacanth groups and tetrapods from the salamander and lizard groups.
The researchers built three-dimensional musculoskeletal models of the various animals in this study by subjecting specimens to a high-resolution CT-scan. A software originally used to study human locomotion was adapted to study fish and tetrapods after it was previously adapted for locomotion studies of the ancestors of modern humans and various dinosaurs. The tetrapods examined in this study are the oldest species examined in this way so far.
This study made it possible to estimate the movement capacity of the joints of the animals examined and the ability of the muscles to move the joints of fins and limbs. The results show that early tetrapods had limbs that were more adapted for propulsion, like fish fins, than to carry their weight. This suggests that they were still tied to water and moved slowly on land, where they could venture out in search of food thanks to the scarce presence of predators.
The researchers intend to continue their studies to get more information on early tetrapods’ hind limbs and to understand how their four limbs worked together. Perhaps, those animals used their forelimbs for propulsion while modern tetrapods mainly use their hind limbs for this purpose. That transition represents another part of the evolution that led to the success of the tetrapods on the mainland.
