Evolution of Terrestrial Movement in Early Tetrapods

The aerial scene depicts two Late Devonian early tetrapods — Ichthyostega and Acanthostega — coming out of the water to move on land. Footprints trail behind the animals to show a sense of movement. Credit: Davide Bonadonna

The water-to-land transition is one of the most important and inspiring major transitions in vertebrate evolution. And the question of how and when tetrapods transitioned from water to land has long been a source of wonder and scientific debate.

Early ideas posited that drying-up-pools of water stranded fish on land and that being out of water provided the selective pressure to evolve more limb-like appendages to walk back to water. In the 1990s newly discovered specimens suggested that the first tetrapods retained many aquatic features, like gills and a tail fin, and that limbs may have evolved in the water before tetrapods adapted to life on land. There is, however, still uncertainty about when the water-to-land transition took place and how terrestrial early tetrapods really were.

A paper published today (November 25, 2020) in Nature addresses these questions using high-resolution fossil data and shows that although these early tetrapods were still tied to water and had aquatic features, they also had adaptations that indicate some ability to move on land. Although, they may not have been very good at doing it, at least by today’s standards.

Lead author Blake Dickson, PhD ’20 in the Department of Organismic and Evolutionary Biology at Harvard University, and senior author Stephanie Pierce, Thomas D. Cabot Associate Professor in the Department of Organismic and Evolutionary Biology and curator of vertebrate paleontology in the Museum of Comparative Zoology at Harvard University, examined 40 three-dimensional models of fossil humeri (upper arm bone) from extinct animals that bridge the water-to-land transition.

Three major stages of humerus shape evolution: from the blocky humerus of aquatic fish, to the L-shape humerus of transitional tetrapods, and the twisted humerus of terrestrial tetrapods. Columns (left to right) = aquatic fish, transitional tetrapod, and terrestrial tetrapod. Rows = Top: extinct animal silhouettes; Middle: 3D humerus fossils; Bottom: landmarks used to quantified shape. Credit: Courtesy of Blake Dickson

“Because the fossil record of the transition to land in tetrapods is so poor we went to a source of fossils that could better represent the entirety of the transition all the way from being a completely aquatic fish to a fully terrestrial tetrapod,” said Dickson.

Two thirds of the fossils came from the historical collections housed at Harvard’s Museum of Comparative Zoology, which are sourced from all over the world. To fill in the missing gaps, Pierce reached out to colleagues with key specimens from Canada, Scotland, and Australia. Of importance to the study were new fossils recently…