They describe how the first vertebrates swam more than 400 million years ago

A recent study has revealed how the first marine vertebrates, such as ostracoderms, swam, and the hydrodynamic mechanisms that allowed them to be the first vertebrates to ascend beyond the sea floor—colonizing the so-called water column—much earlier than, until now. , it was believed.

The work is the work of Francisco Huera from the Rovira i Virgili University (URV) of Tarragona, Héctor Botella, from the Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia, in Spain, and Richard A. Fariña, from the University of the Republic in Montevideo, Uruguay.

After exposing 3D models of these extinct fish to water currents, researchers have determined that, despite their morphology, they were swimmers skilled enough to occupy strata closer to the surface. The results of the study lead to a change in the conception that was had about the first marine vertebrates and help to understand how they evolved to, in the long run, live outside of water.

The Devonian is the geological period in which the first insects appear and when vertebrates, which inhabited the sea, begin to develop limbs. On land, some plants developed woody tissues and the ability to reproduce through seeds, which gave rise to the first forests on Earth. The oceanic ecosystems, populated by bryozoans, brachiopods, corals and trilobites, led to the appearance of the first vertebrates, among which were fish called ostracoderms.

Traditionally, scientific literature has described fish of this class as clumsy swimmers that were limited to crawling along the seabed, due to the configuration of their skeleton. Ostracoderms – a name derived from the Greek “shell skin” – had a rigid exoskeleton that covered the front part of their body, limiting their mobility. These animals, now extinct, did not have a jaw, nor did they have any fin other than the tail, that is, their own tail.

Researchers around the world had long suspected that ostracoderms had played a prominent role in the colonization of the water column of the marine pelagic zone or, in other words, those parts of the sea that are not located on a continental shelf, out to sea. However, the swimming mechanisms they must have used to expand their seafloor habitat to shallower strata were unclear. In fact, biologists attributed the colonization of the water column to more modern fish.

3D model of an Ostracoderm. (Photo: URV / Universitat de Valencia / University of the Republic in Montevideo)

“As soon as I saw an ostracoderm I thought it could swim very well,” confesses Francisco Huera. Huera is not a biologist, but rather he studies fluid dynamics, that is, how air or water behaves under a series of conditions, such as those that occur in the flight of an airplane, in the operation of a wind turbine or in the behavior of fish when they swim. The research team has worked with full-scale models built from fossil remains of ostracoderms to determine to what extent their morphology allowed them to ascend beyond the seabed.

In the water channel of the Laboratory of Fluid-Structure Interaction (LIFE) research group at the URV, Huera exposed the models to water currents and was able to determine the hydrodynamic behavior of prehistoric animals. In this way, they have shown that, although ostracoderms lacked fins, they had a fairly sophisticated control system, which allowed them to swim in a far from primitive way: “They function like a hang glider; The shape of the head generates vorticity—a spiraling rotational current—which exerts additional lift. This same mechanism is what some of the most advanced aircraft exploit,” explains Huera. Researchers have reconstructed the most probable way in which these fish must have swam: “they flapped their caudal fin, in bursts, and tilted their heads to generate more or less lift.”

Colonization of the water column of the pelagic zone represents one of the most important transitions in the evolution of life on Earth. The research results identify and explain the mechanisms by which the first fish managed to expand their habitat to this area and demonstrate that it happened much earlier than previously thought, during the transition between the Silurian and Devonian periods, more than 400 years ago. millions of years.

The study is titled “Delta wing design in earliest nektonic vertebrates.” And it has been published in the academic journal Communications Biology. (Source: URV)