This discovery, made by a research team led by the URV, modifies the previously held conception of primitive fish and demonstrates that they were able to inhabit areas other than the seabed

Research led by the URV has revealed how the first marine vertebrates, such as ostracoderms, were able to swim and how they developed hydrodynamic mechanisms that allowed them to be the first vertebrates to ascend beyond the seabed, colonising the so-called water column much earlier than previously thought. After exposing 3D models of these extinct fish to water currents, Francisco Huera-Huarte, researcher at the Department of Mechanical Engineering, together with biologists from the University of Valencia and the University of the Republic (Uruguay), determined that, despite their morphology, they were skilled enough swimmers to occupy strata closer to the surface. The results of the study mean a change in our understanding of the first marine vertebrates and help to reveal how they eventually evolved to live out of the water.

The Devonian is the geological period when the first insects appeared and when vertebrates living in the sea began to develop limbs. On land, some plants started to develop woody tissues and the ability to reproduce by seed, giving rise to the Earth’s first forests. Ocean ecosystems, populated by bryozoans, brachiopods, corals and trilobites, led to the emergence of the first vertebrates, including 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 for ‘shell skin’ – had a rigid exoskeleton covering the front part of their bodies, limiting their mobility. These animals, now extinct, did not have a jaw, nor did they have any fin other than the caudal fin, i.e. their own tail.

Researchers around the world had long suspected that ostracoderms had played a prominent role in colonising the water column of the marine pelagic zone, or in other words, those parts of the sea that are away from the shore. However, the swimming mechanisms they must have used to expand their habitat from the seafloor to shallower strata were not clear, leading biologists to attribute the colonisation of the water column to more modern fish.

‘As soon as I saw an ostracoderm, I thought it must have been able swim very well,’ confesses Francisco Huera. Huera is not a biologist; instead he studies fluid dynamics, i.e. how air or water behave under a series of conditions, such as those that occur in the flight of an aeroplane, in the operation of a wind turbine or in the behaviour of fish when they swim. The research team worked with full-scale models built from fossil remains of ostracoderms to determine to what extent their morphology allowed them to ascend beyond the seafloor.

In the water channel used by the URV’s Fluid-Structure Interaction Laboratory research group, Huera exposed the models to water currents and was able to determine the hydrodynamic behaviour of the prehistoric animals. In this way, he showed that, although the ostracoderms lacked fins, they had a fairly sophisticated control system, which allowed them to swim in a far from primitive way: ‘They work like a hang-glider; the shape of the head generates vorticity – a rotating spiral current – which exerts an additional lift force. The same mechanism is exploited by some of the most advanced aircraft,’ explains Huera. The researchers reconstructed the most likely way in which these fish must have swum: ‘they flapped their tail fins in bursts and tilted their heads to generate more or less lift.

The colonisation of the pelagic water column 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 were able to expand their habitat into this zone and show that it happened much earlier than previously thought, during the transition between the Silurian and Devonian periods, more than 400 million years ago. This new discovery means a change in our understanding of the first marine vertebrates and helps to reveal how they eventually evolved to live out of the water.

Reference: Botella, H., Fariña, R.A. & Huera-Huarte, F. Delta wing design in earliest nektonic vertebrates. Commun Biol 7, 1153 (2024). https://doi.org/10.1038/s42003-024-06837-8