The colonisation of the pelagic environment-mid-water near the ocean surface-by vertebrates marked a milestone in the establishment of complex aquatic ecosystems. Although this transition has long been associated to the appearance of fins and jaws during the evolutionary process of fish, recent studies suggest that the first active swimmers to occupy the water column were some ostracoderms, primitive jawless fish with no fins except for their caudal fin, their own tail. However, the hydrodynamic mechanism that these agnathans used to rise from benthic zones to the pelagic realm had remained unclear.
The work now published in Nature Communications Biology reveals the hydrodynamic mechanism used by ostracoderms to generate lifting forces through their large headshields by creating vortices or whirlpools known as leading edge vortices (LEVs). The study, combining paleobiology and fluid mechanics, used 3D-printed fossil models over 420 million years old. It also made use of DPIV (digital particle image velocimetry) technology, a technique for accurately measuring two-dimensional velocity fields, applied in water tunnels to study the flow around fish body structures.
"As they swam, the fish generated vortices at the front edges of their headshields, which helped stabilise their swimming and provided a remarkable lifting capacity, similar to that produced in the triangular delta wings used by some of the most modern aircraft and space vehicles", explains Héctor Botella, professor of Palaeontology, researcher at the Cavanilles Institute of Biodiversity and Evolutionary Biology (ICBiBe) of the University of Valencia (UV) and co-author of the article.
Over the last decades, science has shown that LEVs represent a hydro-aerodynamic mechanism universally exploited by many species. This study demonstrates that early vertebrates already exploited LEVs over 400 million years ago to compensate for their lack of mobile appendages and to colonise the water column.
The results of this research open new perspectives on the study of the evolution of early aquatic vertebrates and their ability to adapt to the pelagic marine environment. The discovery that ostracoderms used principles similar to those of delta wings in aeronautics underscores the complexity and diversity of evolutionary strategies. According to the authors, this could also serve as a source of bio-inspiration for designing underwater vehicles.
In addition to the Palaeobiology Unit of the Cavanilles Institute of Biodiversity and Evolutionary Biology (ICBiBe) of the University of Valencia, the research involved the Department of Palaeontology of the University of the Republic (Montevideo, Uruguay) and the Department of Mechanical Engineering of the Rovira i Virgili University (URV, Tarragona).
