Silurian-Devonian Jawless Fish were Ecologically Diversified

Silurian-Devonian Jawless Fish were Ecologically Diversified

Osteostraci, the jawless sister group to all jawed vertebrates, had adaptations for passive control of water flow around the body, according to new research led by the University of Bristol.

Life restoration of Cephalaspis, a typical osteostracan, swimming over the substrate. Image credit: Hugo Salais, Metazoa Studio.

Osteostraci (osteostracans) is a group of extinct bony-armored jawless fish that lived in what are now North America, Europe and Asia from the Middle Silurian to Late Devonian epoch.

These bottom-dwelling creatures were probably relatively good swimmers, possessing dorsal fins, paired pectoral fins, and a strong tail.

The shield of bone covering their head formed a single piece, and so presumably did not grow during adult life. Many possessed bizarre horn-like extensions from the front of their heads.

“The evolution of jaws and fins has classically been seen as the key evolutionary inventions that allowed vertebrates to diversify their lifestyles,” said co-lead author Dr. Humberto Ferron, a postdoctoral researcher at the University of Bristol.

“In this context, jawless ancestors, characterized by the presence of heavy rigid headshields, were assumed to be cumbersome fish-like creatures, living on the bottom of rivers and seas, with poor maneuverability.”

Using computational fluid dynamics, Dr. Ferron, University of Bristol’s Professor Phil Donoghue and their colleagues explored how osteostracans interacted with water currents.

The experiments revealed the bizarre spikes and spines that ornamented the heads of these creatures were actually hydrodynamic adaptations, passively generating lift from water currents flowing over the body.

The varying head shapes of different species allowed them to adapt to different positions, some high, others low, within the water.

They were already ecologically diverse, long before the evolution of their jawed vertebrate relatives.

“The application of computational fluid dynamics, has allowed us to study the swimming performance of ancient vertebrates and learn more about their position in evolutionary history,” said co-author Dr. Imran Rahman, a researcher at the Oxford University Museum of Natural History.

“Our simulations reveal that the different species of osteostracans show equally different hydrodynamic efficiencies,” said co-author Dr. Carlos Martinez Perez, a scientist at the University of Valencia.

“Some of them performed better when moving close to the sea floor or riverbed while others performed better when swimming freely in the water.”

“The different species’ body shapes are adapted to different environments, revealing distinct lifestyles among these groups of jawless early vertebrates,” Professor Donoghue said.

“Our results call into question the prevailing view that these extinct groups of jawless vertebrates were ecologically constrained, and reveal the main evolutionary hypothesis for the origin of jawed vertebrates is more complex than previously thought.”

The study was published in the journal Current Biology.

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Humberto G. Ferrón et al. Computational Fluid Dynamics Suggests Ecological Diversification among Stem-Gnathostomes. Current Biology, published online October 1, 2020; doi: 10.1016/j.cub.2020.09.031

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