Delta wing design in earliest nektonic vertebrates

Area of Science:

• Paleontology
• Evolutionary Biology
• Hydrodynamics

Background:

• Vertebrate colonization of the pelagic realm was a key evolutionary transition.
• Traditionally attributed to jawed vertebrates, early active swimmers were likely jawless "ostracoderms" like Pteraspidiformes.
• The lift and stabilization mechanisms of these early fishes remain unclear due to their lack of conventional control surfaces.

Purpose of the Study:

• To investigate the hydrodynamic mechanisms enabling early vertebrate colonization of the water column.
• To determine how the rigid cephalic shield of Pteraspidiformes generated lift and stability.
• To assess the performance of Pteraspidiformes in pelagic versus benthic conditions.

Main Methods:

• Particle Image Velocimetry (PIV) in a water channel.
• Force measurements on real-sized Pteraspidiformes models.
• Experiments simulating ground effect to compare pelagic and benthic performance.

Main Results:

• The rigid cephalic shield of Pteraspidiformes generates significant hydrodynamic lift, similar to delta wings.
• Flow over models showed leading-edge vortex formation, enhancing lift and delaying stall.
• Pteraspidiformes exhibited superior hydrodynamic performance in simulated pelagic conditions compared to benthic scenarios.

Conclusions:

• Leading-edge vortices generated by the cephalic shield were crucial for Pteraspidiformes to colonize the water column.
• This hydrodynamic mechanism allowed early vertebrates to exploit pelagic environments over 400 million years ago.
• The findings challenge traditional views on the evolution of aquatic vertebrate locomotion.