Abstract
Living in water imposes severe constraints on the evolution of the vertebrate body. As a result of these constraints, numerous extant and extinct aquatic vertebrate groups evolved convergent osteological and soft-tissue adaptations. However, one important suite of adaptations is still poorly understood: dermal cover morphologies and how they influence surface fluid dynamics. This is especially true for fossil aquatic vertebrates where the soft tissue of the dermis is rarely preserved. Recent studies have suggested that the keeled scales of mosasaurids (pelagic lizards that lived during the Late Cretaceous) aided in surface frictional drag reduction in a manner analogous to the riblets on shark placoid scales. However, here we demonstrate that mosasaurid scales were over an order of magnitude too large to have this effect. More likely they increased the frictional drag of the body and may have played a role in controlling flow separation by acting as surface roughness that turbulated the boundary layer. Such a role could have reduced pressure drag and enhanced manoeuvrability. We caution those studying fossil aquatic vertebrates from positing the presence of surface drag reducing morphologies, because as we show herein, to be effective such features need to have a spacing of approximately 0.1 mm or less.
Highlights
SummaryLiving in water imposes severe constraints on the evolution of the vertebrate body. As a result of these constraints, numerous extant and extinct aquatic vertebrate groups evolved convergent osteological and soft-tissue adaptations
Aquatic and fully marine amniotes convergently evolved similar osteological and soft-tissue adaptations as a2015 The Authors
The size of the mosasaurid scales and riblet spacing is such that, contra Lindgren et al [18], it is highly unlikely that they had any drag reduction effect resulting from effects analogous to shark scale riblets; they most likely increased the drag compared with a smooth surface
Summary
Living in water imposes severe constraints on the evolution of the vertebrate body. As a result of these constraints, numerous extant and extinct aquatic vertebrate groups evolved convergent osteological and soft-tissue adaptations. One important suite of adaptations is still poorly understood: dermal cover morphologies and how they influence surface fluid dynamics This is especially true for fossil aquatic vertebrates where the soft tissue of the dermis is rarely preserved. Here we demonstrate that mosasaurid scales were over an order of magnitude too large to have this effect More likely they increased the frictional drag of the body and may have played a role in controlling flow separation by acting as surface roughness that turbulated the boundary layer. Such a role could have reduced pressure drag and enhanced manoeuvrability. We caution those studying fossil aquatic vertebrates from positing the presence of surface drag reducing morphologies, because as we show to be effective such features need to have a spacing of approximately 0.1 mm or less
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