Morphological and hydrodynamic convergence in pelagic vertebrates
Moon, B.C. 2013 Morphological and hydrodynamic convergence in pelagic vertebrates. SVPCA 2013 (Edinburgh)
Abstract
Convergence is a key topic in evolution, and it has bearings on systematics, adaptation and molecular developmental biology. However, much of the study in this area relies on qualitative comparisons between disparate groups; few quantitative studies are available. In pelagic vertebrates, body shape is dominated by the need for streamlining. Ichthyosaurs are noted for their morphological similarity to modern pelagic fishes and cetaceans, and they are here used to test morphological and hydrodynamical convergence between the groups. Their long and comprehensive fossil record includes several konservat-lagerstätten, with skin impressions and body outlines preserved, allowing accurate reconstructions. Previous studies have used regressions between body and caudal fin dimensional ratios; here I extend this to a two-dimensional landmark morphometric technique that reports more of the shape variation. Against the dominant PC axes, ichthyosaurs follow a trend through their phylogeny towards the morphospace occupied by extant fishes, indicating increased similarity in their form. A further test on the hydrodynamical effects of this uses orthogonal two-plane computational fluid dynamics to approximate a three-dimensional body form. Velocity and turbulent kinetic energy were used to quantify the action of fluid flow around the body. The results corroborate the morphometrics: the body forms of derived ichthyosaurs exhibit the properties of extant pelagic vertebrates. Early and Middle Triassic ichthyosaurs show poor dorsal development of the caudal fin and have the long tails characteristic of scyliorhinid sharks, whereas derived ichthyosaurs approximate the morphology of fast- swimming carcharhinid and lamnid sharks, as previously suggested.
References
Gutarra, S., Moon, B.C., Rahman, I.A., Palmer, C.P., Lautenschlager, S., Brimacombe, A.J. and Benton, M.J. 2019. Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs. Proceedings of the Royal Society B: Biological Sciences 286: 20182786. doi:10.1098/rspb.2018.2786