Underwater flight in sea turtles and plesiosaurs

Abstract

This dissertation contributes to our understanding of plesiosaur locomotion by providing foreflipper and hindflipper muscle reconstructions and studying aspects of their muscle physiology (functions, forces, muscle length changes) in comparison to recent sea turtles. This was accomplished by a transdisciplinary biomechanical approach combining knowledge and methods from engineering sciences, comparative anatomy, and paleontology. Plesiosauria belong to a group of extinct reptiles, the Sauropterygia, that adapted to a life in the sea. Plesiosaurs evolved in the Late Triassic and died out at the K/Pg boundary. They are characterized by the increasingly evolving disparity in body form, i.e., either pliosauromorph (large head, short neck) or plesiosauromorph (small head, long neck). Contrastingly, the locomotory apparatus, a fusiform body with a relatively reduced tail and four hydrofoil flippers, experiences little change during over 135 Ma of plesiosaur evolution. So, once the locomotory apparatus of plesiosaurs had evolved, it must have been highly efficient. <br /> In <strong>Chapter 1</strong> flipper osteology and the mode of locomotion of Nothosauria and Plesiosauria are assessed in comparison to recent sea turtles. Plesiosaur locomotion has been disputed for over a century. It has been proposed that plesiosaurs were underwater fliers like penguins and sea turtles, or rowers like e.g., otters, or employing a mixture of both locomotory styles, like sea lions. How the four flippers are coordinated is also still debated. Sea turtles fly underwater. Nonetheless, sea turtles are capable of various rowing motions and even crawling on land. The review concludes that especially joint anatomy and mobilities have largely remained unstudied in all three taxa. Further, osteological evidence mostly corroborates that plesiosaurs were underwater fliers like extant sea turtles while nothosaurs swam partially by tail undulation supported by the foreflippers. <br /> In <strong>Chapter 2</strong> the array of methods (building an analog model of humerus musculature, obtain muscle courses and muscle functions geometrically, pairing up agonistic and antagonistic muscles, finite element structure analysis (FESA) of the humerus) to study underwater flight in plesiosaurs is tested on a recent underwater flying reptile taxon, the sea turtles. This is because for sea turtles muscle attachments and courses can be confirmed by dissection in contrast to the fossil plesiosaurs. To conclude, operating muscle forces during foreflipper up- and downstroke were calculated that show that the downstroke provides more propulsion than the upstroke. Further, the humerus is mostly loaded by compression due to a complex interplay of agonistic and antagonistic muscles and muscle wrappings. This is confirmed by a close match of the compressive stress distribution with the humerus microstructure. <br /> In <strong>Chapter 3</strong> fore- and hindflipper muscles are reconstructed with the extant phylogenetic bracket for the plesiosaur <em>Cryptoclidus eurymerus</em> (IGPB R 324). Additionally, plesiosaur muscle reconstructions are matched with eventually functionally analogous sea turtles, penguins, sea lions, and whales. It turns out that plesiosaurs had complex muscular systems in their fore- and hindflippers that allowed them to twist their flippers along the respective length axis, a feature which has been proven to be crucial for underwater flight by hydrodynamic studies. <br /> In <strong>Chapter 4</strong> <em>Cryptoclidus</em> (IGPB R 324) humerus and femur FESA was computed comparable to Chapter 2. Muscle forces support that the downstroke in plesiosaurs contributed more to propulsion than the upstroke. Further, extensors and flexors that originate from humerus and femur have very high muscle forces corroborating the myological flipper length axis twisting mechanism proposed in chapter 3 and proving its importance for plesiosaur locomotion. <br /> In <strong>Chapter 5</strong> a preliminary FESA of a sea turtle femur, that is part of a rowing and not underwater flying appendage, is presented. The highest muscle forces are obtained for femur pro- and retractors. This highlights that with FESA it is possible to determine differences between limb bones that are employed in different locomotory styles. <br /> <strong>Chapter 6</strong> concludes with a summary of the results of this dissertation placing muscle functions in the context of sauropsid muscle functions and by comparing results for sea turtle and plesiosaur FESA point by point. There are considerable similarities between both underwater flying reptile taxa but also profound differences which highlight the convergently evolved different locomotory muscuskeletal systems but also how similar selective pressures lead to similar adaptations and morphologies.