Mats Eriksson

With their superficially shark-like appearance, the Mesozoic ichthyosaurs provide a classic illustration of major morphological adaptations in an ancestrally terrestrial tetrapod lineage following the invasion of marine habitats^{1, 2–3}. Much of what is known about ichthyosaur soft tissues derives from specimens with body outlines^{4, 5–6}. However, despite offering insights into aspects of biology that are otherwise difficult to envisage from skeletal evidence alone (such as the presence of a crescentic fluke), information on their soft parts has hitherto been limited to a taxonomically narrow sample of small- to dolphin-sized animals^{2,4, 5–6}. Here we report the discovery of a metre-long front flipper of the large-bodied Jurassic ichthyosaur Temnodontosaurus, including unique details of its soft-tissue anatomy. In addition to revealing a wing-like planform, the fossil preserves a serrated trailing edge that is reinforced by novel cartilaginous integumental elements, herein denominated chondroderms. We also document chordwise-parallel skin ornamentations and a protracted fleshy distal tip that presumably acted like a flexible winglet in life. By integrating morphological and numerical data, we show that the observed features probably provided hydroacoustic benefits, and conclude that the visually guided^7,8Temnodontosaurus relied on stealth while hunting in dim-lit pelagic environments. This unexpected combination of control surface modifications represents a previously unrecognized mode of concealment, and underscores the importance of soft-tissue fossils when inferring aspects of palaeoethology and predator–prey palaeoecology.