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Ichthyosaurs are extinct marine reptiles that display a notable external similarity to modern toothed whales. Here we show that this resemblance is more than skin deep. We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius. Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers. These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy. Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment. Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation. Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.

Throughout the animal kingdom, adaptive colouration serves critical functions ranging from inconspicuous camouflage to ostentatious sexual display, and can provide important information about the environment and biology of a particular organism. The most ubiquitous and abundant pigment, melanin, also has a diverse range of non-visual roles, including thermoregulation in ectotherms. However, little is known about the functional evolution of this important biochrome through deep time, owing to our limited ability to unambiguously identify traces of it in the fossil record. Here we present direct chemical evidence of pigmentation in fossilized skin, from three distantly related marine reptiles: a leatherback turtle, a mosasaur and an ichthyosaur. We demonstrate that dark traces of soft tissue in these fossils are dominated by molecularly preserved eumelanin, in intimate association with fossilized melanosomes. In addition, we suggest that contrary to the countershading of many pelagic animals, at least some ichthyosaurs were uniformly dark-coloured in life. Our analyses expand current knowledge of pigmentation in fossil integument beyond that of feathers, allowing for the reconstruction of colour over much greater ranges of extinct taxa and anatomy. In turn, our results provide evidence of convergent melanism in three disparate lineages of secondarily aquatic tetrapods. Based on extant marine analogues, we propose that the benefits of thermoregulation and/or crypsis are likely to have contributed to this melanisation, with the former having implications for the ability of each group to exploit cold environments.

Researcher suggests odd arrangement of marine fossils was stockpiled by enormous, tentacled beast.

Four unrelated groups of large cruising vertebrates (tunas, whales, lamnid sharks and parvipelvian ichthyosaurs) evolved tuna-shaped (thunniform) body plans. Stringent physical constraints, imposed by the surrounding fluids, are probably responsible for this example of evolutionary convergence. Here I present a mathematical model of swimming kinematics and fluid mechanics that specifies and quantifies such constraints, and test the model with empirical data. The test shows quantitatively that morphology, kinematics, and physiology indeed covary tightly in large cruisers. The model enables calculations of optimal cruising speed from external measurements, and also predicts that wide caudal fin spans, typical of thunniform swimmers, are necessary for large cruisers. This finding is contrary to a popular yet rather teleological view that thunniform tails were selected for their high aspect ratios that increased propulsive efficiency. I also show by calculation that Stenopterygius, a Jurassic ichthyosaur, probably had optimal cruising speeds and basal metabolic rates similar to living tunas.

ICHTHYOSAURS are extinct marine reptiles, probably belonging to the Diapsida1, that ranged from the Early Triassic to Late Cretaceous2,3. Post-Triassic ichthyosaurs achieved the highest level of aquatic adaptation among reptiles4, with a streamlined body, lunate tail and a dorsal fin, features exemplified today by thunniform (tuna-like) fishes. However, little is known of how such a body plan evolved from a terrestrial diapsid. Here we report the most complete specimen of the oldest known ichthyosaur, Chensaurus, representing a transition between the two body plans. The specimen, which has a partial skin impression, has a small caudal fin, a long and narrow body, and a high presacral vertebral count. These features all suggest an anguilliform swimming mode. Later ichthyosaurs retained the high vertebral count, but overcame the high swimming costs of this plesiomorphy, achieving a rigid tunniform bauplan by evolving discoidal vertebrae, and a deep fusiform body. Chensaurus therefore seems to be an evolutionary intermediate between the shorter-bodied terrestrial stock from which the group evolved, and advanced thunniform ichthyosaurs.

Ichthyosaurs, extinct marine reptiles superficially similar to sharks and dolphins, flourished throughout most of the Mesozoic, but are best represented in the early Jurassic1–5. Hundreds of skeletons are known, principally from the Lower Liassic (Hettangian, Sinemurian and Lower Pleinsbachian) of south-west England and the Upper Liassic (Toarcian) of southern Germany6–9. Among the most specialized is Eurhinosaurus, unique for its long rostrum and shortened mandible, which confers a striking resemblance to the modern swordfish, Xiphias1,5,10. Known only from the Upper Liassic of Germany, this monotypic genus, like most fossil species, appears suddenly, without antecedents11–14. The recent discovery of a geologically older (Lower Liassic) ichthyosaur with a partially extended rostrum is therefore of considerable interest. As described here, this specimen, which is too large to be part of a growth series of Eurhinosaurus, appears to be ancestral, and could have given rise to Eurhinosaurus through changes in growth rates of the rostrum and mandible.