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.
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The Stenopterygius specimen examined in this study (MH 432) is permanently accessioned in the collections of Urweltmuseum Hauff, Holzmaden, Germany. The data supporting our findings are available from the corresponding author upon reasonable request.
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We thank E. R. Schroeter for scientific advice. S. M. Webb and C. Roach assisted during the SRS-XRF analysis. T. Wigren produced the artistic reconstructions. Financial support was provided by a Grant for Distinguished Young Researchers (642-2014-3773, Swedish Research Council) to J.L., as well as a National Science Foundation INSPIRE grant (EAR-1344198) to M.H.S. and W.Z., and donations from F. M. and S. P. Orr, and V. and G. Mullis. The Paul Scherrer Institute, Switzerland, provided beamtime at the TOMCAT beamline of the Swiss Light Source. SRS-XRF data were collected at the Stanford Synchrotron Radiation Lightsource using beamline 10-2. NanoSIMS measurements were obtained at the Chemical Imaging Infrastructure, Chalmers University of Technology and University of Gothenburg, which is supported by the Knut and Alice Wallenberg Foundation. Part of this work was performed at the Analytical Instrumentation Facility (AIF) of North Carolina State University. The AIF is supported by the State of North Carolina and the National Science Foundation (ECCS-1542015) and is a member of the North Carolina Research Triangle Nanotechnology Network, a site within the National Nanotechnology Coordinated Infrastructure.
Nature thanks A. Houssaye, B. Kessler, S. Kiel and R. N. S. Sodhi for their contribution to the peer review of this work.