Although the timing and speed of terrestrialization within the broader process of the fish-tetrapod transition is debated, it eventually produced fully terrestrial vertebrates. This placed new functional requirements on the skin, and it is self-evident that the transformation of the skin must have been a very important a part of the fish-tetrapod transition. However, until now there has been no direct evidence at all for the condition of the skin in stem tetrapods or related fossil fishes such as elpistostegalians. A comparison between extant bony fish and tetrapods shows that all tetrapods have a keratinous outer layer on the epidermis, which is absent in fish; conversely, mucous and granular glands are present in fish and amphibians, but not in amniotes. This indicates that a keratinous layer must have evolved within the tetrapod stem group, while mucous and granular glands were retained. However, this phylogenetic bracket does not predict the character of stem tetrapod skin very precisely. In particular, it cannot answer whether it resembled the thin and well-vascularized skin of modern lissamphibians, which is adapted for cutaneous respiration.
We present here the first example of three-dimensionally and structurally preserved skin from a Palaeozoic tetrapod. It derives from a new locality in the latest Famennian (latest Devonian) of East Greenland, yielding a unique fossil assemblage with exceptional preservation including, in some cases, cellular-resolution soft-tissue preservation. The preserved skin comes from an essentially articulated but somewhat disturbed area of tetrapod belly scales and tail lepidotrichia, representing the ventral surface of the abdomen of a stem tetrapod. Apparently, the skin adhered to the mud, remaining while most of the body washed away. During collection the blocks split just above (=internal to) the level of the scales, leaving the skin preserved on the surface of thin counterslabs. These have been imaged with synchrotron microtomography at ESRF, with a voxel size of 1.72 µm.
The skin is strikingly thick: the layer external to the squamation is up to 600 µm in thickness. For comparison, in a frog the entire skin is typically only 40-200 µm thick. The external surface carries robust papillae, about 2 mm long and spaced approximately the same distance apart. The interior of the skin contains preserved cellular-scale histology, including aligned 3D spaces of columnar or cuboidal secretory cells that may have formed lumens or acini of glands, as well as associated muscle fibres and probable capillaries. This glandular level must be part of the dermis, but the overall interpretation of the skin hinges on whether the epidermis is preserved. If it is preserved, the aforementioned papillae are features of the skin surface; if it is lost, it could have been quite thick, and the papillae may have been sub-surface structures like the papillate dermis-epidermis boundary of mammals. Further scans at sub-µm resolutions, which will reveal more of the cellular architecture, should allow us to answer this question. However, it is already clear that the skin was much thicker than modern amphibian skin, which must reflect functional differences.