Amniote traits and terrestriality in an early amphibian

Abstract The origin of terrestrial tetrapods is linked to the evolutionary split between amniotes, which occupied dryer upland environments, and stem tetrapods and lissamphibians which diversified into wetlands1-3. Skeletal evidence of early lissamphibian habits includes broad-based skulls with sensory lateral lines, gill bars, pharyngeal teeth, and weakly ossified vertebrae4-6. In contrast, early amniotes generally exhibit narrow-based skulls, well ossified postcrania, and limbs with evidence of claws7,8 implying presence of keratinized, resilient, watertight skin9,10. However, histological and molecular genetic analyses of extant tetrapods find keratinization-linked processes shared throughout the crown clade, suggesting their presence in the stem-group as well11-13. But fossils have thus far provided no direct support for this possibility. Here, we report unusually complete remains of the stem-lissamphibian Eugyrinus wildi from the Upper Carboniferous (~319 million years ago) of Lancashire UK. The specimen displays clawed limbs, regionalized scaly skin, eyes adapted for subaerial vision, and gut contents including terrestrial prey. In addition to supporting the stem-tetrapod keratinization hypothesis11-13, these new data imply that adults of the last common ancestors of living tetrapods were terrestrially adapted even though early life history stages remained tied to aquatic conditions4,6,14. Further, these findings emphasize the derived condition of modern Lissamphibia, suggest emergent behaviours of early temnospondyls in coal swamp habitats, and question the value of early claw-marked fossil trackways as definitive evidence of crown group amniotes and, therefore, as calibrations in tetrapod phylogeny15.