Triassic diapsid shows early diversification of skin appendages in reptiles

Complex integumentary appendages such as avian feathers and mammalian hair play a principal role in tetrapod evolution, with critical functions in insulation, sensation, display and flight. Although feathers and hair originated in the stem-lineages of birds and mammals, respectively^1,2, their underlying gene regulatory network has much deeper amniote roots³. The early evolution of amniote integumentary appendages, however, remains poorly understood because of the absence of fossil evidence. Here we present Mirasaura grauvogeli, a small-sized diapsid from the Middle Triassic epoch (about 247 million years ago) with a distinctive crest formed by elongate integumentary appendages extending serially along its back, similar to those of the poorly understood Triassic reptile Longisquama^{4, 5, 6–7}. Despite its superficially bird-like skull, Mirasaura is not closely related to avemetatarsalians but instead belongs to the exclusively Triassic reptilian clade Drepanosauromorpha⁸. Melanosomes preserved in its integumentary appendages are consistent in geometry with melanosomes of feathers but not those of reptilian skin or mammalian hair. Nevertheless, the morphology of the integumentary appendages and phylogenetic placement of Mirasaura indicate that they are not structurally homologous to feathers or other integumentary appendages in living amniotes. Our findings show that complex integumentary appendages are not restricted to avemetatarsalians and mammaliaforms among amniotes and evolved in a lineage basal to all extant reptiles, challenging our understanding of the evolution of the reptilian integument.