Early Permian synapsid impressions illuminate the origin of epidermal scales and aggregation behavior

Fossil evidence of skin structure¹^,² and group behavior³^,⁴ in the stem group of mammals, the early synapsids, is sparse and restricted to a few occurrences. We describe here exceptional resting trace fossils, Bromackerichnus requiescens n. igen. n. isp., from the early Permian Tambach Formation at the Bromacker locality, Thuringia, Germany.⁵ They are the only definite early synapsid-and, more specifically, sphenacodontid-full-body impressions. They include skin impressions of the limbs, trunk, and tail that show epidermal scales and are associated with Dimetropus leisnerianus footprints. Through a morphological comparison with modern taxa,⁶ we demonstrate for the first time the unequivocal occurrence of epidermal scales in early synapsids. A review of the early amniote and stem amniote trace and body fossil skin record highlights that this constitutes the oldest occurrence of epidermal scales in synapsids, long before the acquisition of hair.⁷ Moreover, we find the first fossil occurrence of epidermal scales in eight different tetrapod groups during the early Permian. This implies an earlier common origin of epidermal scales. The higher chance of preservation and spreading in the early Permian was probably due to a structural reinforcement of the scales as an adaptation to global warming and aridization⁸ at the end of the Late Paleozoic Ice Age. Moreover, sphenacodontid resting, swimming, and locomotion traces of multiple individuals of different sizes co-occur on the same bedding planes, in a relatively small area. This is interpreted as the earliest documented aggregation behavior in sphenacodontid synapsids that gathered in and around small ponds.