A diadectid skin impression and its implications for the evolutionary origin of epidermal scales

Abstract

Corneous skin appendages are not only common and diverse in crown-group amniotes but also present in some modern amphibians. This raises the still unresolved question of whether the ability to form corneous skin appendages is an apomorphy of a common ancestor of amphibians and amniotes or evolved independently in both groups. So far, there is no palaeontological contribution to the issue owing to the lack of keratin soft tissue preservation in Palaeozoic anamniotes. New data are provided by a recently discovered ichnofossil specimen from the early Permian of Poland that shows monospecific tetrapod footprints associated with a partial scaly body impression. The traces can be unambiguously attributed to diadectids and are interpreted as the globally first evidence of horned scales in tetrapods close to the origin of amniotes. Taking hitherto little-noticed scaly skin impressions of lepospondyl stem amniotes from the early Permian of Germany into account, the possibility has to be considered that the evolutionary origin of epidermal scales deeply roots among anamniotes.

Keywords: corneous structures; early tetrapods; palaeozoic; scale pattern; skin appendages.

Figure 1.

Simplified geological map and lithostratigraphy of the Intra-Sudetic Basin focused on the Piaskowiec Czerwony quarry and the ‘Building Sandstone’ member of the Słupiec Formation as the origin of trace fossil specimen Muz.PIG.1732.II.54. Map and lithostratigraphy based on [–17].

Figure 2.

Trace fossil specimen Muz.PIG.1732.II.54 shows footprints of three different diadectid trackways (Ichniotherium cottae) and a partial scaly body impression. (a) Summary three-dimensional scan, arrows mark the position of the body impression; (b) outline drawing of the traces; (c) Ichniotherium cottae trackway with outlined diadectid in motion and bent tail similar to the position of the body impression in Muz.PIG.1732.II.54 based on [22]; (d-e) close-up three-dimensional scan and drawing of the body impression and overprinted Ichniotherium tracks, scale bar measures 2 cm; (f) three-dimensional photogrammetric model of Muz.PIG.1732.II.54.

Figure 3.

Evidence of possible keratinized epidermal scales in early Permian tetrapods from the Thuringian Forest Basin, Germany and diadectid skin synthesis. (a) Diplocaulid abdominal impression assigned to Hermundurichnus fornicatus [39]; (b) diadectid pes-manus couple with scale impression along digits assigned to Ichniotherium cottae [21]; (c) tail impression attributed to synapsid trackmaker [46]; (d) diadectid reconstruction illustrating the abdominal scale pattern based on Muz.PIG.1732.II.54 and Ichniotherium tracks with scale impressions. Scale bars measure 1 cm (a,c), and 3 cm (b).