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Longisquama is a small diapsid reptile with an uncertain phylogenetic position. It is known from an incomplete skeleton with integumentary appendages and isolated appendages. Appendage PIN (for Palaeontological Institute of the Russian Academy of Sciences) 2584/7, preserved as part and counterpart, retains an infilling of fine-grained sediment and high-fidelity impressions of the external left and right surfaces of the appendage ( Fig. 1). This infilling, preserved either on one side of the specimen or on the counterpart, shows that the tubular configuration described for the proximal portion extends along the entire length of the appendage, although the distal portion is expanded anteroposteriorly and flattened transversely. This indicates that in life the two external surfaces were separated from each other by an intervening space (now sediment-filled).

Figure 1: Part and counterpart of an elongated dorsal scale of Longisquama insignis (PIN 2584/7).
figure 1

Where the sedimentary infilling (black circles) is not preserved, sharp impressions of the corrugated external surface of the structure are visible (white circles). Arrows point to corresponding patches of sedimentary infilling on part and counterpart. ab, anterior smooth band; c, corrugations; v, median 'vein'.

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There are no feather-like features on the distal portion of the appendage. Here, two corrugated membrane-like surfaces touch along their leading and trailing edges to form wide, smooth bands. The two membranes were apparently supported by a median vein-like structure extending the length of the appendage. This has been proposed as the homologue of the rhachis of avian feathers2. On either side of this 'vein', the external surfaces of the appendage are corrugated. This corrugation varies along the appendage: proximally, individual rugae are relatively large and widely spaced, but in the distal portion they are smaller and densely packed. The densely arranged distal corrugations have been compared to the pinnae of avian feathers2, but the fossils indicate that these are formed on a membrane-like structure on either side of the 'vein'.

The fossils were split into part and counterpart during collecting, and most of the appendages are now preserved as impressions of their left and right sides, without the intervening sediment core. The surfaces of both the part and counterpart impressions of individual appendages are concave, an indication that these structures are three-dimensional. In contrast, the parts and counterparts of feather impressions in Archaeopteryx are concave and convex, respectively.

We believe that the dorsal appendages of Longisquama are highly modified scales, as suggested previously1,3, rather than feathers. Examination of the holotype of L. insignis (PIN 2584/4) suggests that they were anchored in the skin or epaxial muscles.