The dorsal appendages of the Triassic reptile Longisquama insignis: reconsideration of a controversial integument type

Elongated skin projections of the reptile Longisquama insignis from the Triassic of Kyrgyzstan are preserved as imprints on the only skeletal specimen and on seven additional pairs of fossil slabs and counter-slabs from the same locality and horizon. The integumentary structures became a matter of debate when they were assessed as non-avian feathers homologous to avian feathers. Conflicting interpretations of their morphology and relationship to other appendage types arose from the ambiguity of the fossil skin impressions. On the basis of comparative description of the individual morphology of all yet known Longisquama specimens we address aspects of taphonomy, development, and function and define to what extent Longisquama's appendages share characteristics of avian vaned feathers. We explain the existing feather similarity by their development from a filamentous primordium and a complex sequence of individual processes, some of which are reminiscent of processes observed in feather development. Such an interpretation is in agreement with a set of homologous mechanisms of appendage morphogenesis in an archosauromorph clade including Longisquama and feather-bearing archosaurs but does not necessarily require that the appendages of Longisquama themselves are feathers or high-level feather homologues.