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

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Abstract

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.

Kurzfassung

Längliche Hautvorsprünge des Reptils Longisquama insignis aus der Trias Kirgisistans sind in Zusammenhang mit dem einzigen Fossilskelett sowie isoliert auf der Platte und Gegenplatte von sieben weiteren Exemplaren aus demselben Fundhorizont und von derselben Fundstätte erhalten. Die Hautstrukturen wurden Teil der Debatte um den Ursprung der Federn, als manche Bearbeiter sie als Federn eines Nichtvogels, die homolog zu Vogelfedern seien, auffassten. Im Widerspruch zueinander stehende Interpretationen zu ihrer Morphologie und Vergleichbarkeit mit anderen Typen von Hautanhängen waren besonders auch der Mehrdeutigkeit der fossilen Hauteindrücke geschuldet. Auf der Grundlage eines Vergleichs der Morphologien aller bekannten Longisquama-Anhänge nehmen wir zu Fragen der Taphonomie, Entwicklung und Funktion der Anhänge Stellung, ebenso zu der Frage, inwieweit sie den Vogelfedern mit Federfahne glichen. Wir erklären die bestehende Federähnlichkeit der Longisquama-Anhänge mit der Entwicklung aus einer filamentartigen Anlage und einer komplexen Abfolge von Einzelvorgängen, von denen manche Entwicklungprozessen heutiger Federn geglichen haben könnten. Solch eine Interpretation ist mit der Hypothese in Einklang, dass Mitglieder einer Archosauromorpha-Klade, die Longisquama und federtragende Archosaurier einschließt, eine Reihe homologer Enwicklungsmechanismen für Hautanhänge besitzen, erfordert jedoch nicht, dass die Longisquama-Anhänge selbst Federn oder auf einem hohen Level homolog mit Federn sind.

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Acknowledgments

This work was supported by the State of Saxony (scholarship to M. B.) and by the German Research Foundation (DFG II—VO 1466/1-1). We are grateful to Evgenii N. Kurochkin and Vladimir R. Alifanov for access to the type material in Moscow in February 2007, to Jan Fischer, Daniel Krause, and Robert Georgi for discussion and for their support during the 2007 field season, to Ilja Kogan for his help with translation of Russian publications and reports, and to Christian Foth, Nicholas Fraser, and Jörg W. Schneider for their comments on earlier versions of the manuscript.

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Buchwitz, M., Voigt, S. The dorsal appendages of the Triassic reptile Longisquama insignis: reconsideration of a controversial integument type. Paläontol Z 86, 313–331 (2012). https://doi.org/10.1007/s12542-012-0135-3

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