Squamation and scale microstructure evolution in the Porolepiformes (Sarcopterygii, Dipnomorpha) based on Heimenia ensis from the Devonian of Spitsbergen

ABSTRACT

New material of the “porolepidid” Heimenia ensis (Porolepiformes, Dipnomorpha, Sarcopterygii), sheds light on the evolution of scale morphology in sarcopterygian fishes. This material consists of an articulated, nearly complete body and numerous isolated scales from the Emsian-Eifelian (upper Lower Devonian) of Spitsbergen. Heimenia squamation can be described as ‘transitional’ between that of Porolepis and the more derived Holoptychiidae. The cosmoid scales of Heimenia are divided into three morphotypes: (1) rhombic, (2) intermediate, and (3) rounded. The rounded scales are present in the anterior third of the trunk, intermediate scales occupy its middle third, and rhombic scales extend in the rear and all over the ventral region. The transition between scale morphotypes is progressive along the body. All scales bear traces of a cosmine covering, regardless of their overall shape. Paleohistological study shows that the relative thickness of the cosmine layer decreases whereas the relative thickness of the bony basal layer increases from the rhombic to rounded scales. This unique squamation provides new information about the evolutionary transition from thick/rhombic scales covered with cosmine to thin/rounded scales lacking cosmine in the Porolepiformes. Such morphological and histological changes also occurred by convergence among different groups of Middle–Late Devonian sarcopterygians (e.g., lungfishes and “osteolepiforms”). In Heimenia, the presence of rounded scales in the anterior part of the body is here regarded as an adaptation to a dynamic and agile life style and constitutes a new example to support the hypothesis of the anteroposterior spread of derived characters in fishes.

ACKNOWLEDGMENTS

We are truly grateful to Valentina Karatajute-Talimaa (Vilnius, Lithuania) and Yuri P. Burov (St. Petersburg, Russia) for the collect and the loan of Heimenia material and to Florian Witzmann (MFN, Berlin, Germany) for the loan of holoptychiid scale material for comparisons. We warmly thank Lilian Cazes (UMR 7207, MNHN) for preparing thin sections, Christian Lemzaouda and Philippe Loubry (UMR 7207, MNHN) for the photographs, and Charlène Letenneur (UMR 7207, MNHN) for her advice in picture treatment, and, very specially, Sophie Fernández (UMR 7207, MNHN) for the drawing in Fig. 11. We also thank Philippe Janvier and François Meunier (CNRS, MNHN) and Jean-Yves Sire and Louise Zylberberg (UPMC) for most helpful discussions. The manuscript was greatly improved by the constructive suggestions of two anonymous reviewers. J.M.-F. is supported by a grant from “la Caixa” Foundation (Spain).

Handling editor: Martha Richter