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1 March 2012 Squamation and Scale Microstructure Evolution in the Porolepiformes (Sarcopterygii, Dipnomorpha) Based on Heimenia ensis from the Devonian of Spitsbergen
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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.
© 2012 by the Society of Vertebrate Paleontology
Jorge Mondéjar-Fernández and Gaël Clément "Squamation and Scale Microstructure Evolution in the Porolepiformes (Sarcopterygii, Dipnomorpha) Based on Heimenia ensis from the Devonian of Spitsbergen," Journal of Vertebrate Paleontology 32(2), (1 March 2012). https://doi.org/10.1080/02724634.2012.646836
Received: 12 July 2011; Accepted: 1 November 2011; Published: 1 March 2012
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