Insights into the Evolution of Scales in Tetrapods From Tulerpeton curtum

New insights into the scales of the Devonian tetrapod Tulerpeton curtum Lebedev, 1984

Authors:

Mondéjar-Fernández et al

Abstract:

The Devonian origin of tetrapods and their transition from aquatic to terrestrial habitats is one of the most important episodes in vertebrate evolutionary history (e.g., Coates, 1996; Janvier, 1996; Jarvik, 1996; Daeschler et al., 2006; Clack, 2012). Among the multiple changes that took place during the so-called ‘fish-tetrapod transition,’ those concerning the integumentary dermal skeleton have received little attention. Nonetheless, the skin and associated dermal ossifications of early tetrapods determine key aspects of their metabolism, way of life, and locomotion (e.g., Bystrow, 1947; Castanet et al., 2003; Markey and Marshall, 2007; Witzmann, 2007, 2011).

The main integumentary ossifications of early tetrapods are scales and osteoderms. Osteoderms are plates of dermal bone made by intradermal ossification that often bear a pitted outer surface. By contrast, dermal scales originate from the mesodermal layer of the dermis, are thinner than osteoderms, are often round or elongate ovals in outline, and may overlap (Castanet et al., 2003). Most Palaeozoic tetrapods were broadly covered with ossified dermal scales, mainly in their ventral and lateral regions (Romer, 1956; Janvier, 1996). However, dermal scales were lost in several tetrapod groups during the late Palaeozoic and Mesozoic, and replaced by osteoderms as the main integumentary mineralized structures (Vickaryous and Sire, 2009; Witzmann and Soler-Gijón, 2010).

Devonian tetrapods (such as Ichthyostega and Acanthostega) are considered as mainly aquatic animals, with limited walking abilities (Ahlberg et al., 2005; Pierce et al., 2012), as evidenced by the retention of primitive traits such as a caudal fin supported by osseous fin rays, a lateral line system, internal gills, and a dermal scale covering (Coates, 1996; Jarvik, 1996; Clack, 2012). Within Tetrapoda, the transition to land (or terrestrialization) during the Devonian and Carboniferous (Clack, 2012; Steyer, 2012) affected the squamation, modifying size, shape, overlapping pattern, and the bone tissue of the scales (see Witzmann, 2011, for a thorough review). However, the current lack of knowledge on the squamation of the earliest tetrapods does not allow testing or drawing of broader evolutionary scenarios. Here we furnish the first highly detailed three-dimensional reconstructions of the scales of the Devonian tetrapod Tulerpeton, thereby providing new interpretations on the morphological and microstructural evolution of the squamation in early tetrapods.