Developmental Pattern and Morphology of Salamandrella keyserlingii Limbs (Amphibia, Hynobiidae) Including Some Evolutionary Aspects

Abstract

In this paper, we consider the pattern of skeletal development and morphology in the limbs of the hynobiid species, Salamandrella keyserlingii , with the overall aim of analyzing tetrapod limb evolution in context of the phylogeny. In comparison with anurans the urodeles demonstrate caenogenetic adaptation in distal structures of the limbs. As the most primitive family of Caudata hynobiids are of special interest since a study of early stages is likely to display some plesiomorphic features. In common with other larval salamanders Salamandrella has early development of digits 1 + 2. However, in contrast to them this species displays a number of distinctive characters, including larval adaptations and features which can be considered as phylogenetically primitive for caudates and sometimes for uniseriate sarcopterygians in general. The main peculiarities of Salamandrella can been summarized as follows: 1) early formation of the mesenchyme mass between the zeugopod bifurcation as the base for the proximodistal differentiation of preaxial, median and postaxial columns; 2) initial connection of the median column with postaxial one in the region of intermedium condensation; 3) early formation of the intermedium condensation in close contact with ulnar/fibular condensation; 4) dominance of the posterior or postaxial branch (ulnare/fibulare) in the early limb development; 5) comparatively late formation of the basale commune which arises primarily as a small distal 2 condensation at the base of the digit 2 and later fuses with distal 1 condensation; 6) fusion of the skeletal elements in longitudinal rather than in transverse direction; usually amalgamation of the median elements with postaxial than with preaxial ones; 7) frequent (in 75%) amalgamation of the intermedium with ulnare; 8) presence of two central elements in the standard morphology; 9) presence of a well developed, long temporary epidermal fin between first and second digits used in balance and locomotion of the larvae. This is caenogenetic adaptation. The comparison of Salamandrella with other caudates and some fossil sarcopterygians led us to conclusion that many developmental pattern of its limb (1 – 4) can been considered as an ancestral features. It may be supposed that such characters of urodeles as the precocious appearance of anterior digits and basale commune, the gap between them and proximal (zeugopod/stylopod) portion, distal-to-proximal sequence in development of carpal and tarsal elements have a secondary character, probably correlated with their caenogenetic adaptations, and do not support the hypothesis of the polyphyly of tetrapods.