The evolution of the scalation pattern in temnospondyl amphibians

In most Palaeozoic temnospondyls, thin round-oval scales covering the flanks and the back of the trunk can be distinguished from ventral, elongate gastral scales arranged in a chevron pattern. The extensive growth series of the temnospondyl Sclerocephalus reveals that the morphology of the gastral scales in small larvae corresponds to the round-oval scales of the rest of the body. During subsequent ontogeny, the gastral scales differentiate and attain a spindle-shaped morphology. The tapering end of each gastral scale fits into a dorsal groove on the medial adjacent scale. This arrangement allowed telescoping of the scales and thus provided a high degree of flexibility. In the ontogenetically most advanced specimens of Sclerocephalus the gastral scales attain a rhomboid outline, and the articulation by well-defined facets has reduced the flexibility between them. In most temnospondyls, the gastral scales retain the 'juvenile' spindle-shape or the 'larval' round-oval shape, which can be interpreted as a paedomorphic trait. This suggests that the different types of gastral scales in temnospondyls, as well as the scales of the back and the flanks, can be traced back to the same Anlage of round-oval scales that differentiated early in ontogeny. In the Mesozoic, a complete reduction of dermal scalation occurred independently in distinct dissorophoid, capitosauroid, and trematosauroid temnospondyls. This reduction was probably the result of several factors unique to each group, such as cutaneous respiration, the demand for greater mobility, and the decreased importance of belly protection in fully aquatic temnospondyls.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 150 , 815–834.