The evolution of the scalation pattern in temnospondyl amphibians

Abstract

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.

The bone histology of osteoderms in temnospondyl amphibians and in the chroniosuchian Bystrowiella

“…Among fossil temnospondyl amphibians, the presence of ossified dermal scales that cover the trunk, the tail and the limbs is widespread and can be regarded as a plesiomorphic character of tetrapods (Witzmann 2007). Additionally, different lineages of temnospondyls have evolved the possession of dermal armour consisting of plate-like osteoderms in the postcranial skeleton that vary conspicuously in size, shape and mode of articulation.…”

Dermal bone in early tetrapods: a palaeophysiological hypothesis of adaptation for terrestrial acidosis

“…The heavy dermal cover of some of these tetrapods, bearing a sculptured surface of pits and ridges, may be considered to be osteoderms, similar to those seen today in some reptiles and frogs [19]; but these can be distinguished from thinner, overlapping dermal scales in pockets of the dermis [20] that are an ancestral tetrapod feature [16,17,21]. Dermal bone does not block interaction between the dermis and the superficial integument because blood vessels run through it to the epidermis [22].…”

New insights into the scales of the Devonian tetrapodTulerpeton curtumLebedev, 1984

“…Neither spindle-shaped scales similar to those of Acanthostega (Coates, 1996) nor specialized ventral scales have been identified with confidence in Tulerpeton. Moreover, 'squared' scales, similar to those of temnospondyls (Witzmann, 2007(Witzmann, , 2011, are not known in Tulerpeton or in any other Devonian tetrapod. Their occurrence seems to be restricted to post-Devonian tetrapods and may have evolved during the Carboniferous.…”

“…Morphologically, tetrapod dermal scales are known to be of four different morphotypes: rounded, ovoid, spindle-shaped, and rhombic or 'squared,' the latter being the most frequent in certain Carboniferous forms such as temnospondyls (see Witzmann, 2007: fig. 8, 2011:fig.…”

“…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, 2007Witzmann, , 2011.…”