LOCOMOTION OF LIMBLESS VERTEBRATES: PATTERN AND EVOLUTION

Multiple lines of tetrapods have become secondarily elongate and have reduced or lost their limbs (Gans, 1975, 1985b). The problem of such limb reduction and the associated changes of other systems has been discussed often (Duerden and Essex, 1922; Essex, 1927, 1928; Gasc, 1977; Gasc and Renous, 1974, 1976; Krieg, 1917; Lande, 1978; Raynaud, 1985; Renous, 1977a,b; Sewertzoff, 1908; Steiner and Anders, 1946; Stokely, 1947; Underwood, 1977); this association of reduction and hypertrophy represents a classical topic in zoology (Gasc, 1970; Sewertzoff, 1931). One corollary topic has been the utilization of morphologically intermediate stages; this has recently become involved with questions of adaptation and developmental constraints. Twenty-three years ago, I was asked to discuss limbless movement as part of a symposium on vertebrate locomotion (Gans, 1962; for popular versions, 1966a,b, 1970). It then seemed appropriate to characterize the mechanically most effective locomotor specializations, namely sidewinding, concertina, lateral undulation and rectilinear motion, patterns that exemplify the movements of such derived forms as snakes and amphisbaenians. In redefining their locomotor patterns, I emphasized the transmission of forces from the skin to the exterior, rather than internal motor patterns or the shapes into which the moving animals place their trunk. This simple approach retains its utility for differentiating locomotor patterns.