COMPARATIVE MORPHOLOGY OF LOCOMOTION IN VERTEBRATES

In the present study a unifying framework is proposed for the developm ent of forms of locomotion in vertebrates. Four kinematic processes ar e identified that illustrate the divergence of modes of locomotion, fr om an ancestral undulatory lateral movement to the different modes see n in recent vertebrates: (a) Lateral movements of the segments of the trunk are incorporated, and then removed, in a cephalocaudal order; (b ) a proximodistal transition is then evident, in which appendages (fin s or legs), rather than the segments of the trunk, take over the task of active propulsion; (c) synchronized action of fore- and/or hind leg s follows the primeval alternate action of legs; and (d) movements of the segments of the trunk in the vertical domain are incorporated to f acilitate locomotion. These processes can be illustrated in the ontoge ny of both amphibians and rodents: Lateral undulatory swimming in tadp oles is replaced during metamorphosis by synchronized hind leg strokes in toads and frogs; and pivoting around the hind legs, which is the p rimary quadrupedal activity in rats, is followed by alternate, and the n simultaneous, stepping of the fore- and hind legs. The same processe s may be applied to the development of locomotion in the different ver tebrate classes, from the numerous species of fish that swim with late ral movements, to the widespread use of vertical movements in facilita ting walking and swimming in mammals. When functional reasoning is ign ored and only the form of locomotion is considered, a regular order in development is apparent.