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On the Origin of the Jumping Mechanism in Frogs

Carl Gans and Thomas S. Parsons
Evolution
Vol. 20, No. 1 (Mar., 1966), pp. 92-99
DOI: 10.2307/2406151
Stable URL: http://www.jstor.org/stable/2406151
Page Count: 8
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On the Origin of the Jumping Mechanism in Frogs
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Abstract

Although, in the absence of a more complete fossil record, it is impossible to reach any definite conclusions, we consider it most probable that frogs arose as small riparian forms, living on the invertebrates along the shores of small bodies of fresh water. Saltation was probably first developed as an escape mechanism whereby the pre-frog crossed the air-water interface as rapidly as possible. Such a symmetrical form of locomotion, as opposed to the lateral undulatory and asymmetrical mode typical of primitive tetrapods and their fish ancestors, may also have been used in catching prey, but such an assumption is not necessary for our theories. A saltatory escape mechanism, even with minimal efficiency, would have an obvious selective value, as would any further modifications arising later and tending to refine or perfect it. The initial modifications involved in saltation would not necessarily interfere with the possibility of lateral undulatory locomotion and pre-frogs presumably continued to use the latter when in aquatic surroundings. Some of the later modifications of the saltatory mechanism ultimately would force the complete abandonment of undulatory locomotion, but by this time the selective advantage of well-developed saltatory locomotion on land might well outweigh any disadvantages in the water. Moreover, well-developed saltation is readily adaptable to aquatic locomotion for which, as evidenced by modern frogs, it is quite effective. Some other workers have postulated that pre-frogs developed saltatory locomotion in a more or less purely aquatic or more or less purely terrestrial environment; their theories have been commented on above. Although it is now, and may well remain, impossible to disprove any such theories, we feel that it is easier and more plausible to explain the anatomical changes involved in the origin of frogs, together with their functional "causes" and consequences, on the basis of a primitively riparian habitat. The utility of such arguments as the present rests to some extent in their providing a guide for the interpretation and evaluation of additional fossils that may come to light. We feel that the ideas presented also allow the establishment of simple schemes that may permit evaluation of the several theories utilizing parameters determined from observations and experiments on a variety of Recent forms.

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