The Evolution of Concepts Related to the Origin of the Amphibia

Abstract

Although rhipidistian ancestry of amphibians is now firmly established, there has been considerable difference of opinion as to who originally suggested this relationship. A literature search prompted by another project has revealed what I believe to be main steps in this interesting story. In 1861 T. H. Huxley proposed Crossopterygidae (regarded by him as a suborder of Order Ganoidei in spite of -idae ending) to include Polypterus, various fossil genera later assigned to Rhipidistia and Dipnoi, and coelacanths. In this same paper he also discussed resemblances between his Crossopterygidae and Lepidosiren (at that time only genus assigned to MMuller's Order Dipnoi), and concluded that latter is next of kin to former. Cope attempted to eliminate Agassiz's and Huxley's Ganoidei in 1872 by recognizing three major groups of higher bony fishes -the Dipnoi, Crossopterygia, and Actinopteri. This arrangement was contested by Gill (1872) who could see no valid reason for breaking up Ganoid Series and isolating Crossopterygia. (For an interesting discussion of relative merits of Cope and Gill classifications, see Osborn, 1931:499-502.) Gill concluded, however, that crossopterygians led to dipnoans and latter to batrachiansalthough this is not indicated in his quasigenealogical tree of fishes (ibid., p. xliii). There is no evidence that he favored derivation of amphibians directly from crossopterygians, as mentioned by Osborn (1931:525). Following Gegenbaur's (1865) discussion of archipterygial paired fins and Gunther's (1872) contribution on Ceratodus (Epiceratodus), Huxley (1876) carried out his own investigation of this genus and concluded that dipnoans were nearest allies to Amphibia. The mutual occurrence of autostyly weighed heavily in this opinion. By 1880 Huxley had placed Dipnoi in main line of vertebrate evolution and considered Ganoidei (in which he still included Crossopterygidae) as a side branch of higher bony fishes. In 1885 Cope (p. 237) wrote the crossopterygian fore limb, with its arm-like axis, tells of origin of first limbed vertebrates, Batrachia, whose skull-structure, however, only permits their derivation from Dipnoi or Holocephali. As former subclass has Crossopterygian fin structure, we can safely regard them as ancestors of Batrachia, while Crossopterygian are a sideline from a similar type, probably Ichthyotomi, because these have a free suspensor of lower jaw. Cope was obviously thinking about meaning of autostyly, but like his contemporaries, he concluded that modern amphibians had to arise from an autostylic ancestor. Nevertheless, he felt that crossopterygian fin skeleton was closer to amphibian type than was dipnoan. With one of his characteristic flashes of insight, Cope subsequently broke through dipnoan barrier, partly because of an earlier observation by Huxley. In my opinion, our present understanding of amphibian origin mostly stems from following statement (Cope, 1892:279-280) which is quoted here in its entirety: