RATES OF EVOLUTION IN THE COELACANTH AND DIPNOAN FISHES

The Crossopterygii and the Dipnoi are the only vertebrate orders known to span the long time interval between the Devonian Period and Recent times. Together they make up the subclass Choanichthyes, a group of fishes characterized by the presence of internal nostrils and many features of the skeleton setting them apart from the other major group of bony fishes, the Actinopterygii. Of the two suborders included in the Crossopterygii (Romer, 1945), the Rhipidistia became extinct in the early Permian, while the Coelacanthini have persisted to the present time. The rhipidistians, in spite of their relatively short history, are important, as they gave rise to the tetrapods as well as to the coelacanths. The dipnoans, or lungfishes, were derived either directly from primitive rhipidistians or from a common ancestral group that also gave rise to the rhipidistians. The Coelacanthini were already well defined by the middle Devonian, the time of their first known appearance in the fossil record. They are found in strata of all subsequent geologic periods, except the Tertiary, when apparently they entered the open sea which provided little opportunity for preservation. The single Recent genus, Latimeriac, is known from one specimen obtained in the coastal waters of South Africa. In spite of the long history of the coelacanths, they have not been particularly successful in terms of the total number of described genera (about 27) or in terms of adaptive radiation. On the basis of known evidence, the peak of their diversification was reached in the Triassic, but for this period only 11 genera have been reported and, with a few exceptions, the various morphological specializations in these are of rather minor significance. The Devonian coelacanths, with one probable exception, have been found in marine formations, suggesting a marine origin or, more likely, an early marine radiation. There is only negative evidence for supposing that the coelacanths as a group deserted the marine environment by the Mississippian. It is possible that they have inhabited the seas since their origin, with a few fresh-water invasions. The known Carboniferous and Permian coelacanths occur in fresh-water sediments, while the Triassic genera are about 60 per cent marine. The other Mesozoic genera and, of course, Latimeria are marine. The dipnoans are an equally well differentiated group, with the first known representative occurring in marine rocks of early Devonian age. This genus (Dipnorhynchus) retained a high percentage of primitive rhipidistian characters, and it is probable that the lungfishes arose in the early Devonian. Although the total number of genera is about the same as in the Coelacanthini, the greatest number (15) for any one geologic period is recorded from the Devonian, suggesting that the maximum radiation occurred early in the history of the group. The Devonian genera are found in freshwater, brackish, and marine formations (Romer and Grove, 1935; Westoll, 1949), and there was a short-lived marine invasion at this time if not a marine origin for the group. In later geologic periods the dipnoans are found almost exclusively in fresh-water deposits, and the three living genera are also fresh-water inhabitants. Occasional post-Devonian