EVOLUTION IN THE SUBHOLOSTEAN FISHES

The origin of a higher category involves a structural and functional transition from one adaptive zone to another. During the transition, the character complexes undergoing change do not necessarily evolve at the same rate or at the same time. The evidence for this conclusion is mostly provided by a few examples in the vertebrate fossil record. Actually, the great majority of families, orders and classes of invertebrates, and most higher categories of vertebrates, possess all or most of their distinguishing characters at the time of their first known appearance in the record. Transitional forms frequently combine primitive and advanced characters in such a way that allocation of these forms to an ancestral or a descendent category becomes more or less arbitrary. De Beer (1954), in a recent discussion of the nature of the transition phase, points out that the shift from one major group to another involves animals that possess a mosaic of nearly static ancestral and evolving descendent characters. Mosaic evolution, as he terms it, is thus characteristic of the transition phase. An important consideration in the concept of mosaic evolution is the manner in which morphologic change occurs. De Beer (ibid., p. 169) states that "owing to the genetic make-up and the internal conditions of development in an organism, there may be only a limited number of changes possible, and it is legitimate to think that changes restricted to single organ-systems are more likely to have occurred than progressive changes affecting the entire organism. It is further probahie that chrnngees restricted to single organ-systems at a time were more viable and less likely to succumb to selection. " The purpose of this paper is to consider some aspects of mosaic evolution in a group of extinct ray-finned fishes called the subholosteans by a study of associations between pairs of generic characters tested by the chi square method. This approach is in a sense tentative and exploratory for reasons which will be evident later. Other groups of fossil organisms showing greater taxonomic diversification, particularly among the invertebrates, should permit more thorough statistical treatment and hence more positive conclusions in relation to association. The subholosteans show varying degrees of advancement, from a primitive level, represented by the paleoniscoid fishes, toward a higher level, exemplified by the holostean fishes. The subholosteans, as well as the examples of mosaic evolution cited by De Beer, indicate that the transition changes do not involve single organ-systems but rather functionally integrated structural complexes that may involve components of a number of organ systems (as De Beer implies elsewhere in his paper). For instance, modifications in the feeding mechanism nearly always include changes in the skull, in the jaw musculature, and to some extent in the nerve supply and the circulation. There is also good reason for believing that changes may occur in different parts of the body more or less simultaneously but independently of each other. All alterations must, of course, be integrated in respect to the entire organism, but this does not necessarily involve close functional interdependence of all the complexes undergoing change at any particular time.