EVOLUTIONARY NOVELTY AND ATAVISM IN THE SEMIONOTUS COMPLEX: RELAXED SELECTION DURING COLONIZATION OF AN EXPANDING...

Fishes of the genus Semionotus diversified in the rift lakes of eastern North America during the Mesozoic (Newark Supergroup). Like the well-known cichlid fishes of the African great lakes, diverse complexes of semionotids were apparently endemic to a number of different lakes. Semionotid fishes show considerable morphological diversity in body shape and in a modified row of scales termed "dorsal ridge scales." A number of distinct dorsal-ridge-scale patterns characterize groups of species from the Newark Supergroup. Interestingly, about 5.5% of individuals examined have anomalous scales mixed in with otherwise stereotypic dorsal-ridge-scale patterns. In this study, I take advantage of nearly annual stratigraphic resolution to determine whether dorsal-ridge-scale anomalies are concentrated stratigraphically in the early phase of lake formation and colonization by semionotids. More than 1,700 specimens of semionotid fish were collected from a single lake deposit (cycle P4), representing approximately 21,000 years, in the Early-Jurassic Towaco Formation of the Newark Basin. Dorsal-ridge-scale anomalies are significantly more frequent in older than in younger lake sediments, which I interpret as being the result of relaxed selection during the early colonization of the lake. Anomalous variation parallels variation in dorsal ridge scales between species-groups. Some anomalies are atavisms, while others are unique or foreshadow future evolutionary events. One type of anomaly is incorporated into the dorsal-ridge-scale series of two new species that gave rise to a radiation in a subsequent lake filling the same topographic basin. Because both novelties and atavisms occur in the dorsal-ridge-scale series of single individuals, I argue that the disruptions of the same "developmental program" produced both atavistic and novel traits.