Microstratigraphic study of meristic variation in an Eocene fish from a 10 000-year varved interval at Horsefly, British Columbia

Varved, lacustrine rocks of the Middle Eocene Horsefly deposits in British Columbia are ideal for microstratigraphic studies. Temporal resolution in such varved deposits can theoretically be as small as a year. In the Horsefly beds, specimens can be assigned precisely to their position in the stratigraphic section by comparing the laminations enclosing the fossils with those of a reference section. Each fossil can thus be assigned to a relative year of death. Some 700 specimens of the catostomid fish Amyzon aggregatum from the 10 000-year "H3" varved interval are examined for meristic variation. Very few of the meristic variables are significantly correlated with each other. Meristic series that are the last to develop ontogenetically are also the most phenotypically variable. In the studied interval, meristic variation has a strong temporal component, particularly in the case of fin rays and in the ratio between precaudal and caudal vertebral counts. Much, but not all, of this temporal variation occurs in conjunction with environmental changes in the lake as estimated by taphonomy and is consistent with some combination of ecophenotypic and (or) evolutionary responses of the fish population to the environmental change.