Microstratigraphic study of meristic variation in an Eocene fish from a 10000-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 s uch varved deposits can theoretically be as small as a year. In the Horsefl y beds, specimens can be assigned precisely to their position in the strati graphic section by comparing the laminations enclosing the fossils with tho se of a reference section. Each fossil can thus be assigned to a relative y ear of death. Some 700 specimens of the catostomid fish Amyzon aggregatum f rom the 10 000-year "H3" varved interval are examined for meristic variatio n. Very few of the meristic variables are significantly correlated with eac h other. Meristic series that are the last to develop ontogenetically are a lso the most phenotypically variable. In the studied interval, meristic var iation has a strong temporal component, particularly in the case of fin ray s and in the ratio between precaudal and caudal vertebral counts. Much, but not all, of this temporal variation occurs in conjunction with environment al changes in the lake as estimated by taphonomy and is consistent with som e combination of ecophenotypic and (or) evolutionary responses of the fish population to the environmental change.