STASIS IN BIVALVE GROWTH AND POPULATION ECOLOGY: AGE DISTRIBUTION, GROWTH CURVES, AND BIOMASS OF A POPULATION OF REVUELTIAN (UPPER TRIASSIC: EARLY-MID NORIAN) UNIONIDS FROM WEST TEXAS

Since the Triassic, unionid bivalves have been long-lived and important elements in the ecological and trophic systems of freshwater littoral environments. They often constitute the bulk of biomass in such areas. NMMNH locality 3634 in the Norian-age Bull Canyon Formation of the Chinle Group in Borden County, Texas yields the most extensive and best-preserved sample of unionid bivalves known from the Upper Triassic Chinle Group. The clams are tentatively assigned to Antediplodon sp. based on beak ornamentation and position of maximum width. They were found preserved in growth position as an autochthonous assemblage. Using measurements of 480 near-perfect individuals we performed an allometric analysis that defines relative growth throughout life for these clams.

Additionally, we use probability plotting, a powerful analytical tool adapted from reliability engineering, to separate and define numerous Gaussian (normal) component size distributions that make up the overall multi-modal size distribution of this population. These components appear to represent yearly age groups (annual rings are not present in these fossils).

We use the resolved age group data to produce growth curves and show that length (as a function of age) in these Triassic clams fits the von Bertalanffy formula and has the same growth coefficient as the extant unionid Elliptio dilatata. We substitute the length (age) formula into an empirical expression for mass (length) in modern unionids to produce an expression for ~mass throughout life for the fossil clams. Plots of age distribution, length (age), mass (age), and biomass and yearly production for given population densities are all similar to published parameters of extant unionids. This suggests that unionid growth trajectories evolved early during their evolutionary history. The analysis shows no evidence of sexual dimorphism, or of mixed species.

Probability plotting is a simple statistical method that has widespread applications in invertebrate paleontology where large numbers of individuals from “discrete” populations are available for analysis. It can separate and quantify morphometric groupings arising from any cause. The plots yield population, growth, and ecological information that are otherwise difficult to infer from fossil assemblages.