MASS-MORTALITY LAYERS OF FOSSIL STICKLEBACK FISH: CATASTROPHIC KILLS OF POLYMORPHIC SCHOOLS

Stickleback fish (Gasterosteus doryssus) from late middle-Miocene lacustrine deposits of the Truckee Formation in west-central Nevada are abundant and well preserved. They occasionally occur in unusually high densities on single annual laminations (varves) in "mass-mortality layers." We demonstrate that stickleback mass-mortality layers consist of members of schools and may be used as population samples. Comparison of stickleback mass-mortality samples to time-averaged samples, which accumulated over hundreds or thousands of years in the same deposit, indicates that, for some purposes, the time-averaged samples are acceptable surrogates for instantaneous population samples from the G. doryssus lineage. Mass-mortality and time-averaged samples are similar for variation of pelvic structure and dorsal-spine number, but associations between states of different characters may be weaker in mass-mortality samples than in time-averaged samples. Thus, character variances in time-averaged samples of G. doryssus are comparable to those of living populations, but character correlations are suspect. Character variances and correlations in time-averaged fossil samples must be evaluated on a case-by-case basis and interpreted with caution.