In modern vertebrates, particularly ectotherms in temperate climates, alternating periods of rapid and curtailed growth are often recorded by annual markings in bones and/or scales. By indicating the age at death of individual animals and the time of year when death occurred, such markings permit quantitative studies of growth rates and population dynamics of fossil vertebrates to be made, and aid in interpreting the origin of fossil deposits. Scales of fish collected from the varved, lacustrine Green River Formation (middle Eocene), southwestern Wyoming, show interruptions interpreted as annuli. The position of the most recent annulus relative to the edge of the scales indicates the season of death of individual fish. Perfectly preserved specimens died in early summer, whereas those that died at a later season are disarticulated and scattered. A likely explanation is that early summer mortalities coincided with the yearly summer maximum of carbonate precipitation in the lake, so that carcasses reaching the bottom at this time were protected from disruptive currents. Among a collection of early Pliocene vertebrates from a quarry in Nebraska are many specimens from which individual age and season of death can be determined. Annuli on gar scales and catfish vertebrae indicate that these fish were killed in winter. Among the mammals, merycodonts (aged by their horns and dentitions) and horses (aged by dentitions) apparently died in winter also. Unlike the Green River fish, which accumulated over thousands of years, the Nebraska occurrence probably resulted from a catastrophe.