INITIAL REPORT ON DECAPOD CRUSTACEANS FROM THE MIOCENE BEAR LAKE FORMATION, ALASKA

Population studies of fossil decapod crustaceans are rare due to the low probability of fossilization of a significant sample size. The Bear Lake Formation (Miocene) has yielded a population of 23 specimens of the crab Metacarcinus goederti, allowing collection of morphometric data from the dorsal carapace. Plotting these measurements against one another yields linear trends suggestive of a single population consisting of multiple age classes. In addition to M. goederti, 2 specimens of a new species of Tymolus were collected. Other fossils present in the Bear Lake Formation, including Turritella sagai, indicate a Miocene age. The rocks were deposited in a warm-water, shallow-marine environment, adjacent to a forested area of some relief, as evidenced by non-marine debris flows and terrestrial plant fossils. Crossbedding indicates strong currents in the area (Wisehart, 1971; Nilsen, 1984; Nilsen, 1985; Marincovich and Tomoki, 1986). Extant members of Tymolus range from temperate to warm water as well as from shallow to the upper limits of deep water (Alcock, 1894; Tavares, 1991, 1992; Karasawa, 1993; Tan and Huang, 2000), neither supporting nor refuting the accepted notion of the environment of deposition of the Bear Lake Formation (Alcock, 1894; Tavares, 1991, 1992; Karasawa, 1993; Tan and Huang, 2000). The occurrence of Metacarcinus, however, suggests that the Bear Lake Formation formed in a shallow-marine setting while also providing evidence against a warm-water environment. Extant Metacarcinus live in temperate areas (Feldmann, 2003). This suggests that more work and greater scrutiny be given to the fossil assemblage of the Bear Lake Formation to more accurately refine estimates of water-temperature. If additional fossil evidence strengthens the argument for a warm-water depositional setting for the Bear Lake Formation, this would suggest a change in environmental preference of Metacarcinus since the Miocene.