A middle Eocene hydrocarbon-seep fauna from Ecuador and the lost ecologic niche of large, high-spired gastropods at seeps

Middle Eocene (Bartonian) carbonate rocks found at Punta Ancón in southwestern Ecuador are here identified as an ancient hydrocarbon seep deposit based on its negative δ 13 C signature (as low as −32.4‰). The fauna includes members of the bivalve families Thyasiridae, Lucinidae, and Kalenteridae, and the abyssochrysoid gastropod Ascheria elenensis , which reached an estimated height of 150 mm or more. Such large, high-spired gastropods occurred at Cretaceous through mid-Eocene seeps worldwide but are notably absent today. This geologic range coincides with a period of particularly low marine sulfate concentrations. We discuss the lost ecologic niche of these large, high-spired gastropods and favor an interpretation of them as having been grazers on microbial mats, potentially composed of different kinds of bacteria than those forming mats at modern seeps. As a considerable proportion of the Punta Ancón seep fauna was obligate to this environment, the depositional depth of this site can be described as offshore.