the formation in 1879 and 1947, respectively. These stingrays were later synonymized with Heliobatis. In 1980, a Green River fossil called AMNH P 11557 was described that included a female stingray and two smaller individuals thought to be aborted fetuses. A second specimen called FMNH PF 15166 included a female and a fetus that was still in an embryo within the animal. The genus Asterotrygon was erected in 2004 along with the type species A. maloneyi, and FMNH PF 15166 was designated the holotype specimen. AMNH P 11557 and many other complete fossils were also referred to A. maloneyi. Hundreds of fossils of Heliobatis have been found, while Asterotrygon is represented by a few dozen specimens. Asterotrygon means "star stingray" from the Greek asteros ("star") and trygon ("stingray") after the star-like shape of the base of each denticle. The type species A. maloneyi was named after Thomas Maloney, who donated the paratype specimen AMNH P 11557 to the American Museum of Natural History. Although Asterotrygon is not the earliest stingray (they were present as early as the Early Cretaceous, it is one of the most well-preserved of the early forms. Most earlier stingrays are known only from small teeth, dermal denticles, or stingers, but not entire bodies. Classification Asterotrygon is classified as basal stingray within Myliobatoidei. Only Hexatrygon, the living Sixgill stingray, is more basal. Despite living alongside each other, Asterotrygon and Heliobatis are not closely related. Neither are classified in any living family of stingrays, although a 2004 phylogenetic analysis found Heliobatis to be closely related to a clade including river stingrays and whiptail stingrays. Asterotrygon is more closely related to Urolophidae, the stingarees, and Plesiobatis, the Deepwater stingray. Both types are from the Indo-Pacific. Below is a cladogram showing the phylogenetic relationships of Asterotrygon from Carvalho et al. (2004): Paleobiology Asterotrygon is closely related to Pacific stingrays. Its ancestors probably originated in the western Pacific or Indo-Pacific. In contrast, the contemporaneous Heliobatis probably evolved from stingrays that were already present in the Americas. Because the two genera are not closely related, their ancestors probably colonized the Green River Formation lakes in two separate but closely timed events. Neither Asterotrygon nor Heliobatis are known outside the Green River Formation, so they probably evolved within the lakes. Asterotrygon and other organisms from the Green River Formation lived in subtropical lakes in a mountainous region. These lakes existed for a very