long time, some up to 15 million years. The oldest modern lakes have existed for only a few million years, and the vast majority only a few thousand. Three major lakes are known: Fossil Lake, Lake Gosiute, and Lake Uinta. Although it was the shortest-lived, Fossil Lake preserved the most fossils, including all stingray fossils. Fossil Lake is one of two areas that preserve complete stingray fossils; the other, the Monte Bolca Formation in Italy, preserves stingrays that lived in a marine lagoon behind a coral reef. Monte Bolca preserves a greater diversity of rays, including guitarfishes and electric rays. More rays may have lived in Monte Bolca because it was a marine environment; today, most stingrays live in the ocean and only a few live in fresh water. Fossil Lake was a freshwater lake in a semitropical environment. The modern Gulf Coast and southern Atlantic regions of the United States have been used as modern analogues to the environment. Asterotrygon is found in two types of deposits. One group of fossil localities is called F-1 or the "18-inch layer" and includes limestone that was deposited in the middle of fossil lake over a few hundred years. These sediments were thought to have formed in a deep area where the lake bottom was anoxic. The F-2 layer is thicker, up to , and was deposited over a longer period of time, possibly several thousand years. F-2 limestone was deposited closer to the north and southeastern shorelines of the lake where the water was richer in dissolved oxygen and organic material. New monotypic family Asterotrygonidae Asterotrygon currently contains a single species, Asterotrygon maloneyi, even though there exists several morphological differences in the denticle specimen suggesting that there are a number of undescribed Asterotrygon species. A number of specimens indicate that Fossil Lake was a breeding ground for Asterotrygon.[6 Although there are only a few dozen specimens of this species known, the known sample includes one apparent mating pair, a female specimen with an embryo inside, and a specimen with newly born babies beside it, wherein the specimen with the embryo inside indicates that this species gave live birth or that they were viviparous.[7] Asterotrygon is a relatively primitive type of stingray whose family relationships have not yet been clearly resolved. One study categorized this family outside of all important and popular stingrays’ lineages, so it is placed in the new monotypic