Asterotrygon is an extinct genus of stingray from the Eocene Green River Formation in Wyoming. Several complete skeletons representing juveniles, adults, males and females have been uncovered from the late early Eocene Fossil Butte Member of the formation. The type and only species, A. maloneyi, was named in 2004 on the basis of these fossils. Another stingray, Heliobatis, is also known from the formation. Asterotrygon is a primitive stingray closely related to the living family Urolophidae whose ancestors likely originated in the Indo-Pacific. It lived in Fossil Lake, a body of water that existed in a subtropical mountainous region for only about 2 million years. Description Asterotrygon had a typical stingray shape with a flat, rounded disk formed from the head and pectoral fins and a long, narrow tail with sharp stingers. It ranged in size from in the smallest juvenile (the specimen SMMP 83.25) to in the largest adult (FMNH 15166). The shape of the disk is rounder than those of other extinct stingrays such as Heliobatis, which are more diamond-shaped. The upper surface of the disk is covered in small dermal denticles, each with a small hook. Unlike Heliobatis and modern stingrays, it has a small dorsal fin in front of its stingers. While most stingrays have a cartilaginous rod extending from the stinger to the tip of the tail, Asterotrygon retains separate vertebrae throughout the tail's entire length. The tail is also somewhat thicker at its base than those of other stingrays. Small fin-like tail folds are present at the tip of the tail. The puboischiadic girdle, an element to which the pelvic fins attach, is primitively narrow and arch-shaped. Like other stingrays, Asterotrygon lacks thoracic ribs but possesses cartilage around the spine called thoracolumbar synarcual cartilage. Several details of the neurocranium link Asterotrygon with modern stingrays. Like other stingrays, the hyomandibulae, which allow the jaw to extend outward, are completely separated from the lower jaw. In other cartilaginous fish, the hyomandibulae and lower jaw are fused by the hyomandibular-Meckelian ligament. Despite this loss, Asterotrygon still retains some calcified cartilage in the space where the ligament once was. The postorbital processes behind the eye sockets are broad, flat, and shelf-like. History Before the description of Asterotrygon, Heliobatis was the only known stingray in the Green River Formation. American paleontologist Othniel Charles Marsh named Heliobatis radians in 1877, and Xiphotrygon acutidens and Palaeodasybatis discus were subsequently named from