Allosaurus was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail, and reduced forelimbs. The average length of A. fragilis, the best-known species, has been estimated at 8.5 m (28 ft) and its average mass at 1.7 t (1.9 short tons), with the largest definitive Allosaurus specimen (AMNH 680) estimated at 9.7 m (32 ft) long, with an estimated weight of 2.3–2.7 t (2.5–3.0 short tons). In his 1976 monograph on Allosaurus, James H. Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of 12 to 13 m (39 to 43 ft). As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between 1 and 4 t (1.1 and 4.4 short tons) for modal adult weight (not maximum).