Isanosaurus

Abstract: Five shafts of large long bones of dinosaurs have been found since 1846 in the Rhaetic Bone bed of the Westbury Formation (Upper Triassic) at Aust Cliff near Bristol, Avon, southwestern England. Two bones (1 lost) are Dinosauria incertae sedis and a third (also lost), the longest and best preserved, was probably part of a femur of the melanorosaurid prosauropod Camelotia (Upper Triassic, England). The width of the other two femoral shafts is greater transversely than anteroposteriorly, as in Camelotia. However, these shafts are also straight in lateral view, a derived character that occurs only in sauropods and stegosaurs and some Cretaceous ankylosaurs and ornithopods. However, the shafts are curved in lateral view in basal ankylosaurs and Jurassic ornithopods. Unlike the femora of Upper Jurassic sauropods and stegosaurs, in which the bone is almost solid, the Aust shafts consist of a thin layer of compact cortical bone surrounding a large area, most of which is filled with very lightly constructed cancellous bone. However, cross-sections of the humerus just below the deltopectoral crest of the sauropod Isanosaurus (Upper Triassic, Thailand) and the stegosaur Dacentrurus (Upper Jurassic, England) are intermediate in their histology. The femoral shaft is hollow in some Jurassic stegosaurs, though not to the degree shown by those from Aust. The Aust shafts are truly columnar, agreeing with those of stegosaurs in the apparent lack of a prominent fourth trochanter and of the associated posteromedial depression, structures which are prominent in sauropods. These two shafts, with estimated femoral lengths of about 1000-1100 mm, are tentatively referred to the Stegosauria so stegosaurs probably reached a large size in the Upper Triassic ; the earliest definitive record of the group is Middle Jurassic (Bathonian) of England.