Glacialisaurus is a genus of sauropodomorph dinosaur from the Early Jurassic period of Antarctica. It is known from two specimens; the holotype (name-bearing specimen), a partial tarsus (ankle) and metatarsus, and a partial left femur (thigh bone). The fossils were collected by a team led by paleontologist William R. Hammer during a 1990–91 field expedition to the central region of the Transantarctic Mountains. They come from sedimentary rocks of the Hanson Formation and date to the Pliensbachian stage of the Early Jurassic, around 186 to 182 million years ago. The fossils were described in 2007, and made the basis of the new genus and species Glacialisaurus hammeri. The genus name translates as “icy” or "frozen lizard”, and the specific name honors Hammer. This dinosaur has been classified as a massospondylid, a group of medium-sized, basal (early diverging or "primitive") sauropodomorphs that existed during the Late Triassic and Early Jurassic on every continent except Australia. Its length has been estimated at 6.2 m (20 ft). Glacialisaurus was a large herbivorous dinosaur, though it was average sized for a massospondylid. Glacialisaurus was distinct from other sauropodomorphs in features such as having a robust medial epicondylar ridge on the lower femur, a robust adductor ridge extending from the upper end of the femoral medial condyle, and a second metatarsal with a front border that is weakly convex in the upper end.
Discovery and naming
Fossils of a sauropodomorph dinosaur were discovered by a field team from Augustana College led by paleontologist William R. Hammer during 1990–91 fieldwork in the lower Hanson Formation of Mount Kirkpatrick in the Central Transantarctic Mountains of Antarctica, dating to the Early Jurassic. The fossils were from two different individuals: elements of the right ankle and metatarsus such as the astragalus, two tarsals and four metatarsals preserved in articulation (specimen FMNH PR1823), and the lower part of a left femur (FMNH PR1822, a thigh bone), ending just after the dissipation of the medial epicondylar crest. Several other fossils were collected from the same site, including fossils of the carnivorous theropod dinosaur Cryolophosaurus, a pterosaur humerus (upper arm bone), and a large tooth of a tritylodont, all found at an elevation of about 4,100 m (13,500 ft). The right ankle and tarsus were preserved in a 1 m (3.3 ft) thick layer of strata, while the femur was preserved at the surface weathering next to the Cryolophosaurus specimen. The fossils were sent to the Field Museum of Natural History in Chicago, USA, and were first reported in 1994. This report speculated that cervical vertebrae from Cryolophosaurus found nearby also belonged to the sauropodomorph, but this has since been disproven. The fossils were described by the paleontologists Nathan Smith and Diego Pol, who named the new genus and species Glacialisaurus hammeri, with FMNH PR1823 as holotype specimen. The generic name is derived from the Latin root glacialis meaning 'icy' after its discovery in the Beardmore Glacier region in the Central Transantarctic Mountains and the word sauros meaning 'lizard'. The specific name honors Hammer for his contributions to Antarctic paleontology.
Description
While few remains are known of Glacialisaurus, its leg bones show it was a robust basal (early diverging or "primitive") sauropodomorph. The femur fragment is the larger of the two known specimens, measuring 300 mm (0.98 ft) as preserved, with an estimated total length when intact of 600 mm (2.0 ft). Glacialisaurus is estimated to have been about 6.2 m (20 ft) long. As a basal sauropodomorph, Glacialisaurus would have had a long neck and a proportionally small head with leaf-shaped teeth. The hand would have been short, wide, and robust with a large claw on the thumb.
Leg bones The cross-section of the robust femoral shaft is slightly wider from side to side than from front to back, though not as extreme as in eusauropods. The medial epicondylar crest extends from the medial surface of the lower femoral shaft and is distinct from all other sauropodomorphs in that it is robust, a trait convergently evolved in basal theropods. The front surface of the femur is flat instead of convex from side to side, a feature shared with other basal sauropodomorphs. The top surface of the upper femur lacks any anterior extensor groove. At the lower end, the lateral and medial condyles are separated by a craniocaudal groove that ends abruptly with a popliteal fossa (opening in the bone). Glacialisaurus is distinguished by its robust adductor ridge extending from the upper end of the femoral medial condyle. This ridge starts at the end of the medial condyle and is kidney-shaped with a long axis spanning proximolateral−distomedially.
The astragalus is low and elongate from across side to side and the medial portion lacks the craniocaudally broadening compared to the lateral portion, a trait found in most non-eusauropods. The astragalus is weakly convex at the lower end, though this is not as extreme as in Blikanasaurus and Lessemsaurus. The upper surface of the astragalus is softly convex because it is where the lower end of the tibia (shin bone) articulates with the astragalus. This surface is pierced by two foramina (small openings in bone) that have been interpreted as vascular foramina. The ascending process (protrusion of bone) is mound shaped and its upper articular surface faces proximomedially. The distal tarsals have a laterally elongated triangular shape in when seen from their top ends. The corners of the tarsals are rounded and bulbous, especially in the posteromedial corner. The medial distal tarsal is not confined solely to metatarsal III, but also barely contacts the proximal end of metatarsal II, like in Saturnalia. The lateral distal tarsal has a quadrangular shape and was likely longer mediolaterally than proximodistally.
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