Giganotosaurus (/ˌdʒaɪɡəˌnoʊtəˈsɔːrəs/ jy-gə--noh-tə-sor-əs or gig-ə-not-o-saw-rus meaning "giant southern lizard") is a genus of carcharodontosaurid dinosaurs that lived in what is now Argentina during the early Cenomanian age of the Late Cretaceous Period, approximately some 99.6 to 97 million years ago.

Giganotosaurus was one of the largest theropods, with known individuals equaling or surprassing the size of genera such as Tyrannosaurus and Carcharodontosaurus.

The skeleton of the holotype specimen (MUCPv-Ch1) is about 70% complete and includes parts of the skull, a lower jaw, pelvis, hindlimbs and most of the backbone, missing only the premaxillae, jugals, quadratojugals, the back of the lower jaws and the forelimbs. A second specimen (MUCPv-95) has also been identified, found in 1988 by Jorge Calvo and consisting of a fragment of a lower jaw, said to be 8% larger than the corresponding part in the first specimen.

The skull of Giganotosaurus is large; that of the holotype was in 1995 estimated at 1.53 m (5.0 ft) in length. Even though the original authors briefly claimed the length to be up to 1.8 m (5.9 ft)—leading to an estimate of 1.95 m (6.4 ft) skull length for the referred specimen—this claim was not repeated by subsequent workers and one of the original authors was in 2002 co-writer of an article giving a holotype skull length of 1.6 m (5.2 ft). Some have claimed that even the original estimate was too long and believe the skull to be almost exactly comparable to the one of Tyrannosaurus in length. The skull is slender and elongated in build, with rugose areas on the edges of the snout top and above the eye. The supratemporal openings were overhung by the edges of the skull roof where the jaw muscles of each side directly attached instead of meeting each other at a midline skull crest. The back of the skull as preserved is strongly inclined forwards, bringing the jaw joints far behind the attachment point of the neck. The endocast of Giganotosaurus has a volume of 275 cc (16.8 cu in) and including the olfactory bulbs, it was 19% longer than that of the related theropod, Carcharodontosaurus saharicus.

The shoulder blade was very short and thick, with sudden kinks in its shaft. The ischium had a paddle-shaped end; the thigh bone, 1.43 m (4.7 ft) long in the holotype, had a head that was pointing relatively upwards. The mid dorsal vertebrae carried rather high spines. The total length of the holotype has been estimated between 12.2 and 13 m (40 and 43 ft), the largest specimen at 13.2 m (43 ft), and the weight between 6 and 13.8 tonnes (13,230 and 30,420 lb).

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