| Lythronax Temporal range: Late Cretaceous,
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| Reconstructed skeleton, Milwaukee Public Museum | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Tyrannosauridae |
| Subfamily: | †Tyrannosaurinae |
| Clade: | †Teratophoneini |
| Genus: | †Lythronax Loewen et al., 2013 |
| Type species | |
| †Lythronax argestes Loewen et al., 2013
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Lythronax (LYE-thro-nax) is a genus of tyrannosaurid dinosaur that lived in North America around 81.9-81.5 million years ago during the Late Cretaceous period. The only known specimen was discovered in Utah in the Wahweap Formation of the Grand Staircase–Escalante National Monument in 2009, and it consists of a partial skull and skeleton. In 2013, it became the basis of the new genus and species Lythronax argestes; the generic name Lythronax means "gore king", and the specific name argestes originates from the Greek poet Homer's name for the wind from the southwest, in reference to the specimen's geographic provenance in North America.
Size estimates for Lythronax have ranged between 5 and 8 m (16 and 26 ft) in length, and between 0.5 and 2.5 t (1,100 and 5,500 lb) in weight. It was a heavily built tyrannosaurid, and as a member of that group, it would have had small, two-fingered forelimbs, strong hindlimbs, and a very robust skull. The rear part of the skull of Lythronax appears to have been very broad, with eye sockets that faced forwards to a similar degree as seen in Tyrannosaurus. Lythronax had 11 tooth sockets in the maxilla bone of the upper jaw; most tyrannosaurids had more. The frontmost teeth were the largest, the longest being almost 13 cm (5 in) long. Other details of the skull and skeleton which distinguished Lythronax from other tyrannosaurids included the s-shaped outer margin of the maxilla and a process of the astragalus of the ankle, a projection that expanded further upwards compared to its relatives.
The holotype was found in the Reynolds Point Member of the Wahweap Formation, which dates to the Campanian stage of the Cretaceous. Lythronax is thus the oldest known member of the family Tyrannosauridae, and it is thought to have been more basal than Tyrannosaurus. Due to its age, Lythronax is important for understanding the evolutionary origins of tyrannosaurids, including the development of their anatomical specializations. The forward-facing eyes of Lythronax gave it depth perception, which may have been useful during pursuit or ambush predation.