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 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.

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.

In 2009, Scott Richardson of the U.S. Bureau of Land Management (BLM) was searching for fossils with a co-worker in the Wahweap Formation of the Grand Staircase–Escalante National Monument, southern Utah, when they came across a leg and nasal bone of a theropod dinosaur in the Nipple Butte area.

Richardson contacted a team of paleontologists at the University of Utah, who were excited but initially skeptical, since theropod fossils had not been discovered in the area before.

The fossil remains were carefully excavated over a year by a joint team from the BLM and the Natural History Museum of Utah (UMNH).

Loewen stated that the suffix meaning "king" in the name of Lythronax was intended to allude to its later, similar relative Tyrannosaurus rex.

The prefix meaning "gore" was chosen to exemplify "its presumed lifestyle as a predator with its head covered in the blood of a dead animal".

[11] American paleontologist Scott D. Sampson (a co-describer of Lythronax), who had overseen much of the early research at the monument, expressed fear that such a move would threaten further discoveries.

Like other members of the group, it would have possessed small, two-fingered forelimbs, large and strong hindlimbs, broad jaws, and a very robustly constructed skull.

Each ramus of the dentary (half of the tooth-bearing portion of the lower jaw) was strongly concave towards the outer side (bowing inwards along the length of the skull).

This mirrored the contours of the maxilla of the upper jaw, and the strong expansion of the rear skull; this was similar to Bistahieversor, Tyrannosaurus, and Tarbosaurus, but unlike other tyrannosauroids.

[6] Though the postcranial skeleton of Lythronax is poorly known, the known remains of the pubis (part of the pelvis) and the hindlimb show features typical within Tyrannosauridae.

[2][3][6] Prior to Lythronax being formally named, Zanno and colleagues noted in 2013 that the holotype specimen was likely distinct from Teratophoneus and Bistahieversor, both likewise from southern Utah.

[4] A detailed phylogenetic analysis, conducted by Loewen and colleagues to accompany their 2013 description of Lythronax, based on 303 cranial and 198 postcranial features, placed it and Teratophoneus within the subfamily Tyrannosaurinae.

[6] In 2017, American paleontologists Stephen Brusatte and Thomas D. Carr published a new phylogenetic analysis of Tyrannosauroidea, including a more comprehensive suite of anatomical characteristics and taxa, that disagreed with the results of Loewen and colleagues.

[26] The sequence of interchange events which occurred among Laramidian tyrannosaurids is unclear, and the diverse tyrannosauroids which have been discovered in southern Laramidia (including Lythronax, Teratophoneus, and Bistahieversor) have complicated their evolutionary history further.

[19] Based on their phylogenetic results, Zanno and colleagues proposed that the then-unnamed Lythronax displayed features that united tyrannosaurids from southern Laramidia to the exclusion of other genera.

[18] The hypotheses of Asian-North American migration of Brusatte and Carr were supported by a later run of their analysis by Canadian paleontologist Jared Voris and colleagues in 2020.

Voris and colleagues suggested these morphological differences arose for ecological reasons, possibly including prey composition or feeding strategies.

[2][29] The teeth and jaw muscles of Lythronax would have contributed to strong bite forces, for not just carving out chunks of flesh but also crushing bone.

[30][31] Lythronax was found in terrestrial sedimentary rocks belonging to the lower part of the Reynolds Point Member of the Wahweap Formation.

[32] During the time Lythronax lived, the Western Interior Seaway was at its widest extent, almost completely isolating southern Laramidia from the rest of North America.

[37] Vertebrates present in the Wahweap Formation at the time included freshwater fish, bowfins, abundant rays and sharks, turtles such as Compsemys, crocodilians,[38] and lungfish.