Lythronax is an extinct genus of tyrannosaurid theropod dinosaur that lived around 80.6 to 79.9 million years ago in what is now southern Utah, USA. The generic name is derived from the Greek words lythron meaning “gore” and anax meaning “king”. Lythronax was a large sized, moderately-built, ground-dwelling, bipedal carnivore that could grow up to an estimated 8 m (26.2 ft) in length and weighed 2.5 tonnes (5,500 lb).
L. argestes is the oldest known tyrannosaurid, based on its stratigraphic position. It is known from a specimen thought to be from a single adult that consists of a mostly complete skull, both pubic bones, a tibia, fibula, and metatarsal II and IV from the left hindlimb, as well as an assortment of other bones. Its skull anatomy indicates that, like Tyrannosaurus, Lythronaxhad both eyes facing the front, giving it depth perception.
It has been estimated that Lythronax would have been about 7.3 m (24.0 ft) long, with a weight of around 2.5 tonnes (5,500 lb), based on comparisons to close relatives, and had a large skull filled with sharp teeth. The rostrum of its skull is comparatively short, since it makes up less than two thirds of the total skull length. The whole skull is very broad, and is only about 2.5 times as long as it is wide. Overall, the skull is morphologically most similar to that of Tyrannosaurus and Tarbosaurus. Its robust maxilla possessed a heterodont dentition, as its first five teeth are a lot larger than the other six. Like other tyrannosaurids, Lythronax has large, distally expanded pubic boot which is approximately 60% the length of the pubic bone. The postcranial morphology is similar to that of other tyrannosaurids.
Lythronax is known from the most complete tyrannosaurid specimen discovered from southern Laramidia. The careful excavation took nearly a year. This specimen is housed in the collection of Natural History Museum of Utah in Salt Lake City, Utah. The holotype specimen UMNH VP 20200 was recovered in the UMNH VP 1501 locality of the Wahweap Formation at Grand Staircase-Escalante National Monument (GSENM), in Kane County, southern Utah. It was discovered in 2009 and collected from the lower part of the middle member of this formation in terrestrial sediments. The sediments were radioisotopically dated as being 79.6 to 80.75 million years old, meaning that Lythronax is approximately 80 million years old. Based on its stratigraphic position, L. argestes is the oldest tyrannosaurid dinosaur discovered so far.
Lythronax was named for its likeness to Tyrannosaurus. Loewen et al. wanted to symbolize that the fossil was alike in most features to the much later tyrannosaurid. They decided to keep the ‘king’ in the name and without using ‘rex’, named it Lythronax, the “Gore King”, accordingly. For the specific name they agreed to use argestes, relating to the area where the fossil was found, southwest Utah. The generic name is derived from the Greek λύθρον, lythron, ‘gore’ and ἄναξ, anax, ‘king’. The specific name is the Greek ἀργεστής, “clearing”, used by the poet Homer as the epithet of the southwind.
Lythronax argestes belongs to the family Tyrannosauridae, a family of large-bodied coelurosaurs, with most genera known from North America and Asia. A detailed phylogenetic analysis, based on 303 cranial and 198 postcranial features, places it and Teratophoneus within Tyrannosaurinae. Lythronax is a sister taxon of a clade consisting of the Maastrichtian taxa Tarbosaurus and Tyrannosaurus and the late Campanian Zhuchengtyrannus. Lythronax probably was not a direct ancestor of Tyrannosaurus. If not, they clearly shared a common ancestor that was even older than Lythronax.
A 2013 study edited by Alan L. Titus and Mark A. Loewen on dinosaurs of southern Utah suggested the separation of Teratophoneus, Bistahieversor and Lythronax (UMNH VP 20200 – the Waheap tyrannosaurid). That means that there are three or more tyrannosaurid taxa present in the Western Interior Basin. Their analysis found that the three southern tyrannosaurid form a clade to the exclusion of other tyrannosaurids from northern Campanian formations.
The Wahweap Formation has been radiometrically dated as being between 81 and 76 million years old. During the time that Lythronax lived, the Western Interior Seaway was at its widest extent, almost completely isolating southern Laramidia off from the rest of North America.
Lythronax shared its paleoenvironment with other dinosaurs, such as the hadrosaur Acristavus gagslarsoni and lambeosaur Adelolophus hutchisoni, the ceratopsian Diabloceratops eatoni, and unnamed ankylosaurs and pachycephalosaurs. Vertebrates present in the Wahweap Formation at the time of Lythronax included freshwater fish, bowfins, abundant rays and sharks, turtles like Compsemys, crocodilians, and lungfish. A fair number of mammals lived in this region, which included several genera of multituberculates, cladotherians, marsupials, and placental insectivores.