Lythronax Temporal range: Late Cretaceous, 80 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skeletal reconstruction of Lythronax (A) and Teratophoneus (B)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Tyrannosauridae
Tribe:	†Teratophoneini
Genus:	†Lythronax Loewen et al., 2013
Type species
†Lythronax argestes Loewen et al., 2013

Lythronax ("king of gore", from Greek words lythron meaning "gore" and anax meaning "king") is an extinct genus of tyrannosaurid theropod dinosaur which lived around 80 million years ago in what is now southern Utah. L. argestes is the oldest known tyrannosaurid based on its stratigraphic position.^[1]

Lythronax argestes is known from a partial skeleton and its diagnostic features include a reduced alveoli count, a concave lateral margin of the dentary, its tall cervical neural spine and a broad caudal portion of the skull. The holotype specimen consists of the right maxilla, both nasal bones, the right frontal, the left jugal, the left quadrate, the right laterosphenoid, the right palatine, the left dentary, the left splenial, the left surangular, the left prearticular, a dorsal rib, a caudal chevron, both pubic bones, the left tibia and fibula, and left metatarsals II and IV. Based on the assemblage, the fossils are considered to have come from one individual, which was an adult.^[1]

Comparisons to its close relatives suggest that Lythronax would have been about 8 m (26.2 ft) long, with a weight of around 2.5 tonnes (5,500 lb) and had a large skull filled with sharp teeth.^[2] The rostrum is comparatively short, since it makes up less than two thirds of the total skull length. The whole skull is very broad, it's 0.4 times as wide as long. 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.^[1]

Lythronax is known from the most complete tyrannosaurid specimen discovered from southern Laramidia. 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 in 2009. It was recovered from the lower part of the middle member of this formation in terrestrial sediments that were radioisotopically dated as being approximately 80 million years old.^[3] Based on its stratigraphic position, L. argestes is the oldest tyrannosaurid dinosaur discovered so far.^[1]

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.^[1]

Below is the cladogram by Loewen et al. in 2013.^[1]

Previously, palaeontologists had thought there were multiple exchanges of tyrannosauroids between Asia and North America with various forms moving between the continents via what is now Alaska and the Bering Strait into northern Russia. The new work, however, suggests that almost all of the Asian tyrannosauroids were part of one evolutionary lineage. There were separate evolutionary radiations in North America of northern and southern tyrannosaurids, with Lythronax part of the southern group.^[2]