Lythronax

*Lythronax argestes*

Robert F Walters Artwork

**Lythronax** is an extinct genus of tyrannosaurid theropod dinosaur which lived around 80 to 78 million years ago in what is now southern Utah. 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 9 meters (30 feet) in length and weighed 2.5 tonnes (5,500 lbs).

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, Lythronax had both eyes facing the front, giving it depth perception and like Tyrannosaurus, it possibly had binocular vision and extremely good senses of smell, hearing and night vision.

## Discovery

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 by Scott Richardson of Flagstaff, Arizona, 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.

## Etymology

*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 have argestes, relating to the area where the fossil was found, southwest Utah. The generic name is derived from 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.

## Description

Lythronax was a fairly medium to large tyrannosaur, growing up to 9 metres (30 feet) long from nose to tail and standing over 2 metres tall. It was about the weight of a large crocodile which is around 1-2 tons. It had typical tyrannosauroid features such as the large head, large teeth, small, almost useless arms, the classic *s* feature commonly found in theropods, forward facing eyes and long heavy tail to balance it while it was running and walking. It probably had a very strong bite which is common with tyrannosaurs so its bite might have been similar in force to the American alligator. It was definitely a carnivore from the shape of its teeth. This particular species would not have been a bone crusher like T. rex but a species that was adept at taking huge chunks out of its prey and waiting for it to die of blood loss like modern day wolves.

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.

## Classification

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 was probably not a direct ancestor of Tyrannosaurus, with the two genera sharing a common ancestor that was even older than Lythronax.

## Appearance in other media

### *Jurassic Park*

* *Lythronax* will appear in the game Jurassic World: Alive. While it is accurately portrayed with a coat of feathers, it is shown inaccurately to be able to pronate it's hands.
* *Lythronax* appeared in Jurassic World: The Game.