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.
Discovery and naming
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. They were sent a photo of the nasal bone from which they identified it as belonging to a tyrannosaur, which was likely a new species because it came from an age with no known members of that group. The fossil remains were carefully excavated over a year by a joint team from the BLM and the Natural History Museum of Utah (UMNH). The locality, which is public land, was designated as UMNH VP 1501. Prior to the dinosaur's formal description, it had been referred to as the "Nipple Butte Tyrannosaur" or "Wahweap tyrannosaurid". The specimen, UMNH VP 20200 (with the prefix denoting its storage in the UMNH), was made the holotype of the new genus and species Lythronax argestes by paleontologist Mark A. Loewen and colleagues in 2013. The generic name is derived from the Greek words lythron (λύθρον), meaning "gore", and anax (ἄναξ), meaning "king". The specific name argestes (ἀργεστής) is a Greek name used by the poet Homer for the wind from the southwest, in reference to where the specimen was found within North America. In full, the scientific name can be translated as "gore king (or "king of gore") from the southwest". 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".
The holotype and single known specimen of Lythronax consists of a partial skull and skeleton, which includes the right maxilla, both nasals, 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 second and fourth metatarsals. In the paper that named Lythronax, the authors also described a new specimen of the geologically younger tyrannosaur Teratophoneus (which had been named in 2011); this genus is known from the Kaiparowits Formation of Grand Staircase–Escalante, and the two tyrannosaurs were used to investigate the evolutionary and geographical origins of the family Tyrannosauridae. Based on the paper's conclusions, the UMNH referred to Lythronax as a "great-uncle" of Tyrannosaurus on their website. In 2017, the US government announced plans to shrink the Grand Staircase–Escalante (to little over half its size) and Bears Ears monuments to enable coal mining and other energy development on the land; this was the largest reduction of US national monuments in history. Lythronax itself was one of two dinosaurs from the former monument mentioned in the presidential proclamation, along with Diabloceratops. 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. Media outlets stressed the importance of the area's fossil discoveries—including more than 25 new taxa—while some highlighted Lythronax as one of the significant finds. The US government was subsequently sued by a group of scientists, environmentalists, and Native Americans; in 2021, the monument was restored to its former extent by the subsequent administration.
Description
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