Lythronax

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Lythronax
Temporal range: Late Cretaceous, 80.6–79.9 Ma
Lythronax mount at TyrannosaursMeettheFamily.jpg
Reconstructed skeleton alongside those of other tyrannosaurs, Science Center of Iowa
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: Tyrannosauridae
Subfamily: Tyrannosaurinae
Genus: Lythronax
Loewen et al., 2013
Type species
Lythronax argestes
Loewen et al., 2013

Lythronax (LYE-thro-nax) is a genus of tyrannosaurid 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 possibly grow up to 7.3 m (24.0 ft) in length and a weight of up to 2.5 t (5,500 lb).

The type and only species of Lythronax, 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.

Discovery

Drawn map showing fossil location
Map showing the Nipple Butte area () of Grand Staircase-Escalante National Monument, where Lythronax was found in the Wahweap Formation

In 2009, Scott Richardson of the U.S. Bureau of Land Management 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, confirming it was a new kind, as it came from an age with no known members of that group. Unearthing the fossil remains took a year of careful excavation in 2010, by a joint BLM-Natural History Museum of Utah team. 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".[1][2][3][4][5]

The specimen, UMNH VP 20200 (housed in the Utah Museum) 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.[6] In full, the scientific name can be translated as "gore king" or "king of gore from the southwest". Loewen stated that the describers of Lythronax wanted the word "king" in the name to allude to the later, similar relative Tyrannosaurus rex. The word "gore" was chosen to exemplify "its presumed lifestyle as a predator with its head covered in the blood of a dead animal".[2][7][3]

Skeletal diagrams showing holotype remains of Lythronax (A) and a Teratophoneus specimen (B). N-P show selected bones of the former

The holotype and single known specimen of Lythronax consists of a partial skull and skeleton, which includes 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 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 were used to investigate the evolutionary and geographical origins of the family Tyrannosauridae.[6] In this regard, the Utah Museum referred to Lythronax as a "great-uncle" of Tyrannosaurus on their website.[8]

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, which was the largest reduction of US national monuments in history.[9][10] Lythronax itself was one of two dinosaurs from the monument mentioned in the presidential proclamation.[11] American paleontologist Scott D. Sampson (a co-author of Lythronax), who had overseen much of the early research at the former 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.[10][12][13][14] The US government was subsequently sued by a group of scientists, environmentalists, and Native Americans.[9][14]

Description

Size compared to a human

At the time Lythronax was announced, news sites reported size estimates of about 7.3 m (24.0 ft) in length and around 2.5 t (5,500 lb) in weight, based on comparisons to the much larger relative Tyrannosaurus.[2] American paleontologist Gregory S. Paul gave a lower estimate of 5 m (16 ft) in length and a weight of only 500 kg (1,100 lb) in 2016.[15] Lythronax was a relatively robust tyrannosaurid, a group that possessed strongly reduced two-fingered forelimbs, large and strong hindlimbs, broad jaws and very robustly constructed skulls.[15] Although earlier small-bodied tyrannosauroids possessed protofeathers, their presence could vary with classification in the clade or the age of an individual.[5]

Life restoration showing hypothetical feathers

Lythronax has a short snout and broad skull as in other most other tyrannosaurids. The width of the read skull in Lythronax is comparable only to Tarbosaurus and Tyrannosaurus among tyrannosaurids, the width over 40% of the length. The maxillae of Lythronax are strongly convex along the dental margin, which bears 11 teeth, unique among tyrannosauroids except for Teratophoneus and Bistahieversor. The first 5 maxillary teeth are significantly larger than those following.[6] The teeth were similar to bananas in shape, robust, and bore serrations.[16] Some of the front most teeth were almost 3 cm (1.2 in) long.[1] As in Tyrannosaurus, the palate is well developed and sigmoid along its length. The nasals along the top of the snout are much wider in front, unique to Lythronax. Also diagnostic of Lythronax, the prefrontal and postorbital bones almost exclude the skull roof from participating in the border of the orbit, which faced more anteriorly as in Tyrannosaurus. The jugal was very robust, and shows that the postorbital had a stronger projection into the orbit than any other tyrannosaurid. Mirroring the maxilla of the upper jaw, the dentary of the lower jaw was strongly bowed into the skull along its length, with a high rear end comparable only to Tarbosaurus and Tyrannosaurus. Like other tyrannosaurids, the surangular behind the dentary has a deep shelf just in front of the jaw articulation, although in Lythronax and Tyrannosaurus this shelf is dorsally concave.[6]

The postcranial skeleton of Lythronax is poorly known, but the pubis of the pelvis and the known hindlimb show features typical of Tyrannosauridae. The pubis has a very large expansion on its end termed the pubic boot, which is expanded anteriorly in all tyrannosaurids, although in Lythronax the boot is deep as in more derived tyrannosaurids but unlike Teratophoneus, Gorgosaurus and Albertosaurus. A deep depression is present on the proximal end of the fibula bone of the shin like other tyrannosaurids. Contrasting from the derived Tyrannosaurus and Tarbosaurus however, the astragalus of the ankle has a prominent flange on the anterior side above the articulation for the foot.[6]

Classification

Skull reconstruction (A) and selected skull bones of the holotype
Reconstructed skeleton in front view, Natural History Museum of Utah

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.[6] Lythronax was probably not a direct ancestor of Tyrannosaurus, with the two genera sharing a common ancestor that was even older than Lythronax.[2]

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. New studies have disrupted that hypothesis. It has been recently suggested that almost all of the Asian tyrannosauroids were part of one evolutionary lineage. It is now thought that there were separate evolutionary radiations in North America of northern and southern tyrannosaurids, with Lythronax being placed as part of the southern group.[6] Research on UMNH VP 16690 supports that theory.[5]

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. 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.[5]

Below is the cladogram by Loewen et al. in 2013.[6]

Tyrannosauridae

Gorgosaurus libratus

Albertosaurus sarcophagus

Tyrannosaurinae

Dinosaur Park tyrannosaurid

Daspletosaurus torosus

Daspletosaurus horneri

Teratophoneus curriei

Bistahieversor sealeyi

Lythronax argestes

Tyrannosaurus rex

Tarbosaurus bataar

Zhuchengtyrannus magnus

Paleobiology

Holotype skull reconstructed from 3D scans shown in multiple views

Lythronax is known from a well preserved skull. It had unique characteristics for its age; a short, narrow snout and a wide skull back, with forward-oriented eyes. These characteristics were originally thought to have evolved late in the Cretaceous, but the discovery of Lythronax pushed the origin of the body form back. This skull anatomy would have given Lythronax overlapping vision, allowing it to perceive depth, which is considered to be a predatory condition.[2] Loewen observed that Tyrannosaurus had a similar anatomy.[17]

It is suggested that the teeth were not just for slicing off meat, but also for inflicting damage and crushing bone.[16] The shape of the teeth would have made Lythronax an efficient hunter; it would have been able to carve out huge chunks of flesh and bone and swallow whole.[7]

Paleoenvironment

Reconstructed skull containing original holotype bones, NHMU

Lythronax was collected from the lower part of the middle member of the Wahweap 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.[6] Based on its stratigraphic position, Lythronax is the oldest tyrannosaurid dinosaur discovered so far.[2][3][6] The Wahweap Formation has been radiometrically dated as being between 81 and 76 million years old.[18]

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. It has been suggested that the isolation of Laramidia may have helped tyrannosaurids develop advantageous features, like very powerful jaws. Laramidia may have been the original home of tyrannosaurid theropods. Lythronax was likely the largest predator of its ecosystem.[2] The area where dinosaurs existed included lakes, floodplains, and east-flowing rivers. The Wahweap Formation is part of the Grand Staircase region, an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park through Zion National Park and into the Grand Canyon. The presence of rapid sedimentation and other evidence suggests a wet, seasonal climate.[19]

Lythronax shared its paleoenvironment with other dinosaurs, such as the saurolophine Acristavus and lambeosaurine Adelolophus,[20] the ceratopsian Diabloceratops,[2][21][8] and unnamed ankylosaurs and pachycephalosaurs.[22] Vertebrates present in the Wahweap Formation at the time of Lythronax included freshwater fish, bowfins, abundant rays and sharks, turtles like Compsemys, crocodilians,[23] and lungfish.[24] A fair number of mammals lived in this region, which included several genera of multituberculates, cladotherians, marsupials, and placental insectivores.[25] The mammals are more primitive than those that lived in the area that is now the Kaiparowits Formation. Trace fossils are relatively abundant in the Wahweap Formation, and suggest the presence of crocodylomorphs, as well as ornithischian and theropod dinosaurs.[26] Invertebrate activity in this formation ranged from fossilized insect burrows in petrified logs[27] to various mollusks, large crabs,[28] and a wide diversity of gastropods and ostracods.[29]

See also

References

  1. ^ a b Raloff, J. (2013). "King of Gore". Science News for Students. Retrieved 5 February 2020.
  2. ^ a b c d e f g h Vergano, Dan (Nov 6, 2013). "Newfound 'King of Gore' Dinosaur Ruled Before T. rex". National Geographic. Retrieved 22 November 2013.
  3. ^ a b c "'Gore King of the Southwest', Lythronax argestes". Natural History Museum of Utah. Nov 6, 2013. Retrieved 22 November 2013.
  4. ^ DeBlieux, D. D.; Kirkland, J. I.; Gates, T. A.; Eaton, J. G.; Getty, M. A.; Sampson,, S. D.; Loewen, M. A.; Hayden, M. C. (2013). "Paleontological overview and taphonomy of the Middle Campanian Wahweap Formation in Grand Staircase–Escalante National Monument". In Titus, A.; Loewen, M. (eds.). At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. Bloomington: Indiana University Press. pp. 563–577. ISBN 978-0-253-00883-1.CS1 maint: extra punctuation (link)
  5. ^ a b c d Zanno, L.E.; Loewen, M.A.; Farke, A.A.; Kim, G.-S.; Claessens, L.P.A.M.; McGarrity, C.T. (2013). "Late Cretaceous Theropod Dinosaurs of Southern Utah". In Titus, A.L.; Loewen, M.A. (eds.). At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. Indiana University Press. pp. 540–525. ISBN 978-0-253-00883-1.
  6. ^ a b c d e f g h i j Loewen, M. A.; Irmis, R. B.; Sertich, J. J. W.; Currie, P. J.; Sampson, S. D. (2013). Evans, David C (ed.). "Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans". PLoS ONE. 8 (11): e79420. doi:10.1371/journal.pone.0079420. PMC 3819173. PMID 24223179.
  7. ^ a b "Toothy Dino Terrorized Utah Before T. rex". Discovery NEWS. Jennifer Viegas. Nov 6, 2013. Archived from the original on February 6, 2020. Retrieved 16 November 2013.
  8. ^ a b "Diabloceratops eatoni". Natural History Museum of Utah. Retrieved 16 November 2013.
  9. ^ a b Gonzales, R.; Siegler, K.; Dwyer, C. (2017). "Trump orders largest national monument reduction In U.S. history". NPR.org. Archived from the original on 5 July 2019. Retrieved 25 June 2019.
  10. ^ a b Finnegan, M. (2017). "Remarkable dinosaur discoveries under threat with Trump plan to shrink national monument in Utah, scientists say". latimes.com. Archived from the original on 25 June 2019. Retrieved 24 June 2019.
  11. ^ Trump, D. J. (2017). "Presidential Proclamation Modifying the Grand Staircase-Escalante National Monument". The White House. Retrieved 6 February 2020.
  12. ^ Wiles, T. (30 January 2018). "How Trump's cuts to public lands threaten future dinosaur discoveries". The Guardian. Retrieved 6 February 2020.
  13. ^ Gramling, C. (2017). "Science and politics collide over Bears Ears and other national monuments". Science AAAS. Archived from the original on 26 June 2019. Retrieved 26 June 2019.
  14. ^ a b Wei-Haas, M. (2017). "What shrinking fossil-rich national monuments means for science". Smithsonian. Archived from the original on 24 June 2019. Retrieved 24 June 2019.
  15. ^ a b Paul, G.S. (2016). The Princeton Field Guide to Dinosaurs (Second ed.). Princeton University Press. pp. 108–109, 113. ISBN 978-0-691-16766-4.
  16. ^ a b "King of gore dinosaur was the 'bad grandpa' of tyrannosaurs". Los Angeles Times. Geoffrey Mohan. 6 November 2013. Retrieved 8 November 2013.
  17. ^ White, MacRinac (6 November 2013). "New Dinosaur Species, Lythronax Argestes, Discovered In Utah". LiveScience. Stephanie Pappas. Retrieved 8 November 2013.
  18. ^ M. A. Getty, M. A. Loewen, E. M. Roberts, A. L. Titus, and S. D. Sampson. 2010. Taphonomy of horned dinosaurs (Ornithischia: Ceratopsidae) from the late Campanian Kaiparowits Formation, Grand Staircase-Escalante National Monument, Utah. In M. J. Ryan, B. J. Chinnery-Allgeier, D. A. Eberth (eds.), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press, Bloomington. p. 478–494 ISBN 978-0-253-35358-0
  19. ^ Jinnah, Zubair A. (2009). "Sequence Stratigraphic Control from Alluvial Architecture of Upper Cretaceous Fluvial System - Wahweap Formation, Southern Utah, U.S.A." (PDF). Search and Discovery Article # 30088. Retrieved 10 December 2013.
  20. ^ Terry A. Gates; Zubair Jinnah; Carolyn Levitt; Michael A. Getty (2014). "New hadrosaurid specimens from the lower-middle Campanian Wahweap Formation of Utah". In David A. Eberth; David C. Evans (eds.). Hadrosaurs: Proceedings of the International Hadrosaur Symposium. Indiana University Press. pp. 156–173. ISBN 978-0-253-01385-9.
  21. ^ Gates, T. A.; Horner, J. R.; Hanna, R. R.; Nelson, C. R. (2011). "New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America". Journal of Vertebrate Paleontology. 31 (4): 798. doi:10.1080/02724634.2011.577854.
  22. ^ "New Horned Dinosaurs from the Wahweap Formation" (PDF). Utah Geology. 2007.
  23. ^ Thompson, Cameron R. (2004). "A preliminary report on biostratigraphy of Cretaceous freshwater rays, Wahweap Formation and John Henry Member of the Straight Cliffs Formation, southern Utah". Geological Society of America Abstracts with Programs. 36 (5): 43.
  24. ^ Orsulak, M.; Simpson, E.L.; Wolf, H.I.; Simpson, W.S.; Tindall, S.S.; Bernard, J. Jenesky, T. (2007). "A lungfish burrow in late Cretaceous upper capping sandstone member of the Wahweap Formation Cockscomb ara, Grand Staircase-Escalanta National Monument, Utah". Geological Society of America Abstracts with Programs. 39 (5): 43.CS1 maint: multiple names: authors list (link)
  25. ^ Eaton, J.G; Cifelli, R.L (2005). "Review of Cretaceous mammalian paleontology; Grand Staircase-Escalante National Monument, Utah". Geological Society of America Abstracts with Programs. 37 (7): 115.
  26. ^ Tester, E.; Simpson, E.L.; Wolf, H.I.; Simpson, W.S.; Tindall, S.S.; Bernard, J.; Jenesky T. (2007). "Isolated vertebrate tracks from the Upper Cretaceous capping sandstone member of the Wahweap Formation; Grand Staircase-Escalante National Monument, Utah". Geological Society of America Abstracts with Programs. 39 (5): 42.
  27. ^ De Blieux, D.D. (2007). "Analysis of Jim's hadrosaur site; a dinosaur site in the middle Campanian (Cretaceous) Wahweap Formation of Grand Staircas-Escalante National Monument (GSENM), southern Utah". Geological Society of America Abstracts with Programs. 39 (5): 6.
  28. ^ Kirkland, J.I.; De Blieux, D.D.; Hayden, M. (2005). "An inventory of paleontological resources in the lower Wahweap Formation (lower Campanian), southern Kaiparowits Plateau, Grand Staircase-Escalante National Monument, Utah". Geological Society of America Abstracts with Programs. 37 (7): 114.CS1 maint: multiple names: authors list (link)
  29. ^ Williams, J.A.J.; Lohrengel, C.F. (2007). "Preliminary study of freshwater gastropods in the Wahweap Formation, Bryce Canyon National Park, Utah". Geological Society of America Abstracts with Programs. 39 (5): 43.
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