| Lythronax | |
|---|---|
 | |
| Reconstructed skeleton,Milwaukee Public Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Tyrannosauridae |
| Subfamily: | †Tyrannosaurinae |
| Tribe: | †Teratophoneini |
| Genus: | †Lythronax Loewenet al.,2013 |
| Type species | |
| †Lythronax argestes Loewenet al., 2013 | |
Lythronax (LYE-thro-nax) is agenus oftyrannosauriddinosaur that lived in North America around 81.9-81.5 million years ago during theLate Cretaceousperiod. The only known specimen was discovered inUtah in theWahweap Formation of theGrand Staircase–Escalante National Monument in 2009, and it consists of a partial skull and skeleton. In2013, it became the basis of the new genus and speciesLythronax argestes; the generic nameLythronax means "gore king", and thespecific nameargestes originates from the Greek poetHomer's name for the wind from the southwest, in reference to the specimen's geographic provenance in North America.
Size estimates forLythronax 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 ofLythronax appears to have been very broad, with eye sockets that faced forwards to a similar degree as seen inTyrannosaurus.Lythronax had 11tooth sockets in themaxilla 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 distinguishedLythronax from other tyrannosaurids included thes-shaped outer margin of the maxilla and aprocess of theastragalus of the ankle, a projection that expanded further upwards compared to its relatives.
Theholotype was found in the Reynolds Point Member of the Wahweap Formation, which dates to theCampanianstage of the Cretaceous.Lythronax is thus the oldest known member of thefamily Tyrannosauridae, and it is thought to have been morebasal thanTyrannosaurus. 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 ofLythronax gave itdepth perception, which may have been useful duringpursuit orambush predation.
In 2009, Scott Richardson of theU.S. Bureau of Land Management (BLM) was searching forfossils with a co-worker in theWahweap Formation of theGrand Staircase–Escalante National Monument, southern Utah, when they came across a leg andnasal bone of atheropoddinosaur in the Nipple Butte area. Richardson contacted a team ofpaleontologists at theUniversity 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 atyrannosaur, which was likely a newspecies 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 theNatural History Museum of Utah (UMNH). The locality, which ispublic land, was designated as UMNH VP 1501.[1][2][3][4] Prior to the dinosaur's formal description, it had been referred to as the "Nipple Butte Tyrannosaur" or "Wahweap tyrannosaurid".[5][4]
The specimen, UMNH VP 20200 (with the prefix denoting its storage in the UMNH), was made theholotype of the newgenus and speciesLythronax argestes by paleontologist Mark A. Loewen and colleagues in2013. The generic name is derived from the Greek wordslythron (λύθρον), meaning "gore", andanax (ἄναξ), meaning "king". Thespecific nameargestes (ἀργεστής) is a Greek name used by the poetHomer for the wind from the southwest, in reference to where the specimen was found within North America.[6] In full, thescientific name can be translated as "gore king (or "king of gore") from the southwest". Loewen stated that the suffix meaning "king" in the name ofLythronax was intended to allude to its later, similar relativeTyrannosaurus 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".[2][3][7]
The holotype and single known specimen ofLythronax consists of a partial skull and skeleton, which includes the rightmaxilla, both nasals, the rightfrontal, the leftjugal, the leftquadrate, the rightlaterosphenoid, the rightpalatine, the leftdentary, the leftsplenial, the leftsurangular, the leftprearticular, adorsal rib, a caudalchevron, bothpubic bones, the lefttibia andfibula, and left second and fourthmetatarsals.[6] In the paper that namedLythronax, the authors also described a new specimen of the geologically younger tyrannosaurTeratophoneus (which had been named in 2011); this genus is known from theKaiparowits Formation of Grand Staircase–Escalante, and the two tyrannosaurs were used to investigate the evolutionary and geographical origins of thefamilyTyrannosauridae.[6][8] Based on the paper's conclusions, the UMNH referred toLythronax as a "great-uncle" ofTyrannosaurus on their website.[3]
In 2017, the US governmentannounced plans to shrink the Grand Staircase–Escalante (to little over half its size) andBears Ears monuments to enablecoal mining and other energy development on the land; this was the largest reduction of US national monuments in history.[9][10]Lythronax itself was one of two dinosaurs from the former monument mentioned in thepresidential proclamation, along withDiabloceratops.[11] American paleontologistScott D. Sampson (a co-describer ofLythronax), who had overseen much of the early research at the monument, expressed fear that such a move would threaten further discoveries.[10][6] Media outlets stressed the importance of the area's fossil discoveries—including more than 25 newtaxa—while some highlightedLythronax as one of the significant finds.[12][13][14] The US government was subsequently sued by a group of scientists, environmentalists, andNative Americans; in 2021, the monument was restored to its former extent by the subsequent administration.[9][14][15]
At the timeLythronax was announced, news sites reported size estimates of about 7.3–8 m (24–26 ft) in length and around 2.5 t (5,500 lb) in weight, based on comparisons to the much larger relativeTyrannosaurus; Loewen stated that it may have grown even larger.[1][2] American paleontologistGregory 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.[16]Lythronax was a relatively robust tyrannosaurid. Like other members of the group, it would have possessed small, two-fingered forelimbs, large and strong hindlimbs, broad jaws, and a very robustly constructed skull.[16] Although earlier small-bodied members of thesuperfamily Tyrannosauroidea possessedprotofeathers, their presence could have varied between species or the age of an individual.[4]
Lythronax had a relatively short snout and a broad skull (width over 40% of the length), as in other tyrannosaurids. The nasal bones along the top of the snout were much wider at the front than the middle, unlike in other tyrannosaurids. Viewed from above, the outer margins of the skull (formed by the maxilla and jugal bones) were stronglysigmoid-shaped (or s-shaped). Along with the width of the frontal bone (a bone at the top of the skull), this appeared to have made the rear part ofLythronax's skull very broad, withorbits (eye sockets) that faced nearly forwards. These features are otherwise only known inTarbosaurus andTyrannosaurus; earlier-diverging tyrannosaurids had less forward-facing orbits, and the rears of their skulls were narrower.[6]
Lythronax was also distinct in that the surfaces of the frontal bone that contacted the prefrontal and postorbital bones at its front and rear sides were separated by only a narrow groove. The maxillae ofLythronax were robust and strongly convex along their outer margins, as in all other known tyrannosaurids, but differed in their sigmoid-shaped margins.Lythronax had 11alveoli (tooth sockets) in each maxilla, a trait shared with no tyrannosaurs other thanTeratophoneus andBistahieversor (other tyrannosaurs had 12 or more maxillary alveoli). The maxillary teeth wereheterodont (differentiated), the first five being much larger than those following.[6] Some of the frontmost teeth were almost 13 cm (5 in) long.[1] The teeth were similar to bananas in shape, robust, andserrated.[17] As inTyrannosaurus, the shelf of the palate was well developed.[6]
The jugal bone (or "cheek" bone) was robust, and had a broad postorbitalprocess (which projected upwards from the jugal to contact the postorbital bone), unlike other tyrannosaurs exceptBistahieversor,Tyrannosaurus andTarbosaurus. The front border of the postorbital process had a strong process which indicates thatLythronax had a large subocular flange (a projection into the lower part of the orbit), dissimilar to the smaller ones of other tyrannosaurids. Each ramus of the dentary (half of the tooth-bearing portion of the lower jaw) was strongly concave towards the outer side (bowing inwards along the length of the skull). This mirrored the contours of the maxilla of the upper jaw, and the strong expansion of the rear skull; this was similar toBistahieversor,Tyrannosaurus, andTarbosaurus, but unlike other tyrannosauroids. The dentary was also deep at the rear end, indicating that the following part of the mandible was comparable toTarbosaurus andTyrannosaurus in depth, but not to other tyrannosaurids. Like other tyrannosaurids, thesurangular bone behind the dentary had a deep and well-developed shelf just in front of where the jaw articulated with the skull, andLythronax was similar toTyrannosaurus in that this shelf had a concave upper surface.[6]
Though the postcranial skeleton ofLythronax is poorly known, the known remains of the pubis (part of the pelvis) and the hindlimb show features typical within Tyrannosauridae. The pubic boot, an expansion on the lower end of the pubis, had a large forward-directed process as in all tyrannosaurids. InLythronax, the pubic boot was large and comparatively deep, most similar to those ofTarbosaurus andTyrannosaurus, but dissimilar to the less expanded pubic boots ofTeratophoneus,Albertosaurus,Gorgosaurus, andDaspletosaurus. The fibula, a bone of the lower leg, had a deep midline depression on its upper end, as in other tyrannosaurids. InLythronax, theastragalus of the ankle had an ascending process above its articulation with the foot which was expanded further upwards compared to its relatives.[6]
Lythronax argestes belongs to the family Tyrannosauridae, a family of large-bodiedcoelurosaurs; most tyrannosaurid genera are known from North America and Asia.[6] Based on itsstratigraphic position,Lythronax is the oldest tyrannosaurid discovered so far.[2][3][6] Prior toLythronax being formally named, Zanno and colleagues noted in 2013 that the holotype specimen was likely distinct fromTeratophoneus andBistahieversor, both likewise from southern Utah. This would mean there were at least three tyrannosaurid genera present in theWestern Interior Basin during theCampanianstage. Aphylogenetic analysis conducted by Zanno and colleagues placed all three taxa within a single group of Tyrannosauridae to the exclusion of all other members of the group.[4]
A detailed phylogenetic analysis, conducted by Loewen and colleagues to accompany their 2013 description ofLythronax, based on 303 cranial and 198 postcranial features, placed it andTeratophoneus within thesubfamilyTyrannosaurinae.Lythronax was asister taxon of a group consisting of theMaastrichtian taxaTarbosaurus andTyrannosaurus and the late CampanianZhuchengtyrannus. It was more closely related to this group than other taxa such asDaspletosaurus andTeratophoneus, which were younger thanLythronax but older than the group.[6]
In 2017, American paleontologistsStephen Brusatte andThomas D. Carr published a new phylogenetic analysis of Tyrannosauroidea, including a more comprehensive suite of anatomical characteristics and taxa, that disagreed with the results of Loewen and colleagues. While thetribeAlioramini was outside Tyrannosauridae in the analysis by Loewen and colleagues, Brusatte and Carr placed that group as the mostbasal (early-diverging or "primitive") group within Tyrannosaurinae. Conversely, Loewen and colleagues foundBistahieversor to be aderived ("advanced") tyrannosaurine closely related to the likewise derivedTeratophoneus andLythronax, while Brusatte and Carr placed it in a more basal position directly outside Tyrannosauridae, with bothTeratophoneus andLythronax as basal tyrannosaurines. It was suggested that both of these results stemmed from an over-weighting of some features by Loewen and colleagues, which resulted in the long-snouted alioramin forms being excluded from the short-snouted tyrannosaurines, and the placement ofBistahieversor andLythronax closer toTyrannosaurus than otherwise.[18] The results of the two contrasting analyzes are shown in thecladograms below:[6][18][19]
Topology 1: Loewen and colleagues, 2013 | Topology 2: Brusatte and Carr, 2017
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In a popular book published in 2016, Paul suggested thatLythronax argestes may be a member of the genusTyrannosaurus, and remarked that derived tyrannosaurids "are being badly oversplit at the genus level".[16] Subsequent publications—including both taxonomic and phylogenetic analyzes—have retained the species in the separate genusLythronax.[18][19][20][21] In 2023, paleontologists Charlie R. Scherer and Christian Voiculescu-Holvad movedLythronax to the new clade Teratophoneini along withTeratophoneus andDynamoterror.[22]
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During theLate Cretaceousperiod (around 95 million years ago), theWestern Interior Seaway isolated western North America (Laramidia) from eastern North America (Appalachia), and occasionally isolateddepositional basins from each other.[23] This led to the development of highlyendemic ecosystems in Laramidia; these ecosystems have also roughly been divided into a northern province and a southern province,[6][8][24] but such a clean division is contested.[18][25] Like many Laramidian dinosaur lineages, the evolutionary history of tyrannosaurids—which are limited in distribution to Asia and Laramidia—is characterized by faunal interchange between the two continents.[26] The sequence of interchange events which occurred among Laramidian tyrannosaurids is unclear, and the diverse tyrannosauroids which have been discovered in southern Laramidia (includingLythronax,Teratophoneus, andBistahieversor) have complicated their evolutionary history further.[6][8] In particular, an unresolved question is whetherTyrannosaurus originated from Asian tyrannosaurids or from south Laramidian tyrannosaurids.[19]
Based on their phylogenetic results, Zanno and colleagues proposed that the then-unnamedLythronax displayed features that united tyrannosaurids from southern Laramidia to the exclusion of other genera.[4] While Loewen and colleagues did not recover a unique group of southern taxa, they did resolve all three as being closely related to each other and basal to a group of larger, later forms.[6] From these results, Loewen and colleagues suggested that there was significantbiogeographic division between northern Laramidian and southern Laramidian forms with limited interchange. Also, because they found Alioramini to be placed outside Tyrannosauridae, and the Asian generaTarbosaurus andZhuchengtyrannus in a group excluding all other tyrannosaurids, Loewen and colleagues proposed that there was only a single interchange of tyrannosaurids from North America to Asia. They suggested the interchange took place during the late Campanian, when globalsea levels fell,Tyrannosaurus being descended from North American forms from before such migration took place.[6]
Due to their differing phylogenetic results, the biogeographic conclusions of Loewen and colleagues were disputed by Brusatte and Carr. SinceBistahieversor from southern Laramidia was placed outside Tyrannosauridae, andTeratophoneus from Utah nested closest to theAlaskanNanuqsaurus, Brusatte and Carr instead suggested that there were dynamic and recurrent interchanges of tyrannosaurid fauna between northern and southern Laramidia, and rejected the presence of endemic provinces. The Asian taxaTarbosaurus,Zhuchengtyrannus,Qianzhousaurus, andAlioramus were also placed within Tyrannosaurinae, among North American genera. Brusatte and Carr proposed that at least two continental interchanges occurred, where Tyrannosaurinae originated in Asia and migrated to North America after the divergence of alioramins, and then returned to Asia again withTarbosaurus andZhuchengtyrannus. Another possible scenario suggested by Brusatte and Carr was that two separate migrations to Asia occurred, which separately gave rise to alioramins and larger, later forms. In both scenarios,Tyrannosaurus, nested among Asian taxa, was an "invasive migrant species that spread across Laramidia" from Asia in the Maastrichtian.[18]
The hypotheses of Asian-North American migration of Brusatte and Carr were supported by a later run of their analysis by Canadian paleontologist Jared Voris and colleagues in2020. However, Voris and colleagues amended the original analysis through the additions of the generaDynamoterror from southern Laramidia (New Mexico) andThanatotheristes from northern Laramidia (Alberta), and they were able to replicate the north–south divisions of tyrannosaurids suggested by Loewen and colleagues. The southern taxaTeratophoneus,Dynamoterror, andLythronax formed an exclusive group (to the exclusion ofNanuqsaurus, contrary to Brusatte and Carr) of short- and deep-snouted taxa outside a group of more derived northern Laramidian forms, and the southern Laramidian forms also had a separate skeletal morphotype. Voris and colleagues suggested thesemorphological differences arose for ecological reasons, possibly including prey composition or feeding strategies. As the major prey groups were the same between northern and southern Laramidia when tyrannosaurids lived in those regions, Voris and colleagues concluded the differences in cranial anatomy arose from differences in feeding strategies.[19]
Lythronax differed from most other tyrannosaurids due to its shortened skull with a broadened rear, as well as its forward-directed orbits (which were a direct consequence of its skull morphology). No other tyrannosauroids had such forward-directed orbits except forTyrannosaurus andTarbosaurus,[6] although more derived tyrannosauroids generally had larger and more forward-directed orbits than basal tyrannosauroids.[27] The discovery ofLythronax suggests that these characters had appeared by at least 80 million years ago.[6]
The forward-directed orbits ofLythronax would have enhanced thefield of view of itsbinocular vision by increasing the separation between the orbits and making their lines of sight more parallel to each other (i.e. reducing the optic axis divergence),[27] which would have givenLythronaxdepth perception.[2][28] In 2006, paleontologist Kent Stevens suggested that the similar orbits ofTyrannosaurus would have aided eitherpursuit predation by the observation of distant prey and the three-dimensional detection of obstacles, orambush predation by the ability to judge the timing and direction of lunges.[27]
As a tyrannosaurid,Lythronax would likely have shared the group's other specializations to predatory lifestyles, including large body size; a large skull with powerful jaw muscles and robust teeth; reinforcedsutures holding the skull bones together; and relatively small forelimbs.[2][29] The teeth and jaw muscles ofLythronax would have contributed to strong bite forces, for not just carving out chunks of flesh but also crushing bone.[7][17] The stresses andloads of these bites would have been effectively absorbed by the fused, arched nasal bones and the reinforced sutures.[30][31]
Lythronax was found in terrestrial sedimentary rocks belonging to the lower part of the Reynolds Point Member of the Wahweap Formation. The age of the rocks that yieldedLythronax have been estimated to be 81.49 Ma, with a range of uncertainty between 81.86-81.45 Ma. The overall Wahweap Formation has been radiometrically dated as being between 82.2 and 77.3 million years old.[32] During the timeLythronax lived, the Western Interior Seaway was at its widest extent, almost completely isolating southern Laramidia from the rest of North America.[2] The area where dinosaurs existed included lakes, floodplains, and rivers, which flowed east. The Wahweap Formation is part of theGrand Staircase region, an immense sequence of sedimentary rock layers that stretch south fromBryce Canyon National Park throughZion National Park and into theGrand Canyon. Among other lines of evidence, the presence of rapidly deposited sediments suggests a wet, seasonal climate.[33]
Lythronax was likely the largest predator of its ecosystem.[2] It shared itspaleoenvironment with other dinosaurs, such as thehadrosaursAcristavus andAdelolophus,[34] theceratopsianDiabloceratops,[2][35][36] and unnamedankylosaurs andpachycephalosaurs.[37] Vertebrates present in the Wahweap Formation at the time includedfreshwater fish,bowfins, abundantrays andsharks, turtles such asCompsemys,crocodilians,[38] andlungfish.[39] Numerousmammals lived in this region, which included several genera ofmultituberculates,cladotherians,marsupials, andplacentalinsectivores.[40] The mammals were more primitive than those that lived in the younger Kaiparowits Formation.Trace fossils are relatively abundant in the Wahweap Formation, and suggest the presence ofcrocodylomorphs, as well asornithischian and theropod dinosaurs.[41] Evidence of invertebrate activity in this formation ranged from fossilizedinsect burrows inpetrified logs[42] to fossils ofmollusks, largecrabs,[43] and a wide diversity ofgastropods andostracods.[44]
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