| Tyrannosauroids | |
|---|---|
 | |
| Six tyrannosauroids (top left to bottom right):Tyrannosaurus,Dilong,Alioramus (background) withGuanlong (foreground),Gorgosaurus, and a pair ofYutyrannus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Coelurosauria |
| Clade: | Tyrannoraptora |
| Superfamily: | †Tyrannosauroidea Osborn, 1906 (videWalker, 1964) |
| Type species | |
| †Tyrannosaurus rex Osborn, 1905 | |
| Subgroups | |
Tyrannosauroidea (meaning 'tyrant lizard forms') is asuperfamily (orclade) ofcoelurosauriantheropoddinosaurs that includes thefamilyTyrannosauridae as well as morebasal relatives. Tyrannosauroids lived on theLaurasiansupercontinent beginning in theJurassicPeriod. By the end of theCretaceous Period, tyrannosauroids were the dominant large predators in theNorthern Hemisphere, culminating in the giganticTyrannosaurus.Fossils of tyrannosauroids have been recovered on what are now the continents of North America, Europe and Asia, with fragmentary remains possibly attributable to tyrannosaurs also known from South America and Australia.
Tyrannosauroids werebipedalcarnivores, as were most theropods, and were characterized by numerousskeletal features, especially of theskull andpelvis. Early in their existence, tyrannosauroids were small predators with long, three-fingered forelimbs.Late Cretaceous genera became much larger, including some of thelargest land-based predators ever to exist, but most of these later genera had proportionately small forelimbs with only two digits. Primitivefeathers have been identified in fossils of two species and may have been present in other tyrannosauroids as well. Prominent bony crests in a variety of shapes and sizes on the skulls of many tyrannosauroids may have serveddisplay functions.
Tyrannosauroids varied widely in size, although there was a general trend towards increasing size over time. Early tyrannosauroids were small animals.[3] One specimen ofDilong, almost fully grown, measured 1.6 meters (5.2 feet) in length,[4] and a fully grownGuanlong measured 3 meters (9.8 feet) long.[5] Teeth from Lower Cretaceous rocks (140 to 136 million years old) ofHyogo,Japan, appear to have come from an approximately 5 metres (16 ft) long animal, possibly indicating an early size increase in the lineage.[6] An immatureEotyrannus was over 4 meters (13 feet) in length,[7] and a subadultAppalachiosaurus was estimated at more than 6 meters (20 feet) long,[3] indicating that both genera reached larger sizes. The Late Cretaceous tyrannosaurids ranged from the 9 meters (30 feet)Albertosaurus andGorgosaurus toTyrannosaurus, which exceeded 12 meters (39 feet) in length and may have weighed more than 6,400 kilograms (7 short tons).[3] A 2010 review of the literature concluded that tyrannosaurs were "small- to mid-sized" for their first 80 million years but were "some of the largest terrestrial carnivores to ever live" in their last 20 million years.[8][9]
Skulls of early tyrannosauroids were long, low and lightly constructed, similar to other coelurosaurs, while later forms had taller and more massive skulls. Despite the differences in form, certain skull features are found in all known tyrannosauroids. Thepremaxillary bone is very tall, blunting the front of the snout, a feature whichevolved convergently inabelisaurids. Thenasal bones are characteristically fused, arched slightly upwards and often very roughly textured on their upper surface. The premaxillary teeth at the front of the upper jaw are shaped differently from the rest of the teeth, smaller in size and with aD-shapedcross section. In thelower jaw, a prominent ridge on thesurangular bone extends sideways from just below the jaw joint, except in the basalGuanlong.[3][4][5]
Tyrannosauroids hadS-shaped necks and long tails, as did most other theropods. Early genera had long forelimbs, about 60% the length of the hindlimb inGuanlong, with the typical three digits of coelurosaurs.[5] The long forelimb persisted at least through the Early CretaceousEotyrannus,[7] but is unknown inAppalachiosaurus.[10]Derived tyrannosaurids have forelimbs strongly reduced in size, the most extreme example beingTarbosaurus fromMongolia, where thehumerus was only one-quarter the length of thefemur.[3] The third digit of the forelimb was also reduced over time. This digit was unreduced in the basalGuanlong,[5] while inDilong it was more slender than the other two digits.[4]Eotyrannus also had three functional digits on each hand.[7] Tyrannosaurids had only two, although thevestigialmetacarpal of the third are preserved in some well-preserved specimens.[11] As in most coelurosaurs, the second digit of the hand is the largest, even when the third digit is not present.
Characteristic features of the tyrannosauroid pelvis include aconcave notch at the upper front end of theilium, a sharply defined vertical ridge on the outside surface of the ilium, extending upwards from theacetabulum (hip socket), and a huge "boot" on the end of thepubis, more than half as long as the shaft of the pubis itself.[3] These features are found in all known tyrannosauroids, including basal membersGuanlong[5] andDilong.[4] The pubis is not known inAviatyrannis orStokesosaurus but both show typical tyrannosauroid characters in the ilium.[12] The hindlimbs of all tyrannosauroids, like most theropods, had four toes, although the first toe (thehallux) did not contact the ground. Tyrannosauroid hindlimbs are longer relative to body size than almost any other theropods, and show proportions characteristic offast-running animals, including elongatedtibiae andmetatarsals.[3] These proportions persist even in the largest adultTyrannosaurus,[13] despite its probable inability to run.[14] The third metatarsal of tyrannosaurids was pinched at the top between the second and fourth, forming a structure known as thearctometatarsus.[3] The arctometatarsus was also present inAppalachiosaurus[10] but it is unclear whether it was found inEotyrannus[7] orDryptosaurus.[15] This structure was shared by derivedornithomimids,troodontids andcaenagnathids,[16] but was not present in basal tyrannosauroids likeDilong paradoxus, indicatingconvergent evolution.[4]
Tyrannosaurus was named byHenry Fairfield Osborn in 1905, along with the family Tyrannosauridae.[17] The name is derived from theAncient Greek wordsτυραννοςtyrannos ('tyrant') andσαυροςsauros ('lizard'). The superfamily name Tyrannosauroidea was first published in a 1964 paper by theBritishpaleontologistAlick Walker.[18] Thesuffix-oidea, commonly used in the name of animal superfamilies, is derived from the Greekειδοςeidos ('form').[19]
Scientists have commonly understood Tyrannosauroidea to include the tyrannosaurids and their immediate ancestors.[18][20] With the advent ofphylogenetic taxonomy in vertebrate paleontology, however, theclade has received several more explicit definitions. The first was byPaul Sereno in 1998, where Tyrannosauroidea was defined as a stem-basedtaxon including all species sharing a more recent common ancestor withTyrannosaurus rex than withneornithean birds.[21] To make the family more exclusive,Thomas Holtz redefined it in 2004 to include all species more closely related toTyrannosaurus rex than toOrnithomimus velox,Deinonychus antirrhopus orAllosaurus fragilis.[3] Sereno published a new definition in 2005, usingOrnithomimus edmontonicus,Velociraptor mongoliensis andTroodon formosus as external specifiers.[22] The Sereno definition was adopted in a 2010 review.[8]
Some studies have suggested that the cladeMegaraptora, usually considered to beallosauroids, are basal tyrannosauroids.[23][24] However, other authors disputed the placement of megaraptorans within Tyrannosauroidea,[25][26] and a study of megaraptoran hand anatomy published in 2016 caused even the original scientists suggesting their tyrannosauroid relationships to at least partly reject their prior conclusion.[27]
While paleontologists have long recognized the family Tyrannosauridae, its ancestry has been the subject of much debate. For most of the twentieth century, tyrannosaurids were commonly accepted as members of theCarnosauria, which included almost all large theropods.[28][29] Within this group, theallosaurids were often considered to be ancestral to tyrannosaurids.[20][30] In the early 1990s, cladistic analyses instead began to place tyrannosaurids into the Coelurosauria,[16][31] echoing suggestions first published in the 1920s.[32][33] Tyrannosaurids are now universally considered to be large coelurosaurs.[3][5][34][35][36][37][38]
In 1994, Holtz grouped tyrannosauroids withelmisaurids,ornithomimosaurs andtroodonts into a coelurosaurian clade called Arctometatarsalia based on a common ankle structure where the second and fourth metatarsals meet near thetarsal bones, covering the third metatarsal when viewed from the front.[16] Basal tyrannosauroids likeDilong, however, were found with non-arctometatarsalian ankles, indicating that this feature evolved convergently.[4] Arctometatarsalia has been dismantled and is no longer used by most paleontologists, with tyrannosauroids usually considered to be basal coelurosaurs outsideManiraptoriformes.[3][36][38] While many place tyrannosauroids as basal coelurosaurs, Paul Sereno in his 1990s analysis of theropods would find the Tyrannosaurs to be sister taxa to the Maniraptora with them being closer to birds than Ornithomimosaurs were. He called this groupTyrannoraptora (which in the absence of papers that recover a Tyrannosaur-maniraptoran clade), is a clade which contains most Coelurosaurs.[39] A 2007 analysis found the familyCoeluridae, including the Late Jurassic North American generaCoelurus andTanycolagreus, to be thesister group of Tyrannosauroidea.[34]
The most basal tyrannosauroid known from complete skeletal remains isGuanlong, a representative of the familyProceratosauridae.[5][40] Other early taxa includeStokesosaurus andAviatyrannis, known from far less complete material.[12] The better-knownDilong is considered slightly more derived thanGuanlong andStokesosaurus.[4][5]Dryptosaurus, long a difficult genus to classify, has turned up in several recent analyses as a basal tyrannosauroid as well, slightly more distantly related to Tyrannosauridae thanEotyrannus andAppalachiosaurus.[3][10][41]Alectrosaurus, a poorly known genus from Mongolia, is definitely a tyrannosauroid but its exact relationships are unclear.[3] Other taxa have been considered possible tyrannosauroids by various authors, includingBagaraatan andLabocania.[3]Siamotyrannus from theEarly Cretaceous ofThailand was originally described as an early tyrannosaurid,[42] but is usually considered acarnosaur today.[36][43]Iliosuchus has a vertical ridge on the ilium reminiscent of tyrannosauroids and may in fact be the earliest known member of the superfamily, but not enough material is known to be sure.[12][43]
Below on the left is a cladogram of Tyrannosauroidea from a 2022 study byDarren Naish and Andrea Cau on the genusEotyrannus, and on the right is a cladogram of Eutyrannosauria from a 2020 study by Jared T. Voris and colleagues on the genusThanatotheristes:[44][45]
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In 2018 authors Rafael Delcourt and Orlando Nelson Grillo published a phylogenetic analysis of Tyrannosauroidea which incorporated taxa from the ancient continent ofGondwana (which today consists of the southern hemisphere), such asSantanaraptor andTimimus, whose placement in the group has been controversial.[46] They have found that not onlySantanaraptor andTimimus were placed as tyrannosaurs more derived thanDilong, but they have found in their analysis that tyrannosauroids were widespread inLaurasia and Gondwana since the Middle Jurassic.[46] They have proposed new subclade names for Tyrannosaurioidea. The first isPantyrannosauria referring to all non-proceratosaurid members of the group, whileEutyrannosauria for larger tyrannosaur taxa found in the northern hemisphere such asDryptosaurus,Appalachiosaurus,Bistahieversor, and Tyrannosauridae.[46] Below is their phylogeographic tree they have recovered, in which displays the phylogenetic relationships of the taxa as well as the continents those taxa have been found.[46]
| Tyrannosauroidea |
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In 2021, Chase Brownstein published a research article based on more thorough descriptions of tyrannosauroidmetatarsals and vertebra from theMerchantville Formation inDelaware.[47] This reanalysis of phylogenetic relationships of tyrannosauroids in Appalachia has brought the rediscovery of the cladeDryptosauridae due to the similarities of metatarsals II and IV with the same bones in theDryptosaurus holotype.[47] However. the Merchantville taxon was found to still be different enough to separate it on the genus level fromDryptosaurus. In the phylogentic tree constructed Dryptosauridae is found to be a valid family of non tyrannosaurid eutyrannosaur. It currently sits in apolytomy with the Iren Dabasu taxon and more basal eutryannosaurs.[47]
The tyrannosauroids lived on the supercontinentLaurasia, which split fromGondwana in theMiddle Jurassic. The earliest recognized tyrannosauroids lived in the Middle Jurassic, represented by theproceratosauridsKileskus from the Western Siberia andProceratosaurus from Great Britain. Upper Jurassic tyrannosauroids includeGuanlong from China,Stokesosaurus from the western United States andAviatyrannis andJuratyrant from Europe.
Early Cretaceous tyrannosauroids are known from Laurasia, being represented byEotyrannus from England[7] andDilong,Sinotyrannus, andYutyrannus from northeastern China. Early Cretaceous tyrannosauroid premaxillary teeth are known from theCedar Mountain Formation inUtah[48] and theTetori Group ofJapan.[49]
The Middle Cretaceous record of Tyrannosauroidea is rather patchy. Teeth and indeterminate postcrania of this interval are known from the Cenomanian-ageDakota Formation of western North America andPotomac Formation of New Jersey,[48][50] as well as formations inKazakhstan andTajikistan;[51] two genera,Timurlengia andXiongguanlong, have been found in Asia, while the BrazilianSantanaraptor may belong to this group.[26]Suskityrannus has been found in theMoreno Hill Formation of the Zuni Basin of western New Mexico.[52][53] The first unquestionable remains of tyrannosaurids occur in theCampanianstage of the Late Cretaceous in North America and Asia. Two subfamilies are recognized. The albertosaurines are only known from North America, while the tyrannosaurines are found on both continents.[3] Tyrannosaurid fossils have been found inAlaska, which may have served as a land bridge allowingdispersal between the two continents.[54] Non-tyrannosaurid tyrannosauroids likeAlectrosaurus and possiblyBagaraatan were contemporaneous with tyrannosaurids in Asia, while they are absent from western North America.[3] Eastern North America was divided by theWestern Interior Seaway in the middle of the Cretaceous and isolated from the western portion of the continent. The absence of tyrannosaurids from the eastern part of the continent suggests that the family evolved after the appearance of the seaway, allowing basal tyrannosauroids likeDryptosaurus andAppalachiosaurus to survive in the east as arelict population until the end of the Cretaceous.[10]
Basal tyrannosauroids have also been suggested to be present inAustralia and South America during the Early Cretaceous. NMV P186069, a partialpubis (a hip bone) with a supposed distinctive tyrannosauroid-like form, was discovered inDinosaur Cove inVictoria.[55] However, a response suggested that critical tyrannosauroid characters were absent from the fossil.[56] The Australian taxonTimimus, known from a femur, and the BrazilianSantanaraptor, known from a partial juvenile skeleton, have also been suggested to be tyrannosaurs.[46] However, these placements have been considered questionable, with the supposed tyrannosauroid characters ofSantanaraptor being widely distributed within Coelurosauria, in other characters having similarities tonoasaurids.[57]
A conference paper by Tracy Ford states that there was rough bone texture on the skulls of theropods and higher foramina frequency thanlepidosaurs andmammals which would be evidential for a sensitive snout for theropods.[58][59] A study in 2017 study on a new tyrannosaurid namedDaspletosaurus horneri was published in the journal Scientific Reports, where paleontologist Thomas Carr analyzed the craniofacialtexture ofDaspletosaurus horneri and observed a hummocky rugosity which compared to crocodilian skulls, suggestingDaspletosaurus horneri and with it all tyrannosaurids have flat sensory scales. The subordinate regions were analyzed to have cornified epidermis.[60] However, a 2018 presentation has an alternative interpretation. Crocodilians do not have flat sensory scales, but rather cracked, cornified epidermis due to growth. The hummocky rugosity in the skulls of lepidosaurs have correlation with scales which this bone texture is also present in tyrannosaurid skulls. The foramina frequency in theropod skulls does not exceed 50 foramina, which shows that theropods had lips. It's been proposed that lips are a primitive trait in tetrapods and the soft tissue present in crocodilians are a derived trait because of aquatic or semiaquatic adaptations.[61][62][63][64][65]
Longfilamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early CretaceousYixian Formation and other nearbygeological formations fromLiaoning, China.[66] These filaments have usually been interpreted as "protofeathers,"homologous with the branched feathers found in birds andsome non-avian theropods,[67][68] although other hypotheses have been proposed.[69] A skeleton ofDilong paradoxus was described in 2004 that included the first example of feathers in a tyrannosauroid. Similarly todown feathers of modern birds, the feathers found inDilong were branched but notpennaceous, and may have been used forinsulation.[4] Even large tyrannosauroids have been found with evidence of feathers.Yutyrannus huali, also from the Yixian Formation, is known from three specimens, each preserving traces of feathers on various parts of the body. While not all areas of the body preserve impressions across all three specimens, these fossils demonstrate that even in this medium-sized species, most of the body was covered in feathers.[70]
The presence of feathers in basal tyrannosauroids is not surprising since they are now known to be characteristic of coelurosaurs, found in other basal genera likeSinosauropteryx,[67] as well as all more derived groups.[66] Rare fossilizedskin impressions of some Late Cretaceous tyrannosaurids lack feathers, however, instead showing skin covered in fine, non-overlappingscales.[71] Possibly, feathers were present on other areas of the body: preserved skin impressions are very small and come primarily from the legs, pelvic region, and underside of the tail, which either lack feathers or only covered in a light down in some modern large ground-dwelling birds. Alternatively, secondary loss of feathers in large tyrannosaurids may beanalogous with the similar loss ofhair in the largest modern mammals likeelephants, where a lowsurface area-to-volume ratio slows downheat transfer, making insulation by a coat of hair unnecessary or even detrimental.[4] A scientific publication by Phil Bell and colleagues in 2017 show that tyrannosaurids such asGorgosaurus,Tarbosaurus,Albertosaurus,Daspletosaurus, andTyrannosaurus had scales. The Bell et al. 2017 paper notes that the scale-like integument on bird feet were actually secondarily derived feathers according to paleontological and evolutionary developmental evidence so they hypothesize that the scaly skin preserved on some tyrannosaurid specimens might be secondarily derived from filamentous appendages like on Yutyrannus although strong evidence is needed to support this hypothesis.[72][73][74] However, other paleontologists argue that taphonomy is the possible cause of the lack of filamentous structures in tyrannosaurid fossils.[75]
Bony crests are found on the skulls of many theropods, including numerous tyrannosauroids. The most elaborate is found inGuanlong, where the nasal bones support a single, large crest which runs along the midline of the skull from front to back. This crest was penetrated by several large foramina (openings) which reduced its weight.[5] A less prominent crest is found inDilong, where low, parallel ridges run along each side of the skull, supported by the nasal andlacrimal bones. These ridges curve inwards and meet just behind the nostrils, making the crestY-shaped.[4] The fused nasals of tyrannosaurids are often very rough-textured.Alioramus, a possible tyrannosaurid from Mongolia, bears a single row of five prominent bony bumps on the nasal bones; a similar row of much lower bumps is present on the skull ofAppalachiosaurus, as well as some specimens ofDaspletosaurus,Albertosaurus, andTarbosaurus.[10] InAlbertosaurus,Gorgosaurus andDaspletosaurus, there is a prominent horn in front of each eye on the lacrimal bone. The lacrimal horn is absent inTarbosaurus andTyrannosaurus, which instead have a crescent-shaped crest behind each eye on thepostorbital bone.[3]
These head crests may have been used fordisplay, perhaps for species recognition orcourtship behavior.[3] An example of thehandicap principle may be the case ofGuanlong, where the large, delicate crest may have been a hindrance to hunting in what was presumably an active predator. If an individual was healthy and successful at hunting despite the fragile crest, it would indicate the superior quality of the individual over others with smaller crests. Similarly to the unwieldy tail of a malepeacock or the outsizedantlers of anIrish elk, the crest ofGuanlong may have evolved viasexual selection, providing an advantage in courtship that outweighed any decrease in hunting ability.[5]
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion. You can help byadding to it.(October 2020) |
Neonate sized tyrannosaur fossils have been documented in thescientific literature.[76]
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