Tyrannosauroidea

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Tyrannosauroidea
Skeletal reconstruction of Albertosaurus at the Royal Tyrrell Museum in Alberta

Skeleton reconstruction of Albertosaurus in the Royal Tyrrell Museum in Alberta

Temporal occurrence
Middle Jurassic to Upper Cretaceous ( Bathonian to Maastrichtian )
168.3 to 66 million years
Locations
Systematics
Dinosaur (dinosauria)
Lizard dinosaur (Saurischia)
Theropoda
Tetanurae
Coelurosauria
Tyrannosauroidea
Scientific name
Tyrannosauroidea
Brown , 1914

The Tyrannosauroidea is a superfamily of Coelurosaurier (Coelurosauria) within the theropod dinosaurs . It includes the Tyrannosauridae , which include for instance the well-known Tyrannosaurus rex belongs, as well as the basal, ie standing at the beginning of the development series types . Tyrannosauroids lived on the northern supercontinent Laurasia and first appeared in the Central Jurassic about 168 million years ago. In the Upper Cretaceous they were the dominant great predators of the northern hemisphere . Their fossils were discovered in North America , Europe, and Asia .

Tyrannosauroids, like most theropods, were bipedal carnivores . The group is characterized by numerous mutually derived features ( synapomorphies ), which are particularly found in the skull and pelvic bones. Early in their evolution, tyrannosauroids were small predators with long, three-fingered arms. Upper Cretaceous species grew significantly larger, including some of the largest terrestrial carnivores that have ever existed - but most of these late species had proportionally small arms with only two fingers. Primitive feathers are known from Dilong , an early tyrannosauroid from China , as well as from Yutyrannus, and were perhaps also present in other tyrannosauroids. Many species wore conspicuous ridges of bones in various shapes, perhaps for display purposes.

features

The body size varied considerably between the individual species, but in the course of evolution the group grew larger and larger. Early tyrannosauroids were still relatively small animals - adult specimens of Dilong measured 1.6 meters and Guanlong three meters in length. Cretaceous genera grew larger; a not yet sexually mature Eotyrannus was over four meters long and a not yet fully grown Appalachiosaurus was more than six meters long. Tyrannosaurids of the Upper Cretaceous, on the other hand, sometimes reached gigantic proportions. Albertosaurus and Gorgosaurus were both about nine meters long, and Tyrannosaurus was the largest known genus at over twelve meters in length and perhaps over 6,400 kilograms.

Daspletosaurus skull and cervical vertebrae in the Field Museum of Natural History in Chicago

The skulls of early tyrannosauroids were long, low, and lightly built, resembling those of other coelurosaurs, while later forms had much taller and more massive skulls. Despite these differences, certain cranial features ( synapomorphies ) can be found in all previously known tyrannosauroids: The intermaxillary bone (premaxilla) was very high, which resulted in a blunt snout - a feature that also developed convergently in the Abelisaurids . The paired nasal bone was fused, slightly arched upwards and mostly very rough, often structured like a bark, on the upper side. The teeth of the intermaxillary bone on the front part of the upper jaw were smaller and differently shaped than the rest of the teeth and had a “D” -shaped cross-section. The lower jaw of all tyrannosauroids except Guanlong had a pronounced crest on the surangular , which extended laterally directly below the temporomandibular joint.

Tyrannosauroids, like most other theropods, had "S" -shaped necks and long tails. Early genera had three-fingered, long arms that reached 60 percent of the length of the hind legs in Guanlong . The long arms were characteristic of the group at least as far as the Lower Cretaceous, as Eotyrannus shows, but they have not been preserved in Appalachiosaurus . The arms of later tyrannosauroids were significantly shorter - especially with Tarbosaurus from Mongolia , whose upper arm bone ( humerus ) was only a quarter of the length of the thigh bone ( femur ). The third finger has also receded in the course of evolution: While it was not yet reduced in the basal guanlong , it was already significantly smaller than the other two fingers in Dilong . Eotyrannus still had three functional fingers on each hand; However, tyrannosaurids only had two fingers, although rudiments of a third finger were found in some specimens. As with most other coelurosaurs, the second finger was the largest.

Live reconstruction of Dryptosaurus , a "primitive" tyrannosauroid.

Characteristic features in the pelvic bone include a notch of the lower end of the ilium ( ilium ), a clearly limited vertical comb on the ilium, extending from the hip joint socket stretched (acetabulum) upward, and the enlarged end of the pubis ( pubic bone ) with one that extended "T" -shaped on both sides and was more than half as long as the actual shaft of the pubic bone. These features are found in all known tyrannosauroids, including the basal genera Guanlong and Dilong . The pubic bone is unknown in Aviatyrannis and Stokesosaurus ; however, both genera show characteristics typical of tyrannosauroids in the iliac bone. The hind legs of all tyrannosauroids, like most theropods, had four toes, although the first toe (the hallux ) did not touch the ground. The hind legs of the tyrannosauroids were longer in relation to body size than in almost all other theropods and show proportions that are characteristic of fast-moving animals; thus the shinbone ( os tibia ) and the metatarsal bones were elongated. These proportions existed even in the largest known specimen of Tyrannosaurus , although these animals may not have been able to run. The upper half of the third (i.e., middle) metatarsal bone of the tyrannosaurids was "squeezed" between the other two metatarsals and had only a small portion of the contact area between the metatarsus and the tarsus - a structure known as the arctometatarsus . The Arctometatarsus was also found in Appalachiosaurus , but it is not clear whether it was also present in Eotyrannus or Dryptosaurus . This structure is found in ornithomimids , troodontids and Caenagnathids , but was not present in basal tyrannosauroids such as Dilong , which suggests convergent evolution .

Systematics

Tyrannosaurus was named along with the Tyrannosauridae family by Henry Fairfield Osborn in 1905. The name is derived from the ancient Greek words τύραννος / tyrannos ("tyrant") and σαῦρος / sauros ("lizard"). The name of the superfamily Tyrannosauroidea was first published by the British paleontologist Alick Walker in 1964. The ending -oidea , which is usually used for superfamilies in the animal kingdom, is derived from the ancient Greek εἶδος / eidos ("form").

Scientists mostly understood the Tyrannosauroidea as a taxon that included the Tyrannosauridae and their immediate ancestors. However, since the phylogenetic system was introduced into paleontology, the group received precise definitions. The first was established by Paul Sereno in 1998 and sees the Tyrannosauroidea as a taxon that includes all species more closely related to Tyrannosaurus rex than to neonatal birds . To describe the group even more exclusively, Thomas Holtz redefined it in 2004 to include all species more closely related to Tyrannosaurus rex than to Ornithomimus velox , Deinonychus antirrhopus, or Allosaurus fragilis . Sereno published a new definition in 2005 according to which all species of the Tyrannosauroidea belong to, which were more closely related to Tyrannosaurus rex than to Ornithomimus edmontonicus , Velociraptor mongoliensis and Troodon formosus .

Eotyrannus
Tarbosaurus
Tyrannosaurus rex , the last and largest tyrannosaur
Lythronax
Teratophoneus

classification

Superfamily Tyrannosauroidea

Phylogenetics

In the 20th century, tyrannosaurids were usually considered to be representatives of the Carnosauria , which from the point of view of that time included almost all large theropods. Within this group, the allosaurids were often thought to be the ancestors of the tyrannosaurids. In the early 1990s, cladistic analyzes began to classify the Tyrannosauridae within the Coelurosauria , with the first assumptions being made as early as the 1920s. Tyrannosaurids are generally considered to be large coelurosaurs today.

In 1994 Holtz grouped the tyrannosauroids together with the Elmisaurids , the ornithomimosaurs and the troodontids in a new group within the Coelurosauria, which he called Arctometatarsalia . This group is based on the arctometatarsus, a structure in the metatarsal bones in which the second metatarsal bone is partially covered by the remaining two metatarsal bones when viewed from the front. Basal tyrannosauroids such as Dilong , however, did not show arctometatarsus, which suggests that this trait developed independently ( convergently ) several times in different groups . Therefore, the group Arctometatarsalia was discarded and no longer used by most paleontologists, whereby the Tyrannosauroidea is now mostly classified as a basal group of Coelurosauria outside of the Maniraptoriformes . A new analysis concluded that the Coeluridae family could have been the sister taxon of the Tyrannosauroidea.

The most pristine tyrannosauroid known from complete skeletal material is guanlong . Other early genera include Stokesosaurus and Aviatyrannis , but are known only from far less complete material. The better known Dilong is a bit more modern than Guanlong and Stokesosaurus . Dryptosaurus was long considered difficult to classify, but is now listed as a basal tyrannosauroid, which was somewhat more original than Eotyrannus and Appalachiosaurus . Alectrosaurus , a little-known genus from Mongolia, is definitely a tyrannosauroid, but its exact relationships are unclear. Other taxa have been suggested by some authors as possible tyrannosauroids, including Bagaraatan , Labocania, and a genus incorrectly assigned to the Chilantaisaurus , "C." maortuensis . Siamotyrannus from the Lower Cretaceous Thailand was originally described as an early tyrannosaurid, but is now considered a member of the Carnosauria. Iliosuchus shows a vertical ridge on the iliac bone, which is a feature of the tyrannosauroid. This genus could therefore actually be the earliest known representative of this superfamily, but this can only be confirmed by further bone finds.

distribution

The earliest known tyrannosauroids lived in the Middle Jurassic and include Guanlong from northwestern China, Stokesosaurus from the western United States, and Aviatyrannis from Portugal. Some fossils currently attributed to Stokesosaurus may belong to Aviatyrannis - the dinosaur faunas of Portugal and North America were actually very similar at the time. If Iliosuchus from the Central Jurassic of England was a tyrannosauroid, it would be the earliest known genus; this could indicate that the tyrannosauroids had their origin in Europe.

Tyrannosauroid fossils have only been discovered in the northern land masses. Genera of the Upper Cretaceous are known only from North America and Asia.

Lower Cretaceous tyrannosauroids were also found in all three northern continents. Eotyrannus from England and Dilong from northeast China are the only two genera named from this period; Furthermore, teeth of the intermaxillary bone are known from the Cedar Mountain Formation in Utah (United States) and from the Tetori group in Japan. "Chilantaisaurus" maortuensis from the Dashuigou Formation of Inner Mongolia in China is also sometimes considered to be an early tyrannosauroid of the Lower Cretaceous.

In Europe, tyrannosauroids have disappeared from fossil records since the Middle Cretaceous, suggesting a local extinction on this continent. Teeth and possible body fossils from the Middle Cretaceous are known from the North American Dakota Formation , as well as from formations in Kazakhstan , Tajikistan and Uzbekistan . The first indubitable remains of tyrannosaurids come from the Campanium (late Upper Cretaceous) of North America and Asia. This family is divided into two subfamilies, the albertosaurines were only found in North America, but the tyrannosaurines were discovered on both continents. Tyrannosaurid fossils were also discovered in Alaska , which may have been connected to Asia by a land bridge through which the faunas of North America and Asia could share. The tyrannosauroids Alectrosaurus and perhaps Bagaraatan are classified outside of the Tyrannosauridae, but lived with them in Asia at the same time while they were absent in North America. Since the Middle Cretaceous, eastern North America has been isolated from the western part of the continent by an arm of the sea, the Western Interior Seaway . Since tyrannosaurids were absent in the eastern part of the continent, it is believed that this family did not arise until after the arm of the sea divided the land. This allowed basal tyrannosauroids like Dryptosaurus and Appalachiosaurus to survive in eastern North America to the end of the Upper Cretaceous.

Paleobiology

feathers

Long fibrous structures have been preserved along with the skeletons of numerous coelurosaurs that come from the Lower Cretaceous of the Yixian Formation and other formations from Liaoning (China). These structures are commonly interpreted as "proto-feathers," homologous to the ramified feathers of modern birds, although other hypotheses have been made. A Dilong skeleton described in 2004 is the first known example of proto-feathers in tyrannosauroids. Similar to the down of modern birds, the proto feathers known from Dilong were branched, but they were not contour feathers . Possibly they were used for thermal insulation.

That proto-feathers were found in basal tyrannosauroids is not surprising, since feathers are considered a characteristic feature of the coelurosaurs. They are found in other basal genera such as Sinosauropteryx as well as in more modern genera. However, rare skin prints of large tyrannosaurids show no evidence of feathers, but instead a scaly skin. It is possible that proto-feathers were present on parts of the body that were not passed down through skin prints. Alternatively, the proto-feathers could have been lost in large tyrannosaurids, as the small area-volume ratio of these animals makes thermal insulation superfluous - similar to today's large mammals such as elephants . An exception here is Yutyrannus , in which filaments up to 20 centimeters long have been found that are interpreted as feathers.

Head combs

The head crest of Guanlong , a basal tyrannosauroid from China

Bone ridges have been found on the skulls of many theropods, including numerous tyrannosauroids. The most spectacular example is guanlong , in which the pair of nasal bones supports a single, large ridge that runs along the center line of the skull from the muzzle to the back. The ridge was criss-crossed with large openings that reduced its weight. Less noticeable were the head ornaments of Dilong , which consisted of two low, parallel ridges that ran on each side of the skull and were supported by the nasal bone and tearbone . The combs curved inward just behind the nostrils, creating a "Y" -shaped structure. The fused paired nasal bone of the tyrannosaurids was often very rough textured. Alioramus , a possible tyrannosaurid from Mongolia, showed a single row of five conspicuous bony bumps on the nasal bone; a similar series with much smaller bumps is known from Appalachiosaurus and from some finds from Daspletosaurus , Albertosaurus and Tarbosaurus . The horn sitting on the tear bone is missing in Tarbosaurus and Tyrannosaurus , which instead had a crescent-shaped hood behind each eye on the postorbital bone . The head crests of the tyrannosauroids were probably used for display - perhaps for species recognition or for courtship.

Web links

Commons : Tyrannosauroidea  - Collection of images, videos and audio files

Individual evidence

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