Tyrannosauroidea

Taxobox
name = Tyrannosauroids
fossil_range = Jurassic – Late Cretaceous

---

image_width = 200px
image_caption = "Dryptosaurus"
image_caption = Illustration of "Dryptosaurus aquilunguis".
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = Theropoda
unranked_familia = Coelurosauria
superfamilia = Tyrannosauroidea
superfamilia_authority = Osborn, 1905
subdivision_ranks = Families & Genera
subdivision =
*"Alectrosaurus"
*"Appalachiosaurus"
*"Aviatyrannis"
*?"Bagaraatan"
*"Dilong"
*"Eotyrannus"
*"Guanlong"
*?"Iliosuchus"
*?"Labocania"
*"Stokesosaurus"
*Family Dryptosauridae
*Family Tyrannosauridae
synonyms =
* Deinodontoidea Brown, 1914

Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent beginning in the Jurassic Period. By the end of the Cretaceous Period, tyrannosauroids were the dominant large predators in the Northern Hemisphere, culminating in the gigantic "Tyrannosaurus" itself. Fossils of tyrannosauroids have been recovered on what are now the continents of North America, Europe and Asia.

Tyrannosauroids were bipedal carnivores, as were most theropods, and were characterized by numerous skeletal features, especially of the skull and pelvis. Early in their existence, tyrannosauroids were small predators with long, three-fingered forelimbs. Late Cretaceous genera became much larger, including some of the largest land-based predators ever to exist, but most of these later genera had proportionately small forelimbs with only two digits. Primitive feathers have been found on "Dilong", an early tyrannosauroid from China, and may have been present in other tyrannosauroid genera as well. Prominent bony crests in a variety of shapes and sizes on the skulls of many tyrannosauroids may have served display functions.

Description

Tyrannosauroids varied widely in size, although there was a general trend towards increasing size over time. Early tyrannosauroids were small animals.cite_book |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |year=2004 |chapter=Tyrannosauroidea |editor= Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.). |title=The Dinosauria |edition=Second Edition |publisher=University of California Press |location=Berkeley |pages=111-136 |isbn=0-520-24209-2] One specimen of "Dilong", almost fully grown, measured 1.6 meters (5.3 ft) in length,cite_journal |last= Xu Xing |coauthors= Norell, Mark A.; Kuang Xuewen; Wang Xiaolin; Zhao Qi; & Jia Chengkai. |year=2004 |title=Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids |journal=Nature |volume=431 |issue=7009 |pages=680–684 |doi=10.1038/nature02855] and a full-grown "Guanlong" measured 3 meters (10 ft long).cite_journal |last=Xu Xing |coauthors=Clark, James M.; Forster, Catherine A.; Norell, Mark A.; Erickson, Gregory M.; Eberth, David A.; Jia Chengkai; & Zhao Qi. |year=2006 |title=A basal tyrannosauroid dinosaur from the Late Jurassic of China |journal=Nature |volume=439 |issue=7077 |pages=715–718 |doi=10.1038/nature04511] An immature "Eotyrannus" was over 4 meters (13 ft) in length,cite_journal |last=Hutt |first=Stephen |coauthors=Naish, Darren; Martill, David M.; Barker, Michael J.; & Newberry, Penny. |year=2001 |title=A preliminary account of a new tyrannosauroid theropod from the Wessex Formation (Early Cretaceous) of southern England |journal=Cretaceous Research |volume=22 |issue=2 |pages=227–242 |doi=10.1006/cres.2001.0252] and a subadult "Appalachiosaurus" was estimated at more than 6 meters (20 ft) long, indicating that both genera reached larger sizes. The Late Cretaceous tyrannosaurids ranged from the 9 meter (30 ft) "Albertosaurus" and "Gorgosaurus" to "Tyrannosaurus", which exceeded 12 meters (40 ft) in length and may have weighed more than 6400 kilograms (7 short tons).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. The premaxillary bone is very tall, blunting the front of the snout, a feature which evolved convergently in abelisaurids. The nasal bones are characteristically fused together, 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 than the rest of the teeth, smaller in size and with a "D"-shaped cross section. In the lower jaw, a prominent ridge on the surangular bone extends sideways from just below the jaw joint, except in the basal "Guanlong".

Tyrannosauroids had "S"-shaped necks and long tails, as did most other theropods. Early genera had long forelimbs, about 60% the length of the hindlimb in "Guanlong", with the typical three digits of coelurosaurs. The long forelimb persisted at least through the Early Cretaceous "Eotyrannus", but is unknown in "Appalachiosaurus".cite_journal |last=Carr |first=Thomas D. |coauthors=Williamson, Thomas E.; & Schwimmer, David R. |year=2005 |title=A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=119–143 |url=http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1671%2F0272-4634(2005)025%5B0119%3AANGASO%5D2.0.CO%3B2 |doi=10.1671/0272-4634(2005)025 [0119:ANGASO] 2.0.CO;2 |doilabel=10.1671/0272-4634(2005)025[0119:ANGASO]2.0.CO;2] Derived tyrannosaurids have forelimbs strongly reduced in size, the most extreme example being "Tarbosaurus" from Mongolia, where the humerus was only one-quarter the length of the femur. The third digit of the forelimb was also reduced over time. This digit was unreduced in the basal "Guanlong", but in "Dilong" it was significantly more slender than the other two digits. "Eotyrannus" still had three functional digits on each hand, but tyrannosaurids had only two, although the vestigial remnants of the third are found on some specimens.cite_journal |last=Quinlan |first=Elizibeth D. |coauthors=Derstler, Kraig; & Miller, Mercedes M. |year=2007 |title=Anatomy and function of digit III of the "Tyrannosaurus rex" manus |journal=Geological Society of America Annual Meeting - Abstracts with Programs |pages=77 |url=http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_132345.htm [abstract only] ] 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 a notch at the upper front end of the ilium, a sharply defined vertical ridge on the outside surface of the ilium, extending upwards from the acetabulum (hip socket), and a huge "boot" on the end of the pubis, more than half as long as the shaft of the pubis itself. These features are found in all known tyrannosauroids, including basal members "Guanlong" and "Dilong". The pubis is not known in "Aviatyrannis" or "Stokesosaurus" but both show typical tyrannosauroid characters in the ilium.cite_journal |last=Rauhut |first=Oliver W.M. |year=2003 |title=A tyrannosauroid dinosaur from the Upper Jurassic of Portugal |journal=Palaeontology |volume=46 |issue=5 |pages=903–910 |doi=10.1111/1475-4983.00325] The hindlimbs of all tyrannosauroids, like most theropods, had four toes, although the first toe (the hallux) did not contact the ground. Tyrannosauroid hindlimbs are longer relative to body size than almost any other theropods, and show proportions characteristic of fast-running animals, including elongated tibiae and metatarsals. These proportions persist even in the largest adult "Tyrannosaurus",cite journal |last=Brochu |first=Christopher R. |year=2003 |title=Osteology of "Tyrannosaurus rex": insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull |journal=Society of Vertebrate Paleontology Memoirs |volume=7 |pages=1–138 |url=http://www.vertpaleo.org/publications/memoirs.cfm |doi=10.2307/3889334] despite its probable inability to run.cite_journal |last=Hutchinson |first=John R. |coauthors=& Garcia, Mariano . |year=2002 |title="Tyrannosaurus" was not a fast runner |journal=nature |volume=415 |issue=6875 |pages=1018–1021 |doi=10.1038/4151018a] The third metatarsal of tyrannosaurids was pinched at the top between the second and fourth, forming a structure known as the arctometatarsus. The arctometatarsus was also present in "Appalachiosaurus" but it is unclear whether it was found in "Eotyrannus" or "Dryptosaurus".cite_journal |last=Carpenter |first=Kenneth |authorlink=Ken Carpenter |coauthors=Russell, Dale A.; Baird, Donald; & Denton, Robert |year=1997 |title=Redescription of the holotype of "Dryptosaurus aquilunguis" (Dinosauria: Theropoda) from the Upper Cretaceous of New Jersey |journal=Journal of Vertebrate Paleontology |volume=17 |issue=3 |pages=561–573 |url=http://www.vertpaleo.org/publications/jvp/17-561-573.cfm] This structure was shared by derived ornithomimids, troodontids and caenagnathids,cite_journal |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |year=1994 |title=The phylogenetic position of the Tyrannosauridae: implications for theropod systematics |journal=Journal of Palaeontology |volume=68 |issue=5 |pages=1100–1117 |url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/68/5/1100] but was not present in basal tyrannosauroids like "Dilong", indicating convergent evolution.

Taxonomy and systematics

"Tyrannosaurus" was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae.cite_journal |last=Osborn |first=Henry F. |authorlink=Henry Fairfield Osborn |year=1905 |title="Tyrannosaurus" and other Cretaceous carnivorous dinosaurs |journal=Bulletin of the American Museum of Natural History |volume=21 |pages=259–265 |url=http://digitallibrary.amnh.org/dspace/handle/2246/1464] The name is derived from the Ancient Greek words "τυραννος"/"tyrannos" ('tyrant') and "σαυρος"/"sauros" ('lizard'). The superfamily name Tyrannosauroidea was first published in a 1964 paper by British paleontologist Alick Walker.cite_journal |last=Walker |first=Alick D. |authorlink=Alick Walker |year=1964 |title=Triassic reptiles from the Elgin area: "Ornithosuchus" and the origin of carnosaurs |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=248 |pages=53–134 |doi=10.1098/rstb.1964.0009] The suffix "-oidea", commonly used in the name of animal superfamilies, is derived from the Greek "ειδος"/"eidos" ('form').cite book |last=Liddell |first=Henry G. |authorlink=Henry Liddell |coauthors=& Scott, Robert |year=1980 |title=Greek-English Lexicon |edition=Abridged Edition |publisher=Oxford University Press, |location=Oxford |isbn=0-19-910207-4]

Scientists have commonly understood Tyrannosauroidea to include the tyrannosaurids and their immediate ancestors.cite_journal |last=Bonaparte |first=José F. |authorlink=José Bonaparte |coauthors=Novas, Fernando E.; & Coria, Rodolfo A. |year=1990 |title="Carnotaurus sastrei" Bonaparte, the horned, lightly built carnosaur from the Middle Cretaceous of Patagonia |journal=Contributions in Science (Natural History Museum of Los Angeles County |volume=416 |pages=1–42 |url=http://www.nhm.org/research/publications/Contributions_in_Science/CS416.pdf] With the advent of phylogenetic taxonomy in vertebrate paleontology, however, the clade has received several more explicit definitions. The first was by Paul Sereno in 1998, where Tyrannosauroidea was defined as a stem-based taxon including all species sharing a more recent common ancestor with "Tyrannosaurus rex" than with neornithean birds.cite_journal |last=Sereno |first=Paul C. |authorlink=Paul Sereno |year=1998 |title=A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria |journal=Neues Jahrbuch für Geologie und Paläontologie Abhandlungen |volume=210 |issue=1 |pages=41–83] To make the family more exclusive, 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, using "Ornithomimus edmontonicus", "Velociraptor mongoliensis" and "Troodon formosus" as external specifiers.cite_web |last=Sereno |first=Paul C. |authorlink=Paul Sereno |year=2005 |url=http://www.taxonsearch.org/dev/taxon_edit.php?Action=View&tax_id=350 |title=Stem Archosauria — TaxonSearch, Version 1.0 |accessdate=2007-12-10]

Classification

SUPERFAMILY TYRANNOSAUROIDEA
*?"Iliosuchus" (Middle Jurassic, England)
*"Aviatyrannis" (Late Jurassic, Portugal)
*"Guanlong" (Late Jurassic, western China)
*"Stokesosaurus" (Late Jurassic, western North America)
*"Dilong" (Early Cretaceous, eastern China)
*?"Bagaraatan" (Late Cretaceous, Mongolia)
*"Dryptosaurus" (Late Cretaceous, eastern North America)
*"Eotyrannus" (Early Cretaceous, England)
*"Alectrosaurus" (Late Cretaceous, Mongolia)
*"Appalachiosaurus" (Late Cretaceous, eastern North America)
*?"Labocania" (Late Cretaceous, western Mexico)
*"'Family Tyrannosauridae
**"Albertosaurus" (Late Cretaceous, western North America)
**?"Alioramus" (Late Cretaceous, Mongolia)
**"Daspletosaurus" (Late Cretaceous, western North America)
**"Gorgosaurus" (Late Cretaceous, western North America)
**"Tarbosaurus" (Late Cretaceous, Mongolia)
**"Tyrannosaurus" (Late Cretaceous, western North America)

Phylogeny

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 the Carnosauria, which included almost all large theropods.cite_book |last=Romer |first=Alfred S. |authorlink=Alfred Sherwood Romer |year=1956 |title=Osteology of the Reptiles |location=Chicago |publisher=University of Chicago Press |pages=772pp |isbn=978-0894649851] cite_book |last=Gauthier |first=Jacques |authorlink=Jacques Gauthier |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor=Padian, Kevin. (ed.) |title=The Origin of Birds and the Evolution of Flight |series="Memoirs of the California Academy of Sciences" 8 |pages=1-55 |isbn=978-0940228146] Within this group, the allosaurids were often considered to be ancestral to tyrannosaurids.cite_book |last=Molnar |first=Ralph E. |authorlink=Ralph Molnar |coauthors=Kurzanov, Sergei M.; & Dong Zhiming. |year=1990 |chapter=Carnosauria |editor= Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.). |title=The Dinosauria |edition=First Edition |publisher=University of California Press |location=Berkeley |pages=169-209 |isbn=978-0520067271] In the early 1990s, cladistic analyses instead began to place tyrannosaurids into the Coelurosauria,cite_book |last=Novas |first=Fernando E. |authorlink=Fernando Novas |year=1992 |chapter=The evolution of the carnivorous dinosaurs |language=Spanish |editor=Sanz, José L.; & Buscalioni, Angela D. (eds.) |title=Los Dinosaurios y su Entorno Biotico: Actas del Segundo Curso de Paleontología en Cuenca |location=Cuenca |publisher=Instituto "Juan de Valdez" |pages=125-163] echoing suggestions first published in the 1920s.cite_journal |last=Matthew |first=William D. |authorlink=William Diller Matthew |coauthors=& Brown, Barnum. |year=1922 |title=The family Deinodontidae, with notice of a new genus from the Cretaceous of Alberta |journal=Bulletin of the American Museum of Natural History |volume=46 |pages=367–385 |url=http://digitallibrary.amnh.org/dspace/handle/2246/1300] cite_journal |last=von Huene |first=Friedrich |authorlink=Friedrich von Huene |title=Carnivorous Saurischia in Europe since the Triassic |journal=Geological Society of America Bulletin |volume=34 |issue=3 |pages=449–458] Tyrannosaurids are now universally considered to be large coelurosaurs.cite_journal |last=Sereno |first=Paul C. |authorlink=Paul Sereno |year=1999 |title=The evolution of dinosaurs |journal=Science |volume=284 |issue=5423 |pages=2137–2147 |doi=10.1126/science.284.5423.2137] cite journal |last=Rauhut |first=Oliver W.M. |year=2003 |title=The interrelationships and evolution of basal theropod dinosaurs |journal=Special Papers in Palaeontology |volume=69 |pages=1–213] cite_journal |last=Currie |first=Philip J. |coauthors=Hurum, Jørn H; & Sabath, Karol. |authorlink=Phil Currie |year=2003 |title=Skull structure and evolution in tyrannosaurid phylogeny |journal=Acta Palaeontologica Polonica | volume=48 |issue=2 |pages=227–234 |url=http://app.pan.pl/acta48/app48-227.pdf] cite_journal |last=Norell |first=Mark A. |coauthors=Clark, James M.; Turner, Alan H.; Makovicky, Peter J.; Barsbold, Rinchen; & Rowe, Timothy. |year=2006 |title=A new dromaeosaurid theropod from Ukhaa Tolgod (Ömnögov, Mongolia) |journal=American Museum Novitates |volume= 3545 |pages=1–51 |url=http://digitallibrary.amnh.org/dspace/handle/2246/5823 |doi=10.1206/0003-0082(2006)3545 [1:ANDTFU] 2.0.CO;2 |doilabel=10.1206/0003-0082(2006)3545[1:ANDTFU]2.0.CO;2] cite_journal |last=Senter |first=Phil |year=2007 |title=A new look at the phylogeny of Coelurosauria (Dinosauria, Theropoda) |journal=Journal of Systematic Palaeontology |volume=5 |issue=4 |pages=429–463 |doi=10.1017/S1477201907002143]
Xu "et al." 2006
clade| style=font-size:80%;line-height:80%
label1=Coelurosauria
1=clade
label1=void
1=Compsognathidae
2=clade
label1= Tyrannosauroidea
1=clade
1="Guanlong"
2="Stokesosaurus"
label3=void
3=clade
label1=void
1="Dilong"
2=Tyrannosauridae
2=Maniraptoriformes
Holtz 2004
clade| style=font-size:80%;line-height:80%
label1=Coelurosauria
1=clade
1=Compsognathidae
label2=void
2=clade
label1= Tyrannosauroidea
1=clade
label1=void
1="Bagaraatan"
2=clade
1="Stokesosaurus"
2="Dryptosaurus"
label3=void
3=clade
label1=void
1="Eotyrannus"
2=clade
label1=void
1="Alectrosaurus?
2=clade
label1=void
1="Alioramus?
2=Tyrannosauridae
2=Maniraptoriformes
Carr "et al." 2005
clade| style=font-size:80%;line-height:80%
label1=Tyrannosauroidea
1=clade
label1=void
1="Dryptosaurus"
2=clade
label1=void
1=clade
1="Appalachiosaurus"
2=Tyrannosauridae In 1994, Holtz grouped tyrannosauroids with elmisaurids, ornithomimosaurs and troodonts into a coelurosaurian clade called Arctometatarsalia based on a common ankle structure where the second and fourth metatarsals meet near the tarsal bones, covering the third metatarsal when viewed for the front. Basal tyrannosauroids like "Dilong", however, were found with non-arctometatarsalian ankles, indicating that this feature evolved convergently. Arctometatarsalia has been dismantled and is no longer used by most paleontologists, with tyrannosauroids usually considered to be basal coelurosaurs outside Maniraptoriformes. One recent analysis found the family Coeluridae, including the Late Jurassic North American genera "Coelurus" and "Tanycolagreus", to be the sister group of Tyrannosauroidea.

The most basal tyrannosauroid known from complete skeletal remains is "Guanlong". Other early taxa include "Stokesosaurus" and "Aviatyrannis", known from far less complete material. The better-known "Dilong" is considered slightly more derived than "Guanlong" and "Stokesosaurus". "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 than "Eotyrannus" and "Appalachiosaurus".cite_web |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |date=2005-09-20 |url=http://dml.cmnh.org/2005Sep/msg00345.html |title=RE: Burpee Conference (LONG) |work=Archives of the Dinosaur Mailing List |accessdate=2007-06-18] "Alectrosaurus", a poorly known genus from Mongolia, is definitely a tyrannosauroid but its exact relationships are unclear. Other taxa have been considered possible tyrannosauroids by various authors, including "Bagaraatan", "Labocania" and a species erroneously referred to "Chilantaisaurus", "C." "maortuensis". "Siamotyrannus" from the Early Cretaceous of Thailand was originally described as an early tyrannosaurid,cite_journal |last=Buffetaut |first=Eric |coauthors=Suteethorn, Varavudh; & Tong Haiyan |year=1996 |title=The earliest known tyrannosaur from the Lower Cretaceous of Thailand |journal=Nature |volume=381 |issue=6584 |pages=689–691 |doi=10.1038/381689a0] but is usually considered a carnosaur today.cite_book |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |coauthors=Molnar, Ralph E.; & Currie, Philip J. |year=2004 |chapter=Basal Tetanurae |editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.) |title=The Dinosauria |edition=Second Edition |publisher=University of California Press |location=Berkeley |pages=71–110 |isbn=0-520-24209-2] "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.

Distribution

The earliest recognized tyrannosauroids lived in the Late Jurassic, including "Guanlong" from northwestern China, "Stokesosaurus" from the western United States and "Aviatyrannis" from Portugal. Some fossils currently referred to "Stokesosaurus" may instead belong to "Aviatyrannis", given the great similarities in the dinosaur faunas of Portugal and North America during this time. If "Iliosuchus" from the Middle Jurassic of England is in fact a tyrannosauroid, it would be the earliest known genus and might suggest that the superfamily originated in Europe. Early Cretaceous tyrannosauroids are also found on all three northern continents. "Eotyrannus" from England and "Dilong" from northeastern China are the only two named genera of this age, while Early Cretaceous tyrannosauroid premaxillary teeth are known from the Cedar Mountain Formation in Utahcite journal |last=Kirkland |first=James I. |authorlink=James Kirkland (paleontologist) |coauthors=Britt, Brooks; Burge, Donald L.; Carpenter, Kenneth; Cifelli, Richard; DeCourten, Frank; Eaton, Jeffrey; Hasiotis, Steven; & Lawton, Timothy. |year=1997 |title=Lower to Middle Cretaceous Dinosaur faunas of the central Colorado Plateau: a key to understanding 35 million years of tectonics, sedimentology, evolution, and biogeography |journal=Brigham Young University Geology Studies |volume=42 |issue=II |pages=69–103] and the Tetori Group of Japan.cite_journal |last=Manabe |first=Makoto |year=1999 |title=The early evolution of the Tyrannosauridae in Asia |journal=Journal of Paleontology |volume=73 |issue=6 |pages=1176–1178 |url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/6/1176] "Chilantaisaurus" "maortuensis" from the Dashuigou Formation of Inner Mongolia in China is also sometimes considered to be an Early Cretaceous tyrannosauroid.

By the middle of the Cretaceous, tyrannosauroid fossils are no longer found in Europe, suggesting a localized extinction on that continent. Tyrannosauroid teeth and possible body fossils are known from the North American Dakota Formation, as well as formations in Kazakhstan, Tajikstan and Uzbekistan,cite_book |last=Nessov |first=Lev A. |year=1995 |title=Dinosaurs of Northern Eurasia: new data about assemblages, ecology and paleobiogeography |publisher=Scientific Research Institute of the Earth's Crust, St. Petersburg State University |location=St. Petersburg |pages=156pp |language=Russian] from the middle of the Cretaceous. The first unquestionable remains of tyrannosaurids occur in the Campanian stage 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. Tyrannosaurid fossils have been found in Alaska, which may have served as a land bridge allowing dispersal between the two continents.cite_journal |last=Fiorillo |first=Anthony R. |coauthors=& Gangloff, Roland A. |year=2000 |title=Theropod teeth from the Prince Creek Formation (Cretaceous) of northern Alaska, with speculations on Arctic dinosaur paleoecology |journal=Journal of Vertebrate Paleontology |volume=20 |issue=4 |pages=675–682 |url=http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1671%2F0272-4634%282000%29020%5B0675%3ATTFTPC%5D2.0.CO%3B2 |doi=10.1671/0272-4634(2000)020 [0675:TTFTPC] 2.0.CO;2 |doilabel=10.1671/0272-4634(2000)020[0675:TTFTPC]2.0.CO;2] Non-tyrannosaurid tyrannosauroids like "Alectrosaurus" and possibly "Bagaraatan" were contemporaneous with tyrannosaurids in Asia, while they are absent from western North America. Eastern North America was divided by the Western 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 like "Dryptosaurus" and "Appalachiosaurus" to survive in the east as a relict population until the end of the Cretaceous.

Paleobiology

Feathers

Long filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous Yixian Formation and other nearby geological formations from Liaoning, China.cite_journal |last=Zhou Zhonghe |coauthors=Barrett, Paul M.; & Hilton, Jason. |year=2003 |title=An exceptionally preserved Lower Cretaceous ecosystem |journal=Nature |volume=421 |issue=6925 |pages=807–814 |doi=10.1038/nature01420] These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods,cite_journal |last=Chen Peiji |coauthors=Dong Zhiming; & Zhen Shuonan. |year=1998 |title=An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China |journal=Nature |volume=391 |issue=6663 |pages=147–152 |doi=10.1038/34356] cite_journal |last=Xu Xing |authorlink=Xu Xing |coauthors=Zhou Zhonghe & Prum, Richard A. |year=2003 |title=Branched integumental structures in Sinornithosaurus and the origin of feathers |journal=Nature |volume=410 |issue=6825 |pages=200–204 |doi=10.1038/35065589] although other hypotheses have been proposed.cite_journal |last=Lingham-Soliar |first=Theagarten |coauthors=Feduccia, Alan & Wang, Xiaolin. |year=2007 |title=A new Chinese specimen indicates that ‘protofeathers’ in the Early Cretaceous theropod dinosaur Sinosauropteryx are degradedcollagen fibres |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=270 |issue=1620 |pages=1823–1829 |doi=10.1098/rspb.2007.0352] A skeleton of "Dilong" was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to down feathers of modern birds, the "protofeathers" found in "Dilong" were branched but not pennaceous, and may have been used for insulation.

The presence of "protofeathers" in basal tyrannosauroids is not surprising, since they are now known to be characteristic of coelurosaurs, found in other basal genera like "Sinosauropteryx", as well as all more derived groups. Rare fossilized skin impressions of large tyrannosaurids lack feathers, however, instead showing skin covered in scales.cite_journal |last=Martin |first=Larry D. |authorlink=Larry Martin |coauthors=& Czerkas, Stephan A. |year=2000 |title=The fossil record of feather evolution in the Mesozoic |journal=American Zoologist |volume=40 |issue=4 |pages=687–694 |url=http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1668%2F0003-1569%282000%29040%5B0687%3ATFROFE%5D2.0.CO%3B2 |doi=10.1668/0003-1569(2000)040 [0687:TFROFE] 2.0.CO;2 |doilabel=10.1668/0003-1569(2000)040[0687:TFROFE]2.0.CO;2] It is possible that "protofeathers" were present on areas of the body not preserved with skin impressions. Alternatively, secondary loss of "protofeathers" in large tyrannosaurids may be analogous with the similar loss of hair in the largest modern mammals like elephants, where a low surface area-to-volume ratio slows down heat transfer, making insulation by a coat of hair unnecessary.

Head crests

Bony crests are found on the skulls of many theropods, including numerous tyrannosauroids. The most elaborate is found in "Guanlong", 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. A less prominent crest is found in "Dilong", where low, parallel ridges run along each side of the skull, supported by the nasal and lacrimal bones. This ridges curve inwards and meet just behind the nostrils, making the crest "Y"-shaped. The fused nasals of tyrannosaurid 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 of "Appalachiosaurus", as well as some specimens of "Daspletosaurus", "Albertosaurus", and "Tarbosaurus". In "Albertosaurus", "Gorgosaurus" and "Daspletosaurus", there is a prominent horn in front of each eye on the lacrimal bone. The lacrimal horn is absent in "Tarbosaurus" and "Tyrannosaurus", which instead have a crescent-shaped crest behind each eye on the postorbital bone.

These head crests may have been used for display, perhaps for species recognition or courtship behavior. An example of the handicap principle may be the case of "Guanlong", 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 male peacock or the outsized antlers of an Irish elk, the crest of "Guanlong" may have evolved via sexual selection, providing an advantage in courtship which outweighed any decrease in hunting ability.

References

External links

* [http://home.comcast.net/~eoraptor/Tyrannosauroidea.html List of tyrannosauroid specimens and species] at The Theropod Database.