Byronosaurus

Byronosaurus is a genus of troodontid dinosaur from the Late Cretaceous Period of Mongolia.

Byronosaurus
Temporal range: Late Cretaceous, 80–75 Ma
Byronosaurus
Restoration
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Troodontidae
Genus: Byronosaurus
Norell, Makovicky & Clark, 2000
Species:
B. jaffei
Binomial name
Byronosaurus jaffei
Norell, Makovicky & Clark, 2000

Discovery and naming

Troodontid teeth
Comparison of troodontid teeth; L is Byronosaurus

In 1993, Michael Novacek, a member of an American Museum of Natural History expedition to the Gobi Desert, discovered the skeleton of a small theropod at Ukhaa Tolgod. This was further excavated in 1994 and 1995. The find was illustrated in a publication in 1994.[1] On 15 July 1996, at the Bolor's Hill site, about eight kilometers (five miles) away from the original location, a second specimen was discovered, a skull.

In 2000, Mark Norell, Peter Makovicky and James Clark named and described the type species Byronosaurus jaffei. The species name as a whole honoured Byron Jaffe, "in recognition of his family's support for the Mongolian Academy of Sciences-American Museum of Natural History Paleontological Expeditions".[2]

The holotype, IGM 100/983, was found in a layer of the Djadochta Formation dating from the late Campanian. It consists of a partial skeleton with skull. It contains a partial skull with lower jaws, three neck vertebrae, three back vertebrae, a piece of a sacral vertebra, four partial tail vertebrae, ribs, the lower end of a thighbone, the upper ends of a shinbone and calf bone, a second metatarsal and three toe phalanges. The paratype, specimen IGM 100/984, is the skull found in 1996, of which only the snout has been preserved. Both specimens are of adult individuals.[2]

In 2003, the skeleton was described in detail.[3]

In 2009, two front skulls and lower jaws of very young, perhaps newly hatched, individuals, specimens IGM 100/972 and IGM 100/974, were referred to Byronosaurus, after originally having been identified as Velociraptor exemplars.[4]

Description

Byronosaurus is a troodontid, a group of small, bird-like, gracile maniraptorans. All known troodontids share unique features of the skull, such as closely spaced teeth in the lower jaw, and large numbers of teeth. Troodontids have sickle-claws and raptorial hands, and some of the highest non-avian encephalization quotients, meaning they were behaviourally advanced and had keen senses. Byronosaurus is one of few troodontids that have no serrations on its teeth, similar to its closest relative Xixiasaurus.[5] Byronosaurus was 1.5 m (4.9 ft) long and 50 cm (20 in) tall.[6] It weighed only about 4 kilograms (9 lbs).[6] Unlike most other troodontids, its teeth seem to lack serrations. They are instead needle-like, probably best suited for catching small birds, lizards and mammals. Specifically, they resemble those of Archaeopteryx.

The holotype skull measures about twenty-three centimeters long (nine inches). The snout is pneumatised, with a sinus in each maxilla.[3]

Hatchling skulls

Dromaeosaurid juvenile AMNH 28506 cast
IGM 100/972 at the AMNH

Mark Norell and colleagues described two "perinate" (hatchlings or embryos close to hatching) specimens of Byronosaurus (specimens IGM 100/972 and IGM 100/974) in 1994. The two specimens were found in a nest of oviraptorid eggs in the Late Cretaceous "Flaming Cliffs" of the Djadokhta Formation of Mongolia. The nest is quite certainly that of an oviraptorosaur, since an oviraptorid embryo is still preserved inside one of the eggs. The two partial skulls were first described by Norell et al. (1994) as dromaeosaurids, but reassigned to Byronosaurus after further study.[4][7] The juvenile skulls were either from hatchlings or embryos, and fragments of eggshell are adhered to them although it seems to be oviraptorid eggshell. The presence of tiny Byronosaurus skulls in an oviraptorid nest is an enigma. Hypotheses explaining how they came to be there include that they were the prey of the adult oviraptorid, that they were there to prey on oviraptorid hatchlings, or that an adult Byronosaurus may have laid eggs in a Citipati nest (see nest parasite).[8]

See also

References

  1. ^ Novacek, M.J., Norell, M.A, McKenna, M.C. and Clark, J.M, 1994, "Fossils of the Flaming Cliffs", Scientific American 271(6), 60-69
  2. ^ a b Norell, M.A., Makovicky, P.J. & Clark, J.M., 2000, "A new troodontid theropod from Ukhaa Tolgod, Mongolia", Journal of Vertebrate Paleontology 20(1): 7-11
  3. ^ a b Makovicky, P.J.; Norell, M.A.; Clark, J.M.; Rowe, T.E. (2003). "Osteology and relationships of Byronosaurus jaffei (Theropoda: Troodontidae)" (PDF). American Museum Novitates. 3402: 1–32. doi:10.1206/0003-0082(2003)402<0001:oarobj>2.0.co;2. Archived from the original (PDF) on 2007-03-16. Retrieved 2007-02-04.
  4. ^ a b Bever, G.S. and Norell, M.A. (2009). "The perinate skull of Byronosaurus (Troodontidae) with observations on the cranial ontogeny of paravian theropods." American Museum Novitates, 3657: 51 pp.
  5. ^ Junchang Lü; Li Xu; Yongqing Liu; Xingliao Zhang; Songhai Jia & Qiang Ji (2010). "A new troodontid (Theropoda: Troodontidae) from the Late Cretaceous of central China, and the radiation of Asian troodontids" (PDF). Acta Palaeontologica Polonica. 55 (3): 381–388. doi:10.4202/app.2009.0047.
  6. ^ a b Montague, R. (2006). "Estimates of body size and geological time of origin for 612 dinosaur genera (Saurischia, Ornithischia)". Florida Scientist. 69 (4): 243–257.
  7. ^ Mackovicky, Peter J.; Norell, Mark A. (2004). "Troodontidae". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 184–195. ISBN 0-520-24209-2.
  8. ^ Norell, Mark A.; Clark, James M.; Dashzeveg, Demberelyin; Barsbold, Rhinchen; Chiappe, Luis M.; Davidson, Amy R.; McKenna, Malcolm C.; Perle, Altangerel; Novacek, Michael J. (November 4, 1994). "A theropod dinosaur embryo and the affinities of the Flaming Cliffs dinosaur eggs". Science. 266 (5186): 779–782. doi:10.1126/science.266.5186.779. PMID 17730398.
Archaeornithoides

Archaeornithoides is a genus of maniraptoriform theropod dinosaur of the Late Cretaceous of Mongolia.

Citipati

Citipati (pronounced [ˈtʃiːt̪ɪpət̪i] in Hindi, meaning 'funeral pyre lord') is a genus of oviraptorid theropod dinosaur from the Late Cretaceous Period of what is now Mongolia (specifically, the Djadokhta Formation of Ukhaa Tolgod, in the Gobi Desert). It is one of the best-known oviraptorids, thanks to a number of well-preserved skeletons, including several specimens found in brooding positions atop nests of eggs. These nesting specimens have helped to solidify the link between non-avian dinosaurs and birds.

The type species, Citipati osmolskae, was described in 2001. A second, as yet unnamed species may also exist. Citipati is often confused with the similar Oviraptor.

Gobivenator

Gobivenator is an extinct genus of troodontid theropod dinosaur known from the late Campanian Djadokhta Formation of central Gobi Desert, Mongolia. It contains a single species, Gobivenator mongoliensis. G. mongoliensis is known from a single individual, which represents the most complete specimen of a Late Cretaceous troodontid currently known.

Jianianhualong

Jianianhualong (meaning "Jianianhua dragon") is a genus of troodontid theropod dinosaur from the Early Cretaceous of China. It contains a single species, Jianianhualong tengi, named in 2017 by Xu Xing and colleagues based on an articulated skeleton preserving feathers. The feathers at the middle of the tail of Jianianhualong are asymmetric, being the first record of asymmetrical feathers among the troodontids. Despite aerodynamic differences from the flight feathers of modern birds, the feathers in the tail vane of Jianianhualong could have functioned in drag reduction whilst the animal was moving. The discovery of Jianianhualong supports the notion that asymmetrical feathers appeared early in the evolutionary history of the Paraves.

Jianianhualong possesses a combination of traits seen in basal as well as traits seen in derived troodontids. This is consistent with its phylogenetically intermediate position among the troodontids. This mixture of traits shows a distinct spatial organization, with basal traits being present in the forelimbs and pelvis, and derived traits being present in the skull and hindlimbs. This may represent a case of mosaic evolution, where natural selection acts upon the form of the body in a modular way. Similar transitional patterns of traits are seen in the troodontid Sinusonasus, a close relative of Jianianhualong. Ecologically speaking, Jianianhualong and Sinusonasus are part of the considerable diversity of troodontids that is present within the Yixian Formation.

Mahakala omnogovae

Mahakala (from Sanskrit, named for Mahakala, one of eight protector deities (dharmapalas) in Tibetan Buddhism) is a genus of basal dromaeosaurid dinosaur from the Campanian-age (about 80 million years ago) Upper Cretaceous Djadokhta Formation of Ömnögovi, Mongolia. It is based on a partial skeleton found in the Gobi Desert. Mahakala was a small dromaeosaurid (approximately 70 centimeters long (28 in)), and its skeleton shows features that are also found in early troodontids and avialans. Despite its late appearance, it is among the most basal dromaeosaurids. Its small size, and the small size of other basal deinonychosaurians, suggests that small size appeared before flight capability in birds.

Mark Norell

Mark A. Norell (born July 26, 1957) is an American paleontologist and molecular geneticist, acknowledged as one of the most important living vertebrate paleontologists. He is currently the chairman of paleontology and a research associate at the American Museum of Natural History. He is best known as the discoverer of the first theropod embryo and for the description of feathered dinosaurs. Norell is credited with the naming of the genera Apsaravis, Byronosaurus, Citipati, Tsaagan, and Achillobator. His work regularly appears in major scientific journals (including cover stories in Science and Nature) and was listed by Time magazine as one of the ten most significant science stories of 1993, 1994 and 1996.

Norell is both a fellow of the Explorer's Club and the Willi Hennig Society.

Oviraptoridae

Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between one and two metres long in most cases, though some possible oviraptorids were enormous. Oviraptorids are currently known only from the Late Cretaceous of Asia, with the most well-known species and complete specimens found only in the Gobi Desert of Mongolia and northwestern China.

Paronychodon

Paronychodon (meaning "beside claw tooth") was a theropod dinosaur genus. It is a tooth taxon, often considered dubious because of the fragmentary nature of the fossils, which include "buckets" of teeth but no other remains.

The type species, named by Edward Drinker Cope in 1876, is Paronychodon lacustris, from the Judith River Formation of Montana, dating to 75 million years ago, during the Campanian stage. The holotype is specimen AMNH 3018. It is a tooth about one centimetre long, elongated, recurved, lacking serrations, possessing low vertical ridges and with a D-shaped cross-section, the inner side being flattened. Cope at first thought the tooth belonged to a plesiosaur, but the same year realised it represented a carnivorous dinosaur.A second species, Paronychodon caperatus, is known from the Hell Creek Formation of North Dakota, Montana, and South Dakota and Lance Formation of Wyoming (latest Maastrichtian stage, 66 million years ago) and was originally referred to the mammal genus Tripriodon by Othniel Charles Marsh in 1889, but placed in Paronychodon by George Olshevsky in 1991. It is based on holotype YPM 10624, a tooth close in form to the holotype of P. lacustris but somewhat larger. In 1995 Olshevsky renamed Laelaps explanatus Cope 1876 into a Paronychodon explanatus; today the taxon is seen as based on Saurornitholestes teeth.

A very large number of other specimens matching these teeth in some or all aspects of their anatomy have been referred to Paronychodon. Some of these included serrated teeth, low teeth and teeth without a flattened side. These teeth of the general "Paronychodon" type have been reported from a wide variety of times and places, including the Early Cretaceous Una Formation of Spain, dating to the late Barremian age 125 million years ago.

Paronychodon has been considered a coelurid, an ornithomimosaur, a dromaeosaurid, an archaeopterygid, and a troodontid, though it could also be another kind of coelurosaurian theropod. While most researchers have therefore considered it simply represents indeterminate theropod teeth, a small consensus has found them to be Deinonychosauria. The teeth assigned to Paronychodon are all small, and may have come from various juvenile deinonychosaurs. Jaws from adult individuals bearing identical teeth have never been found. Marsh already suggested such teeth were pathological, having formed when the first teeth of the lower jaws by accident grew back-to-back to each other on the mandible suture. Philip J. Currie in 1990 also concluded to a malformation, thinking the flattened side resulted from the tooth remaining attached too long to the inner wall of the tooth-socket. Serrated specimens of the type would thus simply be deviant dromaeosaurid teeth; however, unserrated teeth might represent a separate taxon or taxa. One study, by Sunny Hwang, showed that the tooth enamel is identical to that found in Byronosaurus, a troodontid known from juveniles with serration-less teeth.Several taxa have on occasion been considered synonyms of Paronychodon, though there is little consensus. Paronychodon was in 1876 by Cope described as being similar to Zapsalis, another tooth taxon, itself often considered synonymous with Richardoestesia, a possible dromaeosaurid. Richardoestesia isosceles would, according to a study by Julia Sankey e.a., be synonymous with the elongated, so-called "Type A", teeth of Paronychodon, to which also the Paronychodon holotype belongs. The Eurasian Euronychodon tooth genus is also sometimes considered a (junior) synonym of Paronychodon.

Philovenator

Philovenator (literally meaning "love hunter") is an extinct genus of troodontid paravian dinosaurs from the Wulansuhai Formation (dated to the Campanian age, sometime between 75 and 71 million years ago) of Inner Mongolia, China. Its specific name honors Phillip J. Currie.

Polyodontosaurus

Polyodontosaurus is a potentially dubious genus of troodontid dinosaur named in 1932 by Gilmore for a left dentary from the Dinosaur Park Formation. It had been considered a synonym of Stenonychosaurus or Troodon for a significant time, before being declared a nomen dubium.

Sinornithoides

Sinornithoides (meaning "Chinese bird form") is a genus of troodontid theropod dinosaurs containing the single species Sinornithoides youngi. S. youngi lived during the Early Cretaceous (Aptian/Albian stage, around 113 million years ago). It measured approximately one meter long (3.3 ft). It lived in Inner Mongolia, China, and probably ate invertebrates and other small prey. They live in what is now Mongolia, which was part of Laurasia.

Sinovenator

Sinovenator (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period.

Talos sampsoni

Talos is an extinct genus of carnivorous bird-like theropod dinosaur, an advanced troodontid which lived during the late Cretaceous period (late Campanian, about 75.95 Ma) in the geographic area that is now Utah, United States.

Timeline of troodontid research

This timeline of troodontid research is a chronological listing of events in the history of paleontology focused on the troodontids, a group of bird-like theropod dinosaurs including animals like Troodon. Troodontid remains were among the first dinosaur fossils to be reported from North America after paleontologists began performing research on the continent, specifically the genus Troodon itself. Since the type specimen of this genus was only a tooth and Troodon teeth are unusually similar to those of the unrelated thick-headed pachycephalosaurs, Troodon and its relatives would be embroiled in taxonomic confusion for over a century. Troodon was finally recognized as distinct from the pachycephalosaurs by Phil Currie in 1987. By that time many other species now recognized as troodontid had been discovered but had been classified in the family Saurornithoididae. Since these families were the same but the Troodontidae named first, it carries scientific legitimacy.Many milestones of troodontid research occurred between the description of Troodon and the resolution of their confusion with pachycephalosaurs. The family itself was named by Charles Whitney Gilmore in 1924. That same year Henry Fairfield Osborn named the genus Saurornithoides. In the 1960s and 1970s researchers like Russell and Hopson observed that troodontids had very large brains for their body size. Both attributed this enlargement of the brain to a need for processing the animal's especially sharp senses. Also in the 1970s, Barsbold described the new species Saurornithoides (now Zanabazar) junior and named the family Saurornithoidae, but as noted this was just a junior synonym of the Troodontidae in the first place.In the 1980s Gauthier classed them with the dromaeosaurids in the Deinonychosauria. That same decade Jack Horner reported the discovery of Troodon nests in Montana. Interest in the life history of Troodon continued in the 1990s with a study of its growth rates based on histological sections of fossils taken from a bonebed in Montana and the apparent pairing of eggs in Troodon nests. This decade also saw the first potential report of European troodontid remains, although this claim has been controversial. A single mysterious tooth from the Late Jurassic Morrison Formation of the United States was described as the oldest known troodontid remains, although this has also been controversial. In the 2000s, several new kinds of troodontid were named, like Byronosaurus and Sinovenator.

Troodontidae

Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens (including specimens which preserve feathers, eggs, embryos, and complete juveniles), have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.

Troodontinae

Troodontinae is a subfamily of troodontid dinosaurs. The subfamily was first used in 2017 for the group of troodontids descended from the last common ancestor of Gobivenator mongoliensis and Zanabazar junior.

Urbacodon

Urbacodon ("URBAC tooth") is a genus of troodontid dinosaur, a type of small carnivore. It lived in Uzbekistan during the early Late Cretaceous Period, about 95 million years ago.

On 9 September 2004, a lower jaw of a small theropod was uncovered by Anton Sergeevich Rezwiy near Itemir in the IT-01 quarry.The type species, Urbacodon itemirensis, was named by Alexandr Averianov and Hans-Dieter Sues in 2007. The first part of the generic name Urbacodon is an acronym, honouring the Uzbek, Russian, British, American and Canadian scientists who participated in its discovery. This acronym was combined with a Greek ὀδών, odon, "tooth". The specific name refers to the provenance from Itemir.The name was based on the holotype ZIN PH 944/16, a single left dentary with preserved replacement teeth from the Cenomanian Dzharakuduk Formation. Averianov and Sues also identified teeth and other material, earlier described by Lev Nesov, as a Urbacodon sp. from the nearby Turonian Bissekty Formation.The holotype dentary of U. itemirensis is 79.2 millimetres long (3.12 in) and has 32 tooth positions. It is rather straight in top view. The teeth are closely packed but between the front twenty-four teeth and the rear eight teeth, a distinctive gap is present, a diastema. This is a unique trait but was not formally designated as an autapomorphy because it might be the result of individual variation. Urbacodon resembles Byronosaurus and Mei but differs from most other Troodontidae in that its teeth lack serrations. Urbacodon is distinguished from Byronosaurus by a less vascularized lateral dentary groove and more bulbous anterior tooth crowns, and from Mei by considerably larger size.Averianov and Sues viewed Urbacodon as more plesiomorphic than Troodon and Saurornithoides in having a straight dentary with fewer teeth, but did not attempt to place it on a cladogram. In 2010, a cladistic analysis showed it as a close relative of Byronosaurus and Xixiasaurus.

Xixiasaurus

Xixiasaurus () is a genus of troodontid dinosaur that lived during the Late Cretaceous Period in what is now China. The only known specimen was discovered in Xixia County, Henan Province, in central China, and became the holotype of the new genus and species Xixiasaurus henanensis in 2010. The names refer to the areas of discovery, and can be translated as "Henan Xixia lizard". The specimen consists of an almost complete skull (except for the hindmost portion), part of the lower jaw, and teeth, as well as a partial right forelimb.

Xixiasaurus is estimated to have been 1.5 metres (5 ft) long and to have weighed 8 kilograms (18 lb). As a troodontid, it would have been bird-like and lightly built, with grasping hands and an enlarged sickle-shaped claw on the second toe. Its skull was long, with a long, low snout that formed a tapering U-shape when seen from below. The frontal bone of the forehead was dome-like in side view, which indicates it had an enlarged braincase. It differed from other troodontids in that the front of the dentary bone of the lower jaw was down-turned. Unlike in most troodontids, the teeth of Xixiasaurus did not have serrations; instead, their carinae (front and back edges) were smooth and sharp. It was distinct among troodontids in having 22 teeth in each maxilla (in other genera the maxillary tooth count was either higher or lower).

The precise relationships of Xixiasaurus with other troodontids are uncertain, but it had some similarities with Byronosaurus. Though troodontids with unserrated teeth were once thought to form a clade, the taxonomic significance of this feature has been questioned. Troodontids had large brains, keen senses, and were probably agile. There has been debate about their diet, with some researchers arguing that they were carnivorous, and others that they were omnivorous or herbivorous. The lack of serrated teeth in Xixiasaurus and some other troodontids indicates these were herbivorous, as they had lost the ability to slice meat. Xixiasaurus is known from the Majiacun Formation, the exact age of which is uncertain. These sedimentary rocks were deposited by braided streams and meandering streams, and are noted for containing abundant dinosaur eggs.

Zanabazar junior

Zanabazar is an extinct genus of troodontid theropod dinosaur from the Late Cretaceous of Mongolia. The genus was originally named by Rinchen Barsbold as a species of Saurornithoides, S. junior. In 2009 it was reclassified as its own genus, named after the first spiritual figurehead of Tibetan buddhism, Zanabazar. The holotype, GIN 100–1, includes a skull, vertebrae, and right hindlimb. Zanabazar was one of the most derived troodontids, and the second largest after Troodon.

Troodontidae

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