Oviraptorosauria

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Oviraptorosaurs
Temporal range: Cretaceous, 130–66  Ma
Oviraptorosauria Diversity.jpg
Oviraptorosauria diversity. Clockwise from top left: GIN 100/42 (which may represent Citipati or a different taxon), "Ronaldoraptor" (undescribed), Avimimus , Anzu and Gigantoraptor
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: Pennaraptora
Clade:† Oviraptorosauria
Barsbold, 1976
Subgroups
Synonyms

Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or without bony crests atop the head. They ranged in size from Caudipteryx , which was the size of a turkey, to the 8-meter-long, 1.4-ton Gigantoraptor . [4] The group (along with all maniraptoran dinosaurs) is close to the ancestry of birds. Some researchers such as Maryanska et al (2002) and Osmólska et al. (2004) have proposed that they may represent primitive flightless birds. [5] [6] The most complete oviraptorosaur specimens have been found in Asia. [7] The North American oviraptorosaur record is sparse. [7]

The earliest and most basal ("primitive") known oviraptorosaurs are Ningyuansaurus wangi , Protarchaeopteryx robusta and Incisivosaurus gauthieri , both from the lower Yixian Formation of China, dating to about 125 million years ago during the Aptian age of the early Cretaceous period. A tiny neck vertebra reported from the Wadhurst Clay Formation of England shares some features in common with oviraptorosaurs, and may represent an earlier occurrence of this group (at about 140 million years ago). [8]

Description

Oviraptorosaurs to scale Oviraptorosauria.png
Oviraptorosaurs to scale

Oviraptorosaurians have shortened rostrums, massive, beaklike mandibles, and long parietal bones. With the exception of the 8-meter long Gigantoraptor , they are generally medium-sized and rarely exceeded 2 meters in length. The most primitive members have four pairs of teeth in the premaxillae, such as in Caudipteryx [9] and in Incisivosaurus they are enlarged and form bizarrely prominent bucktoothed incisors. The more advanced members have no teeth in the jaws.

Pneumatization is extensive in the skulls and vertebrae of the more advanced members. Oviraptorosauria have thick, U-shaped furculae and a large sternal plates that are wider (together) than they are long, unlike in birds and dromaeosaurs. The arms are around half the length of the legs and over half the length of the presacral vertebral column. The hands are long, and tridactyl, with a reduced third finger in Caudipteryx and Ajancingenia . There are between 5 and 8 sacral vertebrae. The pubis is vertical or subvertical, with a concave anterior edge. The tibia is 15%-25% longer than the femur. The tail is short, with the number of vertebrae reduced to 24 or so, and proximally very thick, with broad transverse processes. [6] The ischium retains the primitive character of a prominent, triangular obturator process and lack the proximodorsal process that is found in birds. In advanced oviraptorosaurians, the ischium is curved posteriorly. The pectoral girdle is also primitive; the scapula is a broad blade that is distally expanded, it lies on the lateral aspect of the thorax at an angle to the vertebral column, and the coracoid has the primitive coelurosaur shape with a proximal supracoracoidal nerve foramen and a moderate biceps tubercle. [10]

Anzu wyliei skeleton cast in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado Oviraptorid Clean.png
Anzu wyliei skeleton cast in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado

Oviraptorosaurians are different from most other maniraptorans in the form of their skulls. They have shortened snouts, beak-like jaws with few or no teeth, and a large opening in the lower jaw bone. Some have bony crests atop the skull. The most primitive members have a few teeth in the front of the mouth; in Incisivosaurus , they are enlarged and form bizarrely prominent "bucktoothed" incisors. The arms and hands are generally long (though very reduced in some advanced species) and the shoulder girdle is large and massive, with flexed coracoid bones and prominent attachments for strong arm muscles.

Their tails are very short compared to other maniraptorans. In Nomingia and Similicaudipteryx , the tail ends in four fused vertebrae which Osmólska, He, and others have referred to as a "pygostyle", but which Witmer found was anatomically different and evolved separately from the pygostyle of birds (a bone which serves as the attachment point for a fan of tail feathers). [2] [10]

Feathers

Evidence for feathered oviraptorosaurs exists in several forms. Most directly, four species of primitive oviraptorosaurs (in the genera Caudipteryx , Protarchaeopteryx , and Similicaudipteryx ) have been found with impressions of well developed feathers, most notably on the wings and tail, suggesting that they functioned at least partially for display. Secondly, at least four oviraptorosaur genera (Nomingia, Similicaudipteryx, Citipati , and Conchoraptor ) preserved tails ending in something like a pygostyle, a bony structure at the end of the tail that, in modern birds, is used to support a fan of feathers. [2] [11] Similarly, quill knobs (anchor points for wing feathers on the ulna) have been reported in the oviraptorosaurian species Avimimus portentosus . [12] Additionally, a number of oviraptorid specimens have famously been discovered in a nesting position similar to that of modern birds. The arms of these specimens are positioned in such a way that they could perfectly cover their eggs if they had small wings and a substantial covering of feathers. [13]

Notably, a study on flight feathers has concluded that Caudipteryx was secondarily flightless, implying an ancestral volant ancestor for oviraptorosaurs. [14]

Paleobiology

Diet

The eating habits of these animals are not fully known: they have been suggested to have been either carnivorous, herbivorous, mollusk-eating or egg-eating (the evidence that originally supported the latter is no longer considered valid); these options are not necessarily incompatible.

Some ate small vertebrates. Evidence for this comes from a lizard skeleton preserved in the body cavity of Oviraptor and two baby Troodontid skulls found in a Citipati nest. Evidence in favor of a herbivorous diet includes the presence of gastroliths preserved with Caudipteryx . There are also arguments for the inclusion of mollusks in their diet.

Originally these animals were thought to be egg raiders, based on a Mongolian find showing Oviraptor on top of a nest. Recent studies have shown that the animal was actually on top of its own nest. [15]

A 2022 study of the bite forces of oviraptorosaurs such as Incisivosaurus , Khaan , Citipati , and Conchoraptor suggests that most if not all oviraptorosaurs had a very strong bite force. The moderate jaw gape seen in oviraptorosaurs is indicative of herbivory in the majority of the group, but it is clear they were likely feeding on much tougher or more types of vegetation than other herbivorous theropods in their environment, such as ornithomimosaurs and therizinosaurs were able to. The examinations suggest oviraptorosaurs may have been powerful-biting generalists or specialists that partook of niche partitioning both in body size and cranial function. One particular group, the Caenagnathidae, may have also been more omnivorous or even carnivorous than other oviraptorosaurs. [16]

Reproduction

Hatchling specimen known as "baby Louie" Babylouiethedinosaur.jpg
Hatchling specimen known as "baby Louie"

Several oviraptorosaurian nests are known, with several oviraptorid specimens preserved in a brooding position over large clutches of up to a dozen or more eggs. The eggs are usually arranged in pairs, and forming a circular pattern within the nest. One oviraptorosaurian specimen from China has been found with two unlaid eggs within the pelvic canal. This suggests that, unlike modern crocodilians, oviraptorosaurs did not produce and lay many eggs at the same time. Rather, the eggs were produced within the reproductive organs in pairs, and laid two at a time, with the mother positioned in the center of the nest and rotating in a circle as each pair was laid. This behavior is supported by the fact that the eggs were shaped like highly elongated ovals — the greatest egg elongation among diapsids — with the more pointed end pointing backward from the cloaca and oriented toward the center of the nest. [17] [18] Geochemical analysis also revealed that oviraptorosaurs incubated their eggs in the 35–40 °C (95–104 °F) range, as many modern bird species do today, based on the oxygen isotope ratios in the bones of the fossil embryos of various species during development. [19]

The presence of two shelled eggs within the birth canal shows that oviraptorosaurs were intermediate between the reproductive biology of crocodilians and modern birds. Like crocodilians, they had two oviducts. However, crocodilians produce multiple shelled eggs per oviduct at a time, whereas oviraptorosaurs, like birds, produced only one egg per oviduct at a time. [17]

Relationship to birds

Oviraptorosaurs, like deinonychosaurs, are so bird-like that several scientists consider them to be true birds, more advanced than Archaeopteryx . Gregory S. Paul has written extensively on this possibility, and Teresa Maryańska and colleagues published a technical paper detailing this idea in 2002. [5] [20] [21] Michael Benton, in his widely respected text Vertebrate Paleontology, wrote placement of oviraptorosaurs among birds is highly controversial . [22] However, a number of researchers have disagreed with this classification, retaining oviraptorosaurs as non-avialan maniraptorans slightly more primitive than the deinonychosaurs. [23]

Classification

The internal classification of the oviraptorosaurs has also been controversial. Most studies divide them into two primary sub-groups, the Caenagnathidae and the Oviraptoridae. The Oviraptoridae is further divided into the small, short-armed, and crestless subfamily Ingeniinae, and the larger, crested, long-armed Oviraptorinae. However, some phylogenetic studies have suggested that many traditional members of the Caenagnathidae were more closely related to the crested oviraptorids.

Phylogeny

The 2007 cladistic analysis of Turner and colleagues recovered the Oviraptorosauria as a maniraptoran clade (natural grouping) of maniraptorans more primitive than true birds. They found that the oviraptorosaurs are the sister group to the Therizinosauria and that the two, together, are more basal than any member of Paraves. [23] However, a more recent study by Zanno and colleagues challenged that finding, showing therizinosaurs to be more primitive and not closely related to oviraptorosaurs. [24]

The following cladogram was found by an analysis published with the description of the caenagnathid Anzu. [25]

Oviraptorosauria

Incisivosaurus gauthieri

Caudipteridae

See also

Related Research Articles

<i>Oviraptor</i> Extinct genus of dinosaurs

Oviraptor is a genus of oviraptorid dinosaur that lived in Asia during the Late Cretaceous period. The first remains were collected from the Djadokhta Formation of Mongolia in 1923 during a paleontological expedition led by Roy Chapman Andrews, and in the following year the genus and type species Oviraptor philoceratops were named by Henry Fairfield Osborn. The genus name refers to the initial thought of egg-stealing habits, and the specific name was intended to reinforce this view indicating a preference over ceratopsian eggs. Despite the fact that numerous specimens have been referred to the genus, Oviraptor is only known from a single partial skeleton regarded as the holotype, as well as a nest of about fifteen eggs and several small fragments from a juvenile.

<i>Caudipteryx</i> Genus of oviraptorosaur dinosaurs

Caudipteryx is a genus of small oviraptorosaur dinosaurs that lived in Asia during the Early Cretaceous, around 124.6 million years ago. They were feathered and extremely birdlike in their overall appearance, to the point that some paleontologists suggested it was a bird. Two species have been described: C. zoui, in 1998, and C. dongi, in 2000.

<span class="mw-page-title-main">Oviraptoridae</span> Extinct family of dinosaurs

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.

<span class="mw-page-title-main">Maniraptora</span> Clade of dinosaurs

Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, Oviraptorosauria, and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period, and survive today as living birds.

<i>Avimimus</i> Extinct genus of reptiles

Avimimus, meaning "bird mimic", is a genus of oviraptorosaurian theropod dinosaur, named for its bird-like characteristics, that lived in the late Cretaceous in what is now Mongolia, around 85 to 70 million years ago.

<i>Caenagnathasia</i> Extinct species of reptile

Caenagnathasia is a small caenagnathid oviraptorosaurian theropod from the Late Cretaceous of Uzbekistan.

<i>Nomingia</i> Extinct genus of dinosaurs

Nomingia is a genus of oviraptorosaur theropod dinosaur hailing from the Late Cretaceous Bugin Tsav Beds of Mongolia.

<i>Chirostenotes</i> Extinct genus of dinosaurs

Chirostenotes is a genus of oviraptorosaurian dinosaur from the late Cretaceous of Alberta, Canada. The type species is Chirostenotes pergracilis.

<i>Citipati</i> Genus of oviraptorid dinosaur

Citipati is a genus of oviraptorid dinosaur that lived in Asia during the Late Cretaceous period, about 75 million to 71 million years ago. It is mainly known from the Ukhaa Tolgod locality at the Djadochta Formation, where the first remains were collected during the 1990s. The genus and type species Citipati osmolskae were named and described in 2001. A second species from the adjacent Zamyn Khondt locality may also exist. Citipati is one of the best-known oviraptorids thanks to a number of well-preserved specimens, including individuals found in brooding positions atop nests of eggs, though most of them were initially referred to the related Oviraptor. These nesting specimens have helped to solidify the link between non-avian dinosaurs and birds.

<i>Incisivosaurus</i> Extinct genus of dinosaurs

Incisivosaurus is a genus of small, probably herbivorous theropod dinosaurs from the early Cretaceous Period of what is now the People's Republic of China. The first specimen to be described, IVPP V13326, is a skull that was collected from the lowermost levels of the Yixian Formation in the Sihetun area, near Beipiao City, in western Liaoning Province. The most significant, and highly unusual, characteristic of this dinosaur is its apparent adaptation to an herbivorous or omnivorous lifestyle. It was named for its prominent, rodent-like front teeth, which show wear patterns commonly found in plant-eating dinosaurs. The specific name gauthieri honors Dr. Jacques Gauthier, a pioneer of the phylogenetic method of classification.

<i>Nemegtomaia</i> Extinct genus of dinosaurs

Nemegtomaia is a genus of oviraptorid dinosaur from what is now Mongolia that lived in the Late Cretaceous Period, about 70 million years ago. The first specimen was found in 1996, and became the basis of the new genus and species N. barsboldi in 2004. The original genus name was Nemegtia, but this was changed to Nemegtomaia in 2005, as the former name was preoccupied. The first part of the generic name refers to the Nemegt Basin, where the animal was found, and the second part means "good mother", in reference to the fact that oviraptorids are known to have brooded their eggs. The specific name honours the palaeontologist Rinchen Barsbold. Two more specimens were found in 2007, one of which was found on top of a nest with eggs, but the dinosaur had received its genus name before it was found associated with eggs.

<span class="mw-page-title-main">Caudipteridae</span> Extinct family of dinosaurs

Caudipteridae is an extinct family of oviraptorosaurian dinosaurs known from the Early Cretaceous of China. Found in the Yixian and Jiufotang Formations, the group existed between 125 and 120 million years ago. Distinguishing characteristics of this group have been indicated as including a unique dagger-shaped pygostyle. In 2015, the group was defined as "the most inclusive clade containing Caudipteryx zoui but not Oviraptor philoceratops and Caenagnathus collinsi".

<span class="mw-page-title-main">Caenagnathidae</span> Extinct family of dinosaurs

Caenagnathidae is a family of derived caenagnathoid dinosaurs from the Cretaceous of North America and Asia. They are a member of the Oviraptorosauria, and relatives of the Oviraptoridae. Like other oviraptorosaurs, caenagnathids had specialized beaks, long necks, and short tails, and would have been covered in feathers. The relationships of caenagnathids were long a puzzle. The family was originally named by Raymond Martin Sternberg in 1940 as a family of flightless birds. The discovery of skeletons of the related oviraptorids revealed that they were in fact non-avian theropods, and the discovery of more complete caenagnathid remains revealed that Chirostenotes pergracilis, originally named on the basis of a pair of hands, and Citipes elegans, originally thought to be an ornithomimid, named from a foot, were caenagnathids as well.

<i>Gigantoraptor</i> Extinct genus of dinosaurs

Gigantoraptor is a genus of large oviraptorosaur dinosaur that lived in Asia during the Late Cretaceous period. It is known from the Iren Dabasu Formation of Inner Mongolia, where the first remains were found in 2005.

Zhongornis is a genus of primitive maniraptoran dinosaurs that lived during the Early Cretaceous. It was found in rocks of the Yixian Formation in Lingyuan City (China), and described by Gao et al. in 2008.

<i>Similicaudipteryx</i> Extinct genus of dinosaurs

Similicaudipteryx, meaning "similar to Caudipteryx", is a genus of theropod dinosaur of the family Caudipteridae.

<span class="mw-page-title-main">Caenagnathoidea</span> Extinct superfamily of reptiles

Caenagnathoidea is a group of advanced oviraptorosaurian dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, often with bony crests atop the head. They ranged in size from Caudipteryx, which was the size of a turkey, to the 8 meter long, 1.4 ton Gigantoraptor. The group is close to the ancestry of birds. The most complete specimens have been found in Asia, representing members of the sub-group Oviraptorinae. Notable but fragmentary remains are also known from North America, almost all of which belong to the subgroup Elmisaurinae.

<span class="mw-page-title-main">Timeline of oviraptorosaur research</span>

This timeline of oviraptorosaur research is a chronological listing of events in the history of paleontology focused on the oviraptorosaurs, a group of beaked, bird-like theropod dinosaurs. The early history of oviraptorosaur paleontology is characterized by taxonomic confusion due to the unusual characteristics of these dinosaurs. When initially described in 1924 Oviraptor itself was thought to be a member of the Ornithomimidae, popularly known as the "ostrich" dinosaurs, because both taxa share toothless beaks. Early caenagnathid oviraptorosaur discoveries like Caenagnathus itself were also incorrectly classified at the time, having been misidentified as birds.

<i>Xingtianosaurus</i> Genus of theropod dinosaur

Xingtianosaurus is an extinct genus of oviraptorosaurian theropod dinosaur that lived in what is now China during the Early Cretaceous. The type and only species, X. ganqi, was named and described in 2019. It was placed in the Caudipteridae, alongside Caudipteryx and Similicaudipteryx.

References

  1. ↑ Ji Qiang; Lü Jun-Chang; Wei Xue-Fang; Wang Xu-Ri (2012). "A new oviraptorosaur from the Yixian Formation of Jianchang, Western Liaoning Province, China". Geological Bulletin of China. 31 (12): 2102–2107.
  2. 1 2 3 He, T.; Wang, X.-L.; Zhou, Z.-H. (2008). "A new genus and species of caudipterid dinosaur from the Lower Cretaceous Jiufotang Formation of western Liaoning, China". Vertebrata PalAsiatica. 46 (3): 178–189.
  3. ↑ Funston, G. F.; Tsogtbaatar, C.; Tsogtbaatar, K.; Kobayashi, Y.; Sullivan, C.; Currie, P. J. (2020). "A new two-fingered dinosaur sheds light on the radiation of Oviraptorosauria". Royal Society Open Science. 7 (10): 201184. Bibcode:2020RSOS....701184F. doi: 10.1098/rsos.201184 . PMC   7657903 . PMID   33204472.
  4. ↑ Xu, X.; Tan, Q.; Wang, J.; Zhao, X.; Tan, L. (2007). "A gigantic bird-like dinosaur from the Late Cretaceous of China" (PDF). Nature. 447 (7146): 844–847. Bibcode:2007Natur.447..844X. doi:10.1038/nature05849. PMID   17565365. S2CID   6649123.
  5. 1 2 Maryanska, T., Osmólska, H., & Wolsam, M. (2002). "Avialian status for Oviraptorosauria". Acta Palaeontologica Polonica. 47 (1): 97–116.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. 1 2 Osmólska, Halszka, Currie, Philip J., Brasbold, Rinchen (2004) "The Dinosauria" Weishampel, Dodson, Osmólska. "Chapter 8 Oviraptorosauria" University of California Press.
  7. 1 2 Varricchio, D. J. 2001. Late Cretaceous oviraptorosaur (Theropoda) dinosaurs from Montana. pp. 42–57 in D. H. Tanke and K. Carpenter (eds.), Mesozoic Vertebrate Life. Indiana University Press, Indianapolis, Indiana.
  8. ↑ Naish, D.; Sweetman, S.C. (2011). "A tiny maniraptoran dinosaur in the Lower Cretaceous Hastings Group: evidence from a new vertebrate-bearing locality in south-east England". Cretaceous Research. 32 (4): 464–471. Bibcode:2011CrRes..32..464N. doi:10.1016/j.cretres.2011.03.001.
  9. ↑ Qiang, Ji; Currie, Philip J.; Norell, Mark A.; Shu-An, Ji (June 1998). "Two feathered dinosaurs from northeastern China" (PDF). Nature. 393 (6687): 753–761. Bibcode:1998Natur.393..753Q. doi:10.1038/31635. S2CID   205001388.
  10. 1 2 Witmer, L.M. (2005). "The Debate on Avian Ancestry; Phylogeny, Function and Fossils", "Mesozoic Birds: Above the Heads of Dinosaurs" : 3-30. ISBN   0-520-20094-2
  11. ↑ W. Scott Persons IV; Philip J. Currie; Mark A. Norell (2014). "Oviraptorosaur tail forms and functions". Acta Palaeontologica Polonica. 59 (3). doi: 10.4202/app.2012.0093 .
  12. ↑ Kurzanov, S.M. (1987). "Avimimidae and the problem of the origin of birds." Transactions of the Joint Soviet-Mongolian Paleontological Expedition, 31: 5-92. [in Russian]
  13. ↑ Hopp, Thomas J., Orsen, Mark J. (2004) "Feathered Dragons: Studies on the Transition from Dinosaurs to Birds. Chapter 11. Dinosaur Brooding Behavior and the Origin of Flight Feathers" Currie, Koppelhaus, Shugar, Wright. Indiana University Press. Bloomington, IN. USA.
  14. ↑ https://www.pnas.org/doi/10.1073/pnas.2306639121
  15. ↑ Norell M.A.; Clark J.M.; Chiappe L.M.; Dashzeveg D. (1995). "A nesting dinosaur". Nature. 378 (6559): 774–776. Bibcode:1995Natur.378..774N. doi:10.1038/378774a0. S2CID   4245228.
  16. ↑ Meade, Luke E.; Ma, Waisum (22 February 2022). "Cranial muscle reconstructions quantify adaptation for high bite forces in Oviraptorosauria". Scientific Reports. 12 (1): 3010. Bibcode:2022NatSR..12.3010M. doi:10.1038/s41598-022-06910-4. PMC   8863891 . PMID   35194096.
  17. 1 2 Sato, Tamaki; Cheng, Yen-nien; Wu, Xiao-chun; Zelenitsky, Darla K.; Hsiao, Yu-fu (15 April 2005). "A Pair of Shelled Eggs Inside A Female Dinosaur" (PDF). Science. 308 (5720): 375. doi:10.1126/science.1110578. PMID   15831749. S2CID   19470371.
  18. ↑ Eggs of Earliest Dinosaurs Were Leathery, Fossils Show
  19. ↑ Amiot, Romain; Wang, Xu; Wang, Shuo; Lécuyer, Christophe; Mazin, Jean-Michel; Mo, Jinyou; Flandrois, Jean-Pierre; Fourel, François; Wang, Xiaolin; Xu, Xing; Zhang, Zhijun; Zhou, Zhonghe (September 2017). "δ 18 O-derived incubation temperatures of oviraptorosaur eggs". Palaeontology. 60 (5): 633–647. Bibcode:2017Palgy..60..633A. doi: 10.1111/pala.12311 .
  20. ↑ Paul, G.S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press.
  21. ↑ Paul, G.S. (1988). Predatory Dinosaurs of the World. New York: Simon & Schuster.
  22. ↑ Benton, M. J. (2004). Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd.
  23. 1 2 Turner, Alan H.; Pol, Diego; Clarke, Julia A.; Erickson, Gregory M.; Norell, Mark A. (7 September 2007). "A Basal Dromaeosaurid and Size Evolution Preceding Avian Flight". Science. 317 (5843): 1378–1381. Bibcode:2007Sci...317.1378T. doi: 10.1126/science.1144066 . PMID   17823350. S2CID   2519726.
  24. ↑ Zanno, L.E.; Gillette, D.D.; Albright, L.B.; Titus, A.L. (2009). "A new North American therizinosaurid and the role of herbivory in 'predatory' dinosaur evolution". Proceedings of the Royal Society B. 276 (1672): 3505–11. doi:10.1098/rspb.2009.1029. PMC   2817200 . PMID   19605396.
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