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Byronosaurus

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Byronosaurus
Temporal range: Late Cretaceous, 80–75 Ma
↓
Diagram showing known remains
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Troodontidae
Subfamily: †Troodontinae
Genus: †Byronosaurus
Norell, Makovicky & Clark, 2000
Species:
†B. jaffei
Binomial name
†Byronosaurus jaffei
Norell, Makovicky & Clark, 2000

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

Discovery and naming

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

Life restoration

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]

Classification

Maxillae of Byronosaurus compared with relatives

The following cladogram shows the position of Byronosaurus within Troodontidae according to a 2017 analysis by the palaeontologist Caizhi Shen and colleagues:[7]

Palaeobiology

Troodontids had some of the highest encephalization quotients (a measure of the ratio between actual brain size and the brain size predicted from body size) among non-avian dinosaurs. As suggested by their large eye-sockets and well-developed middle-ears, they appear to have had keen senses. They also had proportionately long legs, which indicates they were agile.[8][5]

Diet

Comparison of troodontid teeth; L is Byronosaurus

Due to their large brains, possible stereoscopic vision, grasping hands, and enlarged sickle-claws, troodontids were generally assumed to have been predatory. In 1998, the palaeontologist Thomas R. Holtz and colleagues pointed out that the serrations on the teeth of troodontids were different from those of typical, carnivorous theropods in their large size and wide spacing, which is similar to the condition in herbivorous dinosaurs (including therizinosaurid theropods) and lizards rather than carnivorous dinosaurs. They suggested that this difference in coarseness may be related to the size and resistance of plant and meat fibres, and that troodontids may have been herbivorous or omnivorous. They also pointed out that some features that had been interpreted as predatory adaptations in troodontids were also found in herbivorous and omnivorous animals, such as primates and raccoons.[9][5]

In 2001, the palaeontologists Philip J. Currie and Dong Zhiming rejected the idea that troodontids could have been herbivorous. They stated that troodontid anatomy was consistent with a carnivorous lifestyle, and pointed out that the structure of their serrations was not much different from those of other theropods. They noted that troodontid features such as sharply pointed serrations that curved up towards the tip of the teeth, razor sharp enamel between the serrations, and blood grooves at the bases, were not seen in herbivorous dinosaurs, which had simpler, cone shaped serrations.[10] Lü and colleagues discussed the previous studies of troodontid diet, and suggested that the loss of serrations in the teeth of Byronosaurus and some other troodontids was related to a change in their diet. Since the teeth would appear to have lost their typical ability to slice meat, at least these troodontids may therefore have been either herbivorous or omnivorous.[5] In 2015, the palaeontologist Christophe Hendrickx and colleagues suggested that basal (or "primitive") troodontids with unserrated teeth were herbivorous, whereas more derived troodontids with serrated teeth were carnivorous or omnivorous.[11]

Reproduction

Hatchling specimen 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 Djadochta 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][12] 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 was considered an enigma. Hypotheses explaining how they came to be there included 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).[13] These have all been shown to not be the case, as later a Byronosaurus nest was found two metres uphill from the oviraptorid nest, with the oviraptorid nest at the end of a drainage course from the Byronosaurus nest. The baby Byronosaurus skulls must have been washed from one nest to the other.[14] The eggs of Byronosaurus are not paired, suggesting the layer had only one egg tube, the same as in modern birds.[15] This differs from the condition in Citipati and Gigantoraptor where the eggs are laid in pairs.[16]

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. S2CID 51824767. 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. ^ a b c d 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. doi:10.1043/0098-4590(2006)069[0243:EOBSAG]2.0.CO;2 (inactive 31 July 2022). JSTOR 24321451.{{cite journal}}: CS1 maint: DOI inactive as of July 2022 (link)
  7. ^ Shen, C.; Lü, J.; Liu, S.; Kundrát, M.; Brusatte, S.L.; Gao, H. (2017). "A new troodontid dinosaur from the Lower Cretaceous Yixian Formation of Liaoning Province, China" (PDF). Acta Geologica Sinica – English Edition. 91 (3): 763–780. doi:10.1111/1755-6724.13307. hdl:20.500.11820/dc010682-fce0-4db4-bef6-7b2b29f5be8a. S2CID 129939153.
  8. ^ Makovicky, P.J.; Norell, M.A. (2004). "Troodontidae". In Weishampel, D.B.; Dodson, P.; Osmolska, H. (eds.). The Dinosauria (2 ed.). University of California Press. pp. 184–195. ISBN 978-0-520-24209-8.
  9. ^ Holtz, T.R.; Brinkman, D.L.; Chandler, C.L. (1998). "Denticle morphometrics and a possibly omnivorous feeding habit for the theropod dinosaur Troodon". GAIA. 15: 159–166. CiteSeerX 10.1.1.211.684. ISSN 0871-5424.
  10. ^ Currie, P.J.; Zhiming, D. (2001). "New information on Cretaceous troodontids (Dinosauria, Theropoda) from the People's Republic of China". Canadian Journal of Earth Sciences. 38 (12): 1753–1766. Bibcode:2001CaJES..38.1753C. doi:10.1139/cjes-38-12-1753.
  11. ^ Hendrickx, C.; Hartman, S.A.; Mateus, O. (2015). "An overview on non-avian theropod discoveries and classification". PalArch's Journal of Vertebrate Palaeontology (1): 1–73. ISSN 1567-2158. Archived from the original on 2018-06-22. Retrieved 2018-12-04.
  12. ^ 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.
  13. ^ 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. Bibcode:1994Sci...266..779N. doi:10.1126/science.266.5186.779. PMID 17730398. S2CID 22333224.
  14. ^ http://dml.cmnh.org/2011Jul/msg00114.html
  15. ^ "Byronosaurus nest - Photo: AMNH/M. Ellison".
  16. ^ "Prehistoric Beast of the Week: Byronosaurus: Beast of the Week". 16 April 2017.
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