Temporal range: Late Cretaceous, 80–75Ma
Norell, Makovicky & Clark, 2000
Byronosaurus is a genus of troodontid dinosaur which lived during the Late Cretaceous Period. It was named for Byron Jaffe, "in recognition of his family's support for the Mongolian Academy of Sciences-American Museum of Natural History Paleontological Expeditions." The first example of Byronosaurus was discovered in 1993 at Ukhaa Tolgod, Gobi Desert, Mongolia; a second was found in 1996 at Bolor's Hill, about 8 kilometers away (5 miles).
Byronosaurus is a troodontid, a group of small, bird-like, gracile maniraptorans. All troodontids have many 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. Bryonosaurus is one of few troodontids that have no serrations on its teeth, similar to its closest relative Xixiasaurus. Bryonosaurus was 1.5 m (4.9 ft) long and 50 cm (20 in) tall. It weighed only about 4 kilograms (9 lbs). 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 remains of two adult individuals have been found, including two skulls. One, measuring 23 centimeters long (8 inches), is better preserved than any other troodontid skull found to date. It has a chamber in the snout where air enters from the nostrils before passing through to the mouth - another feature similar to that found in birds. Additionally, two hatchling skulls have been identified as juvenile Byronosaurus.
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. 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).
- Junchang Lü, Li Xu, Yongqing Liu, Xingliao Zhang, Songhai Jia, and 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.
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- Mackovicky, Peter J.; Norell, Mark A. (2004). "Troodontidae". In Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 184–195. ISBN 0-520-24209-2.
- 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.
- 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.