Avimimus ( ), meaning "bird mimic" (Latin avis = bird + mimus = 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, during the Maastrichtian.
Discovery and species
thumb|left|Skeletal diagram showing some known elements of A. portentosus
The remains of Avimimus were recovered by Russian paleontologists and officially described by Dr. Sergei Kurzanov in 1981. The Avimimus fossils were initially described as having come from the Djadokta Formation by Kurzanov; however, in a 2006 description of a new specimen, Watabe and colleagues noted that Kurzanov was probably mistaken about the provenance, and it is more likely that Avimimus hailed from the more recent Nemegt Formation. The type species is A. portentosus.
A variety of isolated bones that have been attributed to Avimimus were considered to be distinct from A. portentosus, and were initially referred to as Avimimus sp. In 2008, a team of Canadian, American, and Mongolian paleontologists headed by Phil Currie reported in 2006 an extensive bonebed of Avimimus sp. fossils. The bonebed is in the Nemegt Formation, 10.5 meters above the Barun Goyot Formation, in the Gobi Desert. The team reported finding abundant bones of at least ten individuals of Avimimus, but the deposit may hold more. All individuals were either adult or subadult, and the adults showed little variation in size, suggesting determinate growth. The team also suggests that the individuals were found together because they were gregarious in life, providing possible indications that Avimimus formed age-segregated groups for either lekking or flocking purposes. The adults showed a greater degree of skeletal fusion in the tarsometatarsus and tibiotarsus, and also more prominent muscle scars. The preservation of the bonebeds suggest that they were buried rapidly, uncovered by rapid flow of water, and then buried again a short distance away. In 2018, Avimimus sp. was formally described as a new species, A. nemegtensis.
Description
thumb|left|Size of A. portentosus compared to a human
Avimimus was a small, bird-like dinosaur with a length of 1.5 m (5 ft). Kurzanov, in 1987, also reported the presence of quill knobs, and while Chiappe confirmed the presence of bumps on the ulna, their function remained unclear. Kurzanov was so convinced they were attachment points for feathers that he concluded that Avimimus may have been capable of weak flight.
Kurzanov placed Avimimus in its own family, Avimimidae, in 1981. In 1991, Sankar Chatterjee erected the Order Avimimiformes to include Avimimus. Neither of these group names is used frequently by paleontologists as they include only a single species. More recent studies have shown that Avimimus is best grouped within the Oviraptoridae, within the subgroup Elmisaurinae.
The following cladogram follows an analysis by Phil Senter, 2007.
Paleoecology
thumb|left|Excavation at a [[bonebed of A. nemegtensis reported in 2006]]
The Nemegt Formation of Mongolia, is estimated to date back to the Maastrichtian. During the Late Cretaceous period, the land that is now the Nemegt Formation had a continental semi-arid climate, with average annual temperatures estimated at 7-8 Degrees C annually. It is interpreted as having wet summers and dry winters, with summer rainfall sourced from the East Asian monsoon.
thumb|Skull diagram of A. nemegtensis
The Nemegt formation is characterized by a floodplain ecosystem, considered similar to the Okavango Delta today. In the Nemegt it coexisted with other dinosaurs like, the tyrannosaurid Tarbosaurus, hadrosaurid Saurolophus, and a variety of other dinosaurs.
Both species of Avimimus come from different layers in the Nemegt formation. A. portentosus is known from the Bugiin Tsav and Shar Tsav localities, correlated to the Middle & Upper Nemegt formation. A. nemegtensis comes from the Nemegt locality, correlated with the Lower Nemegt formation.