Avimimus nemegtensis, meaning “bird mimic”, was a very birdy oviraptorosaur and the type species of its own family Avimimidae, though some paleontologists put it within Caenognathidae. It had a long neck with a small head, though its braincase and eyes were comparatively large. Its legs were long and slender, and it was likely a fast runner. There are at least two species of Avimimus, A. portentosus (described in 1981) and A. nemegtensis (described in 2018 and depicted here). The tail was missing on the original Avimimus portentosus fossil, leading its describer to conclude that it must have lacked a tail in life! However, later specimens with caudal vertebrae and hips designed for carrying a long tail suggested otherwise. Due to its small to nonexistent teeth, it’s likely Avimimus was a herbivore and/or insectivore. Avimimus nemegtensis specimens have been found in a bonebed containing at least 10 individual adults, suggesting they may have flocked together or at least formed leks.
Once thought to have come from the Djadochta Formation, it is now considered more likely that both Avimimus species lived in the more recent Nemegt Formation. While currently in the middle of the Gobi Desert, the Nemegt Formation of Late Cretaceous Mongolia was once more lush and green, with forests of tall conifer trees, streams, and shallow lakes covered in duckweed and pondweed. Rivers carved out steep cliffs and canyons. This habitat supported a diverse amount of fauna, and Avimimus nemegtensis would have lived alongside a variety of dinosaurs, including true birds. While it was seemingly the only avimimid here (aside from A. portentosus), it lived alongside both oviraptorids like Nemegtomaia and caenognathids like Elmisaurus. Another “bird-mimic”, the speedy ornithomimosaur Gallimimus lived in the Nemegt as well. It would seem speed was necessary for these animals, as the tyrannosaur Tarbosaurus would have been the apex predator. If it could catch it, Avimimus would have been an easier meal for Tarbosaurus than the giant-clawed Therizinosaurus, the hadrosaurs Barsboldia and Saurolophus angustirostris, and the titanosaur Nemegtosaurus. But there was still a wealth of smaller prey Tarbosauruses both young and old could have pursued, such as the little pachycephalosaurs Homalocephale and Prenocephale, the troodontid Tochisaurus, and even the smaller tyrannosauroid Bagaraatan. Avimimus would have had to use its quick wits and quicker legs to stay alive.
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