A small alvarezsaurid from the eastern Gobi Desert offers insight into evolutionary patterns in the Alvarezsauroidea

ABSTRACT

A partial postcranial skeleton of a small alvarezsaurid from the Late Cretaceous of the Mongolian eastern Gobi Desert locality of Khugenetslavkant represents the first reported articulated theropod material from that locality. The specimen is recognized as the holotype of a new taxon herein named Albinykus baatar, gen. et sp. nov. Phylogenetic analysis places Albinykus within Alvarezsauridae as the sister taxon of Shuvuuia, another Late Cretaceous Mongolian taxon from the slightly younger Djadokhta Formation. The complete coossification of the proximal tarsals with the tibia, and of the distal tarsals and proximal metatarsals, present in Albinykus are previously unknown in Alvarezsauridae. Extensive fusion is consistent with histological data from the tibia indicating that the individual was a subadult. These results are striking given that Albinykus is among the smallest known non-avian dinosaurs with a body mass no greater than 1 kg and ranks among the smallest known alvarezsaurids. Alvarezsauridae shows a decreasing size trend throughout its evolutionary history, a rarity among dinosaurian clades. Within maniraptoran dinosaurs, such a trend has thus far only been recognized within Avialae and at the origin of Paraves with respect to other coelurosaurs. The holotype was recovered articulated in a ‘seated’ position, with hind limbs aligned and the feet tucked under the body. This body posture, which is present in Aves, has been previously noted in other maniraptoran clades (i.e., Oviraptoridae Troodontidae) and is now recognized in Alvarezsauridae.

ACKNOWLEDGMENTS

We thank J. Choiniere and J. Hutchinson for discussion. P. Brinkman, North Carolina Museum of Natural Science, and M. Brown, The University of Texas at Austin, skillfully prepared the specimen. S. Werning expertly prepared the histological section and North Carolina State University undergraduate Z. Boles assisted in study of the specimen. The holotype was collected thanks to the work of the 2004 field crew of the Mongolian Academy of Sciences/American Museum of Natural History Joint Paleontological Expedition. We thank Fernando Novas and an anonymous reviewer for improving the paper. The phylogenetic analysis was conducted using TNT, a program made freely available thanks to a subsidy by the Willi Hennig Society. This project was funded by the American Museum of Natural History, the National Science Foundation (ATOL 0228693), and The University of Texas at Austin. Specimen preparation was also supported by the North Carolina Museum of Natural Science and North Carolina State University.

Handling editor: Hans-Dieter Sues