[D] Albinykus baatar [~/~]
Nesbitt, Clarke, Turner & Norell 2011
Cretaceous Late Santonian Campanian
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Alvarezsauridae
Khugenetslavkant sandstone, Gobi Desert, Mongolia
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 Ukhaa Tolgod [Djadokhta Formation].
Other dinosaurian taxa from Khugenetslavkant include the ceratopsian Yamaceratops dorngobiensis (Makovicky and Norell, 2006), an as yet undescribed hypsilophodontid (Eberth et al., 2009), and possible ornithomimid (Khand et al., 2000) dinosaurs as well as undescribed non-avian theropod fragments (Eberth et al., 2009). Two new taxa of eutherian mammals were also recently reported from the locality (Giallombardo and Novacek, 2006; Giallombardo, 2007).
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.
Holotype: IGM 100/3004 (field number: MAE PSS 04–18). Articulated hind limbs consisting of portions of the right ilium, partial ischium, fragmentary right femur, nearly complete right tibia and nearly complete left, proximal portion of left and right fibulae, complete right and left pedes.
Locality and horizon: The age of the formation has most recently been considered ?Santonian–Campanian (Eberth et al., 2009). The range of possible ages for the formation does not allow precise chronostratigraphic relationships with other famous Asian Cretaceous deposits (Eberth et al., 2009). However, the exposed deposits have been considered older than those of Djadokhta localities such as Ukhaa Tolgod in the western Gobi Desert (Dingus et al., 2008).
Etymology: Albin, referring to ‘wandering lights’ as used by Mongolian Shamans to describe light phenomena in the Gobi Desert (see Heissig, 2004), and onyx, Greek for claw; and baatar, Mongolian for hero.
Diagnosis: Albinykus is placed within Alvarezsauridae based on the derived presence of the following characters (see Phylogenetic Results): an extreme arctometatarsalian condition with metatarsal III abbreviated and excluded from the proximal surface (character 203(3) of Turner et al., 2007), and the anterior surface of metatarsal III concave (character 69(1) of Longrich and Currie, 2009).
We use the stem definition of Alvarezsauroidea (Bonaparte, 1991) and the node defintion of Alvarezsauridae (Bonaparte, 1991) following Choiniere et al. (2010). The combination of the following unique character state combination within the clade further differentiates the new taxon: a short metatarsal I with a rounded proximal tip (unknown in Alvarezsaurus and Patagonykus); well-pronounced and knob-like crest for attachment of the M. iliofibularis on the fibula proportionally larger than other alvarezsaurids; phalanx IV-4 longer than both IV-2 and IV-3; and the presence of a deep groove on the anterior face of the ascending process of the astragalus.
A small flange on the lateral side of the distal end of metatarsal IV is shared exclusively with Parvicursor, Shuvuuia, and Mononykus within Alvarezsauridae. Albinykus differs from all other alvarezsaurids by the presence of complete fusion between the tibia and proximal tarsals but because little is known about alvarezsaurid ontogeny, we elect to exclude this character state from the main body of fragments and small pieces of dinosaur egg shell and isolated juvenile Yamaceratops elements.