Abstract
The theropod dinosaur Aniksosaurus darwini gen. et sp. nov. has been recovered from the Upper Cretaceous, Bajo Barreal Formation, of Central Patagonia. Aniksosaurus darwini gen. et sp. nov. was a small tetanurine, approximately 2 meters long. Aniksosaurus exhibits several unique traits (e.g., cranial cervical vertebrae with dorsoventrally deep neural arches, provided with a pair of cavities at their cranial surfaces; neural canal wide; cranial caudals with ventral sagittal keel, and transverse processes triangular-shaped in dorsal view; manual ungual phalanges robust; ilium with extremely expanded brevis shelf; femur with deep notch for M. Iliotrochantericus; metatarsal and digit IV of pes transversely narrow). Available postcranial bones of Aniksosaurus exhibit derived features of Coelurosauria (e.g., ilium with well developed cuppedicus fossa; femur with anterior trochanter proximally projected, almost reaching the level of the articular head; greater trochanter craniocaudally expanded; femoral head rectangular-shaped in cranial aspect; and fibular shaft craniocaudally narrow), as well as characteristics suggesting that the new Patagonian taxon is more derived than some basal coelurosaurians such as compsognathids, Ornitholestes, and coelurids. Comparisons with maniraptoriforms (a clade including Ornithomimosauria, Tyrannosauridae, Oviraptorosauria, Alvarezsauridae and Paraves) support that Aniksosaurus is less derived than these theropods. In sum, Aniksosaurus is here considered as a Late Cretaceous survivor of a basal coelurosaurian radiation.
Highlights
The theropod dinosaur Aniksosaurus darwini gen. et sp. nov. has been recovered from the Upper Cretaceous, Bajo Barreal Formation, of Central Patagonia
They are members of a variety of lineages including, among the most conspicuous, the compsognathid Mirischia asymmetrica (Naish et al, 2004), and the maniraptoriform Santanaraptor placidus (Kellner, 1999, 2001), both from the ?Albian Santana Formation of NE Brazil, the alvarezsaurids Alvarezsaurus calvoi and Patagonykus puertai (Bonaparte, 1991; Novas, 1996, 1997), possible oviraptorosaurs from NW Argentina (Frankfurt & Chiappe, 1999), derived maniraptorans of unknown affiliation from the Maastrichtian Marilia Formation (Novas et al, 2005), and the dromaeosaurids Neuquenraptor argentinus (Novas & Pol, 2005), Unenlagia comahuensis (Novas & Puerta, 1997; Novas, 2003), Buitreraptor gonzalezorum (Makovicky et al, 2005), and possibly Unquillosaurus ceibali (Novas & Agnolín, 2004). We add to this list of coelurosaurians a new predatory dinosaur, Aniksosaurus darwini gen. et sp. nov., from the Cenomanian Bajo Barreal Formation of central Patagonia (Fig. 1)
The recovered bones includes five tibiae of the right side, indicating that the fossil assemblage is made up, at least, by five different specimens. We interpret that they belong to a single species for the following reasons: 1) all the bones correspond to small animals of approximately the same size; 2) at least some bones were preserved in articulation; 3) duplicate hindlimb elements are anatomically identical to the holotype; 4) available bones exhibit theropod, tetanurine and coelurosaurian synapomorphies; and, 5) there is no evidence of any other dinosaurian taxon preserved in the quarry
Summary
The theropod dinosaur Aniksosaurus darwini gen. et sp. nov. has been recovered from the Upper Cretaceous, Bajo Barreal Formation, of Central Patagonia. They are members of a variety of lineages including, among the most conspicuous, the compsognathid Mirischia asymmetrica (Naish et al, 2004), and the maniraptoriform Santanaraptor placidus (Kellner, 1999, 2001), both from the ?Albian Santana Formation of NE Brazil, the alvarezsaurids Alvarezsaurus calvoi and Patagonykus puertai (Bonaparte, 1991; Novas, 1996, 1997), possible oviraptorosaurs from NW Argentina (Frankfurt & Chiappe, 1999), derived maniraptorans of unknown affiliation from the Maastrichtian Marilia Formation (Novas et al, 2005), and the dromaeosaurids Neuquenraptor argentinus (Novas & Pol, 2005), Unenlagia comahuensis (Novas & Puerta, 1997; Novas, 2003), Buitreraptor gonzalezorum (Makovicky et al, 2005), and possibly Unquillosaurus ceibali (Novas & Agnolín, 2004). Comparative Zoology, Cambridge; MDT-PV, Museo Desiderio Torres -Paleovertebrados, Sarmiento, Chubut, Argentina; MOR, Museum of the Rockies, Bozeman; NMC, Canadian Museum of Nature, Ottawa; YPM, Yale Peabody Museum, New Haven
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