A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda) Author Norell, Mark A. Author Makovicky, Peter J. Author Bever, Gabe S. Author Balanoff, Amy M. Author Clark, James M. Author Barsbold, Rinchen Author Rowe, Timothy text American Museum Novitates 2009 2009-06-25 3654 1 64 http://www.bioone.org/doi/abs/10.1206/648.1 journal article 10.1206/648.1 0003-0082 5357410 Saurornithoides Osborn, 1924 TYPE SPECIES : Saurornithoides mongoliensis Osborn, 1924 , by original designation . INCLUDED SPECIES : Only the type species is currently referred to Saurornithoides . LOCALITIES AND AGE: Saurornithoides is known with certainty only from the location of the holotype of Saurornithoides mongoliensis —Djadokhta Formation of Bayan Zag, upper Cretaceous of Mongolia (fig. 2). DIAGNOSIS: Differs from Zanabazar junior in its smaller overall size, lower number of teeth (108 versus 118), the presence of a jugal that is straight beneath the round orbit rather than curved, and the possession of a pneumatic recess on the lateral surface of the prootic dorsal to the trigeminal fenestra (the pneumatic recess is also absent in Troodon formosus ). The maxillary teeth of Saurornithoides exhibit some increase in crown height posteriorly but to a lesser degree than in Zanabazar junior . Also in contrast to Zanabazar junior , the maxillary teeth of Saurornithoides lack replacement gaps. In these latter two characters, Saurornithoides agrees with Mei long , Sinovenator changii , and basal avialans among paravians, whereas Zanabazar junior is more similar to Byronosaurus jaffei and dromaeosaurs. COMMENT: The monophyly of Saurornithoides as historically recognized—including Saurornithoides mongoliensis and Saurornithoides junior —has not previously been questioned. A sister-taxon relationship between these two taxa, however, is difficult to support empirically. This may be due in part to the overall poor preservation of the holotype of Saurornithoides mongoliensis . Morphological disparity does exist between the known specimens of these two taxa and, although this list of differences is less than we might normally expect when taxonomically separating specimens at the generic level, the retention of junior in Saurornithoides implies a privileged relationship with mongoliensis that cannot currently be supported. The one polarized character previously considered to support the monophyly of mongoliensis + junior was the position of the facial foramen (CN VII) within the lateral depression of the braincase ( Turner et al., 2007b ). It now appears likely that this feature also is present in Troodon formosus and therefore is derived at a slightly more inclusive position on the troodontid tree (see below).