Sinusonasus [SEP] parent taxon
ischium is elongated. "Sinusonasus" has a long lower leg, indicating a good running capacity. The foot is "arctometatarsal", with a 'pinched' upper third metatarsal. The second metatarsal is distinctly shorter than the fourth. The second toe bears a retractable 'sickle claw'.
"Sinusonasus" was in 2004 placed in the Troodontidae. It was presumed to have had a rather derived position, despite living in the Early Cretaceous. This was by the describing authors not interpreted as an indication for a long
In 2019, the palaeontologist Scott Hartman and colleagues recovered "Xixiasaurus" as the sister taxon of "Sinusonasus", in a clade with "Daliansaurus" and "Hesperornithoides" (sharing features such as a straight ulna and having an upwards projected curve on the claw of the first finger).
Troodontids have mainly been discovered in the northern hemisphere, largely restricted to Asia and North America. They appear to have reached their greatest diversity during the Campanian stage of the Late Cretaceous in Asia. Troodontids probably originated in Asia,