Hesperonychus, meaning “west claw,” is a genus of small dromaeosaurid dinosaur from the Campanian stage of the Late Cretaceous Period (75 million years ago). There is one described species, H. elizabethae, which was named in honor of Dr. Elizabeth Nicholls who discovered it in 1982. It was found in Dinosaur Park Formation in Alberta.
Hesperonychus is known from a partial pelvic girdle. The remains were described by Nick Longrich and Phil Currie in 2009. A collection of finger, toe bones, and claws in the collection of the Royal Tyrrell Museum may also belong to Hesperonychus.
Longrich and Currie’s phylogenetic analysis of the partial remains concluded that Hesperonychus is a member of the Microraptorinae, a clade (grouping) of small dromaeosaurids previously thought to be restricted to the Early Cretaceous of Asia. The previously youngest known Microraptorinae was Microraptor itself from the Aptian age of the Early Cretaceous. The findings pushed the fossil range forward by 45 million years.
Though known only from partial remains, it is estimated that the total length of Hesperonychus is less than 42 inches and weighed about 4 pounds. This suggests that it would have been the smallest carnivorous dinosaur found in North America.
Microraptorines are well-known for their small size and, in some species, the ability to fly or glide. It is unlikely for Hesperonychus to exhibit four wings or gliding behavior, though, and Longrich and Currie speculated that Hesperonychus was more similar to Sinornithosaurus given their close similarity in size. However, Microraptorines did not vary much in size, remaining small relative to other dromaeosaurids throughout their history.