Hesperonychus

Description

The dinosaur Hesperonychus is known primarily from a single partial pelvis, catalogued as UALVP 48778, which Dr. Elizabeth Nicholls collected in Dinosaur Park in 1982. This fossil was not formally studied until Nick Longrich and Phil Currie described it in 2009. Several tiny toe bones, including sickle claws, from the Dinosaur Park and Oldman Formations have been tentatively assigned to this genus. These slender bones suggest Hesperonychus was not a member of the Eudromaeosauria group. Although the bones are small, the fused pubic bones show the animal was an adult, not a juvenile of a larger species. Based on the partial remains, researchers estimate Hesperonychus was about one meter long and weighed between 1.5 and 2 kilograms, making it one of the smallest non-avian dinosaurs known from North America.

Classification

Longrich and Currie's phylogenetic work originally placed Hesperonychus within Microraptorinae, a group of small dromaeosaurids that had previously only been known from the Early Cretaceous of Asia. They called this discovery "remarkable" because the youngest known microraptorine before this was Microraptor from the Aptian stage of the Early Cretaceous. Finding Hesperonychus in the Campanian stage of the Late Cretaceous extended the group's known fossil record by 45 million years. Bambiraptor, a North American species from the Late Cretaceous, had sometimes been grouped with microraptorines, but Longrich and Currie, along with other researchers, found it to be more closely related to Saurornitholestes. Hesperonychus was placed in Microraptoria based on several features: a spatulate, or rounded, pubic symphysis, a strong backward curve in the lower part of the pubis, and lateral tubercles on the pubes that in Hesperonychus expanded into wing-like structures.

Later studies have challenged this classification. Some researchers have excluded Hesperonychus from phylogenetic analyses because its remains are too fragmentary, while others have placed it in various positions either within or outside dromaeosaurids. In 2012, Martyniuk considered it a eudromaeosaurian. In 2013, microraptorines were thought to be averaptorans outside dromaeosaurids. In 2014, Brusatte and colleagues removed five "wildcard" taxa, including the paravians Pyroraptor and Hesperonychus, from their analyses due to a severe lack of data, concluding that their classifications remain uncertain. Other studies have followed this exclusion. In 2019, Hartman and colleagues suggested Hesperonychus might actually be an avialan close to modern birds like Balaur bondoc, though they disagreed that microraptorines themselves were avialans. That same year, Rauhut and colleagues listed Hesperonychus among various theropod genera as a "problematic taxon" because of its unstable phylogenetic position. In 2020, Hesperonychus was recovered as a dromaeosaurid and a sister taxon to the polytomic microraptorines, but not a member of that group.