S. jiashanensisZhou et al. 1995[1] Sinonyx ("Chinese claw") is a genus of extinct, superficially wolf-like mesonychid mammals from the late Paleocene of China (about 56 million years ago).

[2] Sinonyx was about 1.5 m (5 ft) long, about the size of a modern grey wolf with a large elongated head, short legs, digitigrade feet adapted for running, and tiny hooves on all of its toes.

[6] Some studies have found Andrewsarchus, once considered a mesonychid, to form the sister group to the clade composed of Cetacea and Hippopotamidae along with Entelodontidae.

[7] Mesonychidae as defined by Szaly and Gould (1966) is probably paraphyletic or polyphyletic, with Hapalodectinae and Andrewsarchinae (represented by the lone skull of Andrewsarchus) wrongly assigned to the group.

Sinonyx is no longer considered ancestral to Cetacea,[7] but has convergent features to early land-dwelling whales that suggest it may have had some similar adaptations.