Thaumastocyoninae is an extinct subfamily of amphicyonids, large terrestrial carnivores, which inhabited what is now Europe during the Miocene epoch.

The subfamily was erected by Hürzeler (1940), and is defined by the complete suppression of m1 metaconid, reduction of the premolars, except the p4, which is reinforced, and the oblique abrasion of the teeth, and the possession of hypercarnivorous tendencies.

[3] The first thaumastocyonines appear during the earliest Miocene epoch in the form of Crassidia intermedia, which was already the largest predator in its habitat.

One possible explanation are the effects of the Vallesian Crisis, a major local extinction event as a result of the reduction of forests in favor of open habitats, which reshaped Europe's fauna.

[3] Below is a cladogram based on cranial, mandibular and dental characters, after Morales et al., 2021,[3] showing the phylogenetic relationships between the species of the subfamily: Pseudocyonopsis landesquei Daphoenodon superbus Cynelos lemanensis "Ysengrinia" americana Crassidia intermedia Ysengrinia gerandia Peignecyon felinoides Tomocyon grivensis "Ysengrinia" valentiana Agnotherium antiquum Ammitocyon kainos Thaumastocyon bourgeoisi Thaumastocyon dirus T. dirus Y. depereti Y. tolosana Y. valentiana