Chthonophis (meaning "snake beneath the earth", from the Greek chthonios ("beneath the earth") and ophis ("snake")) is an extinct genus of amphisbaenian lizard with only one known species, Chthonophis subterraneus, from the earliest Paleocene of northeastern Montana. Chthonophis was named in 2015 on the basis of a partial lower jaw from an outcrop of the Fort Union Formation in the Bug Creek Anthills. The surfaces of the bone are well-rounded, suggesting that the remains had been partially digested by another animal before the jaw had been buried and fossilized. Chthonophis is the oldest known amphisbaenian, yet phylogenetic analysis shows that it was not the most basal. Longrich et al. (2015) classified Chthonophis in its own family, Chthonophidae, finding it to be more derived than Rhineuridae but more basal than other clades such as Blanidae and Amphisbaenidae (which together make up Amphisbaeniformes). The existence of a derived amphisbaenian soon after the Cretaceous-Paleogene extinction event suggests that Amphisbaenia has its origins in the Cretaceous, although no Cretaceous amphisbaenians are currently known. Below is a cladogram from Longrich et al. (2015) showing the phylogenetic relationships of Chthonophis:^[1]

Chthonophis Temporal range: Early Paleocene
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Family:	†Chthonophidae Longrich et al., 2015
Genus:	†Chthonophis Longrich et al., 2015
Type species
†Chthonophis subterraneus Longrich et al., 2015