''Chthonophis'' (meaning "snake beneath the earth", from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''chthonios'' ("beneath the earth") and ''ophis'' ("snake")) is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

amphisbaenia Amphisbaenia (called amphisbaenians or worm lizards) is a group of typically legless lizards, comprising over 200 extant species. Amphisbaenians are characterized by their long bodies, the reduction or loss of the limbs, and rudimentary eyes. A ...

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

with only one known species, ''Chthonophis subterraneus'', from the earliest

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

of northeastern Montana. ''Chthonophis'' was named in 2015 on the basis of a partial lower jaw from an outcrop of the

Fort Union Formation The Fort Union Formation is a geologic unit containing sandstones, shales, and coal beds in Wyoming, Montana, and parts of adjacent states. In the Powder River Basin, it contains important economic deposits of coal, uranium, and coalbed methane. ...

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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 Rhineuridae is a family of amphisbaenians (commonly called worm lizards) that includes one living genus and species, '' Rhineura floridana'', as well as many extinct species belonging to both ''Rhineura'' and several extinct genera. The living '' ...

but more basal than other

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s such as

Blanidae ''Blanus'', also known as ''worm lizards'', are a genus of amphisbaenians found in the Mediterranean region of Europe and North Africa. Like other amphisbaenians, ''Blanus'' species are specialized for a subterranean existence, with long, slend ...

and

Amphisbaenidae The Amphisbaenidae (common name: worm lizards) are a Family (biology), family of amphisbaenians, a group of limbless vertebrates. There are 12 genera, containing 183 species. Geographic range Amphisbaenids occur in South America, some Caribbean ...

(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 The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

, although no Cretaceous amphisbaenians are currently known. Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from Longrich ''et al.'' (2015) showing the phylogenetic relationships of ''Chthonophis'':

References

Amphisbaenia Paleocene lepidosaurs Paleocene reptiles of North America Fossil taxa described in 2015 Paleontology in Montana {{paleo-lizard-stub