Rhineuridae is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 ...

ns (commonly called worm lizards) that includes one living

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 ...

and

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, ''

Rhineura floridana ''Rhineura floridana'', known commonly as the Florida worm lizard, graveyard snake, or thunderworm, is a species of amphisbaenian in the family Rhineuridae. The species is the only extant member of the genus ''Rhineura'', and is found primarily ...

'', as well as many extinct species belonging to both ''Rhineura'' and several extinct genera. The living ''R. floridana'' is found only in

Georgia Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...

and

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

, but extinct species ranged across North America, some occurring as far west as Oregon. The family has a fossil record stretching back 60 million years to the

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), ...

and was most diverse in the continental interior during the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

and

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

Fossil record

The fossil record of the Rhineuridae extends back almost to the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, with the oldest rhineurid, '' Plesiorhineura tsentasai'', occurring in the Early Paleocene. ''Plesiorhineura'' is only known from a partial jaw, but it shares many features with modern rhineurids. Eocene rhineurids, such as ''

Spathorhynchus fossorium ''Spathorhynchus'' is an extinct genus of amphisbaenians or worm lizards that existed from the Middle Eocene to the Early Oligocene in what is now Wyoming. It includes two species, the type species ''S. fossorium'', named in 1973 from the Middle ...

'', are remarkably similar to the modern ''Rhineura'', suggesting very conservative evolution within the family. Unlike the

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 include round-headed, keeled, and shovel-snouted forms, the fossil rhineurids are exclusively of the shovel-snouted variety. The fossil rhineurids are known exclusively from North America, but show that the group once had a much wider distribution than the current range of ''R. floridana'', with species known from Colorado, Wyoming, South Dakota, Montana, and as far west as Oregon.Berman, David S. "A new amphisbaenian (Reptilia: Amphisbaenia) from the Oligocene-Miocene John Day formation, Oregon." Journal of Paleontology (1976): 165-174. The wide distribution probably reflects the fact that the climate was much warmer in the past. The initial rhineurid radiation in North America occurred when most of the continent was humid and subtropical. Rhineurids persisted through a significant climatic transition around the time of the Eocene-Oligocene boundary in which most of North America became semi-arid and covered by

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

s. The decline of rhineurids began with an abrupt period of cooling called

Middle Miocene disruption The Middle Miocene Climatic Transition (MMCT) was a relatively steady period of climatic cooling that occurred around the middle of the Miocene, roughly 14 million years ago (Ma), during the Langhian stage, and resulted in the growth of ice sheet ...

. The distribution range of rhineurids shrank southward as global temperatures continued to fall, leaving ''R. floridana'' isolated in Florida as the only living representative of the group. Macrorhineura skinneri

In 2007, paleontologist Daniel Hembree reviewed all North American fossil amphisbaenians and found every species to belong to Rhineuridae. He also showed that many extinct rhineurids cannot be justified as their own species and are instead

synonyms A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

of earlier named species, reducing the number of valid species from twenty-two to nine. For example, ''Jepsibaena minor'', ''Rhineura amblyceps'', ''R. attenuatus'' ''R. minutus'', and ''R. wilsoni'' are sometimes considered synonyms of '' Protorhineura hatchetri''. Since the late nineteenth century, paleontologists had been naming new species of extinct North American amphisbaenians on the basis of single fragmentary specimens that often preserved too few anatomical features to support their status as distinct species. By synonymizing many of these species, Hembree showed that rhineurid diversity in North America was lower than once thought.

Relationships

Hembree's 2007 review of North American amphisbaenians also included a

phylogenetic analysis 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 data ...

of rhineurids and other amphisbaenian lizards. The analysis found that ''Rhineura floridana'' is the most basal species within the family, meaning its lineage was the first to diverge from the group. However, the species itself is quite young, with the oldest fossils coming from the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

of Florida. The ancestor of ''R. floridana'' likely migrated into what is now Florida from the continental interior, while the remaining rhineurids remained there to radiate and diversify.

References

{{Taxonbar, from=Q13416851 Amphisbaenians Reptiles of the United States Natural history of Florida Lizard families Taxa named by Paulo Vanzolini Extant Paleocene first appearances