Rhynchonkos
Encyclopedia
Rhynchonkos is an extinct genus
of microsaur. It is the only member of the family
Goniorhynchidae. Originally known as Goniorhynchus, it was renamed in 1981. The type
and only species
is R. stovalli, found from the Early Permian Hennesey Formation in Cleveland County
, Oklahoma
. Rhynchonkos shares many similarities with Eocaecilia
, an early caecilian
from the Early Jurassic
of Arizona
. The close similarity between Rhynchonkos and Eocaecilia may be an indication that caecilians are descendants of microsaurs. However, such a relationship is no longer widely accepted.
of Rhynchonkos lacks ribs. Both Rhynchonkos and Euryodus
have atlases that bear a strong resemblance to those of nectridea
ns. Like nectrideans, the arch of the atlas is attached to the centrum, although this is likely the result of convergence
.
The skull is triangular in dorsal view. The limbs are very small. It has a pointed, overhanging snout that extends beyond the tooth row. There are five or six premaxilla
ry teeth and sixteen maxilla
ry teeth, all of which are narrow and peg-shaped. The palate
also has teeth, with rows on the ectopterygoid, palatine
, and vomer
. The bones of the skull roof
are similar to those of gymnarthrids
. There are two rows of teeth adjacent to one another in the lower jaw, with a marginal row on the dentary and an inner row on the coronoid
.
n moth
named in 1896. Because of this preoccupation, it was renamed Rhynchonkos by Schultze and Foreman (1981). Rhynchonkos was first described as a gymnarthrid, but has since been placed in its own family, Goniorhynchidae. Goniorhynchidae was named before Rhynchonkos was given as a replacement name for the genus, and the name Rhynchonkidae has been used as a replacement name for the family in some studies.
Rhynchonkos shares many features with the early caecilian Eocaecilia, including an elongated trunk, small limbs, and a similar skull. Based on these features, caecilians seemed to have originated from Rhynchonkos or another closely related microsaur. Carroll and Currie (1975), the first to suggest this possible relationship, noted similarities in temporal fenestration, palatal structure, braincase composition, and mandibular dentition. In the temporal region of the skulls of Rhynchonkos and caecilians, the number of bones are reduced. Both Rhynchonkos and caecilians possess a primitive combination of palatal bones, including the ectopterygoid. The two taxa also have rows of teeth on the palate in addition to the marginal rows on the maxilla and premaxilla. Carrol and Currie also mentioned that Rhynchonkos and caecilians have a pleurosphenoid that joins the otic-occipital portion of the braincase with the sphenethmoid, a characteristic which they considered unique among amphibians. The adjacent tooth rows on the coronoid and the dentary of Rhynchonkos were also considered a characteristic that linked it with caecilians.
Despite these similarities, many of the characteristics that suggested a close relationship between Rhynchonkos and caecilians have since been considered primitive, convergent, or indeterminate. For example, tooth rows on the palate have evolved multiple times independently in early amphibians. The general appearance of Rhynchonkos is similar to that of caecilians, but is also similar to other amphibians that have independently developed elongated bodies. Adelospondyl
s, lysorophia
ns, aïstopods, and some salamander
s all have an increased number of vertebrae that lengthen the body. Reduced limbs are seen in many aquatic or burrowing amphibians, and are not unique to microsaurs and caecilians.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of microsaur. It is the only member of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Goniorhynchidae. Originally known as Goniorhynchus, it was renamed in 1981. The type
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
and only species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
is R. stovalli, found from the Early Permian Hennesey Formation in Cleveland County
Cleveland County, Oklahoma
Cleveland County is a county in the central part of the U.S. state of Oklahoma. The population was 255,755 at the 2010 census. Its county seat is Norman. Cleveland County is part of the Oklahoma City Metropolitan Statistical Area.-History:...
, Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
. Rhynchonkos shares many similarities with Eocaecilia
Eocaecilia
Eocaecilia micropodia is an extinct species of caecilian from the Early Jurassic Period in Arizona, USA. It shared some characteristics with salamanders and the now extinct Microsauria...
, an early caecilian
Caecilian
The caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade Apoda. They are mostly...
from the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
of Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
. The close similarity between Rhynchonkos and Eocaecilia may be an indication that caecilians are descendants of microsaurs. However, such a relationship is no longer widely accepted.
Description
Rhynchonkos has an elongated body with at least 37 presacral vertebrae. Most vertebrae have ribs. Unlike other microsaurs, the atlasAtlas (anatomy)
In anatomy, the atlas is the most superior cervical vertebra of the spine.It is named for the Atlas of Greek mythology, because it supports the globe of the head....
of Rhynchonkos lacks ribs. Both Rhynchonkos and Euryodus
Euryodus
Euryodus is an extinct genus of microsaur within the family Gymnarthridae.-See also:* Prehistoric amphibian* List of prehistoric amphibians...
have atlases that bear a strong resemblance to those of nectridea
Nectridea
Nectridea is an extinct order of lepospondyl amphibians from the Carboniferous and Permian periods, which included animals such as Diplocaulus. In appearance, they would have resembled modern newts or aquatic salamanders. They had long flattened tails to aid in swimming, and well-developed hind...
ns. Like nectrideans, the arch of the atlas is attached to the centrum, although this is likely the result of convergence
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
The skull is triangular in dorsal view. The limbs are very small. It has a pointed, overhanging snout that extends beyond the tooth row. There are five or six premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
ry teeth and sixteen maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
ry teeth, all of which are narrow and peg-shaped. The palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
also has teeth, with rows on the ectopterygoid, palatine
Palatine bone
The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
, and vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
. The bones of the skull roof
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
are similar to those of gymnarthrids
Gymnarthridae
Gymnarthridae is an extinct family of tuditanomorph microsaurs. Gymnarthrids are known from Europe and North America and existed from the Late Carboniferous through the Early Permian. Remains have been found from the Czech Republic, Nova Scotia, Illinois, Texas, and Oklahoma.Gymnarthrids are...
. There are two rows of teeth adjacent to one another in the lower jaw, with a marginal row on the dentary and an inner row on the coronoid
Coronoid process of the mandible
The mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size....
.
Classification
Rhynchonkos was first described as Goniorhynchus by Olson (1970). However, the name Goniorhynchus was preoccupied by an IndiaIndia
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
n moth
Moth
A moth is an insect closely related to the butterfly, both being of the order Lepidoptera. Moths form the majority of this order; there are thought to be 150,000 to 250,000 different species of moth , with thousands of species yet to be described...
named in 1896. Because of this preoccupation, it was renamed Rhynchonkos by Schultze and Foreman (1981). Rhynchonkos was first described as a gymnarthrid, but has since been placed in its own family, Goniorhynchidae. Goniorhynchidae was named before Rhynchonkos was given as a replacement name for the genus, and the name Rhynchonkidae has been used as a replacement name for the family in some studies.
Relationship to caecilians
Despite these similarities, many of the characteristics that suggested a close relationship between Rhynchonkos and caecilians have since been considered primitive, convergent, or indeterminate. For example, tooth rows on the palate have evolved multiple times independently in early amphibians. The general appearance of Rhynchonkos is similar to that of caecilians, but is also similar to other amphibians that have independently developed elongated bodies. Adelospondyl
Adelospondyli
Adelospondyli are an order of elongate, presumably aquatic, Carboniferous amphibians. The skull is solidly roofed, and elongate, with the orbits located very far forward. The limbs are well developed. There is a single family, the Adelogyrinidae...
s, lysorophia
Lysorophia
Lysorophia is an order of aquatic Carboniferous and Permian amphibians within the extinct subclass Lepospondyli. Lysorophians resembled small snakes, as their bodies are extremely elongate. There is a single family, the Lysorophidae...
ns, aïstopods, and some salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...
s all have an increased number of vertebrae that lengthen the body. Reduced limbs are seen in many aquatic or burrowing amphibians, and are not unique to microsaurs and caecilians.
See also
- Prehistoric amphibian
- List of prehistoric amphibians