# Rhynchonkos

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***Rhynchonkos*** is an extinct [genus](/source/Genus) of [rhynchonkid](/source/Rhynchonkidae) [microsaur](/source/Microsaur). Originally known as *Goniorhynchus*, it was renamed in 1981 because the name had already been given to another genus;[1] the family, likewise, was originally named Goniorhynchidae but renamed in 1988.[2] The [type](/source/Type_species) and only known [species](/source/Species) is *R. stovalli*, found from the [Early Permian](/source/Early_Permian) [Fairmont Shale](/source/Fairmont_Shale) in [Cleveland County](/source/Cleveland_County,_Oklahoma), [Oklahoma](/source/Oklahoma).[3][4] *Rhynchonkos* shares many similarities with *[Eocaecilia](/source/Eocaecilia)*, an early [caecilian](/source/Caecilian) from the [Early Jurassic](/source/Early_Jurassic) of [Arizona](/source/Arizona). Similarities between *Rhynchonkos* and *Eocaecilia* have been taken as evidence that caecilians are descendants of microsaurs.[5] However, such a relationship is no longer widely accepted.[6][7]

## Description

*Rhynchonkos* has an elongated body with at least 37 presacral [vertebrae](/source/Vertebra). Most vertebrae have ribs. Unlike other microsaurs, the [atlas](/source/Atlas_(anatomy)) of *Rhynchonkos* lacks ribs. Both *Rhynchonkos* and *[Euryodus](/source/Euryodus)* have atlases that bear a strong resemblance to those of [nectrideans](/source/Nectridea). Like nectrideans, the arch of the atlas is attached to the [centrum](/source/Vertebra#Structure), although this is likely the result of [convergence](/source/Convergent_evolution).[8]

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 [premaxillary](/source/Premaxilla) teeth and sixteen [maxillary](/source/Maxilla) teeth, all of which are narrow and peg-shaped. The [palate](/source/Palate) also has teeth, with rows on the [ectopterygoid](/source/Ectopterygoid), [palatine](/source/Palatine_bone), and [vomer](/source/Vomer). The bones of the [skull roof](/source/Skull_roof) are similar to those of [gymnarthrids](/source/Gymnarthridae). There are two rows of teeth adjacent to one another in the lower jaw, with a marginal row on the [dentary](/source/Dentary) and an inner row on the [coronoid](/source/Coronoid_process_of_the_mandible).[3]

## Classification

*Rhynchonkos* was first described as *Goniorhynchus* by Olson (1970).[9] However, the name *[Goniorhynchus](/source/Goniorhynchus)* was preoccupied by an Indian [moth](/source/Moth) named in 1896.[10] Because of this preoccupation, it was renamed *Rhynchonkos* by Schultze and Foreman (1981).[1] *Rhynchonkos* was first described as a gymnarthrid, but was soon placed in its own family, which was at first called Goniorhynchidae.[3] However, Goniorhynchidae was named before *Rhynchonkos* was given as a replacement name for the genus. Citing what is currently [Article 39](http://www.nhm.ac.uk/hosted-sites/iczn/code/includes/page.jsp?article=39&nfv=) of the [International Code of Zoological Nomenclature](/source/International_Code_of_Zoological_Nomenclature), Zanon (1988)[2] pointed out that if any family is to bear the name Goniorhynchidae, it must be based on the valid genus *Goniorhynchus* Hampson, 1896, not on the invalid *Goniorhynchus* Olson, 1970; he therefore coined the replacement name Rhynchonkidae, which has been used in some works.[11][12]

### Relationship to caecilians

*Rhynchonkos* shares many features with the early caecilian *Eocaecilia*, including an elongated snout, small limbs, and a similar skull. Based on these features, it has been suggested that caecilians originated from *Rhynchonkos* or another closely related microsaur.[5] Carroll and Currie (1975), the first to suggest this possible relationship, noted similarities in temporal fenestration, palatal structure, braincase composition, and mandibular dentition.[13] In the temporal region of the skulls of *Rhynchonkos* and caecilians, the number of bones is 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. Carroll and Currie also mentioned that *Rhynchonkos* and caecilians have a [pleurosphenoid](/source/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.[14]

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.[14] The general appearance of *Rhynchonkos* is similar to that of caecilians, but is also similar to other amphibians that have independently developed elongated bodies. [Adelospondyls](/source/Adelospondyli), [lysorophians](/source/Lysorophia), [aïstopods](/source/A%C3%AFstopod), and some [salamanders](/source/Salamander) 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.[6]

## References

1. Schultze, H.-P. & Foreman, B. (1981). "A new gymnarthrid microsaur from the Lower Permian of Kansas with a review of the tuditanomorph microsaurs (Amphibia)". *Occasional Papers of the Museum of Natural History, the University of Kansas*. **91**: 1–25.

1. Zanon, R. T. (1988). "A replacement name for the family Goniorhynchidae Carroll and Gaskill, 1978 (Amphibia: Microsauria)". *[Journal of Paleontology](/source/Journal_of_Paleontology)*. **62** (2): 317. [doi:10.1017/S0022336000030031](https://doi.org/10.1017/S0022336000030031). [S2CID 87467293](https://api.semanticscholar.org/CorpusID:87467293)

1. Carroll, R. L. & Gaskill, P. (1978). "The order Microsauria". *Memoirs of the American Philosophical Society*. **126**

1. Szostakiwskyj, M.; Pardo, J. D.; Anderson, J. S. (2015). "Micro-CT study of *Rhynchonkos stovalli* (Lepospondyli, Recumbirostra), with description of two new genera". *PLOS ONE*. **10** (6). [Bibcode:2015PLoSO..1027307S](https://ui.adsabs.harvard.edu/abs/2015PLoSO..1027307S). [doi:10.1371/journal.pone.0127307](https://doi.org/10.1371/journal.pone.0127307). [PMC 4465623](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4465623). [PMID 26061187](https://pubmed.ncbi.nlm.nih.gov/26061187)

1. Carroll, R. L. (2001). ["The origin and early radiation of terrestrial vertebrates"](http://doc.rero.ch/record/14744/files/PAL_E1871.pdf). *[Journal of Paleontology](/source/Journal_of_Paleontology)*. **75** (6): 1202–1213. [doi:10.1666/0022-3360(2001)075<1202:toaero>2.0.co;2](https://doi.org/10.1666/0022-3360(2001)075%3C1202:toaero%3E2.0.co;2). [S2CID 59359868](https://api.semanticscholar.org/CorpusID:59359868)

1. Jenkins, F. A.; Walsh, D. M.; Carroll, R. L. (2007). ["Anatomy of *Eocaecilia micropodia*, a limbed caecilian of the Early Jurassic"](http://www.mcz.harvard.edu/Publications/pubs/mczbull-158-6-285.pdf). *Bulletin of the Museum of Comparative Zoology*. **158** (6): 285–366. [doi:10.3099/0027-4100(2007)158\[285:AOEMAL](https://doi.org/10.3099/0027-4100(2007)158%5B285:AOEMAL%5D2.0.CO;2)2.0.CO;2]. [S2CID 86379456](https://api.semanticscholar.org/CorpusID:86379456)

1. Cannatella, D. C.; Vieites, D. R.; Zhang, P.; Wake, M. H.; Wake, D. B. (2009). "Amphibians (Lissamphibia)". *The Timetree of Life*. Hedges, S. B. & Kumar, S. (eds.). Oxford University Press. pp. 353–356.

1. Carroll, R. L. & Chorn, J. (1995). "Vertebral development in the oldest microsaur and the problem of "lepospondyl" relationships". *Journal of Vertebrate Paleontology*. **15** (1): 37–56. [doi:10.1080/02724634.1995.10011206](https://doi.org/10.1080/02724634.1995.10011206)

1. Olson, E.C. (1970). "New and little known genera and species of vertebrates from the Lower Permian of Oklahoma". *Fieldiana*. **18** (3): 359–434.

1. Hampson, G. F. (1896). [*The Fauna of British India, Including Ceylon and Burma: Moths Volume IV*](https://www.biodiversitylibrary.org/item/180032#page/5/mode/1up). Taylor and Francis. – via Biodiversity Heritage Library.

1. Laurin, M. & Reisz, R. R. (1997). "A new perspective on tetrapod phylogeny". *Amniote Origins: Completing the Transition to Land*. Sumida, S.S. & Martin, K.L.M. (eds.). Academic Press. pp. 9–60. ISBN 978-0-12-676460-4.

1. Marjanović, David & Laurin, Michel (2013). ["The origin(s) of extant amphibians: a review with emphasis on the "lepospondyl hypothesis"](https://zenodo.org/record/5375982). *Geodiversitas*. **35** (1): 207–272. [doi:10.5252/g2013n1a8](https://doi.org/10.5252/g2013n1a8). [S2CID 67823991](https://api.semanticscholar.org/CorpusID:67823991)

1. Carroll, R. L. & Currie, P. J. (1975). ["Microsaurs as possible apodan ancestors"](http://doc.rero.ch/record/14582/files/PAL_E2243.pdf). *Zoological Journal of the Linnean Society*. **57** (3): 229–247. [doi:10.1111/j.1096-3642.1975.tb00817.x](https://doi.org/10.1111/j.1096-3642.1975.tb00817.x)

1. Bolt, J.R. (1991). "Lissamphibian origins". *Origins of the Higher Groups of Tetrapods: Controversy and Consensus*. Schultze, H.-P. & Trueb, L. (eds.). Cornell University Press. pp. 194–222. ISBN 978-0-8014-2497-7.

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Adapted from the Wikipedia article [Rhynchonkos](https://en.wikipedia.org/wiki/Rhynchonkos) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Rhynchonkos?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.