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{{Short description|Extinct order of amphibians}}
{{Automatic taxobox
| image = Diplocaulus - skeleton and model.jpg
| image_caption = Skeleton and model of ''[[Diplocaulus]]''.
| fossil_range = {{Fossil range|Late Carboniferous|Lopingian}}
| taxon = Nectridea
| authority = Miall, [[1875 in paleontology|1875]]
| subdivision_ranks = Subgroups
| subdivision = *''[[Arizonerpeton]]''
*[[Diplocaulidae]]
*[[Scincosauridae]]
*[[Urocordylidae]]
}}
'''Nectridea''' is an extinct order of [[lepospondyl]] [[
==
[[File:Urocordylus.jpg|left|thumb|191x191px|A life restoration of ''[[Urocordylus]]'', a [[Urocordylidae|urocordylid]]]]
Nectrideans are a diverse group of tetrapods, including the aquatic [[Urocordylidae]], the presumably terrestrial [[Scincosauridae]], and the bizarre horned members of [[Diplocaulidae]] (also known as Keraterpetonidae), which includes the "boomerang-headed" ''Diplocaulus'', one of the most famous genera of prehistoric amphibians (in the traditional sense of the word). By the time the earliest known nectrideans appeared in the Late Carboniferous fossil record, they had already diversified into these families, indicating that basal nectrideans are unknown. These different families are united primarily by features of the [[Vertebral column|spinal column]] rather than the skull.
=== Vertebrae ===
In many groups of early tetrapods, each [[vertebra]] is formed by three parts: an intercentrum at the front, a pleurocentrum at the back, and a plate-like neural spine jutting out from the top of the vertebrae which may be positioned between the two or fused to the pleurocentrum. In nectrideans, only the pleurocentra are present, and the neural spines are completely fused to them. Their vertebrae also possess an extra set of joints connecting the vertebrae (apophyses), just above the typical set (zygapophyses). Furthermore, their caudal (tail) vertebrae also possess haemal spines which jut out from the bottom of each vertebra. While most early tetrapods have slanted haemal spines which are positioned between the vertebrae, the haemal spines of nectrideans are completely vertical in orientation and fused to the pleurocentra, directly opposite the neural spines. Both the neural spines and haemal spines of the tail are characteristically fan-shaped. As a whole, nectrideans have rather short bodies and long and paddle-like tails, adapted for swimming.<ref name=":1">{{Cite book|title=The rise of amphibians|last=Carroll|first=R. L.|publisher=The Johns Hopkins University Press|year=2009}}</ref><ref name=":0">{{Cite journal|last=Milner|first=Andrew R.|date=1993|title=The Paleozoic Relatives of Lissamphibians|jstor=1466948|journal=Herpetological Monographs|volume=7|pages=8–27|doi=10.2307/1466948}}</ref>
=== Other features ===
Almost all early tetrapods have three coronoid bones lining the inside edge of each side of the jaw. Nectrideans, however, only have one. Their [[Clavicle|clavicular blades]] (shoulder girdles) are wide and possess a short rear extension, while the [[scapulocoracoid]]s (shoulder blades) are weakly ossified.<ref name=":1" />
They also had well-developed hind limbs, with a full set of five toes each. Their forelimbs were slightly reduced, but not to the same extent as in [[Microsauria|microsaurs]] and other lepospondyls. Although ''Urocordylus'' retained five fingers, most nectridieans had only four, similar to modern amphibians.<ref name=":1" /><ref name="EoDP">{{cite book |editor=Palmer, D.|year=1999 |title= The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals|publisher= Marshall Editions|location=London|page= 54|isbn= 1-84028-152-9}}</ref>
[[File:Scincosaurus.jpg|thumb|Life restoration of ''[[Scincosaurus]]'', a [[Scincosauridae|scincosaurid]]|left]]
== Paleobiology ==
Both diplocaulids and urocordylids are considered to be completely aquatic, although only diplocaulids possess [[lateral line]] canals on their skulls. However, this does not mean that urocordylids lacked lateral lines, as several modern amphibians possess that organ without any skeletal indication. Scincosaurids have much more well-ossified limbs and thinner tails than other nectrideans, and are usually considered to be terrestrial. However, some have been reported to possess lines of pits on the skull, which may be an indication that they were amphibious rather than purely terrestrial.<ref>{{Cite journal|last=Vallin|first=Grégoire|last2=Laurin|first2=Michel|title=Cranial morphology and affinities ofMicrobrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians|url=http://www.bioone.org/doi/pdf/10.1671/5.1|journal=Journal of Vertebrate Paleontology|language=en-US|volume=24|issue=1|pages=56–72|doi=10.1671/5.1}}</ref>
A trace fossil known as ''Hermundurichnus fornicatus'' of a tetrapod resting on a lake bed may have been attributed to ''Diplocaulus'' or an animal like it. This trace indicates that the underside of nectrideans was covered in small, diamond-shaped scales and that the "horns" of the skull were connected with the body by flaps of skin.<ref>{{Cite book|title=Dinosaur Tracks and other Fossil Footprints of Europe|last=Lockey|first=Martin|last2=Meyer|first2=Christian|publisher=Columbia University Press|year=2000}}</ref>
==Classification==
There has been some controversy over the precise classification of Nectridea over the past century, and even whether it constitutes a valid monophyletic clade. A few older studies consider nectrideans to be very basal tetrapods, related to ''[[Ichthyostega]]'' or [[Colosteidae|colosteids]]. However, most studies generally place nectrideans inside the subclass [[Lepospondyli]]. [[Aistopoda]], a group of legless lepospondyls, have often been allied with particular families of nectrideans, but not the order as a whole. For example, Anderson (2001) positioned diplocaulids as close relatives of Aistopods with urocordylids and scincosaurids as progressively more primitive members of the group. On the other hand, Ruta ''et al''. (2003) claimed that urocordylids were closer to aistopods. As aistopods do not possess most nectridean features, they have traditionally been excluded from the group. However, this would make Nectridea an invalid [[Paraphyly|paraphyletic group]] as the aistopods, may have been descended from nectrideans, are not considered nectrideans themselves.<ref name=RCQ03>{{cite journal |author=Marcello Ruta, Michael I. Coates and Donald L. J. Quicke |year=2003 |title=Early tetrapod relationships revisited |url=http://pondside.uchicago.edu/oba/faculty/coates/5.RutCoaQuick2003.pdf |journal=Biological Reviews |volume=78 |issue=2 |pages=251–345 |doi=10.1017/S1464793102006103 |pmid=12803423}}</ref>
The following is a cladogram from the Ruta ''et al.'' (2003):
{{Barlabel|size=11|at1=10.5|label1="Nectridea"|colour1=black|style=font-size:100%;line-height:85%
|cladogram=
{{clade
|label1=[[Lepospondyli]]
|1={{clade
|1={{clade
|1=''[[Batropetes fritschi]]''
|2=[[Tuditanomorpha]]}}
|2={{clade
|1={{clade
|1=''[[Microbrachis pelikani]]''
|2={{clade
|1=''[[Hyloplesion longicostatum]]''
|2=''[[Odonterpeton triangulare]]''}} }}
|2={{clade
|1=[[Lysorophia]]
|2={{clade
|1=[[Adelospondyli]]
|label2=[[Holospondyli]]
|2={{clade
|1={{clade
|1=[[Aistopoda]]
|2=[[Urocordylidae]] |barbegin2=black
}}
|2={{clade
|1=[[Diplocaulidae]] |bar1=black
|2=[[Scincosauridae]] |barend2=black
}}
}}
}}
}}
}}
}}
}}
}}
==References==
{{
*Ahlberg, P. E. & Milner, A. R. 1994: The origin and early diversification of tetrapods. Nature: vol. 368, 7 April, pp.
*Anderson, J. S. 1998: Phylogenetic analysis of the Lepospondyli (Tetrapoda). Journal of Vertebrate Paleontology. Vol. 18, #3, Suppl. to #3, pp.
*Carroll, R. L., 1988: Vertebrate paleontology and evolution. W. H. Freeman and company, New York, 1988, 698.
*Carroll, R. L., 1988: Appendix. 594-648. ''in'' Carroll, R. L., 1988: Vertebrate paleontology and evolution. W. H. Freeman and company, New York, 1988, 698.
*Carroll, R. L. 1996: Revealing the patterns of macroevolution. Nature: vol. 381, 2 May, pp.
*Clack, J. A., 1998: A new Early Carboniferous tetrapod with a mélange of crown-group characters. Nature: vol. 394, 2 July, pp.
*Shubin, N., 1998 [preview]: Evolutionary cut and paste. Nature: vol. 394, 2 July, pp.
*Vallin, G. & Laurin, M., 2004: Cranial morphology and affinities of Microbrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians. Journal of Vertebrate Paleontology: Vol. 24, #1, pp.
==External links==
*[http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/amphibia/lepospondyli/nectridae.html Nectridae Phylogeny.]
{{Tetrapodomorpha|St.}}
{{Nectridea|H.}}
{{Taxonbar|from=Q135007}}
[[Category:Nectrideans| ]]
[[Category:Pennsylvanian first appearances]]
[[Category:
[[Category:Tetrapodomorph orders]]
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