ichthyostegalia

• Ichthyostegalia
• Acanthostegidae
• Acanthostega
• Crassigyrinidae
• Crassigyrinus
• Densignathidae
• Densignathus
• Elginerpetontidae
• Elginerpeton
• Obruchevichthys
• Ichthyostegidae
• Hynerpeton
• Ichthyostega
• Jakubsonidae
• Jakubsonia
• Metaxygnathidae
• Metaxygnathus
• Sinostegidae
• Sinostega
• Tulerpetontidae
• Tulerpeton
• Ventastegidae
• Ventastega
• Ymeridae
• Ymeria

As first described, the order's sole member was Ichthyostega, from which the group takes its name. Ichthyostega was seen as transitional between fish and the early Stegocephalians, in that it combines a flat, heavily armoured stegocephalian skull with a fishlike tail bearing fin rays.{{cite journal | last1 = Jarvik | first1 = E | year = 1996 | title = The Devonian tetrapod Ichthyostega | journal = Fossils & Strata | volume = 40 | pages = 1–213 }} Later work on Ichthyostega and other Devonian Labyrinthodonts shows that they also had more than 5 digits to each foot, in fact the whole foot being fin-like.{{cite journal | last1 = Coates | first1 = M.I. | last2 = Clack | first2 = J.A. | year = 1990 | title = Polydactyly in the earliest known tetrapod limbs | journal = Nature | volume = 347 | issue = 6288| pages = 66–67 | doi=10.1038/347066a0| bibcode = 1990Natur.347...66C | s2cid = 4319165 }} Acanthostega, later found in the same locations, appears to have had a soft operculum and both it and Ichthyostega possessed functional internal gills as adults.{{cite journal | last1 = Ruta | first1 = M | last2 = Coates | first2 = MI | last3 = Quicke | first3 = DLJ | year = 2003 | title = Early tetrapod relationships revisited | journal = Biological Reviews | volume = 78 | issue = 2| pages = 251–345 | doi=10.1017/s1464793102006103 | pmid=12803423| s2cid = 31298396 }}{{cite journal | last1 = Clack | first1 = J.A. | title = & al (2003) A uniquely specialized ear in a very early tetrapod | journal = Nature | volume = 425 | issue = 6953| pages = 65–69 | doi=10.1038/nature01904| pmid = 12955140 | bibcode = 2003Natur.425...65C| year = 2003 | s2cid = 4411060 }}

The feet are only known from Ichthyostega, Acanthostega, and Tulerpeton, but appear to be polydactyl in all forms with more than the usual five digits for tetrapods and were paddle-like.{{cite journal | last1 = Coates | first1 = M. I. | last2 = Clack | first2 = J. A. | year = 1990 | title = Polydactyly in the earliest known tetrapod limbs | journal = Nature | volume = 347 | issue = 6288| pages = 66–67 | doi=10.1038/347066a0| bibcode = 1990Natur.347...66C | s2cid = 4319165 }} The tail bore true fin rays like those found in fish.{{cite journal | last1 = Jarvik | first1 = E. | author-link = Erik Jarvik | year = 1996 | title = The Devonian tetrapod Ichthyostega | journal = Fossils & Strata | volume = 40 | pages = 1–213 }}

The ichthyostegalians were large to medium-sized, with an adult size form most genera on the order of a meter or more. Their heads were flat and massive, with a host of labyrinthodont teeth. They were carnivorous and probably mainly ate fish, but may also have fed on washed-up carcasses of fish and other marine life, and hunted unwary arthropods and other invertebrate life along the tidal channels of the coal swamps. The vertebrae were complex and rather weak. At the close of the Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults. As adults, the animals would have been heavy and clumsy on land, and would probably appear more as fish that occasionally went ashore rather than proper land animals. All were however predominately aquatic and some spent all or nearly all their lives in water.

The order Ichthyostegalia was erected for Ichthyostega, and contained until the 1980s only three genera (Ichthyostega, Acanthostega and Tulerpeton). While "Ichthyostegalia" in principle contain the most basal of animals with toes rather than fins, Clack and Ahlberg uses it for all finds more advanced than Tiktaalik (the closest relative of tetrapods known to have retained paired fins rather than feet).{{cite web|title=New Fossils Of Extremely Primitive 4-Legged Creatures Close The Gap Between Fish And Land Animals|url=https://www.sciencedaily.com/releases/2008/06/080625140643.htm|work=Science News|publisher=Science Daily|access-date=28 February 2012}} Under this use, the number of known Devonian tetrapods have increased dramatically, so that the group now contain 12 genera:{{cite journal |last=Clack |first=J.A. |author2=Ahlberg, P.E. |author3=Blom, H. |author4= Finney, S.M. |year=2012 |title=A new genus of Devonian tetrapod from North-East Greenland, with new information on the lower jaw of Ichthyostega |journal=Palaeontology |volume=55 |issue=1 |pages=73–86 |doi=10.1111/j.1475-4983.2011.01117.x |doi-access=free }} Most of the newer finds are redescriptions of very fragmentary finds, usually just the lower jaw. These were thought to have been from fish when found, but cladistic analyses indicate they are more advanced than Tiktaalik, though whether they actually had feet rather than fins is unknown. In order of discovery:

• Ichthyostega{{cite journal|last=BLOM|first=H.|title=Taxonomic revision of the Late Devonian tetrapod Ichthyostega from East Greenland|journal=Palaeontology|date=1 January 2005|volume=48|issue=1|pages=111–134|doi=10.1111/j.1475-4983.2004.00435.x|doi-access=free}}
• Acanthostega{{cite journal|last=Jarvik|first=E.|title=On the fish‐like tail in the ichthyostegid stegocephalians|journal=Meddelelser om Grønland|year=1952|volume=114|pages=1–90}}
• Tulerpeton{{cite journal|last=Lebedev|first=O.A.|title=The first find of a Devonian tetrapod vertebrate in the USSR|journal=Doklady Akademii Nauk SSSR|year=1984|volume=278|series=Palaeontology|pages=1470–1473|language=ru}}
• Metaxygnathus{{cite journal|last=Campbell|first=K.S.W.|author2=Bell, M.V.|title=A primitive amphibian from the Late Devonian of New South Wales|journal=Alcheringa|year=1977|volume=1|issue=4|pages=369–381|doi=10.1080/03115517708527771}}
• Elginerpeton{{cite journal|last=Ahlberg|first=Per E.|title=Elginerpeton pancheni and the earliest tetrapod clade|journal=Nature|year=1995|volume=373|issue=6513|pages=420–425|doi=10.1038/373420a0|bibcode=1995Natur.373..420A|s2cid=4344655}}
• Obruchevichthys{{cite journal|last=Ahlberg|first=P. E.|title=Tetrapod or near-tetrapod fossils from the Upper Devonian of Scotland|journal=Nature|year=1991|volume=354|issue=6351|pages=298–301|doi=10.1038/354298a0|bibcode=1991Natur.354..298A|s2cid=4364221}}
• Ventastega
• Hynerpeton{{cite journal|last=Daeschler|first=EB |author2=Shubin, NH |author3=Thomson, KS |author4=Amaral, WW|title=A devonian tetrapod from north america.|journal=Science|date=Jul 29, 1994|volume=265|issue=5172|pages=639–42|doi=10.1126/science.265.5172.639 |pmid=17752761|bibcode=1994Sci...265..639D|s2cid=2835592 }}
• Densignathus{{cite journal|last=Daeschler|first=Edward B.|author2=Clack, Jennifer A. |author3=Shubin, Neil H. |title=Late Devonian tetrapod remains from Red Hill, Pennsylvania, USA: how much diversity?|journal=Acta Zoologica|date=1 May 1009|volume=90|pages=306–317|doi=10.1111/j.1463-6395.2008.00361.x}}
• Sinostega{{cite journal|last=Zhu|first=Min|author2=Ahlberg, Per E. |author3=Zhao, Wenjin |author4=Jia, Liantao |title=Palaeontology: First Devonian tetrapod from Asia|journal=Nature|date=19 December 2002|volume=420|issue=6917|pages=760–761|doi=10.1038/420760a |pmid=12490937|bibcode=2002Natur.420..760Z|s2cid=4420615}}
• Jakubsonia{{cite journal |last=Lebedev |first=O.L. |year=2004 |title=A new tetrapod Jakubsonia livnensis from the Early Famennian (Devonian) of Russia and palaeoecological remarks on the Late Devonian tetrapod habitats |journal=Acta Universitatis Tatviensis |volume=679 |pages=79–98}}
• Ymeria

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Category:Stegocephali

Category:Tetrapodomorph orders

Category:Paleozoic amphibians

Category:Paraphyletic groups