Ichthyostegalia Explained

Ichthyostegalia is an order of extinct Tetrapodomorphs, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade.^[1] While the group are recognized as having feet rather than fins, most, if not all, had internal gills in adulthood and lived primarily as shallow water fish and spent minimal time on land.

The group evolved from elpistostegalian fish in the late Devonian,^[2] or possibly in the middle Devonian.^{[3] [4]} They continued to thrive as denizens of swampland and tidal channels throughout the period. They gave rise to the Temnospondyli and then disappeared during the transition to the Carboniferous.^[5]

Classification

• 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

Description

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.^[6] 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.^[7] 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.^{[8] [9]}

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.^[10] The tail bore true fin rays like those found in fish.^[11]

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.

Genera

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).^[12] Under this use, the number of known Devonian tetrapods have increased dramatically, so that the group now contain 12 genera:^[13] 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^[14]
• Acanthostega^[15]
• Tulerpeton^[16]
• Metaxygnathus^[17]
• Elginerpeton^[18]
• Obruchevichthys^[19]
• Ventastega^[18]
• Hynerpeton^[20]
• Densignathus^[21]
• Sinostega^[22]
• Jakubsonia^[23]
• Ymeria^[13]

Notes and References

1. Book: Parsons, [[Alfred Romer|Alfred Sherwood Romer]], Thomas S.. The vertebrate body. 1986. Saunders College Pub.. Philadelphia. 978-0-03-910754-3. 6th.
2. Web site: Fossil Record of the Amphibia . 2022-10-03 . ucmp.berkeley.edu.
3. Niedźwiedzki . 2010. Tetrapod trackways from the early Middle Devonian period of Poland . Nature. 463. 7277. 43–48 . 10.1038/nature08623 . 20054388. 2010Natur.463...43N. 4428903.
4. Uppsala University (2010, January 8). Fossil footprints give land vertebrates a much longer history. ScienceDaily. Retrieved January 8, 2010, from https://www.sciencedaily.com/releases/2010/01/100107114420.htm
5. Anderson . J.S. . Reisz . R.R. . Scott . D. . Fröbisch . N.B. . Sumida . S.S. . 2008 . A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders . Nature . 453 . 7194. 515–518 . 10.1038/nature06865 . 18497824. 2008Natur.453..515A . 205212809 .
6. Jarvik . E . 1996 . The Devonian tetrapod Ichthyostega . Fossils & Strata . 40 . 1–213 .
7. Coates . M.I. . Clack . J.A. . 1990 . Polydactyly in the earliest known tetrapod limbs . . 347 . 6288. 66–67 . 10.1038/347066a0. 1990Natur.347...66C . 4319165 .
8. Ruta . M . Coates . MI . Quicke . DLJ . 2003 . Early tetrapod relationships revisited . . 78 . 2. 251–345 . 10.1017/s1464793102006103 . 12803423. 31298396 .
9. Clack . J.A. . & al (2003) A uniquely specialized ear in a very early tetrapod . . 425 . 6953. 65–69 . 10.1038/nature01904. 12955140 . 2003Natur.425...65C. 2003 . 4411060 .
10. Coates . M. I. . Clack . J. A. . 1990 . Polydactyly in the earliest known tetrapod limbs . . 347 . 6288. 66–67 . 10.1038/347066a0. 1990Natur.347...66C . 4319165 .
11. Jarvik . E. . Erik Jarvik . 1996 . The Devonian tetrapod Ichthyostega . Fossils & Strata . 40 . 1–213 .
12. Web site: New Fossils Of Extremely Primitive 4-Legged Creatures Close The Gap Between Fish And Land Animals. Science News. Science Daily. 28 February 2012.
13. Clack . J.A. . Ahlberg, P.E. . Blom, H. . Finney, S.M. . 2012 . A new genus of Devonian tetrapod from North-East Greenland, with new information on the lower jaw of Ichthyostega . Palaeontology . 55 . 1 . 73–86 . 10.1111/j.1475-4983.2011.01117.x . free .
14. BLOM. H.. Taxonomic revision of the Late Devonian tetrapod Ichthyostega from East Greenland. Palaeontology. 1 January 2005. 48. 1. 111–134. 10.1111/j.1475-4983.2004.00435.x. free.
15. Jarvik. E.. On the fish‐like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland. 1952. 114. 1–90.
16. Lebedev. O.A.. The first find of a Devonian tetrapod vertebrate in the USSR. Doklady Akademii Nauk SSSR. 1984. 278. Palaeontology. 1470–1473. ru.
17. Campbell. K.S.W.. Bell, M.V.. A primitive amphibian from the Late Devonian of New South Wales. Alcheringa. 1977. 1. 4. 369–381. 10.1080/03115517708527771.
18. Ahlberg. Per E.. Elginerpeton pancheni and the earliest tetrapod clade. Nature. 1995. 373. 6513. 420–425. 10.1038/373420a0. 1995Natur.373..420A. 4344655.
19. Ahlberg. P. E.. Tetrapod or near-tetrapod fossils from the Upper Devonian of Scotland. Nature. 1991. 354. 6351. 298–301. 10.1038/354298a0. 1991Natur.354..298A. 4364221.
20. Daeschler. EB . Shubin, NH . Thomson, KS . Amaral, WW. A devonian tetrapod from north america.. Science. Jul 29, 1994. 265. 5172. 639–42. 10.1126/science.265.5172.639 . 17752761. 1994Sci...265..639D. 2835592 .
21. Daeschler. Edward B.. Clack, Jennifer A. . Shubin, Neil H. . Late Devonian tetrapod remains from Red Hill, Pennsylvania, USA: how much diversity?. Acta Zoologica. 1 May 1009. 90. 306–317. 10.1111/j.1463-6395.2008.00361.x.
22. Zhu. Min. Ahlberg, Per E. . Zhao, Wenjin . Jia, Liantao . Palaeontology: First Devonian tetrapod from Asia. Nature. 19 December 2002. 420. 6917. 760–761. 10.1038/420760a . 12490937. 2002Natur.420..760Z. 4420615.
23. Lebedev . O.L. . 2004 . A new tetrapod Jakubsonia livnensis from the Early Famennian (Devonian) of Russia and palaeoecological remarks on the Late Devonian tetrapod habitats . Acta Universitatis Tatviensis . 679 . 79–98.