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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


Tetrapods

  

          The earliest landliving vertebrates were the Ichthyostegalia, an order of early tetrapods who are considered amphibians in the widest sense (sensu latu). This group is considered an evolutionary grade rather than a clade; in other words, it contains a mixture of descent groups, and is not monophyletic. While all members of the group had limbs or 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. They all had labyrinth-teeth with infolded dentine and enamel layers; i.e., are labyrinthodonts, another mixed or paraphyletic group on the level of subclass.

            The Ichthyostegalia evolved during the Early to Middle Devonian from Tetrapodomorph fish in the order Elpistostegalia, similar to but earlier than Panderichthys or Tiktaalik. They successfully colonized swampland and tidal channels or coastal rivers throughout the Devonian period. The Ichthyostegalia gave rise to the amphibian order called Temnospondyli,  and then disappeared at the transition to the Mississippian period, when a major extinction event occurred at the end of the Devonian (ca. 360-359 mya).

            The first Ichthyostegalia emerged in the fossil record at about 375 mya, during the Late Devonian period. Two of the most completely studied are Ichthyostega and Acanthostega, both found in Greenland.

            Ichthyostega (Greek: "fish roof", referring to armored skull bones; see below for “Stegocephalian”) was an early tetrapod genus from the end of the Upper Devonian (374 – 359 mya). It was one of the first tetrapods known in the fossil record. Ichthyostega was first described in 1932 by Gunnar Save-Soderbergh, who named five species from a collection of 14 fossil specimens collected the previous year in Late Devonian rock strata by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955. The species were distinguished on the basis of skull proportions and skull bone patterns   (Jarvik 1996). 

            Ichthyostega was at first seen as transitional between fish and the early Stegocephalians (a term meaning “roofed head”, coined in 1868 by Edward Drinker Cope for large amounts of dermal armor on the skulls of early amphibians; and which became the name of another paryphyletic or mixed group). Ichthyostega combines such a flat, heavily armoured stegocephalian skull with a fishlike tail bearing fin-rays. Later work on Ichthyostega and other Devonian Labyrinthodonts shows that they also had more than 5 digits to each foot (e.g., Acanthostega has 8 digits), with the whole foot being somewhat fin-like. Acanthostega (“spiky roof”), later found in the same East Greenland locations as Ichthyostega, also appears to have had a soft operculum (gill plate, which in amphibians changed into a pectoral bone), and both it and Ichthyostega possessed functional internal gills as adults.

            Ichthyostegalians were relatively large, with adults up to a meter or more in length. Their flat, massive heads were filled with labyrinthodont teeth, used for carnivorous subsistence which presumably concentrated on fish as a staple.  They probably also ate arthropods and other invertebrates life along tidal channels. Their 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.

    Ichthyostega shows a fully transitional form between fish and amphibians, combining a fishlike tail and gills with an amphibian skull and limbs. Ichthyostega was a labyrinthodont, with air-breathing lungs and flexible, muscular limbs that helped it navigate through shallow water in swamps. Although undoubtedly of amphibian build and habit, however, it is not considered a true member of the group in the narrow sense, as the first true amphibians appeared in the Mississppian  period.

            The genus is closely related to Acanthostega, also first found in East Greenland in 1931-2 by Save-Soderbergh. Ichthyostega's skull seems more fish-like than that of Acanthostega, but its shoulder and hip girdles appear more robust and better adapted to movement on land. Ichthyostega also had more supportive ribs and stronger vertebrae,  with more developed process (called zygaophyses) for rib attachment. All of this strongly indicates that Ichthyostega sometimes walked on land .

            This contrasts with other early tetrapods, including Elginerpeton and Obruchevichthys.

            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 use it for all finds more advanced than Tiktaalik (the closest relative of tetrapods known to have retained paired fins rather than feet). (Coates and Clack  1990)



Glossary 
.

References:

Chang & Smith 1992

Coates, M. I. and Clack, J. A. 1990. "Polydactyly in the earliest known tetrapod limbs". Nature 347: 66–67.

Janvier 1996

Jarvik, E. 1980

Jarvik, E  1996 . "The Devonian tetrapod Ichthyostega". Fossils & Strata 40: 1–213.)

Laurin et al. 2007

Long  1995


Schultze and Trueb 1991


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