name = "Ichthyostega"
fossil_range = Fossil range|367|362.5
image_width = 250px
image_caption = Life restoration of "Ichthyostega" after Ahlberg, 2005.
phylum = Chordata
ordo = Ichthyostegalia
familia = Ichthyostegidae
genus = "Ichthyostega"
genus_authority = Säve-Söderbergh, 1932
subdivision_ranks = Species
subdivision = "I. stensioei"
"Ichthyostega" (Greek: "fish roof") is an early
tetrapod genusthat lived in the Upper Devonian( Famennian) period, 367-362.5 million years ago that represents an intermediate form between fishand amphibians. "Ichthyostega" possessed lungs and limbs that helped it navigate through shallow water in swamps. Though sometimes referred to as an "amphibian", it is not considered a true member of the group, as the first true amphibians appeared in the Carboniferousperiod.
History and systematics
Gunnar Säve-Söderberghdescribed four "Ichthyostega" species from the Upper Devonian of East Greenlandand one species belonging to the genus "Ichthyostegopsis", "I. wimani". These species could be synonymous (in which case only "I. stensioei" would remain), because their morphological differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.
The genus is closely related to "Acanthostega gunnari", also from East Greenland. "Ichthyostega"'s skull seems more fish-like than that of "Acanthostega", but its
girdle(shoulder and hip) morphology seems stronger and better adapted to land-life. "Ichthyostega" also had more supportive ribs and stronger vertebrae with more developed zygapophyses. The first tetrapods (who probably didn't walk on land) were " Elginerpeton" and " Obruchevichthys".
"Ichthyostega" was about convert|5|ft|m long and had seven digits on each hind foot. The exact number of digits on the hand is not yet known, but was probably about the same as on the foot. It had a fin containing fin rays on its tail.
Adaptations for land life
Early tetrapods like "Ichthyostega" and "Acanthostega" differed from animals like Crossopterygians (for instance "
Eusthenopteron" or " Panderichthys") in their increased adaptations for life on land. Though Crossopterygians possessed lungs, they used gills as their primary means of acquiring oxygen; "Ichthyostega" appears to have relied on its lungs as its primary apparatus for breathing. The skinof early tetrapods, unlike that of Crossopterygians, helped retain bodily fluids and deter desiccationCrossopterygians used their bodyand tailfor locomotion and their fins for balance; "Ichthyostega" used its limbs for locomotion and its tail for balance.
" – exhibited a sequence of adaptations:
Panderichthys", suited to muddy shallows;
Tiktaalik" with limb-like fins that could take it onto land;
- Early tetrapods in weed-filled swamps, such as:
Acanthostega" which had feet with eight digits,
- "Ichthyostega" with limbs.
The size of an adult "Ichthyostega" (1.5 m or 4 ft) precluded much completely terrestrial locomotion. Juveniles, though, could have moved about on land much more easily. The massive ribcage, though. was made up of overlapping ribs and the creatures possessed a stronger skeletal structure, a more rigid spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications clearly evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have born full weight in an adult.
Jennifer A. Clacksuggests that "Ichthyostega" and its relatives spent time basking in the sun to raise their body temperatures, much as some animals do today: the Marine Iguanas on the GalapagosIsland, seals or the Gharial. They would have returned to the water to cool themselves, hunt for food and reproduce. In that case, they would need strong forelimbs to pull at least their anterior part out of the water, and a stronger ribcage and spine to support them while sunbathing on their abdomen like modern crocodiles. The greater mobility of juveniles on land would have helped them avoid aquatic predators.
Water was also still a requirement, because the
gel-like eggs of the earliest terrestrial tetrapods couldn't survive out of water, so reproductioncould not occur without it. Water was also needed for their larvae and external fertilization. Most land-dwelling vertebrates have since developed two methods of internal fertilization; either direct as seen in all amniotes and a few amphibians, or indirect for many salamanders by placing a spermatophoreon the ground which then is picked up by the female salamander.
Elginerpeton", " Acanthostega", "Ichthyostega", etc.) were "succeeded" by temnospondyls and anthracosaurs, such as " Eryops", an amphibian that truly developed the ability to walk on land. There is a gap of 20-30 million years between both groups. This gap, a classic in vertebrate paleontology, is known as Romer's Gap, after the American paleontologist Alfred Sherwood Romer. In 2002 a 350 million year old fossil named " Pederpes finneyae" was found.
List of transitional fossils
* [http://tolweb.org/tree?group=Ichthyostega_stensioei&contgroup=Terrestrial_Vertebrates Excellent site on early tetrapods]
* [http://www.bio.miami.edu/tom/bil160/bil160goods/20_verts2.html Course site]
* [http://ijolite.geology.uiuc.edu/00FallClass/geo143/lect/lect12.html Course site]
* [http://www.livescience.com/animalworld/050831_four_legs.html First Four-Legged Animals Inched Along]
* [http://www.scientificamerican.com/print_version.cfm?articleID=000DC8B8-EA15-137C-AA1583414B7F0000 "Getting a Leg Up on Land"] "Scientific American" Nov. 21, 2005, article by
Jennifer A. Clack.
*Blom, H. (2005) — Taxonomic Revision Of The Late Devonian Tetrapod Ichthyostega from East Greenland. "Palaeontology", 48, Part 1:111–134
*Westenberg, K. (1999) — From Fins to Feet. "National Geographic", 195, 5:114–127
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Look at other dictionaries:
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Ichthyostega — Ich|thy|os|te|ga* der; Gen. od. s, Plur. ...g’alia <zu ↑ichthyo... u. gr. stégos »Dach« (wegen des beschuppten Körpers)> ausgestorbener, erdgeschichtlich ältester Lurch des Oberdevons … Das große Fremdwörterbuch
genus Ichthyostega — noun second earliest fossil amphibian ever found; of the Devonian; found in Greenland • Hypernyms: ↑amphibian genus • Member Holonyms: ↑amphibia, ↑class Amphibia • Member Meronyms: ↑Ichyostega … Useful english dictionary