Ichthyostega

Ichthyostega (meaning 'roof fish') is an extinct fishapod. It lived in Greenland during the Late Devonian. Ichthyostega was one of the first vertebrates with four limbs. It was also one of the first that could hold its weight on land. Ichthyostega had lungs and limbs that helped it live in shallow water in swamps. While Ichthyostega is often called a 'tetrapod' because of its limbs and fingers, it evolved long before true tetrapods, and is better called a 'stem tetrapod'. Also, while Ichthyostega looked like an amphibian, it is not really one, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic Period. Until other early fishapods were found in the late 20th century, Ichthyostega was the only transitional fossil between fish and tetrapods, with features of both. Scientists thought it moved similar to a salamander, but newer research has shown that it moved more similar to a seal.[3]

Ichthyostega
Temporal range: Late Devonian,
Reconstructed skeleton in Moscow Paleontological Museum
Drawing of the skeleton
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum:
Vertebrata
Superclass:
Osteichthyes
Class:
Sarcopterygii
Family:
Ichthyostegidae

Säve-Söderbergh, 1932
Genus:
Ichthyostega

Säve-Söderbergh, 1932
Type species
†Ichthyostega stensioei
Säve-Söderbergh, 1932
Other species[1][2]
  • †I. eigili
    Säve-Söderbergh, 1932
  • †I. kochi?
    Säve-Söderbergh, 1932
  • †I. watsoni
    Säve-Söderbergh, 1932
Synonyms
Genus names
  • Ichthyostegopsis
    Säve-Söderbergh, 1932
Species names
  • Ichthyostega stensiöi
    Säve-Söderbergh, 1932
  • Ichthyostegopsis wimani
    Säve-Söderbergh, 1932

Classification
In the Late Devonian, descendants of pelagic lobe-finned fish – like Eusthenopteron – adapted for life on land step-by-step:
  • Panderichthys, which lived in muddy shallows, not on land
  • Tiktaalik with limb-like fins that could take it on land
  • Vertebrates with real limbs in weed-filled swamps, such as:
    • Acanthostega which had feet with eight digits,
    • Ichthyostega, with an oval-shaped neck and limbs.
Descendants also include pelagic lobe-finned fish such as coelacanths.

Ichthyostega used to be grouped in the order "Ichthyostegalia", which was named after it. However, this group of fishapods does not include all of its descendants and is not used by many researchers today. Scientists have found by studying phylogeny that Ichthyostega is between other early four-limbed vertebrates. The evolutionary tree below comes from a study by Swartz in 2012.[4]

Simplified phylogeny of the fish–tetrapod transition.
Elpistostegalia

Panderichthys





Tiktaalik



Elpistostege



Stegocephali

Elginerpeton




Ventastega




Acanthostega




Ichthyostega




Whatcheeriidae




Colosteidae




Crassigyrinus




Baphetidae



Crown group Tetrapoda












Biology

Early vertebrates with limbs such as Ichthyostega and Acanthostega were more adapted to living on land than earlier fishapods such as Eusthenopteron or Panderichthys. Fishapods had lungs, but used gills to breathe out carbon dioxide. Fishapods swam with their bodies and tails, and steered with their fins. Ichthyostega may have walked on land with its front limbs and swam with its tail.

Drawing of Ichthyostega.

Ichthyostega had a large ribcage with ribs covering each other, and a stronger skeleton than earlier fishapods. Its spine was similar to that of a fish. Its front limbs were probably strong enough to help the animal get out of the water. This may have evolved so Ichthyostega could hold its weight on land. The back limbs were smaller than the front limbs and could not hold its weight. Ichthyostega could not move side-to-side because of its large ribs.[5] The front limbs could move enough to push the body up and forward, probably allowing the animal to drag itself across flat land by "crutching" with both front limbs at the same time, similar to a mudskipper[3] or a seal.[6][7] It could not walk the same way that most quadrupeds do because its forelimbs could not turn around enough.[3]

References
Wikimedia Commons has media related to Ichthyostega.
  1. Haaramo, Mikko. "Taxonomic history of the genus †Ichthyostega Säve-Söderbergh, 1932". Mikko's Phylogeny Archive. Blom, 2005. Retrieved 24 October 2015.
  2. "Ichthyostega". Paleofile. Retrieved 24 October 2015.
  3. Stephanie E. Pierce; Jennifer A. Clack; John R. Hutchinson (2012). "Three-dimensional limb joint mobility in the early tetrapod Ichthyostega". Nature. 486 (7404): 524–527. Bibcode:2012Natur.486..523P. doi:10.1038/nature11124. PMID 22722854. S2CID 3127857.
  4. Swartz, B. (2012). "A marine stem-tetrapod from the Devonian of Western North America". PLOS ONE. 7 (3): e33683. Bibcode:2012PLoSO...733683S. doi:10.1371/journal.pone.0033683. PMC 3308997. PMID 22448265.
  5. "Devonian Times – Tetrapods Answer". Archived from the original on 2021-07-26. Retrieved 2024-06-08.
  6. Williams, James J. (May 24, 2012). "Ichthyostega, one of the first creatures to step on land, could not have walked on four legs, say scientists". BelleNews. Retrieved 4 November 2013.
  7. Mosher, Dave (May 23, 2012). "Evolutionary Flop: Early 4-Footed Land Animal Was No Walker?". National Geographic News. Archived from the original on May 25, 2012. Retrieved 4 November 2013.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.