Ichthyostega

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Ichthyostega
Temporal range:
Ma
Reconstructed skeleton in Moscow Paleontological Museum
Skeletal reconstruction
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Sarcopterygii
Clade: Tetrapodomorpha
Clade: Stegocephali
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 synonymy
  • Ichthyostegopsis
    Säve-Söderbergh, 1932
Species synonymy
  • Ichthyostega stensiöi
    Säve-Söderbergh, 1932
  • Ichthyostegopsis wimani
    Säve-Söderbergh, 1932

Ichthyostega (from

stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not a true member of the group in the narrow sense, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish and tetrapod features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.[3]

History

Zoological Museum of Copenhagen

In 1932

Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.[4]

Description

Size comparison

Ichthyostega was a fairly large animal for its time, as it was broadly built and about 1.5 m (4.9 ft) long. The skull was low, with dorsally placed eyes and large

labyrinthodont teeth. The posterior margin of the skull formed an operculum covering the gills. The spiracle was situated in an otic notch
behind each eye.

Postcranial skeleton

The legs were large compared to contemporary relatives and it

had seven digits on each hind leg. The exact number of digits on the forelimb is not yet known, since fossils of the hand have not been found.[5] While in water, the foot would have functioned like a fleshy paddle more than a fin.[6]

The

fin rays). The tail fin was not as deep as in Acanthostega, and would have been less useful for swimming.[4]

Ichthyostega is related to

zygapophyses. Whether or not these traits were independently evolved in Ichthyostega is debated. It does, however, show that Ichthyostega may have ventured onto land on occasions, unlike contemporaneous limbed vertebrates, such as Elginerpeton and Obruchevichthys.[citation needed
]

Classification

Tail with fin rays, counterpart of specimen A.69
In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:
  • Panderichthys, suited to muddy shallows;not on land
  • Tiktaalik with limb-like fins that could take it onto land;
  • Fully limbed vertebrates in weed-filled swamps, such as:
    • Acanthostega which had feet with eight digits,
    • Ichthyostega, with an oval-shaped neck and limbs.
Descendants also included pelagic lobe-finned fish such as coelacanth species.

Traditionally, Ichthyostega was considered part of an order named for it, the "

stegocephalian stem-tetrapods. The evolutionary tree of early stegocephalians below follows the results of one such analysis performed by Swartz in 2012.[7]

Elpistostegalia

Panderichthys

Stegocephalia

Tiktaalik

Elpistostege

Elginerpeton

Ventastega

Acanthostega

Ichthyostega

Whatcheeriidae

Colosteidae

Crassigyrinus

Baphetidae

Tetrapoda

Paleobiology

Model reconstruction

Early limbed vertebrates like Ichthyostega and Acanthostega differed from earlier

tetrapodomorphs such as Eusthenopteron or Panderichthys in their increased adaptations for life on land. Though tetrapodomorphs possessed lungs, they used gills as their primary means of discharging carbon dioxide
. Tetrapodomorphs used their bodies and tails for locomotion and their fins for steering and braking; Ichthyostega may have used its forelimbs for locomotion on land and its tail for swimming.

Its massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a largely fishlike spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications may have evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have borne full weight in an adult, while the broad, overlapping ribs would have inhibited side-to-side movements.[8] The forelimbs had the required range of movement to push the body up and forward, probably allowing the animal to drag itself across flat land by synchronous (rather than alternate) "crutching" movements, much like that of a mudskipper[3] or a seal.[9][10] It was incapable of typical quadrupedal gaits as the forelimbs lacked the necessary rotary motion range.[3]

See also

  • Evolutionary history of life
  • Hynerpeton
  • List of transitional fossils
  • List of prehistoric amphibians
  • Ymeria

References

  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. ^
    S2CID 3127857
    .
  4. ^ .
  5. ^ .
  6. .
  7. ^ Devonian Times – Tetrapods Answer
  8. ^ 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.
  9. ^ 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.

Further reading

External links