Eusthenopteron

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Eusthenopteron Fossil range: Late Devonian
life restoration of Eusthenopteron foordi
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Sarcopterygii Subclass: Tetrapodomorpha Superorder: Osteolepidida Family: Tristichopteridae Genus: Eusthenopteron Whiteaves, 1881
Species
E. foordi (type) E. savesoderberghi E. waengsjoei E. wenjukowi

Eusthenopteron was a genus of lobe-finned fish which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a pelagic animal. The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 million years ago. Eusthenopteron was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha, Quebec.

Anatomically, Eusthenopteron shares many unique features in common with the earliest known tetrapods. It shares a similar pattern of skull rooting bones with forms such as Ichthyostega and Acanthostega. Eusthenopteron, like other tetrapodomorph fishes, had internal nostrils, (or a choana) which are found only in land animals and sarcopterygians. It also had labyrinthodont teeth, characterized by infolded enamel, which characterizes all of the earliest known Tetrapods as well. Like other basal sarcopterygians, Eusthenopteron possessed a two-part cranium, which hinged at mid-length along an intracranial joint. Eusthenopteron's notoriety comes from the pattern of its fin endoskeleton, which bears a distinct humerus, ulna, and radius (in the fore-fin) and femur, tibia, and fibula (in the pelvic fin). This is the characteristic pattern seen in tetrapods. It is now known to be a general character of fossil sarcopterygian fins.

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – 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.

Descendants also included pelagic lobe-finned fish such as coelacanth species.

Eusthenopteron differs significantly from later Carboniferous tetrapods in the apparent absence of a recognized larval stage and a definitive metamorphosis. In even the smallest known specimen of Eusthenopteron foordi (at 29 mm), the lepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera like Polyodon. This might indicate that Eusthenopteron developed directly, with the hatchling already attaining the general body form of the adult (Cote et. al, 2002).

[edit] See also

• Tiktaalik, an even more tetrapod-like sarcopterygian.
• Gogonasus

[edit] References

• S. Cote, R. Carroll, R. Cloutier, and L. Bar-Sagi (September 2002). "Vertebral development in the Devonian Sarcopterygian fish Eusthenopteron foordi and the polarity of vertebral evolution in non-amniote tetrapods". Journal of Vertebrate Paleontology 22 (3): 487–502. doi:10.1671/0272-4634(2002)022[0487:VDITDS]2.0.CO;2. 

• R. Cloutier (1996). "Taxonomic review of Eusthenopteron foordi.", Devonian Fishes and Plants of Miguasha, Quebec, Canada. Dr. Friedrich Pfeil, München, 487–502. 

• E. Jarvik (1980). Basic Structure and Evolution of Vertebrates. Academic Pess. 

[edit] External References

• http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.860.html