Eusthenopteron

Eusthenopteron is a genus of prehistoric 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. Some 2,000 Eusthenopteron specimens have been collected from Miguasha, one of which was the object of intensely detailed study and several papers from the 1940s to the 1990s by paleoichthyologist Erik Jarvik.

Description
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.

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).

External References

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