Eusthenopteron

Eusthenopteron (from εὖ, 'good', σθένος , 'strength', and πτερόν 'wing' or 'fin') is a genus of prehistoric sarcopterygian (often called "lobe-finned") fish known from several species that lived during the Late Devonian period, about 385 million years ago. It has attained an iconic status from its close relationship to tetrapods. Early depictions of animals of this genus show them emerging onto land, but paleontologists now widely agree that eusthenopteron species were strictly aquatic animals.

The genus was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha, Quebec, Canada. Some 2,000 Eusthenopteron specimens have been collected from Miguasha, one of which was the object of intensely detailed study and several papers by paleoichthyologist Erik Jarvik between the 1940s and the 1990s.

Description
Eusthenopteron is a medium- to large-sized tristichopterid. The species E. foordi is estimated to have exceeded 1.5 m in length, while the species E. jenkinsi probably reached 2.1 m.

The earliest known fossilized evidence of bone marrow has been found in Eusthenopteron, which may be the origin of bone marrow in tetrapods. Eusthenopteron shares many unique features among fishes but in common with the earliest-known tetrapods. It shares a similar pattern of skull roofing bones with stem tetrapoda forms such as Ichthyostega and Acanthostega. Eusthenopteron, like other tetrapodomorph fishes, had internal nostrils (or a choana), one of the defining traits of tetrapodomorphs, including tetrapods. It also had labyrinthodont teeth, characterized by infolded enamel, which characterizes all of the earliest known tetrapods as well.

Unlike the early tetrapods, Eusthenopteron did not have larval gills.

Classification
Like other fish-like 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. These appendicular long bones had epiphyseal growth plates that allowed substantial longitudinal growth through endochondral ossification, as in tetrapod long bones. These six appendicular bones also occur in tetrapods and are a synapomorphy of a large clade of sarcopterygians, possibly Tetrapodomorpha (the humerus and femur are present in all sarcopterygians). Similarly, its elasmoid scales lack superficial odontodes composed of dentine and enamel; this loss appears to be a synapomorphy with more crownward tetrapodomorphs.

Eusthenopteron differs significantly from some 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, with a length of 29 mm, the lepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera like Polydon (the American paddlefish). This might indicate that Eusthenopteron developed directly, with the hatchling already attaining the adult's general body form (Cote et al., 2002).
 * 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.