Athena Review Image Archive ™
Eusthenopteron fordii skeleton
Skeleton of Eusthenopteron fordii (after Jarvik 1980).
Eusthenopteron
fordii is a species of sarcopterygian (lobe-finned) fishes dating from the Late Devonian period (385 mya), with a close
relationship to tetrapods. It is in the class
Sarcoptergerii, the subclass Tetrapodomorpha, the superorder Osteolepidida, and the family Tristichopteridae.
Eusthenopteron was first described by J. F. Whiteaves in 1881 as part of a large collection of fishes from Miguasha, Quebec. At least 2,000 Eusthenopteron fossils have been found at Miguasha, including those studied between the1940s and 1990s by paleoichthyologist Erik Jarvik, focusing on their close linkage to early tetrapods.
While Eusthepteron was a strictly aquatic animal, it is classed as a Tetrapodomorph, considered the most direct precursors to tetrapods. The largest individuals grew up to 1.8 m (6.5 ft) in length. Eusthenopteron shares many unique features with the earliest known tetrapods. It has a similar pattern of skull roofing bones with Ichthyostega and Acanthostega. Importantly, its fin endoskeleton also bears a distinct humerus, ulna, and radius.
Eusthenopteron had labyrinthodont teeth, characterized by infolded enamel, which characterizes all of the earliest known tetrapods. Like other fish-like sarcopterygians, Eusthenopteron possessed a two-part cranium, which hinged at mid-length along an intracranial joint. It also had internal nostrils (or a choana); which are found only in land animals and sarcopterygians.
The reptilian
stapes or middle ear ossicle seems to have evolved from the hyomandibular bone of
sacropterygians like Eusthenopteron, which has prominent hyomandibular
facets. Differing morphology of the stapes vs. the hyomandibular relates
to the orbital (stapedial) artery.
References:
Clack 1994
Janvier 1996
Jarvik 1980
Smithson, T.R. and K.S. Thompson 1982 The hyomandibular of Eusthenopteron foordi Whiteaves (Pisces: Crossopterygii) and the early evolution of the tetrapod stapes
Werstoll 1943
.Copyright © 1996-2020 Rust Family Foundation (All Rights Reserved).