Eusthenopteron; pronounced YOU-sthen-OP-teh-ron
Oceans of the northern hemisphere
Late Devonian (385 million years ago)
Size and Weight:
About 4 feet long and 50 pounds
Large size; limb-like fin structure
To the untrained eye, the fossil of Eusthenopteron might resemble that of an ordinary, albeit 385-million-year-old, fish.
What makes this early tetrapod a prize catch among paleontologists is that the skeletal structure of its fins includes primitive versions of various bones (femurs, ulnae, etc.) found in the arms and legs of modern, terrestrial animals. There are other similarities as well, in similarly primitive form: internal nostrils, enameled teeth, and a two-part cranium, to name just three.
So was Eusthenopteron an ocean-dwelling prehistoric fish, or an amphibious tetrapod? Opinion has seesawed back and forth, but it's now believed that this creature spent all of its time in the shallow seas of the Devonian period, and represented an important intermediate stage (the more dramatic term would be "missing link") between fish and early tetrapods.