Acanthostega

Name:

Acanthostega (Greek for "spiky roof"); pronounced ah-CAN-tho-STAY-gah

Habitat:

Rivers and swamps of the northern latitudes

Historical Period:

Late Devonian (360 million years ago)

Size and Weight:

About two feet long and 5-10 pounds

Diet:

Probably fish

Distinguishing Characteristics:

Stubby legs; long tail; eight digits on front flippers

About Acanthostega

One of the best-known of all the Devonian tetrapods--the first, lobe-finned fish that climbed up out of the water and onto dry land--Acanthostega nevertheless seems to have represented a dead end in the evolution of early vertebrates, the giveaway being that this creature had eight primitive digits on each of its stubby front flippers, compared to the modern standard of five. Also, despite its classification as an early tetrapod, it's possible to oversell the extent to which Acanthostega was a land animal. To judge by certain anatomical features--such as its fish-like teeth and the "lateral line" sensory apparatus running along the length of its slender body--this tetrapod probably spent most of its time in shallow water, using its rudimentary legs only to crawl from puddle to puddle.

There's another, alternate, explanation for Acanthostega's anatomy: perhaps this tetrapod didn't walk, or crawl, at all, but rather used its eight-digit forelimbs to navigate weed-choked swamps (during the Devonian periods, land plants began, for the first time, to shed leaves and other detritus into nearby pools of water) in pursuit of prey. In this case, the forelimbs of Acanthostega would be a classic example of "pre-adaptation": they didn't evolve specifically for the purpose of walking on land, but came in handy (if you'll excuse the pun) when later tetrapods, descended from Acanthostega, finally made that evolutionary leap. (This scenario would also account for Acanthostega's internal gills, as well as its weak ribs, which made it unable to poke its chest fully out of the water.)