Acanthostega gunnari

Acanthostega gunnari from Wikipedia

Name: Acanthostega gunnari [Gunnar's spiny-plate]

When: Latest Devonian Period, about 365 million years ago

Where: South Celsus Berg, Greenland

Claim to fame: Acanthostega is the most primitive known vertebrate with fingers and toes, but don't get too excited. In most respects, they lived the lives of fresh-water fish. With open gill slits, they wouldn't have survived long out of water. Maybe they were capable of crawling onto land briefly to sun themselves or snap up a morsel, but in most respects, but that would be all. Indeed, the joints of their arms and legs show that they could only have been used as paddles, not for walking.

So why fingers and toes, instead of fin-rays? Apparently this resulted from a genetic accident that reversed the domains of two genes (hoxd11 and hoxd13) that control limb development. Actually, Acanthostega was an overachiever where digits were concerned, with eight fingers and seven toes. Only later did the maximum number of vertebrate digits stabilize at five. Evolving lineages don't have feelings, but if they did, Acanthostega's might have been looking at its fingers and toes asking, "What am I supposed to do with these?" It took vertebrates about twenty million years to answer that question, for it was only in the Carboniferous that we find fossils of vertebrates capable of planting their hands and feet on the ground, lifting their bodies, and walking purposefully.

At the rear of the skull, framed by embayments of the skull roof, are openings for the spiracles, accessory breathing openings that correspond to our middle ears and eustachian tubes. Eventually, land vertebrates would skin over these openings to create ear-drums and proper middle ears, but for Acanthostega they were used fish-style for taking in or expelling water or air in breathing. If you've ever watched sting-rays breathing in an aquarium, you've seen how this works.