Tiktaalik (indigenous Indian for a freshwater fish); pronounced tick-TAH-lick
Shallow seas of the northern hemisphere
Late Devonian (375 million years ago)
Size and Weight:
About 3 to 9 feet long and 10 to 50 pounds
Long, thin body; possible lungs
One of the more famous tetrapod discoveries of recent times, the well-preserved remains of Tiktaalik were unearthed in late Devonian deposits in northern Canada in 2004. Its claim to fame is that it neatly fills in a troublesome gap in tetrapod evolution, perched midway between true fish with faintly tetrapod-like features (such as Eusthenopteron and Panderichthys) and true early tetrapods like Acanthostega. For this reason, it's referred to by some experts as a "fishapod."
The most remarkable feature of Tiktaalik is that its "fins" were supported by tetrapod-like bones--for example, the front pair had primitive, wrist-like structures, which would have enabled this fish to prop itself up and take quick gasps of air. Which leads to another remarkable thing about Tiktaalik, and lobe-finned fish and tetrapods in general: the presence of spiracles (primitive nostrils) on top of its head, which point to its having had primitive lungs as well as gills. Tiktaalik also seems to have been be the first vertebrate with what we can recognizably call a neck, which would have endowed it with much-needed flexibility during its short jaunts onto dry land.
Recently, researchers unearthed the rear portion of a Tiktaalik individual (previous fossils had only revealed the head and front limbs). Unexpectedly, this specimen was found to possess unusually well-developed hind legs, with especially strong hip bones, thus making it only a short evolutionary leap from Tiktaalik to the more advanced tetrapods that succeeded it a few million years later. It's even possible, experts surmise, that the earliest tetrapods may have possessed hind limbs (and walked underwater) even as they led a fully aquatic existence!