Tiktaalik

Tiktaalik /tɪkˈtɑːlɨk/ is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, with many features akin to those of tetrapods (four-legged animals).[1] The Tiktaalik is understood as representative of the evolutionary transition from fish to amphibians. It is an example from several lines of ancient sarcopterygian fish developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods.[2] The Tiktaalik and animals similar to the Tiktaalik are understood to be the common ancestors of a wide swathe of all terrestrial fauna: amphibians, reptiles, birds, and mammals.[3] Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.

Description
Tiktaalik provides insights on the features of the extinct closest relatives of the tetrapods. Unlike many previous, more fishlike transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. The homology of distal elements is uncertain; there have been suggestions that they are homologous to digits, although this is incompatible with the digital arch developmental model because digits are supposed to be postaxial structures, and only three of the (reconstructed) eight rays of Tiktaalik are post-axial.[4] However, the proximal series can be directly compared to the ulnare and intermedium of tetrapods. The fin was clearly weight bearing, being attached to a massive shoulder with expanded scapular and coracoid elements and attached to the body armor, large muscular scars on the ventral surface of the humerus, and highly mobile distal joints. The bones of the fore fins show large muscle facets, suggesting that the fin was both muscular and had the ability to flex like a wrist joint. These wrist-like features would have helped anchor the creature to the bottom in fast moving current.

Also notable are the spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. This would have been useful in shallow water, where higher water temperature would lower oxygen content. This development may have led to the evolution of a more robust ribcage, a key evolutionary trait of land living creatures.[2] The more robust ribcage of Tiktaalik would have helped support the animal’s body any time it ventured outside a fully aquatic habitat. Tiktaalik also lacked a characteristic that most fishes have—bony plates in the gill area that restrict lateral head movement. This makes Tiktaalik the earliest known fish to have a neck, with the pectoral girdle separate from the skull. This would give the creature more freedom in hunting prey either on land or in the shallows.[6]

Tiktaalik is sometimes compared to gars (esp. Atractosteus spatula, the alligator gar) of the Lepisosteidae family, with whom it shares a number of characteristics:
 * diamond-shaped scale patterns common to the Crossopterygii class (in both species scales are rhombic, overlapping and tuberculated);
 * teeth structured in two rows;
 * both internal and external nostrils;
 * tubular and streamlined body;
 * absence of anterior dorsal fin;
 * broad, dorsoventrally compressed skull;
 * dorsally placed eyes;
 * paired frontal bones;
 * marginal nares;
 * subterminal mouth;
 * lung-like organ

Paleobiology
Tiktaalik generally had the characteristics of a lobe-finned fish, but with front fins featuring arm-like skeletal structures more akin to a crocodile, including a shoulder, elbow, and wrist. The fossil discovered in 2004 did not include the rear fins and tail. It had rows[8] of sharp teeth of a predator fish, and its neck could move independently of its body, which is not common in other fish (Tarrasius, Mandageria, placoderms,[9][10] and extant seahorses being some exceptions; see also Lepidogalaxias and Channallabes apus[11]). The animal had a flat skull resembling a crocodile's; eyes on top of its head, suggesting that it spent a lot of time looking up; a neck and ribs similar to those of tetrapods, with the ribs being used to support its body and aid in breathing via lungs; well developed jaws suitable for catching prey; and a small gill slit called a spiracle that, in more derived animals, became an ear.

The fossils were found in the "Fram Formation", deposits of meandering stream systems near the Devonian equator, suggesting a benthic animal that lived on the bottom of shallow waters and perhaps even out of the water for short periods, with a skeleton indicating that it could support its body under the force of gravity whether in very shallow water or on land.[13] At that period, for the first time, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in.[2] The discoverers said that in all likelihood, Tiktaalik flexed its proto-limbs primarily on the floor of streams and may have pulled itself onto the shore for brief periods.[14] Neil Shubin and Ted Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 2000

“ We're making the hypothesis that this animal was specialized for living in shallow stream systems, perhaps swampy habitats, perhaps even to some of the ponds. And maybe occasionally, using its very specialized fins, for moving up overland. And that's what is particularly important here. The animal is developing features which will eventually allow animals to exploit land. ”

—Ted Daeschler, ==Classification and evolution==