Tiktaalik (/tɪkˈtɑ:lɪk/; Inuktitut ᑎᒃᑖᓕᒃ [tikta:lik]) is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian Period, about 375 Mya (million years ago), having many features akin to those of tetrapods (four-legged animals).[1] Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) based on various specimens.[2]

Unearthed in Arctic Canada, Tiktaalik is a non-tetrapod member of Osteichthyes (bony fish), complete with scales and gills—but it has a triangular, flattened head and unusual, cleaver-shaped fins. Its fins have thin ray bones for paddling like most fish, but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. Those fins and other mixed characteristics mark Tiktaalik as a crucial transition fossil, a link in evolution from swimming fish to four-legged vertebrates.[3] This and similar animals might be the common ancestors of all vertebrate terrestrial fauna: amphibians, reptiles, birds and mammals.[4]

The first Tiktaalik fossils were found in 2004 on Ellesmere Island in Nunavut, Canada. The discovery, made by Edward B. Daeschler of the Academy of Natural Sciences, Neil H. Shubin from the University of Chicago, and Harvard University Professor Farish A. Jenkins Jr., was published in the April 6, 2006, issue of Nature[1] and quickly recognized as a transitional form.

Discovery

Discovery site of Tiktaalik fossils
In 2004, three fossilized Tiktaalik skeletons were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island, Nunavut, in northern Canada.[5][6] Estimated ages were reported at 375 Ma, 379 Ma and 383 Ma. At the time of the species' existence, Ellesmere Island was part of the continent Laurentia (modern eastern North America and Greenland),[7] which was centered on the equator and had a warm climate. When discovered, one of the skulls was found sticking out of a cliff. Upon further inspection, the fossil was found to be in excellent condition for a 375-million-year-old specimen.[8][9]

The discovery by Daeschler, Shubin and Jenkins was published in the April 6, 2006, issue of Nature[1] and quickly recognized as a transitional form. Jennifer A. Clack, a Cambridge University expert on tetrapod evolution, said of Tiktaalik, "It's one of those things you can point to and say, 'I told you this would exist,' and there it is."[10]


Neil Shubin, one of the paleontologists who discovered Tiktaalik, holding a cast of its skull
After five years of digging on Ellesmere Island, in the far north of Nunavut, they hit pay dirt: a collection of several fish so beautifully preserved that their skeletons were still intact. As Shubin's team studied the species they saw to their excitement that it was exactly the missing intermediate they were looking for. 'We found something that really split the difference right down the middle,' says Daeschler.

— [11]
Tiktaalik is an Inuktitut word meaning "large freshwater fish".[4] The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered.[7] The specific name roseae honours an anonymous donor.[12] Taking a detailed look at the internal head skeleton of Tiktaalik roseae, in the October 16, 2008, issue of Nature,[13] researchers show how Tiktaalik was gaining structures that could allow it to support itself on solid ground and breathe air, a key intermediate step in the transformation of the skull that accompanied the shift to life on land by our distant ancestors.[14] More than 60 specimens of Tiktaalik have been discovered, though the holotype remains the most complete and well-described fossil.[15]

Description
Tiktaalik provides insights on the features of the extinct closest relatives of the tetrapods. Tiktaalik was a large fish: the largest known fossils have an estimated length of 2.75 m (9.02 feet),[2] with the longest lower jaws reaching a length of 31 centimetres (1.0 ft).[1]

Skull and neck

Skull showing otic notches above the eyes
The skull of Tiktaalik was low and flat, more similar in shape to that of a crocodile than most fish. The rear edge of the skull was excavated by a pair of indentations known as otic notches. These notches may have housed spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. Tiktaalik also lacked a characteristic 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 (shoulder) girdle separate from the skull. This would give the creature more freedom in hunting prey on land or in the shallows.[10]

Forelimbs
The "fins" of Tiktaalik have helped to contextualize the origin of weight-bearing limbs and digits. The fin has both a robust internal skeleton, like tetrapods, surrounded by a web of simple bony fin rays (lepidotrichia), like fish.[1] The lepidotrichia are thickest and most extensive on the front edge and upper side of the fin, leaving more room for muscle and skin on the underside of the fin.[2] The pectoral fin was clearly weight bearing, being attached to a massive shoulder girdle with expanded scapular and coracoid elements attached to the body armor. Moreover, there are large muscle scars on the underside of the forefin bones, and the distal joints of the wrist are highly mobile. Together, these suggest 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 a fast current.[8][10]


Pectoral fin of the Australian lungfish (Neoceratodus fosteri), showing an anatomy common to many lobe-finned fish. Note the midline metapterygial axis (with rectangular axials), branching radials, and very thin fin rays (lepidotrichia)

Comparisons between Devonian tetrapodomorphs during the transition from pectoral fins to forelimbs. Tiktaalik is at the middle.
One of the persistent questions facing paleontologists is the evolution of the tetrapod limb: specifically, how the internal bones of lobed fins evolved into the feet and toes of tetrapods. In many lobe-finned fish, including living coelacanths and the Australian lungfish, the fin skeleton is based around a straight string of midline bones, making up the metapterygial axis. The component bones of the axis are known as axials or mesomeres. The axis is flanked by one or two series of rod-like bones known as radials. Radials can be characterized as preaxial (in front of the axials) or postaxial (behind the axials). This semi-symmetrical structure is difficult to homologize with the more splayed lower limbs of tetrapods.

Tiktaalik retains a metapterygial axis with distinctly enlarged axial bones, a very fish-like condition. Even Panderichthys, which is otherwise more fish-like, seems to be more advanced towards a tetrapod-like limb.[16] Nevertheless, the internal skeleton of the pectoral fin can still be equated to the forelimb bones of tetrapods. The first axial, at the base of the fin, has developed into the humerus, the single large bone making up the stylopodium (upper arm). This is followed by the two bones of the zeugopodium (forearm): the radius (i.e., the first preaxial radial) and ulna (i.e., the second axial). The radius is much larger than the ulna, and its front edge thins into a sharp blade like that of Panderichthys.[1][16]

Further down, the internal skeleton transitions into the mesopodium, which in tetrapods contains the bones of the wrist. Tiktaalik has two large wrist bones: the narrow intermedium (i.e., the second preaxial radial) and the blocky ulnare (i.e., the third axial). In tetrapods, the wrist is followed by the hand and finger bones. The origin of these bones has long been a topic of contention.[17][18][19]


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