Tiktaalik (/tɪkˈtɑːlɪk/; Inuktitut ᑎᒃᑖᓕᒃ [tiktaːlik]) 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).

[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.

Those fins and other mixed characteristics mark Tiktaalik as a crucial transition fossil, a link in evolution from swimming fish to four-legged vertebrates.

In 2004, three fossilized Tiktaalik skeletons were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island, Nunavut, in northern Canada.

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.

"[10] 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.

[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.

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.

As predicted by the digital arch model, there are multiple (at least eight) rectangular distal radials arranged in a dispersed pattern, similar to fingers.

[15] As with other regions of the body, the pelvis (hip) was intermediate in form between earlier lobe-finned fish (like Gooloogongia and Eusthenopteron) and tetrapods (like Acanthostega).

There is a broad upper iliac blade continuous with a low semi-cartilaginous pubic process in front of the acetabulum (hip socket).

In addition, in tetrapods the left and right pelvises often connect to each other or the spinal column, while in Tiktaalik each side of the pelvis is fully separate.

[1] Strong lungs (as supported by the plausible presence of a spiracle) may have led to the evolution of a more robust ribcage, a key evolutionary trait of land-living creatures.

Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod".

Tiktaalik was thus inserted below Acanthostega and Ichthyostega, acting as a transitional form between limbless fish and limbed vertebrates ("tetrapods").

[1] Some press coverage also used the term "missing link", implying that Tiktaalik filled an evolutionary gap between fish and tetrapods.

Rather, its fossils help to illuminate evolutionary trends and approximate the hypothetical true ancestor to the tetrapod lineage, which would have been similar in form and ecology.

In response, Daescler et al. (2006) redefined Elpisostegalia as a clade, including all vertebrates descended from the common ancestor of Panderichthys, Elpistostege and tetrapods.

Nevertheless, they still retained the phrase "elpistostegalian fish" to refer to the grade of early elpisostegalians which had not acquired limbs, digits, or other specializations which define tetrapods.

"[38] The proposed origin of tetrapods among elpistostegalian fish was called into question by a discovery made in the Holy Cross Mountains of Poland.

In January 2010, a group of paleontologists (including Ahlberg) published on a series of trackways from the Eifelian stage of the Middle Devonian, about 12 million years older than Tiktaalik.

Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution,[40] while Shubin argued that Tiktaalik could have produced very similar footprints.

[41] In a later study, Shubin expressed a significantly modified opinion that some of the Zachelmie footprints, those which lacked digits, may have been made by walking fish.

[35] Narkiewicz, co-author of the article on the Zachelmie trackways, claimed that the Polish "discovery has disproved the theory that elpistostegids were the ancestors of tetrapods",[44] a notion partially shared by Philippe Janvier.

One approach maintains that the first pulse of elpistostegalian and tetrapod evolution occurring in the Middle Devonian, a time when body fossils showing this trend are too rare to be preserved.

[50] Tiktaalik generally had the characteristics of a lobe-finned fish, but with front fins featuring arm-like skeletal structures more akin to those of a crocodile, including a shoulder, elbow and wrist.

The animal had a flat skull resembling a crocodile's; eyes on top of its head; 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.

[57] Neil Shubin and Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 2000:[8][9]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.

[59] 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.