Tiktaalik (; ) is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian Period, about 375 Mya (million years ago), having many features similar to those of tetrapods (four-legged animals). Tiktaalik is estimated to have had a total length of on the basis of various specimens.
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. This and similar animals might be the common ancestors of all vertebrate terrestrial fauna.
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 and quickly recognized as a transitional form. Tiktaalik has subsequently become one of the most widely known fossils of the fish-tetrapod transition, along with Acanthostega and Ichthyostega from Greenland.
Discovery
In 2004, three fossilized Tiktaalik remains were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island, Nunavut, in northern Canada. 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), 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.
The discovery by Daeschler, Shubin and Jenkins was published in the April 6, 2006 issue of Nature 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."
Tiktaalik is an Inuktitut word meaning "large freshwater fish". The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered. The specific name roseae honours an anonymous donor. Taking a detailed look at the internal head skeleton of Tiktaalik roseae, in the October 16, 2008, issue of Nature, 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. More than 60 specimens of Tiktaalik have been discovered, though the holotype remains the most complete and well-described fossil.
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 , with the longest lower jaws reaching a length of .
Skull and neck The skull of Tiktaalik was low and flat, more similar in shape to that of a salamander 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.
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. 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. 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.
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 a unit known as the metapterygial axis. The component bones of this axis, arranged in a single line, 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). The semi-symmetrical form of a lobed fin is difficult to homologize with the less linear construction of tetrapod lower limbs.
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. Nevertheless, the internal skeleton of Tiktaalik's pectoral fin can still be equated to the forelimb bones of tetrapods. The first and largest 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.
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. In the early 20th century, most paleontologists considered the digits to develop symmetrically from radials at the tip of the fin. Another school of thought, popularized in the 1940s, argued that the hand was neomorphic. In other words, it was an entirely new structure that spontaneously evolved after the distal axials and radials were reduced. A third hypothesis, emphasized by Shubin and Alberch (1986), is that digits are homologous to postaxial radials in particular. This interpretation, better known as the digital arch model, is supported by numerous developmental studies. A consistent set of Hox genes are responsible for moderating both the rear edge of the fin (in several modern fish) and the digits of modern tetrapods as their embryos develop. The digital arch model posits that the metapterygial axis was bent forwards at a sharp angle near the origin of tetrapods. This allowed the axials to transform into wrist bones, while the narrower postaxial radials splay out and evolve into fingers.
Tiktaalik presents a contradictory set of traits. As predicted by the digital arch model, there are multiple (at least eight) rectangular distal radials arranged in a dispersed pattern, similar to fingers. Some of the radials are even arranged sequentially, akin to finger joints. However, the metapterygial axis is straight and runs down the middle of the fin. Only three of the finger-like radials appear to be postaxial, while the model predicts that most or all of the radials should be postaxial. It remains to be seen whether any of the distal radials of Tiktaalik are homologous to fingers. Finger-like distal radials are also known in other elpistostegalians: Panderichthys (which has at least four) and Elpistostege (which has 19).
Hip and hindlimbs 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). The pelvis was much larger than in other fish, nearly the same size as the shoulder girdle, like tetrapods. In terms of shape, the pelvis is a single bone, much more similar to fish. There is a broad upper iliac blade continuous with a low semi-cartilaginous pubic process in front of the acetabulum (hip socket). This contrasts with the more complex pelvis of tetrapods, which have three separate bones (the ilium, pubis and ischium) making up the hip. 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. The orientation of the hip socket is halfway between the rear-facing socket of other fish and the sideways-facing socket of tetrapods.
The hindlimbs, also known as pelvic fins, appear to be almost as long as the forelimbs. This is yet another trait more similar to tetrapods than to other fish. Though not all bones are preserved in the fossil, it is clear that the hindlimbs of Tiktaalik had lepidotrichia and at least three large rod-like ankle bones. If fully preserved, the pelvic fins would probably have been internally and externally very similar to the pectoral fins. The Specimen MCZ 11916 from the Museum of Comparative Zoology reveals a well-preserved femur with broad, well defined preaxial and postaxial margins. The femur expands distally into three articular facets, each connecting to separate radial elements with varying shapes. The anterior radial tapers distally and doesn't appear to articulate further, the middle radial has a constructed shaft, and the posterior radial displays a pronounced flange. Both the middle and posterior radials exhibit expanded distal ends, and all three radials are elongate, cylindrical bones with well-developed outer bone layers. The ends of the femur and radial have become completely ossified, so this preservation indicates a likelihood of a previous presence of cartilaginous joint surfaces. The fin, in general, is associated with a plentiful number of scales and fin rays (Lepidotrichia), the basal segments of which are elongated and overlap a decent portion of the internal skeleton beyond the femur. This condition can also be seen in rhizodontid fins.
Torso The torso of Tiktaalik is elongated by the standards of most Devonian tetrapodomorphs. Although the vertebrae are not ossified, there are about 45 pairs of ribs between the skull and the hip region. The ribs are larger than in earlier fish, imbricating (overlapping) via blade-like flanges. Imbricating ribs are also known in Ichthyostega, though in that taxon the ribs are more diverse in shape.
Tiktaalik most likely lacked dorsal fins, like other elpistostegalians as well as tetrapods. The shape of the tail and caudal fin are unknown, since that portion of the skeleton has not been preserved. Many lobe-finned fish have a single anal fin on the underside of the tail, behind the pelvic fins. While not reported in Tiktaalik, an anal fin can be observed in Elpistostege, a close relative.