| Tiktaalik | |
|---|---|
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| Cast of theTiktaalikholotype in theField Museum, Chicago | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Elpistostegalia |
| Genus: | †Tiktaalik Daeschler,Shubin &Jenkins,2006 |
| Type species | |
| †Tiktaalik roseae Daeschler, Shubin & Jenkins, 2006 | |
Tiktaalik (/tɪkˈtɑːlɪk/;Inuktitut:ᑎᒃᑖᓕᒃ[tiktaːlik]) is amonospecificgenus ofextinctsarcopterygian (lobe-finned fish) from the lateDevonian Period, about 375 Mya (million years ago), having many features similar to those oftetrapods (four-legged animals).[1]Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) on the basis of various specimens.[2]
Unearthed inArctic Canada,Tiktaalik is a non-tetrapod member ofOsteichthyes (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 allowedTiktaalik 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 markTiktaalik as a crucialtransition fossil, a link inevolution from swimming fish to four-legged vertebrates.[3] This and similar animals might be the common ancestors of allvertebrate terrestrialfauna: amphibians, reptiles, birds and mammals.[4]
The firstTiktaalik fossils were found in 2004 onEllesmere Island inNunavut, Canada. The discovery, made byEdward B. Daeschler of theAcademy of Natural Sciences,Neil H. Shubin from theUniversity of Chicago, andHarvard University ProfessorFarish A. Jenkins Jr., was published in the April 6, 2006 issue ofNature[1] and quickly recognized as a transitional form.
In 2004, three fossilizedTiktaalik skeletons were discovered in the Late Devonian fluvialFram Formation onEllesmere Island,Nunavut, innorthern 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 continentLaurentia (modern easternNorth America andGreenland),[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 ofNature[1] and quickly recognized as a transitional form.Jennifer A. Clack, aCambridge University expert on tetrapod evolution, said ofTiktaalik, "It's one of those things you can point to and say, 'I told you this would exist,' and there it is."[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. 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 anInuktitut word meaning "large freshwater fish".[4] The "fishapod" genus received this name after a suggestion byInuit elders ofCanada'sNunavut Territory, where the fossil was discovered.[7] The specific nameroseae honours an anonymous donor.[12] Taking a detailed look at the internal head skeleton ofTiktaalik roseae, in the October 16, 2008, issue ofNature,[13] researchers show howTiktaalik 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 ofTiktaalik have been discovered, though the holotype remains the most complete and well-described fossil.[15]
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]
The skull ofTiktaalik was low and flat, more similar in shape to that of asalamander than most fish. The rear edge of the skull was excavated by a pair of indentations known asotic notches. These notches may have housedspiracles 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 makesTiktaalik the earliest-known fish to have a neck, with thepectoral (shoulder) girdle separate from the skull. This would give the creature more freedom in hunting prey on land or in the shallows.[10]
The "fins" ofTiktaalik have helped to contextualize the origin of weight-bearing limbs anddigits. The fin has both a robust internal skeleton, like tetrapods, surrounded by a web of simple bony finrays (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 expandedscapular andcoracoid 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]
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 livingcoelacanths and theAustralian 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 tohomologize with the less linear construction of tetrapod lower limbs.
Tiktaalik retains a metapterygial axis with distinctly enlarged axial bones, a very fish-like condition. EvenPanderichthys, which is otherwise more fish-like, seems to be more advanced towards a tetrapod-like limb.[16] Nevertheless, the internal skeleton ofTiktaalik'spectoral 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 thehumerus, the single large bone making up thestylopodium (upper arm). This is followed by the two bones of thezeugopodium (forearm): theradius (i.e., the first preaxial radial) andulna (i.e., the second axial). The radius is much larger than the ulna, and its front edge thins into a sharp blade like that ofPanderichthys.[1][16]
Further down, the internal skeleton transitions into themesopodium, which in tetrapods contains the bones of the wrist.Tiktaalik has two large wrist bones: the narrowintermedium (i.e., the second preaxial radial) and the blockyulnare (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]
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.[18][20][21][22] A third hypothesis, emphasized by Shubin and Alberch (1986), is that digits are homologous to postaxial radials in particular.[23][24] This interpretation, better known as the digital arch model, is supported by numerous developmental studies. A consistent set ofHox 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.[19][25][26][27][28][29] 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.[18][23]
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 ofTiktaalik are homologous to fingers.[18] Finger-like distal radials are also known in other elpistostegalians:Panderichthys (which has at least four)[16] andElpistostege (which has 19).[15]
As with other regions of the body, thepelvis (hip) was intermediate in form between earlier lobe-finned fish (likeGooloogongia andEusthenopteron) and tetrapods (likeAcanthostega). 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 upperiliac blade continuous with a low semi-cartilaginouspubic process in front of theacetabulum (hip socket). This contrasts with the more complex pelvis of tetrapods, which have three separate bones (the ilium, pubis andischium) making up the hip. In addition, in tetrapods the left and right pelvises often connect to each other or the spinal column, while inTiktaalik 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.[30]
The hindlimbs, also known aspelvic 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 ofTiktaalik 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.[30]
The torso ofTiktaalik 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 inIchthyostega, though in that taxon the ribs are more diverse in shape.[1]
Tiktaalik most likely lackeddorsal fins, like other elpistostegalians as well as tetrapods. The shape of the tail andcaudal fin are unknown, since that portion of the skeleton has not been preserved. Many lobe-finned fish have a singleanal fin on the underside of the tail, behind the pelvic fins. While not reported inTiktaalik, an anal fin can be observed inElpistostege, a close relative.[15]
Tiktaalik was covered by rhombic (diamond-shaped) bonyscales, most similar toPanderichthys among lobe-finned fish. The scales are roughly textured, slightly broader than long, and overlap from front-to-back.[1]
Strong lungs (as supported by the plausible presence of a spiracle) may have led to the evolution of a more robustribcage, a key evolutionary trait of land-living creatures.[31] The more robust ribcage ofTiktaalik would have helped support the animal's body any time it ventured outside a fully aquatic habitat.[10]
Tiktaalik is sometimes compared togars (especially thealligator gar), with whom it shares a number of characteristics:[32]
Tiktaalik roseae is the onlyspecies classified under the genus.Tiktaalik lived approximately 375 million years ago. It is representative of thetransition between non-tetrapod vertebrates (fish) such asPanderichthys, known from fossils 380 million years old, and early tetrapods such asAcanthostega andIchthyostega, known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterizeTiktaalik as a "fishapod".[8][33]
Tiktaalik is atransitional fossil; it is to tetrapods whatArchaeopteryx is tobirds. While it may be that neither is ancestor to any living animal, they serve as evidence that intermediates between very different types of vertebrates did once exist. The mixture of both fish and tetrapod characteristics found inTiktaalik include these traits:
Thephylogenetic analysis of Daeschler et al. (2006) placedTiktaalik as asister taxon toElpistostege and directly abovePanderichthys, which was preceded byEusthenopteron.Tiktaalik was thus inserted belowAcanthostega andIchthyostega, acting as a transitional form between limbless fish and limbed vertebrates ("tetrapods").[1] Some press coverage also used the term "missing link", implying thatTiktaalik filled an evolutionary gap between fish and tetrapods.[34] Nevertheless,Tiktaalik has never been claimed to be a direct ancestor to 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 its original description,Tiktaalik was described as a member ofElpistostegalia, a name previously used to refer to particularly tetrapod-like fish such asElpistostege andPanderichthys. Daeschler et al. (2006) recognized that this term referred to aparaphyleticgrade of fish incrementally closer to tetrapods. Elpistostegalian fish have few unique traits which are not retained from earlier fish or inherited by later tetrapods.
In response, Daescler et al. (2006) redefined Elpisostegalia as aclade, including all vertebrates descended from thecommon ancestor ofPanderichthys,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. In this sense,Tiktaalik is an elpistostegalian fish.[1] Later papers also use the term "elpisostegid" for the same category of Devonian fish.[35][36]
This order of thephylogenetic tree was initially adopted by other experts, most notably byPer Ahlberg andJennifer Clack.[37] However, it was questioned in a 2008 paper by Boisvert et al., who noted thatPanderichthys, due to its morederiveddistal forelimb structure, might be closer to tetrapods thanTiktaalik or even that it wasconvergent with tetrapods.[16] Ahlberg, co-author of the study, considered the possibility ofTiktaalik's fin having been "an evolutionary return to a more primitive form."[38]
The proposed origin of tetrapods among elpistostegalian fish was called into question by a discovery made in theHoly Cross Mountains ofPoland. In January 2010, a group of paleontologists (including Ahlberg) published on a series of trackways from theEifelian stage of the Middle Devonian, about 12 million years older thanTiktaalik.[35][39] These trackways, discovered at theZachełmie quarry, appear to have been created by fully terrestrial tetrapods with a quadrupedal gait.[35]
Tiktaalik's discoverers were skeptical about the Zachelmie trackways. Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution,[40] while Shubin argued thatTiktaalik 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.[42] However, Ahlberg insisted that those tracks could not have possibly been formed either by natural processes or by transitional species such asTiktaalik orPanderichthys.[35][43] Instead, the authors of the publication suggested that "ichthyostegalian"-grade tetrapods were the responsible trackmakers, based on availablepes morphology of those animals.[35]
Narkiewicz, co-author of the article on the Zachelmie trackways, claimed that the Polish "discovery has disproved the theory thatelpistostegids were the ancestors of tetrapods",[44] a notion partially shared byPhilippe Janvier.[45] To resolve the questions posed by the Zachelmie trackways, several hypotheses have been suggested. 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. This maintains the elpistostegalian–tetrapod ancestor–descendant relationship apparent in fossils, but also introduces longghost lineages required to explain the apparent delay in fossil appearances.[35] Another approach is that elpistostegalian and tetrapod similarities are a case ofconvergent evolution. In this interpretation, tetrapods would originate in the Middle Devonian while elpisostegalians originate independently in the Late Devonian, before going extinct near the end of the period.[46][47][48][49]
Estimates published after the discovery of Zachelmie tracks suggested that digited tetrapods may have appeared as early as 427.4 Mya and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy.[47]
However, a reanalysis of the Zachelmie trackways in 2015 suggested that they do not constitute movement trackways, but should rather be interpreted as fish nests or feeding traces.[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 acrocodile, including ashoulder,elbow andwrist. The fossil discovered in 2004 did not include the rear fins and tail, which were found in other specimens. It had rows[51] of sharp teeth indicative of a predator fish, and its neck could move independently of its body, which is not common in other fish (Tarrasius,Mandageria,placoderms[52][53] and extantseahorses being some exceptions; see alsoLepidogalaxias andChannallabes apus[54]). 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 inbreathing vialungs; well developed jaws suitable for catching prey; and a smallgill slit called a spiracle that, in morederived animals, became anear. Spiracles would have been useful in shallow water, where higher water temperature would lower oxygen content.[55]
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.[56] In 2014, the discovery of the animal's pelvic girdle was announced; it was strongly built, indicating the animal could have used them for moving in shallow water and across mudflats.[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. 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.[58]
The fossils ofTiktaalik were found in theFram Formation, deposits of meandering stream systems near the Devonian equator, suggesting abenthic 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.[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.[31]
VariousInternet memes criticizeTiktaalik for its evolutionary adaptations, construing its venture onto land as the critical cause for a chain of events that eventually led to all human suffering.[60]
Other lobe-finned fish found in fossils from the Devonian Period:
Given that recent phylogenies consistently placePanderichthys belowTiktaalik in the tetrapod stem group, it is surprising to discover that its pectoral fin skeleton is more limb-like than that of its supposedly more derived relative. [...] It is difficult to say whether this character distribution implies thatTiktaalik is autapomorphic, thatPanderichthys and tetrapods areconvergent, or thatPanderichthys is closer to tetrapods thanTiktaalik.
{{citation}}: CS1 maint: work parameter with ISBN (link){{citation}}: CS1 maint: work parameter with ISBN (link)Curiously, the radial bones ofPanderichthys are more finger-like than those ofTiktaalik, a fish with stubby leg-like limbs that lived about five million years later. Many scientists regardTiktaalik as a "missing link": the crucial transitional animal between fish and the first tetrapods. One possibility, Ahlberg said, is that finger development took a step backward withTiktaalik, and thatTiktaalik's fins represented an evolutionary return to a more primitive form.
It follows that the attribution of some of the nondigited Devonian fossil trackways to limbed tetrapods may need to be revisited.
It is possible that the close similarity between elpistostegids and tetrapods might have been the result of evolutionary convergence. The common ancestor of elpistostegids and tetrapods wouldn't have to have looked like Tiktaalik – it could have been a more undifferentiated, tetrapodomorph fish. Elpistostegids and tetrapodomorphs, each following their own paths, grew to look more and more like one other.
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