Temporal range: Late Devonian,
|Tiktaalik in the Field Museum, Chicago|
Daeschler, Shubin & Jenkins, 2006
Daeschler, Shubin & Jenkins, 2006
Tiktaalik (//; 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).
Unearthed in Arctic Canada, Tiktaalik is technically a fish, complete with scales and gills – but it has a triangular, flattened head and unusual 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 a suite of other characteristics set Tiktaalik apart as something special; it has a combination of features that show the evolutionary transition between swimming fish and their descendants, the four-legged vertebrates – a clade which includes amphibians, reptiles, birds and mammals.
The first well-preserved 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 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 postaxial.
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 forefins 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 attribute 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. 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.
- 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;
- paired frontal bones;
- marginal nares;
- subterminal mouth;
- lung-like organ.
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 fossil discovered in 2004 did not include the rear fins and tail. It had rows 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, and extant seahorses being some exceptions; see also Lepidogalaxias and Channallabes apus). 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.
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. 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. 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. 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. Neil Shubin and 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.
Classification and evolution
Tiktaalik roseae is the only species classified under the genus. Tiktaalik lived approximately 375 million years ago. It is representative of the transition between non-tetrapod vertebrates (fish) such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, 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 characterize Tiktaalik as a "fishapod".
Tiktaalik is a transitional fossil; it is to tetrapods what Archaeopteryx is to birds, troodonts and dromaeosaurids. 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 in Tiktaalik include these traits:
- fish gills
- fish scales
- fish fins
- half-fish, half-tetrapod limb bones and joints, including a functional wrist joint and radiating, fish-like fins instead of toes
- half-fish, half-tetrapod ear region
- tetrapod rib bones
- tetrapod mobile neck with separate pectoral girdle
- tetrapod lungs
2006 – 2010
The phylogenetic analysis by Daeschler et al. placed Tiktaalik as a sister taxon to Elpistostege and directly above Panderichthys preceded by Eusthenopteron. Tiktaalik was thus inserted below Acanthostega and Ichthyostega as a transitional form and a true "missing link".
Such order of the phylogenetic tree was initially adopted by other experts, most notably by Per Ahlberg and Jennifer Clack. However, it was questioned in a 2008 paper by Boisvert et al., who noted that Panderichthys, due to its more derived distal portion, might be closer to tetrapods than Tiktaalik or even that it was convergent with tetrapods. Ahlberg, co-author of the study, considered the possibility of Tiktaalik's fin having been "an evolutionary return to a more primitive form."
2010 – now
In January 2010, a group of paleontologists including Ahlberg published a paper accompanied by extensive supplementary material (discussed also in a Nature documentary) which showed that the first tetrapods appeared long before Tiktaalik and other elpistostegids. Their conclusions were based on numerous trackways (esp. Muz. PGI 1728.II.16) and individual footprints (esp. Muz. PGI 1728.II.1) discovered at the Zachełmie quarry in the Holy Cross Mountains (Poland). A tetrapod origin of those tracks was suggested based on:
- distinct digits and limb morphology;
- trackways reflecting quadrupedal gait and diagonal walk;
- no body or tail drag marks;
- very wide stride in relation to body length (much beyond that of Tiktaalik or any other fish);
- various size footprints with some unusually big (up to 26 cm wide) indicating body lengths of over 2.5 m.
Track-bearing layers were assigned to the lower-middle Eifelian based on conodont index fossil samples (costatus Zone) and "previous biostratigraphic data obtained from the underlying and overlying strata" with subsequent studies confirming this dating.
Both 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, while Shubin argued that Tiktaalik could have produced very similar footprints (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). However, Ahlberg insisted that those tracks could not have possibly been formed either by natural processes or by transitional species such as Tiktaalik or Panderichthys. Instead, the authors of the publication suggested ichthyostegalians as trackmakers, based on available pes morphology of those animals. However, a paper published in 2015 that undertook a critical review of Devonian tetrapod footprints called into question the designation of the Zachelmie marks and instead suggested an origin as fish nests/feeding traces. An earlier study in 2012 indicated that Zachelmie trackmakers were even more advanced than Ichthyostega in terms of quadrupedalism. Grzegorz Niedźwiedzki's reconstruction of one of the trackmakers was identical to that of Tulerpeton.
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", a notion partially shared by Philippe Janvier. There have been a number of new hypotheses suggested as to a possible origin and phylogenetic position of the elpistostegids (including Tiktaalik):
- their phylogenetic position remains unchanged and the footprints found in the Holy Cross Mountains are attributed to tetrapods but as a result there are at least six long ghost lineages separating Zachelmie trackmakers from various elpistostegalian and ichthyostegalian species;
- they were "late-surviving relics rather than direct transitional forms";
- they were "an evolutionary dead-end";
- they were a result of convergent or parallel evolution so that apomorphies and striking anatomical similarities found in both digited tetrapods and elpistostegalians evolved at least twice.
Convergency is considered responsible for uniquely tetrapod features found also in other non-elpistostegalian fish from the period like Sauripterus (finger-like jointed distal radial bones) or Tarrasius (tetrapod-like spine with 5 axial regions).
Estimates published after the discovery of Zachelmie tracks suggested that digited tetrapods may have appeared as early as 427.4 Ma ago and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy.
Until more data become available, the phylogenetic position of Tiktaalik and other elpistostegids remains uncertain.
In 2004, three fossilized Tiktaalik skeletons were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island, Nunavut, in northern Canada. Estimated ages reported at 375 MYA, 379 MYA, and 383 MYA. 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."
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.— 
The name 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 cryptically 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.
Other lobe-finned fish found in fossils from the Devonian Period:
- Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins Jr. (6 April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan". Nature. 440 (7085): 757–763. Bibcode:2006Natur.440..757D. doi:10.1038/nature04639. PMID 16598249.
- "What has the head of a crocodile and the gills of a fish?". evolution.berkeley.edu. Archived from the original on 2018-06-12. Retrieved 2018-06-06.
- Shubin, Neil (2008). Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: University of Chicago Press. ISBN 9780375424472.
- Laurin M (2006). "Scanty evidence and changing opinions about evolving appendages". Zoologica Scripta. 35 (6): 667–668. doi:10.1111/zsc.2006.35.issue-6.
- Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 978-0-375-42447-2.
- Holmes, Bob (2007). "Meet Your ancestor, the Fish that crawled". New Scientist. Archived from the original on 2016-04-13. Retrieved 2007-02-07.
- Jennifer A. Clack, Scientific American, Getting a Leg Up on Land Archived 2013-12-07 at the Wayback Machine November 21, 2005.
- Spitzer, Mark (2010). Season of the Gar: Adventures in Pursuit of America's Most Misunderstood Fish. University of Arkansas Press. pp. 65–66. ISBN 978-1-55728-929-2. Archived from the original on 2014-01-07. Retrieved 2016-10-29.
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- Sam Van Wassenbergh; Anthony Herrel; Dominique Adriaens; Frank Huysentruyt; Stijn Devaere & Peter Aerts (13 April 2006). "Evolution: A catfish that can strike its prey on land". Nature. 440 (7086): 881. Bibcode:2006Natur.440..881V. doi:10.1038/440881a. PMID 16612372. S2CID 4423295.
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- The Academy of Natural Sciences, Philadelphia, press release April 3, 2006. (doc)
- Neil H. Shubin, Edward B. Daeschler and Farish A. Jenkins Jr (6 April 2006). "The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb". Nature. 440 (7085): 764–771. Bibcode:2006Natur.440..764S. doi:10.1038/nature04637. PMID 16598250. S2CID 4412895.
- Shubin, N. H.; Daeschler, E. B.; Jenkins, F. A. (2014). "Pelvic girdle and fin of Tiktaalik roseae". Proceedings of the National Academy of Sciences. 111 (3): 893–899. Bibcode:2014PNAS..111..893S. doi:10.1073/pnas.1322559111. PMC 3903263. PMID 24449831.
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- NewsHour, Fossil Discovery Archived 2014-01-22 at the Wayback Machine, April 6, 2006.
- John Noble Wilford, The New York Times, Scientists Call Fish Fossil the Missing Link Archived 2017-11-15 at the Wayback Machine, Apr. 5, 2006.
- Daeschler, Edward B.; Shubin, Neil H.; Jenkins, Farish A. Jr (6 April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan" (PDF). Nature. 440 (7085): 757–763. Bibcode:2006Natur.440..757D. doi:10.1038/nature04639. PMID 16598249. S2CID 4413217. Archived (PDF) from the original on 12 February 2015. Retrieved 24 January 2015.
- Rex Dalton (5 April 2006). "The fish that crawled out of the water". Nature. doi:10.1038/news060403-7. Archived from the original on 24 January 2015. Retrieved 24 January 2015.
- Ahlberg, Per Erik; Clack, Jennifer A. (6 April 2006). "A firm step from water to land". Nature. 440 (7085): 747–749. Bibcode:2006Natur.440..747A. doi:10.1038/440747a. PMID 16598240. S2CID 4392361.
- Boisvert, Catherine A.; Mark-Kurik, Elga; Ahlberg, Per E. (4 December 2008). "The pectoral fin of Panderichthys and the origin of digits". Nature. 456 (7222): 636–638. Bibcode:2008Natur.456..636B. doi:10.1038/nature07339. PMID 18806778. S2CID 2588617. Archived from the original on 4 January 2014. Retrieved 24 January 2015.
Given that recent phylogenies consistently place Panderichthys below Tiktaalik 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 that Tiktaalik is autapomorphic, that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik.
- Ker Than (September 24, 2008). "Ancient Fish Had Primitive Fingers, Toes". National Geographic News. National Geographic Society. Archived from the original on September 27, 2008.
Curiously, the radial bones of Panderichthys are more finger-like than those of Tiktaalik, a fish with stubby leg-like limbs that lived about five million years later. Many scientists regard Tiktaalik 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 with Tiktaalik, and that Tiktaalik's fins represented an evolutionary return to a more primitive form.
- Niedźwiedzki, Grzegorz; Szrek, Piotr; Narkiewicz, Katarzyna; Narkiewicz, Marek; Ahlberg, Per E. (7 January 2010). "Tetrapod trackways from the early Middle Devonian Period of Poland". Nature. 463 (7277): 43–48. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. PMID 20054388. S2CID 4428903.
- Niedźwiedzki, Grzegorz; Szrek, Piotr; Narkiewicz, Katarzyna; Narkiewicz, Marek; Ahlberg, Per E. (2010). "Tetrapod trackways from the early Middle Devonian Period of Poland. Supplementary information". Nature. 463 (7277): 43–8. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. PMID 20054388. S2CID 4428903.
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- Narkiewicz, Katarzyna; Narkiewicz, Marek (January 2015). "Middle Devonian invertebrate trace fossils from the marginal marine carbonates of the Zachełmie tetrapod tracksite, Holy Cross Mountains, Poland". Lethaia. 48 (1): 10–12. doi:10.1111/let.12083.
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- [Neil Shubin] says that a model of Tiktaalik's skeleton would produce a print much like the one in the paper if it's mushed into sand, and different consistencies or angles would produce an even closer match. He adds, "There is nothing in Tiktaalik's described anatomy that suggests it didn't have a stride." in Ed Yong (January 6, 2010). "Fossil tracks push back the invasion of land by 18 million years". Discover. Archived from the original on May 16, 2010.
- King, Heather M.; Shubin, Neil H.; Coates, Michael I.; Hale, Melina E. (December 27, 2011). "Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes". PNAS. 108 (52): 21146–21151. Bibcode:2011PNAS..10821146K. doi:10.1073/pnas.1118669109. PMC 3248479. PMID 22160688.
It follows that the attribution of some of the nondigited Devonian fossil trackways to limbed tetrapods may need to be revisited.
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- The 2007 artistic restoration of Tulerpeton by Dmitry Bogdanov available at Wikimedia is virtually identical to the 2008 rendering of a Zachelmie trackmaker by Grzegorz Niedźwiedzki.
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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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|Wikiquote has quotations related to: Tiktaalik|
|Wikimedia Commons has media related to Tiktaalik.|
- University of Chicago website dedicated to the discovery
- Interview with Neil Shubin on The Inoculated Mind, February 12, 2008.
- Interview with Neil Shubin on Tech Nation where he discusses the discovery of the Tiktaalik, February 14, 2008.
- on YouTube
- Fishapod stars in music video, YouTube. Accessed on December 27, 2008.
- Finding Tiktaalik: Interview with Neil Shubin, Royal Institution video, February 2013
- A today's fish with tetrapod anatomy, able to move like an early tetrapod – Cryptotora thamicola