Temporal range: Late Devonian, 375Ma
|Tiktaalik in the Field Museum, Chicago.|
Daeschler, Shubin & Jenkins, 2006
Daeschler, Shubin & Jenkins, 2006
Tiktaalik // is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, about 360 Mya, with many features akin to those of tetrapods (four-legged animals). Tiktaalik may be representative of the evolutionary transition from fish to amphibians. It is an example from several lines of ancient sarcopterygian fish developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods. It and similar animals may be the common ancestors of a wide swathe of all terrestrial fauna: amphibians, reptiles, birds, and mammals. The first well-preserved Tiktaalik fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.
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 post-axial. 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 fore fins 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 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;
- dorsally placed eyes;
- 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 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 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, suggesting that it spent a lot of time looking up; 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 in shallow water and across mudflats. Neil Shubin and Ted 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.||”|
—Ted Daeschler, 
Classification and evolution
Tiktaalik roseae is the only species classified under the genus. Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that 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 Aurornis 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 at al. placed Tiktaalik as a sister taxon to Elpistostege and directly below Panderichthys preceded by Eusthenopteron. Tiktaalik was thus inserted above Acanthostega and Ichthyostega as a transitional fossil 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 at 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 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 established 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 lenghts 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 Titaalik's discoverers were skeptical about the Zachelmie trackways. Edward Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution, while Neil 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 2012 study 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.
Prof. 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 "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.
It should be noted that 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 Myr ago and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy.
Until more data becomes available the phylogenetic position of Tiktaalik and other elpistostegids remains uncertain.
In 2004, three fossilized Tiktaalik skeletons were discovered in rock formed from late Devonian river sediments 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), 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 383-million-year-old specimen.
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. 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 "burbot", a freshwater fish related to true cod. 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. doi:10.1038/nature04639. PMID 16598249.
- Jennifer A. Clack, Scientific American, Getting a Leg Up on Land Nov. 21, 2005.
- 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.
- "Meet Your ancestor, the Fish that crawled". New Scientist Magazine. Retrieved 2007-02-07.
- 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.
- "Fossil Suggests Missing Link From Fish to Land". NPR (National Public Radio). Retrieved 2006-11-27.
- K. Trinajstic et al. (12 July 2013). "Fossil Musculature of the Most Primitive Jawed Vertebrates". Science 341 (6142): 160–164. doi:10.1126/science.1237275.
- "Primitive fish could nod but not shake its head: Ancient fossils reveal surprises about early vertebrate necks, abdominal muscles". Science News. June 13, 2013.
- Sam Van Wassenbergh, Anthony Herrel, Dominique Adriaens, Frank Huysentruyt, Stijn Devaere, and Peter Aerts (13 April 2006). "Evolution: A catfish that can strike its prey on land". Nature 440 (7086): 881. doi:10.1038/440881a. PMID 16612372.
- "The fish that crawled out of the water". Nature. Retrieved 2006-04-06.
- 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. doi:10.1038/nature04637. PMID 16598250.
- Shubin, N. H.; Daeschler, E. B.; Jenkins, F. A. (2014). "Pelvic girdle and fin of Tiktaalik roseae". Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1322559111.
- Peterson, Britt (April 5, 2006). "An Evolutionary Finding". Seed. Retrieved 2006-04-05.
- NewsHour, Fossil Discovery, April 6, 2006.
- John Noble Wilford, The New York Times, Scientists Call Fish Fossil the Missing Link, 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". Nature 440 (7085): 757–763. Bibcode:2006Natur.440..757D. doi:10.1038/nature04639. PMID 16598249.
- Rex Dalton (5 April 2006). "The fish that crawled out of the water". Nature. doi:10.1038/news060403-7.
- 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.
- 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.
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.
- 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). Archived from the original on September 1, 2014.
- Walking with tetrapods (FLV). Nature. January 6, 2010. Archived from the original on December 20, 2014.
- Jonathan Amos (6 January 2010). "Fossil tracks record 'oldest land-walkers'". BBC. Archived from the original on January 7, 2010.
- Narkiewicz, Katarzyna; Narkiewicz, Marek (1 March 2010). "Mid Devonian carbonate platform development in the Holy Cross Mts. area (central Poland): new constraints from the conodont Bipennatus fauna". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 255 (3): 287–300. doi:10.1127/0077-7749/2009/0025.
- Niedźwiedzki, Grzegorz; Narkiewicz, Marek; Szrek, Piotr (2014). "The age of the oldest tetrapod tracks from Zachełmie, Poland". Bulletin of Geosciences 89 (3): 593–606.
- 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.
- "Trace evidence is not enough for me to change my mind about accepted theories on tetrapod evolution" - Edward Daeschler as quoted in Rex Dalton (January 6, 2010). "Discovery pushes back date of first four-legged animal". Nature. doi:10.1038/news.2010.1. Archived from the original on March 8, 2014. "I am not ready to discard the established paradigm for the fish-tetrapod transition" - Edward Daeschler as quoted in Jef Akst (January 6, 2010). "Tetrapods' old age revealed". The Scientist. "With all respect to the scientists involved in this study, there may be other explanations for these suggestive tracks." - Edward Daeschler as quoted in Dan Vergano (January 6, 2010). "Four-legged finding muddies paleontological waters". USA Today. Archived from the original on December 24, 2014.
- [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. doi:10.1073/pnas.1118669109.
It follows that the attribution of some of the nondigited Devonian fossil trackways to limbed tetrapods may need to be revisited.
- "You can see anatomical details consistent with a footprint, including sediments displaced by a foot coming down", "There is no way these could be formed by a natural process." - Per Ahlberg as quoted in Rex Dalton (January 6, 2010). "Discovery pushes back date of first four-legged animal". Nature. doi:10.1038/news.2010.1. Archived from the original on March 8, 2014.
- Pierce, Stephanie E.; Clack, Jennifer A.; Hutchinson, John R. (28 June 2012). "Three-dimensional limb joint mobility in the early tetrapod Ichthyostega" (PDF). Nature 486 (7404): 523–526. doi:10.1038/nature11124. PMID 22722854.
- Niedźwiedzki, Grzegorz; Szrek, Piotr (2010). "Way to Go!" (PDF). ACADEMIA (Polish Academy of Sciences) 2 (26): 28–31. ISSN 1731-7401. OCLC 786293607. Archived from the original on January 19, 2015.
- 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.
- W.Ż. (February 4, 2010). "A Creature That Time Forgot". The Warsaw Voice (Warsaw). Archived from the original on December 22, 2014.; "W Polsce odkryto ślady najstarszych kopalnych czworonogów" [Oldest tetrapod fossil footprints discovered in Poland]. Science & Scholarship in Poland (Polish Press Agency) (in Polish) (Warsaw). January 7, 2010. Archived from the original on December 22, 2014.
- "We now have to invent a common ancestor to the tetrapods and elpistostegids." - Philippe Janvier as quoted in Karen McVeigh (January 6, 2010). "Footprints show tetrapods walked on land 18m years earlier than thought". The Guardian (London). Archived from the original on March 2, 2014.
- Editor's summary (7 January 2010). "Four feet in the past: trackways pre-date earliest body fossils". Nature 463 (7277). Archived from the original on November 3, 2012.
- "Ancient Four-Legged Beasts Leave Their Mark". Science. 6 January 2010. Archived from the original on September 30, 2013.
- Janvier, Philippe; Clément, Gaël (7 January 2010). "Muddy tetrapod origins". Nature 463 (7277): 40–41. doi:10.1038/463040a. PMID 20054387.
- Friedman, Matt; Brazeau, Martin D. (7 February 2011). "Sequences, stratigraphy and scenarios: what can we say about the fossil record of the earliest tetrapods?". Proceedings of the Royal Society B 278 (1704): 432–439. doi:10.1098/rspb.2010.1321. PMID 20739322. Archived from the original on December 22, 2014.
- Gee, Henry (January 6, 2010). "First Footing". SciLogs. Archived from the original on December 22, 2014.
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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- Nunavut Living Dictionary. Entry for tiktaalik
- Coyne, Jerry (2009). Why Evolution is True. Viking. ISBN 978-0-670-02053-9.
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- "Fishapod" Reveals Origins of Head and Neck Structures of First Land Animals Newswise, Retrieved on October 15, 2008.
|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.
- Lecture (presentation) by Neil Shubin about the discovery of Tiktaalik on YouTube
- Fishapod stars in music video, YouTube. Accessed on December 27, 2008.
- Finding Tiktaalik: Interview with Neil Shubin, Royal Institution video, February 2013