Tiktaalik is a Wikipedia:monospecific Wikipedia:genus of extinct sarcopterygian (lobe-finned fish) from the late Wikipedia:Devonian period, with many features akin to those of Wikipedia:tetrapods (four-legged animals).
Pronunciation: IPAc-English: t|ɪ|k|ˈ|t|ɑː|l|ɨ|k}}
The Tiktaalik is understood as representative of a evolutionary transition from fish to amphibians. It is an example from several lines of ancient Wikipedia:sarcopterygian fish developing adaptations to the oxygen-poor shallow-water habitats of its time, leading to the evolution of tetrapods. The Tiktaalik and animals similar to the Tiktaalik are understood to be the common ancestors of a wide swathe of all terrestrial fauna: amphibians, reptiles, birds, and mammals. Well-preserved fossils were found in 2004 on Wikipedia:Ellesmere Island in Wikipedia:Nunavut, Wikipedia:Canada.
However, if recent research is correct, and the fossil record is not incomplete, then the evolution of the first four-limbed land animals and fishes were separate, concurrent, or the reverse of what is currently understood. While a gradual transition of forms is the norm for evolution, this is only because the capacity for genes to express forms more commonly falls within a limited range. Moreover, there is the problem of the classification of species being influenced by visible forms, rather than the genes that indicate more profound classifications. Humans group things readily and eagerly and with a very high success rate, but not always correctly. Biological classifications (Wikipedia:taxonomy) have been struck sometimes to the core by genetic evidence, and for the concept of evolution as always following our preconceptions about which animal is more evolved than another to be struck down would be only another idol fallen from its pedestal.
With the advent of such fields of study as Wikipedia:phylogenetics, Wikipedia:cladistics, and Wikipedia:systematics, the Linnaean Wikipedia:Taxonomy classification system has progressed to a system of modern biological classification based on the Wikipedia:evolutionary relationships between organisms, both living and extinct. An example of a modern classification is the one published in 2009 by the Wikipedia:Angiosperm Phylogeny Group for all living Wikipedia:flowering plant families (the Wikipedia:APG III system).
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.
[[Wikipedia:File:Cymbacephalus beauforti3.jpg|thumb|right|Tiktaalik with its eyes placed on top of a flat head resembled extant Wikipedia:Crocodilefish: Wikipedia:Cymbacephalus beauforti and Wikipedia:Papilloculiceps longiceps (flatheads)]] [[Wikipedia:File:Alligator gar.png|thumb|Wikipedia:Alligator gar is an extant fish which resembles Tiktaalik most]] Also notable are the Wikipedia: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 Wikipedia: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 Wikipedia:pectoral girdle separate from the skull. This would give the creature more freedom in hunting prey either on land or in the shallows.
Tiktaalik is sometimes compared to Wikipedia:gars (esp. Wikipedia:Atractosteus spatula, the Wikipedia:alligator gar) of the Wikipedia:Lepisosteidae family, with whom it shares a number of characteristics:
- diamond-shaped scale patterns common to the Wikipedia: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 Wikipedia: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 Wikipedia:crocodile, including a Wikipedia:shoulder, elbow, and Wikipedia: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 (Wikipedia:Tarrasius, Wikipedia:Mandageria, Wikipedia:placoderms, and extant Wikipedia:seahorses being some exceptions; see also Wikipedia: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 Wikipedia:breathing via Wikipedia:lungs; well developed jaws suitable for catching prey; and a small Wikipedia:gill slit called a Wikipedia:spiracle that, in more Wikipedia:derived animals, became an Wikipedia:ear. [[Wikipedia:File:Tiktaalik skull front.jpg|thumb|Skull showing Wikipedia:spiracle holes above the eyes]] The fossils were found in the "Wikipedia:Fram Formation", deposits of meandering stream systems near the Devonian equator, suggesting a Wikipedia: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, Wikipedia: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. Wikipedia:Neil Shubin and Wikipedia:Ted Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 2000
Classification and evolutionEdit
Tiktaalik roseae is the only Wikipedia: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 Wikipedia:Panderichthys, known from fossils 380 million years old, and early tetrapods such as Wikipedia:Acanthostega and Wikipedia:Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Wikipedia:Neil Shubin, to characterize Tiktaalik as a "fishapod".
Tiktaalik is a Wikipedia:transitional fossil; it is to tetrapods what Wikipedia: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
Putative tetrapod footprints found in Wikipedia:Poland and reported in Wikipedia:Nature in January 2010 were "securely dated" at 10 million years older than the oldest known elpistostegids. If this is a true tetrapod record, Tiktaalik was a "late-surviving relic" rather than the original transitional form. An alternative interpretation is that the Polish trackways, which do not have digital impressions, were made by walking fish 
In 2004, three fossilized Tiktaalik skeletons were discovered in rock formed from late Devonian river sediments on Wikipedia:Ellesmere Island, Wikipedia:Nunavut, in Wikipedia:northern Canada. At the time of the species' existence, Ellesmere Island was part of the continent Wikipedia:Laurentia (modern eastern Wikipedia:North America and Wikipedia: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 Wikipedia:Academy of Natural Sciences, Neil H. Shubin from the Wikipedia:University of Chicago, and Wikipedia:Harvard University Professor Farish A. Jenkins, Jr, was published in the April 6, 2006 issue of Nature and quickly recognized as a transitional form. Wikipedia:Jennifer A. Clack, a Wikipedia: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."
Template:Cquote The name Tiktaalik is an Wikipedia:Inuktitut word meaning "Wikipedia:burbot", a freshwater fish related to true Wikipedia:cod. The "fishapod" genus received this name after a suggestion by Wikipedia:Inuit elders of Wikipedia: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.
Evolution skipped fish? Edit
After the initial discovery of Tiktaalik Roseae in 2004, a successive discovery of preserved footprints/tracks by Grzegorz Niedzwiedzki was found on the surface of large limestone slabs from the Zachelmie Quarry in Poland. After further investigation via radiocarbon dating Scientists determined the age of the slabs and their subsequent footprints to be dated at 395 million years old, about the same age as early fish fossils. This finding generated worldwide interest since Tiktaalik Roseae was determined to have lived approximately 375 million years ago and was purported to be among the first of initial ancestors that crossed the boundary from fish to land animals; thus being a direct descendant of all future amphibians, reptiles, birds, and mammals (including humans). Conversely, either the fossil record is incomplete, or 
See also Edit
Other lobe-finned fish found in fossils from the Devonian period:
- Tiktaalik Wikiquote
- University of Chicago website dedicated to the discovery
- Interview with Neil Shubin on Wikipedia:The Inoculated Mind, February 12, 2008.
- Interview with Neil Shubin on Tech Nation where he discusses the discovery of the Tiktaalik, February 14, 2008.
- Fishapod stars in music video, YouTube. Accessed on December 27, 2008.
- Finding Tiktaalik: Interview with Neil Shubin, Wikipedia:Royal Institution video, February 2013
- ↑ 1.0 1.1 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". 757–763. Template:Citation/identifier. Template:Citation/identifier. http://www.nature.com/nature/journal/v440/n7085/abs/nature04639.html.
- ↑ 2.0 2.1 2.2 Wikipedia: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. Template:Citation/identifier.
- ↑ Angiosperm Phylogeny Group (2009), "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III", Botanical Journal of the Linnean Society 161 (2): 105–121, Template:Citation/identifier, http://www3.interscience.wiley.com/journal/122630309/abstract, retrieved 2010-12-10
- ↑ Laurin M (2006). "Scanty evidence and changing opinions about evolving appendages". 667–668. Template:Citation/identifier. http://onlinelibrary.wiley.com/doi/10.1111/zsc.2006.35.issue-6/issuetoc.
- ↑ 6.0 6.1 6.2 6.3 Shubin, Neil (2008). "Your Inner Fish". Pantheon. Template:Citation/identifier.
- ↑ 7.0 7.1 7.2 "Meet Your ancestor, the Fish that crawled". New Scientist Magazine. http://www.newscientist.com/channel/life/mg19125681.500-meet-your-ancestor--the-fish-that-crawled.html;jsessionid=NDHPCECNAGNA. 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. Template:Citation/identifier. http://books.google.com/books?id=Yepqz2i3Iv4C&pg=PA65.
- ↑ "Fossil Suggests Missing Link From Fish to Land". NPR (National Public Radio). http://www.npr.org/templates/story/story.php?storyId=5325720. Retrieved 2006-11-27.
- ↑ K. Trinajstic et al. (12 July 2013). "Fossil Musculature of the Most Primitive Jawed Vertebrates". 160–164. Template:Citation/identifier. http://www.sciencemag.org/content/341/6142/160.
- ↑ "Primitive fish could nod but not shake its head: Ancient fossils reveal surprises about early vertebrate necks, abdominal muscles". Wikipedia:Science News. June 13, 2013. https://www.sciencenews.org/article/primitive-fish-could-nod-not-shake-its-head.
- ↑ 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". 881. Template:Citation/identifier. Template:Citation/identifier. http://www.nature.com/nature/journal/v440/n7086/abs/440881a.html.
- ↑ "The fish that crawled out of the water". Nature. http://www.nature.com/news/2006/060403/full/060403-7.html. 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". 764–771. Template:Citation/identifier. Template:Citation/identifier. http://www.nature.com/nature/journal/v440/n7085/abs/nature04637.html.
- ↑ 16.0 16.1 Peterson, Britt (April 5, 2006). "An Evolutionary Finding". Seed. http://www.seedmagazine.com/news/2006/04/an_evolutionary_finding.php. Retrieved 2006-04-05.
- ↑ John Noble Wilford, The New York Times, Scientists Call Fish Fossil the Missing Link, Apr. 5, 2006.
- ↑ Niedzwiedzki G., Szrek P., Narkiewicz K., Narkiewicz M, Ahlberg P. (2010). "Tetrapod trackways from the early Middle Devonian period of Poland" (PDF). 43–48. Template:Citation/identifier. http://nature.com/nature/journal/v463/n7277/pdf/nature08623.pdf.
- ↑ King, H. et al.. "Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes". http://www.pnas.org/content/early/2011/12/08/1118669109.
- ↑ Gorner, Peter (2006-04-05). "Fossil could be fish-to-land link". Chicago Tribune.
- ↑ Easton, John (2008-10-23). "Tiktaalik’s internal anatomy explains evolutionary shift from water to land". University of Chicago Chronicle. Wikipedia:University of Chicago. http://chronicle.uchicago.edu/081023/tiktaalik.shtml. Retrieved 2009-07-19
- ↑ 22.0 22.1 Spotts, Peter (April 6, 2006). "Fossil fills gap in move from sea to land". The Christian Science Monitor. http://www.csmonitor.com/2006/0406/p02s01-stss.html. Retrieved 2006-04-05.
- ↑ Nunavut Living Dictionary. Entry for tiktaalik
- ↑ Coyne, Jerry (2009). "Why Evolution is True". Viking. Template:Citation/identifier.
- ↑ Jason P. Downs, Edward B. Daeschler, Farish A. Jenkins & Neil H. Shubin (16 October 2008). "The cranial endoskeleton of Tiktaalik roseae". 925–929. Template:Citation/identifier. Template:Citation/identifier.
- ↑ "Fishapod" Reveals Origins of Head and Neck Structures of First Land Animals Newswise, Retrieved on October 15, 2008.
- ↑ http://news.nationalgeographic.com/news/2010/01/100106-tetrapod-tracks-oldest-footprints-nature-evolution-walking-land/
- ↑ NPR Science Friday Interview on Tiktaalik Roseae
- ↑ Palaeontologist Jennifer Clack, University of Cambridge, UK; in: Curry, M., Ancient four-legged beasts leave their mark, ScienceNOW Daily News, http://news.sciencemag.org/evolution/2010/01/ancient-four-legged-beasts-leave-their-mark 6 January 2010.
- ↑ "Here we present well-preserved and securely dated tetrapod tracks from Polish marine tidal flat sediments of early Middle Devonian (Eifelian stage) age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids. They force a radical reassessment of the timing, ecology and environmental setting of the fish–tetrapod transition, as well as the completeness of the body fossil record." Tetrapod trackways from the early Middle Devonian period of Poland Grzegorz Niedźwiedzki1, Piotr Szrek2,3, Katarzyna Narkiewicz3, Marek Narkiewicz3 & Per E. Ahlberg4 Nature International weekly journal of science