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Tiktaalik

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Tiktaalik
Temporal range: Late Devonian (Frasnian), 375 Ma
Possible record in the Middle Devonian
Tiktaalik Chicago.JPG
Tiktaalik in the Field Museum, Chicago
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Stegocephalia
Genus: Tiktaalik
Daeschler, Shubin & Jenkins, 2006
Type species
Tiktaalik roseae
Daeschler, Shubin & Jenkins, 2006

Tiktaalik (/tɪkˈtɑːlɪk/; 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).[1] It may have grown up to 2.5 m (8.2 ft) in length.

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.[2] This and similar animals might be the common ancestors of all vertebrate terrestrial fauna: amphibians, reptiles, birds, and mammals.[3]

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[1] and quickly recognized as a transitional form.

Description[edit]

Tiktaalik provides insights on the features of the extinct closest relatives of the tetrapods.

Skull and neck[edit]

Skull showing otic notches above the eyes

The skull of Tiktaalik was low and flattened, more similar in shape to that of a crocodile than most fish. The rear of the skull was excavated by a pair of indentations known as otic notches. These notches likely housed spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. 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 on land or in the shallows.[4]

Limbs[edit]

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: exactly how did the bones of the lower limbs evolve into the feet and toes of vertebrates? There have been suggestions that the fin rays of Tiktaalik are homologous to digits, although this is incompatible with the digital arch developmental model, a hypothesis describing how the two parallel bones of lower limbs evolved to produce feet. According to this model, digits are supposed to be postaxial structures, but only three of the (reconstructed) eight rays of Tiktaalik are postaxial.[5]

However, the proximal series (bones of the upper limb) 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 a fast current.[6][4]

Torso[edit]

Restoration

Strong lungs (as supported by the plausible presence of a spiracle) may have led to the evolution of a more robust ribcage, a key evolutionary trait of land-living creatures.[7] The more robust ribcage of Tiktaalik would have helped support the animal's body any time it ventured outside a fully aquatic habitat.[4]

Tiktaalik is sometimes compared to gars (esp. Atractosteus spatula, the alligator gar) of the family Lepisosteidae, with whom it shares a number of characteristics:[8]

  • 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.

Paleobiology[edit]

Limb from shoulder (bottom) to fin (top)
The alligator gar is an extant fish that bears some resemblance to Tiktaalik.

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, which were found in other specimens. It had rows[9] 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,[10][11] and extant seahorses being some exceptions; see also Lepidogalaxias and Channallabes apus[12]). 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. Spiracles would have been useful in shallow water, where higher water temperature would lower oxygen content.[13]

Tiktaalik's discoverers believe the animal ventured onto land just as present day mudskippers do, propping up on their fins.

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.[14] 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.[15] Neil Shubin and Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 2000:[6][16]

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.[17]

Paleoecology[edit]

The fossils of Tiktaalik 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.[18] 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.[7]

Classification and evolution[edit]

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:
Descendants also included pelagic lobe-finned fish such as coelacanth species. In 2000 P. Ahlberg et al. described a transitional form from fish to tetrapod, Livoniana. This creature dates 374 – 391 million years ago, a successor to Panderichthys.

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".[6][19]

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
    • Fish gills
    • Fish scales
    • Fish fins
  • "Fishapod"
    • 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
    • Tetrapod rib bones
    • Tetrapod mobile neck with separate pectoral girdle
    • Tetrapod lungs

Classification[edit]

2006–2010: Elpistostegids as tetrapod ancestors[edit]

Life restoration of Tiktaalik

The phylogenetic analysis of Daeschler et al. (2006) placed Tiktaalik as a sister taxon to Elpistostege and directly above Panderichthys, which was preceded by Eusthenopteron. Tiktaalik was thus inserted below Acanthostega and Ichthyostega, acting as a transitional form between limbless fish and limbed vertebrates ("tetrapods").[1] Some press coverage also used the term "missing link", implying that Tiktaalik filled an evolutionary gap between fish and tetrapods.[20] 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 of Elpistostegalia, a name previously used to refer to particularly tetrapod-like fish such as Elpistostege and Panderichthys. Daeschler et al. (2006) recognized that this common usage referred to a paraphyletic grade, since elpistostegalian fish have few unique traits not also inherited by later tetrapods. In response, they redefined Elpisostegalia as a clade, including all vertebrates descended from the common ancestor of Panderichthys, 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.[21][22]

Such order of the phylogenetic tree was initially adopted by other experts, most notably by Per Ahlberg and Jennifer Clack.[23] However, it was questioned in a 2008 paper by Boisvert et al., who noted that Panderichthys, due to its more derived distal forelimb structure, might be closer to tetrapods than Tiktaalik or even that it was convergent with tetrapods.[24] Ahlberg, co-author of the study, considered the possibility of Tiktaalik's fin having been "an evolutionary return to a more primitive form."[25]

2010–present: Doubts over tetrapod ancestry[edit]

Zachełmie trackmakers predate not only ichthyostegids and elpistostegids (including Tiktaalik) but also a number of tetrapodomorph fish which until 2010 were unanimously considered ancestors of tetrapods.

The proposed origin of tetrapods among elpistostegalian fish was called into question by a discovery made in the Holy Cross Mountains of Poland. In January 2010, a group of paleontologists (including Ahlberg) published on a series of trackways from the Eifelian stage of the Middle Devonian, about 12 million years older than Tiktaalik.[21][26] These trackways, discovered at the Zachełmie quarry, appear to have been created by fully terrestrial tetrapods with a quadrupedal gait.[21]

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,[27] while Shubin argued that Tiktaalik could have produced very similar footprints.[28] 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.[29] 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.[21][30] Instead, the authors of the publication suggested that "ichthyostegalian"-grade tetrapods were the responsible trackmakers, based on available pes morphology of those animals.[21]

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",[31] a notion partially shared by Philippe Janvier.[32] 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 long ghost lineages required to explain the apparent delay in fossil appearances.[21] Another approach is that elpistostegalian and tetrapod similarities are a case of convergent 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.[33][34][35][36]

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.[34]

However, a reanalysis in 2015 of the Zachelmie trackways find that it fails the criteria for it being identified as Devonian tetrapod trackways and were instead reinterpreted as fish nests or feeding traces.[37]

Until more data become available, the phylogenetic position of Tiktaalik and other elpistostegids remains uncertain.

Discovery[edit]

Discovery site of Tiktaalik fossils

In 2004, three fossilized Tiktaalik skeletons were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island, Nunavut, in northern Canada.[38][39] Estimated ages 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),[40] 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.[6][16]

The discovery by Daeschler, Shubin, and Jenkins was published in the April 6, 2006, issue of Nature[1] 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."[4]

Neil Shubin, one of the paleontologists who discovered Tiktaalik, holding a cast of its skull

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.

— [41]

The name Tiktaalik is an Inuktitut word meaning "large freshwater fish".[3] The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered.[40] The specific name roseae cryptically honours an anonymous donor.[42] Taking a detailed look at the internal head skeleton of Tiktaalik roseae, in the October 16, 2008, issue of Nature,[43] 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.[44]

Cultural significance[edit]

This image by Zina Deretsky has been used in many memes.

Tiktaalik has been used as the subject of various Internet memes. The images generally humorously criticize Tiktaalik for its evolutionary adaptations, construing them as playing a critical role in the chain of events that would eventually lead to all human suffering.[45]

See also[edit]

Other lobe-finned fish found in fossils from the Devonian Period:

References[edit]

  1. ^ a b c d e 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" (PDF). Nature. 440 (7085): 757–763. Bibcode:2006Natur.440..757D. doi:10.1038/nature04639. PMID 16598249.
  2. ^ "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.
  3. ^ a b 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.
  4. ^ a b c d Holmes, Bob (2007). "Meet Your ancestor, the Fish that crawled". New Scientist. Archived from the original on 2016-04-13. Retrieved 2007-02-07.
  5. ^ 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.
  6. ^ a b c d Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 978-0-375-42447-2.
  7. ^ a b Jennifer A. Clack, Scientific American, Getting a Leg Up on Land Archived 2013-12-07 at the Wayback Machine November 21, 2005.
  8. ^ 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.
  9. ^ "Fossil Suggests Missing Link From Fish to Land". NPR (National Public Radio). Archived from the original on 2006-10-13. Retrieved 2006-11-27.
  10. ^ K. Trinajstic et al. (12 July 2013). "Fossil Musculature of the Most Primitive Jawed Vertebrates". Science. 341 (6142): 160–164. Bibcode:2013Sci...341..160T. doi:10.1126/science.1237275. PMID 23765280. S2CID 39468073.{{cite journal}}: CS1 maint: uses authors parameter (link)
  11. ^ "Primitive fish could nod but not shake its head: Ancient fossils reveal surprises about early vertebrate necks, abdominal muscles". Science News. June 13, 2013. Archived from the original on December 15, 2013. Retrieved December 14, 2013.
  12. ^ 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.
  13. ^ Dalton, Rex (2006). "The fish that crawled out of the water". Nature: news060403–7. doi:10.1038/news060403-7. S2CID 129031187. Archived from the original on 2006-04-11. Retrieved 2006-04-06.
  14. ^ 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.
  15. ^ 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.
  16. ^ a b Peterson, Britt (April 5, 2006). "An Evolutionary Finding". Seed. Archived from the original on April 11, 2006. Retrieved April 5, 2006.{{cite news}}: CS1 maint: unfit URL (link)
  17. ^ NewsHour, Fossil Discovery Archived 2014-01-22 at the Wayback Machine, April 6, 2006.
  18. ^ The Academy of Natural Sciences, Philadelphia, press release April 3, 2006. (doc)
  19. ^ 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.
  20. ^ 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.
  21. ^ a b c d e f 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.
  22. ^ Ahlberg, Per E. (2018). "Follow the footprints and mind the gaps: a new look at the origin of tetrapods". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 109 (1–2): 115–137. doi:10.1017/S1755691018000695. ISSN 1755-6910.
  23. ^ 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.
  24. ^ 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.
  25. ^ 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.
  26. ^ Niedźwiedzki, Grzegorz; Narkiewicz, Marek; Szrek, Piotr (2014). "The age of the oldest tetrapod tracks from Zachełmie, Poland". Bulletin of Geosciences. 89 (3): 593–606. Archived from the original on 2015-05-11. Retrieved 2015-01-24.
  27. ^ "Trace evidence is not enough for me to change my mind about accepted theories on tetrapod evolution" – Daeschler as quoted in Rex Dalton (January 6, 2010). "Discovery pushes back date of first four-legged animal". Nature: news.2010.1. 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" – Daeschler as quoted in Jef Akst (January 6, 2010). "Tetrapods' old age revealed". The Scientist. Archived from the original on March 4, 2016. Retrieved November 10, 2019. "With all respect to the scientists involved in this study, there may be other explanations for these suggestive tracks." – 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.
  28. ^ [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.
  29. ^ 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.
  30. ^ "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." - Ahlberg as quoted in Rex Dalton (January 6, 2010). "Discovery pushes back date of first four-legged animal". Nature: news.2010.1. doi:10.1038/news.2010.1. Archived from the original on March 8, 2014.
  31. ^ 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.
  32. ^ "We now have to invent a common ancestor to the tetrapods and elpistostegids." – 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.
  33. ^ Janvier, Philippe; Clément, Gaël (7 January 2010). "Muddy tetrapod origins". Nature. 463 (7277): 40–41. Bibcode:2010Natur.463...40J. doi:10.1038/463040a. PMID 20054387. S2CID 447958.
  34. ^ a b 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. PMC 3013411. PMID 20739322.
  35. ^ 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.
  36. ^ "Ancient Four-Legged Beasts Leave Their Mark". Science. 6 January 2010. Archived from the original on September 30, 2013.
  37. ^ Lucas, Spencer G. (2015-10-02). "Thinopus and a Critical Review of Devonian Tetrapod Footprints" (PDF). Ichnos. 22 (3–4): 136–154. doi:10.1080/10420940.2015.1063491. ISSN 1042-0940.
  38. ^ Gorner, Peter (2006-04-05). "Fossil could be fish-to-land link". Chicago Tribune.
  39. ^ Easton, John (2008-10-23). "Tiktaalik's internal anatomy explains evolutionary shift from water to land". University of Chicago Chronicle. University of Chicago. 28 (3). Archived from the original on 2012-04-07. Retrieved 2009-07-19.
  40. ^ a b Spotts, Peter (April 6, 2006). "Fossil fills gap in move from sea to land". The Christian Science Monitor. Archived from the original on April 6, 2006. Retrieved 2006-04-05.
  41. ^ Holmes, Bob (5 April 2006). "First fossil of fish that crawled onto land discovered". New Scientist News. Archived from the original on 6 April 2006. Retrieved 2006-04-07.
  42. ^ Coyne, Jerry (2009). Why Evolution is True. Viking. ISBN 978-0-670-02053-9. Archived from the original on 2019-10-07. Retrieved 2019-09-03.
  43. ^ Jason P. Downs, Edward B. Daeschler, Farish A. Jenkins & Neil H. Shubin (16 October 2008). "The cranial endoskeleton of Tiktaalik roseae". Nature. 455 (7215): 925–929. Bibcode:2008Natur.455..925D. doi:10.1038/nature07189. PMID 18923515. S2CID 4411801.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  44. ^ "Fishapod" Reveals Origins of Head and Neck Structures of First Land Animals Archived 2008-10-19 at the Wayback Machine Newswise, Retrieved on October 15, 2008.
  45. ^ Imbler, Sabrina (29 April 2022). "Started Out as a Fish. How Did It End Up Like This?". The New York Times. Retrieved 29 April 2022.

External links[edit]