This page has been archived and is no longer updated. Published online 5 April Nature doi Rex Dalton. A crucial fossil that shows how animals crawled out from the water, evolving from fish into land-loving animals, has been found in Canada. The creature, described today in Nature 1 , 2 , lived some million years ago. Palaeontologists are calling the specimen from the Devonian a true 'missing link', as it helps to fill in a gap in our understanding of how fish developed legs for land mobility, before eventually evolving into modern animals including mankind.
The crew found the samples in a river delta on Ellesmere Island in Arctic Canada; these included a near-complete front half of a fossilized skeleton of a crocodile-like creature, whose skull is some 20 centimetres long. The beast has bony scales and fins, but the front fins are on their way to becoming limbs; they have the internal skeletal structure of an arm, including elbows and wrists, but with fins instead of clear fingers. The team is still looking for more complete specimens to get a better picture of hind part of the animal. When Neil joined the University of Pennsylvania as a new assistant professor in the late s, he was determined to uncover some of the missing links among some of our earliest backboned ancestors.
The key period for vertebrates coming onto land is within the late Devonian, specifically from about to million years ago. Prior to this time, vertebrates were represented only by fish. By the end of the Devonian about million years ago , life on land was changing in dramatic ways—vertebrates had evolved limbs and started to walk, and insects and spiders had also entered terrestrial habitats. In the vertebrate fossil record, a few key fossils mark some of the stages of the fin-to-limb transition and the origin of four-legged vertebrates tetrapods.
But these ancient fish do not have the equivalent of wrists or fingers. By roughly million years ago, the full suite of tetrapod limb characteristics are present in animals such as Acanthostega.
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Discovered on the east coast of Greenland, one of the most surprising features of this animal are the eight digits on its hands and at least that many on its feet , which reveal that the early condition in tetrapods was the possession of more than the five digits common to all recent forms. But as great a fossil as Acanthostega is, at the time Neil began his studies, there was quite a gap between its fully formed limbs and the fins of earlier fish.
That was the gap that Neil, and his new graduate student Ted Daeschler, aimed to close. Fortunately for Neil and Ted, the commonwealth of Pennsylvania sits on a lot of Devonian rock. Three-hundred-eighty million years ago, the Acadia mountain range was drained by a series of meandering rivers that emptied into the inland Catskill Sea.
The resulting Catskill Delta now lies in the present day Appalachians, with ancient floodplain deposits extending from southeastern New York through Pennsylvania, Maryland, and West Virginia. So, luckily, Neil and Ted would not have to go far to go prospecting. But, just as in Connecticut years earlier, the rock was largely covered with greenery and urban development, and there was no seacoast.
The best they could do was to scout highway road cuts, which at least had the advantages of being easily accessible and very inexpensive field trips. For several years over the first half of the s, Neil and Ted embarked on a series of roadside adventures. They first found a few fish scales there and then, while Neil was off in Greenland on another expedition, Ted returned to the site and found the shoulder of a tetrapod—the first late Devonian tetrapod discovered outside of Greenland. Comprising about 75 vertical feet of rock formed from about , years of Devonian deposits, the hill was loaded with fossils.
With such easy access, Neil and Ted could haul anything back to the lab at Penn for closer inspection. Route 15 provided another bonanza. Combing through some freshly blasted boulders, Neil and Ted carted a few big hunks back for further analysis. They spotted a large fish fin poking out of one boulder, but it was not the sort of fin they usually found. This one had bones inside. Those eight rods looked like they could be forerunners of digits, the eight digits that appeared in animals like Acanthostega.
Since tetrapods and various fish co-existed at Red Hill, it became clear that the rocks were too recent Red Hill was million years old. The key transition had already taken place some time earlier. If they were to find transitional fossils, they had to look at rocks that were a bit older. They had learned what the right kinds of rocks were from Red Hill and other road cuts. Fossils were best preserved in deposits at the margins or overbanks of ancient streams that were part of delta systems. But where in the world might they find such rocks that others had not already explored?
They considered China, South America, and Alaska, but the prospects did not look encouraging. Then one day, in the course of settling an entirely unrelated geological squabble, they opened an old undergraduate geology textbook and happened upon a map showing several late Devonian deposits in North America. It showed East Greenland—but Neil and many others had been there and done that. It showed the Catskill Formation, where they had now toiled for years. And it showed the Canadian Arctic Islands—a vast and, paleontology-wise, virgin territory.
Excited, they went off to discuss a possible expedition over lunch at their favorite Chinese restaurant. If Neil was to find his fortune at the top of the world, he and Ted would need to do their homework.
The first order of business was to find a damn good map. The Arctic Islands encompass tens of thousands of square miles and include a number of uninhabited islands in one of the most remote parts of the planet. That is a lot of territory to cover and, because of the extreme climate, they would have a very short window of time to scout and to decide where to dig. They needed to narrow their search somehow. Fortunately, many of the remote places of the world are of interest to more than just a small tribe of paleontologists. For many decades, the major oil and gas companies and various governments have surveyed them for prospective natural resources.
And, fortunately for Neil and Ted, the Geological Survey of Canada and a host of major oil companies had sponsored an extensive survey of the Arctic Islands. Weighing in at pages, the paper by Ashton Embry and J. Edward Klovan was the fruit of four years of work in the early s mapping out the geological formations of the various islands. For two months each summer between late June and late August, the only months during which field work was possible, and constantly hampered by fog, snow, rain, and high winds, Embry and Klovan made their way across the Canadian Arctic by helicopter and light airplane, taking measurements and samples at every step.
Sandstone units are interpreted to be point-bar and channel-fill deposits, whereas the shale siltstone units are of overbank origin. But this terrain was even better than the Catskills, for in this part of the world there was virtually no vegetation covering the rocks. They had endless exposures they could survey.
So the Fram Formation was their target, but the next question was how to get there? They figured the best way to find out was to track down Ashton Embry, some 23 years after his survey. They explained their Catskill work and presented their ideas for exploring the Arctic. Getting around the islands was complex.
There were very few settlements or airfields. The distances involved were beyond the normal range of helicopters and required a system of fuel depots that enabled helicopters to island-hop. Beyond these logistics, there were the matters of funding and planning the expedition. They soon received a generous commitment from an anonymous donor that would cover all of their costs. By spring of , plans were well along for a six-week trip that summer. The team was buying supplies and working out the travel logistics. With the certain uncertainty of the weather, every contingency had to be considered.
There was also the matter of permits. The expedition was headed into Nunavut Territory, which was controlled by the Inuit people, and they required permission from the local Ministry and the Hamlet of Grise Fiord. It was a big setback, but the team was determined to go to the Arctic. They shifted their plans away from Ellesmere Island to Melville Island in the west.
With about 20o inhabitants, it serves as an aviation hub and houses a grocery store and three hotels—which as it turns out was fortunate for the six-person field team, as they were grounded there by several days of bad weather. They were also a bit unsettled by small talk with the locals. Time was precious and they had to be ready to go at the slightest break in the weather. When the veteran bush pilot finally got them aloft in his twin engine Otter plane, they were wishing they had waited longer.
It was a scary ride. They were enshrouded in fog the whole way, looking for a fog-covered island. The pilot made several passes before plunking the plane down on the tundra. They unloaded all of the gear, the pilot wished them luck, and he was gone. It and similar animals may possibly be the common ancestors of the broad swath of all vertebrate terrestrial fauna : amphibians, reptiles, birds, and mammals.
The first well-preserved Tiktaalik fossils were found in 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 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.
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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.
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. Tiktaalik is sometimes compared to gars esp.
Discovery of new Tiktaalik roseae fossils reveals key link in evolution of hind limbs
Atractosteus spatula , the alligator gar of the Lepisosteidae family, with whom it shares a number of characteristics: . 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 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 fish that crawled out of the water : Nature News
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. 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. Tiktaalik roseae is the only species classified under the genus. Tiktaalik lived approximately million years ago. It is representative of the transition between non-tetrapod vertebrates fish such as Panderichthys , known from fossils million years old, and early tetrapods such as Acanthostega and Ichthyostega , known from fossils about million years old.
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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:.
The phylogenetic analysis by Daeschler et al. 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. In January , 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. PGI A tetrapod origin of those tracks was suggested based on:. 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. 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.
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. 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 Until more data become available, the phylogenetic position of Tiktaalik and other elpistostegids remains uncertain. In , 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 million-year-old specimen. The discovery, made by Edward B. Jenkins, Jr , was published in the April 6, , issue of Nature  and quickly recognized as a transitional form. Jennifer A.