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Found in the Canadian Arctic, the new fossil boasts leglike fins, scientists say. The creature is being hailed as a crucial missing link between fish and land animals—including the prehistoric ancestors of humans. Researchers say the fish shows how fins on freshwater species first began transforming into limbs some 380 million years ago. The change was a huge evolutionary step that opened the way for vertebrates—animals with backbones—to emerge from the water.  "This animal represents the transition from water to land—the part of history that includes ourselves," said paleontologist Neil Shubin of the University of Chicago. Shubin was co-leader of a team that uncovered three nearly complete fossils measuring up to nine feet (three meters) long on Ellesmere Island in 2004. The new species, Tiktaalik roseae, had a flattened, crocodile-like head and strong, bony fins. The large fish probably flexed and extended these fins like legs to help it move through shallow, subtropical waters or even on land, the team says. The discovery marked the culmination of a five-year, 400-mile (650-kilometer) fossil hunt across the Arctic's frozen tundra. The National Geographic Society partially funded the project, which is to be detailed tomorrow in the journal Nature. The fish shows other features characteristic of land animals, including ribs, a neck, and nostrils on its snout for breathing air. The previously unknown creature is the closest known fish ancestor of land vertebrates, Shubin said. It likely used its fins "to prop its body, much like we do when we do a push-up," he said. Likewise, the animal's broad ribs would have supported its long, scaly trunk, adds team member Farish Jenkins of Harvard University in Cambridge, Massachusetts. Land Excursions Water supports the bodies of submerged fish, making strong ribs largely unecessary, "so this animal must have developed these structures for life in the shallows and making excursions on to land," Jenkins says. Shubin says the fish's wide head and sharp teeth suggest it hunted much like a crocodile and that it also breathed air. "Look at the side of the snout. It has a nice big pair of external nostrils," he said. Tiktaalik could become an icon of evolution in action, write paleontologists Per Ahlberg of Sweden's Uppsala University and Jennifer A. Clack of the University of Cambridge in the United Kingdom in an accompanying commentary. The paleontologists say the new fish form goes a long way toward filling the evolutionary gap between fish and the earliest amphibians. "Our remote ancestors were large, flattish, predatory fishes," they write. "Strong limblike pectoral fins enabled them to haul themselves out of the water." Evolutionary scientists agree that all four-limbed land vertebrates, including dinosaurs and mammals, are descended from lobe-fins, a group of primitive fishes with fins suggesting limbs. Living lobe-fins include lungfish, which have gills but can also breathe air using modified swim bladders that act as lungs. Tiktaalik would have breathed like a lungfish, says Clack, senior assistant curator at Cambridge's University Museum of Zoology. "It's increasing its reliance on air, so it's not purely a gill-breather," she said. This freshwater fish needed a large, wide head to pump air into its simple lungs. "It's a sort of bellows arrangement," Clack explained. "The more air you can get in with a mouthful, the better." Like a Croc The creature's long snout seems to be adapted for snapping at prey and hunting with its head above water like a crocodile, Clack said. "Snapping underwater is less efficient, because water pressure gets in the way," she added. "There would probably have been some large invertebrates around the water margins, or it might still have been feeding on small fishes and things in the water." The creature also lacked the rigid bony covering over the head and shoulders that most fish have, effectively giving Tiktaalik a neck.
"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding [such] fossils," said Daeschler, curator of vertebrate biology at the Academy of Natural Sciences in Philadelphia, Pennsylvania. Ellesmere Island is more than 600 miles (970 kilometers) north of the Arctic Circle in Canada's Nunavut territory. Polar bears roam the now frigid region. But Nunavut's fossil-bearing rocks were formed when North America was part of a giant supercontinent that straddled the Equator. Huge predators would have lurked in Tiktaalik's rivers and lakes, study co-leader Shubin says—perhaps one reason why Tiktaalik appears to have been headed for land. "Land had no predators, and it also had food in the form of invertebrates," Shubin said. "Put this all together and the shallows and mudflats might have been a good place to make a living." 
The oldest fossils of land plants visible with the naked eye are about 425 million years old. They are miniscule plants from the Mid-Silurian of Ireland en are called Cooksonia. Cooksonia plants had dichotomously divided little stems with small knobs at the end. These knobs were sporangia and they were filled with spores. During about 20 or 30 millions of years these were the most common plants.
They became extinct at the end of the Early Devonian. In the photo on the left my finest specimen of Cooksonia. It comes from South Wales and measures 3.5 cm. Recently microscopic remains have been found in Oman indicating that land plants existed already 475 million years ago. Probably land plants have developed from green algae living in the sea. During the Early Devonian, from about 410 million years ago, more variation came into the flora. New groups of plants came into existence, but still the plants were small (up to 50 cm). Flowers developed only 250 millions of years later. Leaves didn't exist either, except for the scalelike leaves, like those of the clubmosses. Several plants had spines. These were not for defending the plant, for animal life on the land was still minimal and the land animals were very small (millipedes, collembolans, trigonotarbids ('spiders'), very small crustaceans, etc.). Probably the function of the spines was to enlarge the green surface and so to enhance the assimilation. Furthermore the spines could have given more hold to the plants in forming small bushes of plants of the same species, hanging more or less on each other. The first tetrapods are now thought to have evolved in shallow and swampy freshwater habitats, towards the end of the Devonian, a little more than 360 million years ago. By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities.
Early tetrapods were, like early land plants, tied to the water by their reproductive mechanisms. Like most "fish" they had external fertilization in water and laid eggs in water which developed into aquatic larvae. The larvae metamorphosed into land-living adults. Living "amphibians" have inherited this primitive mode of reproduction. Sometime during the Carboniferous, one group of tetrapods developed the amniotic egg. Primitively, the amniotic egg has a shell hardened by calcium carbonate which is impermeable to water but allows gases to be transpired. The embryo lies floating in the amniotic fluid, which is formed by the embryo. The shell is formed by the mother.  Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines—they could not have lived in freshwater only. The common ancestor of all present gnathostomes lived in freshwater, and later migrated back to the sea. To deal with the much higher salinity in sea water, they evolved the ability to turn the nitrogen waste product ammonia into harmless urea, storing it in the body to make the blood as salty as the sea water without poisoning the organism. Ray-finned fishes later returned to freshwater and lost this ability. Since their blood contained more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they could not recover their old trick of turning ammonia to urea, and they had to evolve salt excreting glands instead. Lungfishes do the same when they are living in water, making ammonia and no urea, but when the water dries up and they are forced to burrow down in the mud, they switch to urea production. Like cartilaginous fishes, the coelacanth can store urea in its blood, as can the only known amphibians which can live for long periods of time in salt water (the toad Bufo marinus and the frog Rana cancrivora). These are traits they have inherited from their ancestors.
If early tetrapods lived in freshwater, and if they lost the ability to produce urea and used ammonia only, they would have to evolve it from scratch again later. Not a single species of all the ray-finned fishes living today has been able to do that, so it is not likely the tetrapods would have done so either. Terrestrial animals that can only produce ammonia would have to drink constantly, making a life on land impossible (a few exceptions exist, as some terrestrial woodlice can excrete their nitrogenous waste as ammonia gas). This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater (unless they first migrated into freshwater habitats and then migrated onto land so shortly after that they still hadn't forgot how to make urea), even if some who never went to land (or extinct primitive species that returned to water) of course could have adapted to freshwater lakes and rivers. Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones (the "living fossil" coelacanth is a related marine lobe-finned fish without these shallow-water adaptations). Even closer related was Panderichthys, who even had a choana. These fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. Their fins could also have been used to attach themselves to plants or similar while they were laying in ambush for prey. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water. It is now clear that the common ancestor of the bony fishes had a primitive air-breathing lung (later evolved into a swim bladder in most ray-finned fishes). This suggests that it evolved in warm shallow waters, the kind of habitat the lobe finned fishes were living and made use of their simple lung when the oxygen level in the water became too low. The lungfishes are now considered as being the closest living relatives of the tetrapods, even closer than the coelacanth. Fleshy lobe fins supported on bones rather than ray-stiffened fins seems to have been an original trait of the bony fishes (Osteichthyes). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned (Actinopterygii) fishes evolved their fins in the opposite direction. The most primitive group of the ray-fins, the bichirs, still have fleshy frontal fins. Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All of them were from the European-North American supercontinent, which comprised Europe, North America and Greenland. The only exception is a single Gondwanan genus, Metaxygnathus, which has been found in Australia. The first Devonian tetrapod identified from Asia was recognized from a fossil jawbone reported in 2002. The Chinese tetrapod Sinostega pani was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China. This finding substantially extended the geographical range of these animals and has raised new questions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time. These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous deposits, some 20 million years later. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.
Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their metamorphosis had what it took to make use of what land had to offer. Already adapted to breathe air and move around in shallow waters near land as a protection (just as modern fish (and amphibians) often spent the first part of their life in the comparative safety of shallow waters like mangrove forests), two very different niches partially overlapped each other, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous while the other was much safer and much less crowded, offering less competition over resources. The terrestrial niche was also a much more challenging place for primary aquatic animals, but because of the way evolution and the selection pressure works, those juveniles who could take advantage of this would be rewarded. Once they gained a small foothold on land, evolution took care of the rest, thanks to all their preadaptations and being at the right place at the right time. At this time there were a lot of invertebrates crawling around on land and near water, in moist soil and wet litter, more than big enough to give the small ones a good meal. Some were even big enough to eat small tetrapods, but land would still be a much safer place and offer more than the waters if they knew how to make use of it. Adults would be too heavy and slow and demand bigger prey. Small juveniles were much lighter, faster and was satisfied with relatively small invertebrates. Modern mudskippers are said to be able to snap insects in flight while on land, so maybe we shouldn't underestimate the early juvenile tetrapods either. The fully grown obviously kept most of the anatomical and other forms of adaptations from their juvenile stage, giving them modified limbs and other traits of terrestrial properties. To be successful adults they first had to be successful juveniles. The adults of some of the smaller species were in that case probably able to move on land too when sufficiently evolved. If some sort of neoteny or dwarfism occurred, making the animals sexually mature and fully grown while still living on land, they would only need to visit water to drink and reproduce. |
