Acanthostega

Acanthostega
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Acanthostega is an extinct tetrapod genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian (Famennian) about 360 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto land.

Description
It had eight digits on each hand and foot linked by webbing, it lacked wrists, and was generally poorly adapted to come onto land. Acanthostega also had a remarkably fish-like shoulder and forelimb. The front foot of Acanthostega could not bend forward at the elbow, and thus could not be brought into a weight-bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. It had lungs, but its ribs were too short to give support to its chest cavity out of water, and it also had gills which were internal and covered like those of fish, not external and naked like those of some modern amphibians which are almost wholly aquatic.

Therefore, paleontologists surmise that it probably lived in shallow, weed-choked swamps, the legs having evolved for some other purpose than walking on land. Jennifer A. Clack interprets this as showing that this was primarily an aquatic creature descended from fish that had never left the sea, and that tetrapods had evolved features which later proved useful for terrestrial life, rather than crawling onto land and then gaining legs and feet as had previously been surmised. 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. Clack remarks on how the lower jaw of Acanthostega shows a change from the jaws of fish which have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some small teeth in the inner row. It differs, having a small number of larger teeth in the outer row and smaller teeth in the inner row, and she suggests that this change probably went with a shift in early tetrapods from feeding exclusively in water to feeding with the head above water or on land.

Research based on analysis of the suture morphology in its skull indicates that the species may have bitten directly on prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical of terrestrial animals. Their results indicate that Acanthostega was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water.

Discovery
The fossilized remains are generally well preserved, with the famous fossil by which the significance of this species was discovered being found by Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik.
Related species


Acanthostega is seen as part of widespread speciation in the late Devonian period, starting with purely aquatic lobe-finned fish, with their successors showing increased air breathing capability and related adaptions to the jaws and gills, as well as more muscular neck allowing freer movement of the head than fish have, and use of the fins to raise the body of the fish. These features are displayed by the earlier Tiktaalik, which like the Ichthyostega living around the same time as Acanthostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic.
Sources

http://tolweb.org/tree?group=Acanthostega&contgroup=Terrestrial_Vertebrates
http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/150.150.html#Acanthostega
http://faculty.uca.edu/~benw/biol4402/lecture8c/sld004.htm
http://gatito.valdosta.edu/fossil_pages/fossils_dev/a1.html
http://faculty.evansville.edu/de3/b39903/PDFs/12_Land_InvasionII.pdf
http://www.wfiu.indiana.edu/amos/library/scripts/acanthostega.html
http://theclacks.org.uk/jac/acanthostega.htm



Ichthyostega
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Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived in the Upper Devonian (Famennian) period, 367-362.5 million years ago, and the first to intermediate between fish and amphibians. Ichthyostega had legs but its limbs probably weren't used for walking as once believed, but were used instead to negotiate its way through the swamps of the time. It is sometimes referred to as an "amphibian", but it is not a true member. The first true amphibians appeared in the Carboniferous period.

History and systematics
In 1932 Gunnar Säve-Söderbergh described four Ichthyostega species from the Upper Devonian of East Greenland and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be synonymous (in which case only I. stensioei would remain), because their morphological differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.

The genus is closely related to Acanthostega gunnari, also from East Greenland. Ichthyostega's skull seems more fish-like than that of Acanthostega, but its girdle (shoulder and hip) morphology seems stronger and better adapted to land-life. Ichthyostega also had more supportive ribs and stronger vertebrae with more developed zygapophyses. The first tetrapods (who probably didn't walk on land) were Elginerpeton and Obruchevichthys.

Characteristics
Ichthyostega was about 1.5 meter long or 5 feet long and had seven digits on each hind foot. The exact number of digits on the hand is not yet known, but was probably about the same as on the foot. It had a fin containing fin rays on its tail.
Adaptations for land-life
Primitive tetrapods like Ichthyostega and Acanthostega differed from animals like Crossopterygians (for instance Eusthenopteron or Panderichthys) in that although Crossopterygians had lungs, they used their gills as the primary means of acquiring oxygen. Ichthyostega probably used lungs as its primary means of breathing. Primitive tetrapods had a special type of skin that helped them retain bodily fluids and deter desiccation, whereas Crossopterygians did not. Moreover, Crossopterygians used their body and tail to move about and their fins for balance while, Ichthyostega instead used its limbs for locomotion and its tail for balance.
The adult animals were so big and heavy (1.5 m or 4 ft) and poorly adapted for terrestrial locomotion that there probably wouldn't have been much benefit to their being on land. However, the massive ribcage is made up of overlapping ribs -- and compared to their ancestors, the body has a stronger skeletal structure, a more advanced spine, and forelimbs with possibly enough power to pull the body from the water. These anatomical modifications are clearly evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller and so weak they couldn't have been able to bear the weight of an adult. Jennifer A. Clack suggests that Ichthyostega and relatives were spending time basking in the sun to raise their temperature, perhaps adopting a similar lifestyle to that of the Marine Iguana on Galapagos, seals or the Gharial -- mostly returning to water for food, to drink, to reproduce, or to cool down. In that case, they would need strong forelimbs to pull at least their anterior part out of the water, and a stronger ribcage and spine to support them while sunbathing on their abdomen like modern crocodiles. The juveniles, on the other hand, would have been able to move around on land much more easily.

Water was also still a requirement, because the gel-like eggs of the earliest terrestrial tetrapods couldn't survive out of water, so reproduction could not occur without it. Water was also needed for their larvae and external fertilization. Most land-dwelling vertebrates have since developed two methods of internal fertilization; either direct as seen in all amniotes and a few amphibians, or indirect for many salamanders by placing a spermatophore on the ground which then is picked up by the female salamander.

Ichthyostegoids (Elginerpeton, Acanthostega, Ichthyostega, etc.) were "succeeded" by temnospondyls and anthracosaurs, such as Eryops, an amphibian that truly developed the ability to walk on land. There is a gap of 20-30 million years between both groups. This gap, a classic in vertebrate paleontology, is known as Romer's Gap, after the American paleontologist Alfred Sherwood Romer. In 2002 a 350 million year old fossil named Pederpes finneyae was found. The ages, and relationship to other species is of course only an educated guess.

See also

Prehistoric amphibian
Prehistoric life

External links

Excellent site on early tetrapods
Course site
Course site
First Four-Legged Animals Inched Along
Getting a Leg Up on Land Scientific American Nov. 21, 2005, article by Jennifer A. Clack.

References

Blom, H. (2005) — Taxonomic Revision Of The Late Devonian Tetrapod Ichthyostega from East Greenland. Palaeontology, 48, Part 1:111–134
Westenberg, K. (1999) — From Fins to Feet. National Geographic, 195, 5:114–127


Eusthenopteron
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Eusthenopteron was a genus of lobe-finned fish which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a pelagic animal. The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 million years ago. Eusthenopteron was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha, Quebec.

Anatomically, Eusthenopteron shares many unique features in common with the earliest known tetrapods. It shares a similar pattern of skull rooting bones with forms such as Ichthyostega and Acanthostega. Eusthenopteron, like other tetrapodomorph fishes, had internal nostrils, (or a choana) which are found only in land animals and sarcopterygians. It also had labyrinthodont teeth, characterized by infolded enamel, which characterizes all of the earliest known Tetrapods as well. Like other basal sarcopterygians, Eusthenopteron possessed a two-part cranium, which hinged at mid-length along an intracranial joint. Eusthenopteron's notoriety comes from the pattern of its fin endoskeleton, which bears a distinct humerus, ulna, and radius (in the fore-fin) and femur, tibia, and fibula (in the pelvic fin). This is the characteristic pattern seen in tetrapods. It is now known to be a general character of fossil sarcopterygian fins.


Eusthenopteron differs significantly from later Carboniferous tetrapods in the apparent absence of a recognized larval stage and a definitive metamorphosis. In even the smallest known specimen of Eusthenopteron foordi (at 29 mm), the lepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera like Polyodon. This might indicate that Eusthenopteron developed directly, with the hatchling already attaining the general body form of the adult (Cote et. al, 2002).

See also

Tiktaalik, an even more tetrapod-like sarcopterygian.
Gogonasus

Panderichthys
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Panderichthys is a 90–130 cm long fish from the Late Devonian period (Frasnian epoch) of Latvia. It has a large tetrapod-like head. Panderichthys exhibits transitional features between lobe-finned fishes and early tetrapods such as Acanthostega. The evolution from fish to land dwelling tetrapods required many changes in physiology, most importantly the legs and their supporting structure, the girdles. Well preserved fossils of Panderichthys clearly show these transitional forms, making Panderichthys a rare and important find in the history of life.


Fish like Panderichthys were the ancestors of the first tetrapods, air-breathing, terrestrial animals from which the land vertebrates, including humans, are descended. The most notable characteristic of Panderichthys was its spiracle, a vertical tube used to breathe water through the top of its head, while its body was submerged in mud. This spiracle is a transitional organ that led, through evolution, to the development of the stirrup bone, one of the three bones (stirrup, hammer, and anvil) in the human middle ear.

Elpistostege
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Elpistostege is an extinct genus of tetrapod-like fish that lived in the Late Devonian period (Late Givetian to Early Frasnian). Fossils of skull and a part of the backbone have been found at Escuminac Formation in Quebec, Canada.

External links

Elpistostege at Palaeos
History of Elpistostege

Tiktaalik
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Tiktaalik is a genus of extinct sarcopterygian (lobe-finned) fish from the late Devonian period, with many features akin to those of tetrapods (four-legged animals) . 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 amphibians. Well preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it was an intermediate form between fish such as Panderichthys, which lived about 385 million years ago, and early tetrapods such as Acanthostega and Ichthyostega, which lived about 365 million years ago. Its mixture of fish and tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod" .

Description
Tiktaalik appears to be a transitional form between fish and amphibian. Unlike many previous, more fishlike transitional fossils, Tiktaalik 'fins' have basic wrist bones and simple fingers, showing that they were weight bearing. Close examination of the joints show that although they probably were not used to walk, they were more than likely used to prop up the creature’s body, much like a pushup action. 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 were speculated to evolve, if not from land excursions, then as a useful adaptation to anchor the creature to the bottom in fast moving current.

A more robust ribcage is also a feature of Tiktaalik, which would have been very helpful in supporting the animal’s body if it did indeed venture from the water. Tiktaalik also lacked a characteristic that most fishes have - bony plates in the gill area that restrict lateral head movement. This means Tiktaalik is currently the earliest fish with a neck, which would give it more freedom in hunting prey either on land or in the shallows.

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.


Tiktaalik is a transitional fossil; it is to tetrapods what Archaeopteryx is to birds.

Its mixture of both fish and tetrapod characteristics include these traits:


Fish

fish gills
fish scales
"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
tetrapod lungs

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 rear fins and tail have not yet been found. It had rows of sharp teeth of a predator fish, and its neck was able to move independently of its body, which is not possible in other fish. The animal also had a flat skull resembling a crocodile's; eyes on top of its head, suggesting it spent a lot of time looking up; a neck and ribs similar to those of tetrapods, with the latter 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 . Neil Shubin and Ted Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 1999. In an interview, Ted Daeschler stated that "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.

The name Tiktaalik is an Inuktitut word meaning "burbot", a shallow-water fish. The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered .

Discovery
The 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 Laurentia continent, which was centered on the equator and had a warm climate.

The remarkable find was made by a paleontologist who noticed the skull sticking out of a cliff. On further inspection, the ancient animal was found to be in fantastic shape for a 383-million-year-old specimen .

The discovery was published in the April 6 2006 issue of Nature and quickly recognized as a classic example of 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." According to a New Scientist article,

"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."
Images: casts of Tiktaalik fossils
See also
Other lobe-finned fish found in fossils from the Devonian period:

Eusthenopteron
Panderichthys
Ichthyostega
Coelacanth
Gogonasus

References
External links

University of Chicago website dedicated to the discovery
Associated Press, Fossil shows how fish made the leap to land, Apr. 5, 2006.
Alok Jha, The Guardian, Discovered: the missing link that solves a mystery of evolution, Apr. 6, 2006.
BBC news, Arctic fossils mark move to land, Apr. 5, 2006.
NewsHour, Fossil Discovery, April 6, 2006. (Interview with Ted Daeschler)
Quirks and Quarks, Missing Link Fish Fossil, April 8, 2006. (Interview with Ted Daeschler)
Time Magazine, "Our Cousin the Fishapod", April 10,2006
Science Museum (London), "Fish fingers: how our limbs came from fins" April 6, 2006.
Harvard Gazette Missing link crawls out of muck
Video interview with Neil Shubin Tiktaalik: Fish out of Water
Video Building Tiktaalik



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