Ichthyostega, which has been found in the Upper Devonian formation (Celsius Bjerg) of East Greenland, is one of the earliest tetrapods in tetrapodomorphs 4), 7), 8).
Ichthyostega is about 1.5 m in length 4). The forelimb has a humerus, rarius and ulna like exant tetrapods, but distal potion (hand) is unknown. Hindlimb also has a femur, tibia, fibula and, according to Jarvik's description, five digits 7), but actually seven digits 1), 4), 10). Its tail has a fin 4), 7).
The body transeverse-section is an upright oval shape and the thoracic ribs are broadened in the middle and deeply overlapping adjacent ones, suggesting that the lateral flexion of thorax may be ristricted considerably 1), 4). The lumbar region may have been able to flex dorsoventrally (like an inchworm) 1).
The hind limbs were paddle-shape and played a role of oars in swimming or had a stabilizing function during substrate movement, rather than legs 4), 9).
Presumably, even though Ichthyostega could lift its head with its forelimbs and hauled itself to the shore, Ichthyostega could not walk like modern terrestrial tetrapods, and was basicaly an aquatic tetrapods swimming in shallow water, or moved on land/substrate by alternating its lumbar extension and flexion like inchworms or by synchronous 'crutching' motion of its forelimbs like mudskippers 1), 4), 9), 10).
Most specimens of Ichthyostega derived from flood deposits so that the infomations about their original habitat are little 2), 8). Ichthyostega may have lived in the freshwater environments or brackish water near estuaries, like other Devonian tetrapods 8).
Clearly, Ichthyostega was a predator 8). Presumably, it was ambushing its preys in the shallow, sometimes, putting a part of its head out of the water.
Devonian tetrapods have been found exclusively from East Greenland, which were a part of Euramerica (Laurussia, or Old Red Snadstone continent, NOT Laurasia!) and located near the equator of those times. However, a fossil jaw that closely resembles Ichthyostega has also been found in remote areas from there, suggesting that it may have been swimming near shore and moving far away 3), 5), 8). Additionally, Devonian tetrapods have been collected in the Antarctic Circle of those days, suggesting that they lived not only warm habitats but also distributed globally 5).
Although Ichthyostega is the closest to modern terresterial vertebrates amongst the tetrapod stem groups, it could not use quadrupedal gaits like exant four-legged vertebrates, finally, it could not (or would not) transit to land completely 1), 2), 9), 10).
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- Devonian Times - Ichthyostega spp.
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