<P> A 2012 study using 3D reconstructions of Ichthyostega concluded that it was incapable of typical quadrupedal gaits . The limbs could not move alternately as they lacked the necessary rotary motion range . In addition, the hind limbs lacked the necessary pelvic musculature for hindlimb - driven land movement . Their most likely method of terrestrial locomotion is that of synchronous "crutching motions", similar to modern mudskippers . (Viewing several videos of mudskipper "walking" shows that they move by pulling themselves forward with both pectoral fins at the same time (left & right pectoral fins move simultaneously, not alternatively). The fins are brought forward and planted; the shoulders then rotate rearward, advancing the body & dragging the tail as a third point of contact . There are no rear "limbs" / fins, and there is no significant flexure of the spine involved .) </P> <P> The first tetrapods probably evolved in coastal and brackish marine environments, and in shallow and swampy freshwater habitats . Formerly, researchers thought the timing was towards the end of the Devonian . In 2010, this belief was challenged by the discovery of the oldest known tetrapod tracks, preserved in marine sediments of the southern coast of Laurasia, now Świętokrzyskie (Holy Cross) Mountains of Poland . They were made during the Eifelian stage at the end of the Middle Devonian . The tracks, some of which show digits, date to about 395 million years ago--18 million years earlier than the oldest known tetrapod body fossils . Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like Tiktaalik . The animal that produced the tracks is estimated to have been up to 2.5 metres (8.2 ft) long with footpads up to 26 centimetres (10 in) wide, although most tracks are only 15 centimetres (5.9 in) wide . The new finds suggest that the first tetrapods may have lived as opportunists on the tidal flats, feeding on marine animals that were washed up or stranded by the tide . Currently, however, fish are stranded in significant numbers only at certain times of year, as in alewife spawning season; such strandings could not provide a significant supply of food for predators . There is no reason to suppose that Devonian fish were less prudent than those of today . According to Melina Hale of University of Chicago, not all ancient trackways are necessarily made by early tetrapods, but could also be created by relatives of the tetrapods who used their fleshy appendages in a similar substrate - based locomotion . </P> <Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2012) (Learn how and when to remove this template message) </Td> </Tr>

What current organism is the descendant of the lobe-finned fish from which tetrapods evolved