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In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish — like Eusthenopteron — exhibited a sequence of adaptations: *Panderichthys, suited to muddy shallows *Tiktaalik with limb-like fins that could take it onto land *Early tetrapods in weed-filled swamps, such as: **Acanthostega, which had feet with eight digits **Ichthyostega with limbs Descendants also included pelagic lobe-finned fish such as coelacanth species.
Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water.The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles.Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs.In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians).The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period.
The early types resembled their cartilaginous ancestors in many features of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.They did, however, have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms.With the exception of a pair of spiracles, the gills did not open singly to the exterior as they do in sharks; rather, they were encased in a gill chamber stiffened by membrane bones and covered by a bony operculum, with a single opening to the exterior.The cleithrum bone, forming the posterior margin of the gill chamber, also functioned as anchoring for the pectoral fins.The cartilaginous fishes do not have such an anchoring for the pectoral fins.This allowed for a movable joint at the base of the fins in the early bony fishes, and would later function in a weight bearing structure in tetrapods.