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By the Upper Devonian period, the fin-limb transition as well as other skeletal changes such as gill arch reduction, opercular series loss, mid-line fin loss, and scale reduction were already completed in many aquatic organisms. [3] As aquatic tetrapods began their transition to land, several skeletal changes are thought to have occurred to ...
Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least part of the time in water. These animals are called "secondarily aquatic" because although their ancestors lived on land for hundreds of millions of years, they all originally descended from ...
Thus, those very few tetrapod fossils found in this "gap" are all the more prized by palaeontologists because they document these significant changes and clarify their history. The transition from an aquatic, lobe-finned fish to an air-breathing amphibian was a significant and fundamental one in the evolutionary history of the vertebrates.
The vertebrate land invasion refers to the transition of vertebrate animals from being aquatic/semiaquatic to predominantly terrestrial during the Late Devonian period. This transition allowed some vertebrates to escape competitive pressure from other aquatic animals and explore niches on land, [1] which eventually established the vertebrates as the dominant terrestrial phylum.
The early tetrapod Acanthostega had at least three and probably four pairs of gill bars, each containing deep grooves in the place where one would expect to find the afferent branchial artery. This strongly suggests that functional gills were present. [96] Some aquatic temnospondyls retained internal gills at least into the early Jurassic. [97]
Phylogenetic analysis has shown Ichthyostega is intermediate between other primitive stegocephalian stem-tetrapods. The evolutionary tree of early stegocephalians below follows the results of one such analysis performed by Swartz in 2012. [9] Simplified phylogeny of the fish–tetrapod transition.
Fossils of both aquatic and terrestrial tetrapods are known from these localities, providing an important record of the transition between life in water and life on land [19] and filling some of the lacunae in Romer's gap.
Tulerpeton is one of the early transition tetrapods – a marine animal capable of living on land. The separation of the pectoral-shoulder girdle from the head allowed the head to move up and down, and the strengthening of the legs and arms allowed the early tetrapods to propel themselves on land. Tulerpeton is important in the study of dactyly.