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The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
The tongue is anchored to the hyoid bone, which was once the lower half of a pair of gill bars (the second pair after the ones that evolved into jaws). [89] [90] [91] The tongue did not evolve until the gills began to disappear. Acanthostega still had gills, so this would have been a later development. In an aquatically feeding animals, the ...
However, this group represents a paraphyletic grade of primitive stem-tetrapods and is not used by many modern researchers. 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 ...
Stegocephalians include both the modern lineage of limbed vertebrates (the crown group tetrapods, including modern amphibians, reptiles, birds and mammals) as well as a portion of the stem group, the earliest limbed tetrapodomorphs such as Ichthyostega and Acanthostega, which evolved in the Devonian period long before the origin of the crown group.
Polydactyly in early tetrapod aquatic animals, such as in Acanthostega gunnari (Jarvik 1952), one of an increasing number of genera of stem-tetrapods known from the Upper Devonian, which are providing insights into the appearance of tetrapods and the origin of limbs with digits. It also occurs secondarily in some later tetrapods, such as ...
The lungfish is the closest relative to the fish from which all four-limbed animals descended, making the information in its massive genetic code vital to understanding the last 400 million years ...
In ichthyology the difference between Euteleostomi and Osteichthyes is that the former presents a cladistic view, i.e. that the terrestrial tetrapods evolved from lobe-finned fish (Sarcopterygii). Until recently, the view of most ichthyologists has been that Osteichthyes were paraphyletic and include only bony fishes. [ 8 ]
A natural animal lover, She has an affinity for cute, furry creatures and has a bond with Murf. Rok later become the ship's science officer and her affinity for Murf causes a Starfleet Academy professor to suggest that she major in xenobiology. In season 2, Rok is assigned to the Cetacean Ops on board Voyager for her training. She is at first ...