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An anal fissure is a break or tear in the skin of the anal canal. Anal fissures may be noticed by bright red anal bleeding on toilet paper and undergarments, or sometimes in the toilet. If acute they are painful after defecation , [ 1 ] but with chronic fissures, pain intensity often reduces and becomes cyclical.
However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous. The key innovation in amniotes over amphibians is the amnion , which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water.
Acanthostega (meaning "spiny roof") is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs.It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.
Livyatan is an extinct genus of macroraptorial sperm whale containing one known species: L. melvillei.The genus name was inspired by the biblical sea monster Leviathan, and the species name by Herman Melville, the author of the famous novel Moby-Dick about a white bull sperm whale.
[18] [19] Tetrapods can be divided into four classes: amphibians, reptiles, birds and mammals. Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as the Carboniferous Period [ 20 ] whereas other returns occurred as recently as the Cenozoic , as in cetaceans ...
The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the Devonian. [23] 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.
An informal term with a broader meaning is stem tetrapod, a stem group consisting of all species more closely related to modern tetrapods than to lungfish, but excluding the crown group. This group includes both traditional "labyrinthodonts" as well as more basal tetrapodomorph fish, though its total content is a matter of some uncertainty, as ...
Evolution of tetrapods (3 C, 14 P) R. Reptiliomorphs (5 C, 11 P) T. Tetrapod taxa (3 C) Pages in category "Tetrapods" The following 4 pages are in this category, out ...