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Fishes are a paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal taxonomy.Traditional classification divides fish into three extant classes (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes: [1]
The evolution of fishes took place over a timeline which spans the Cambrian to the Cenozoic, including during that time in particular the Devonian, which has been dubbed the "age of fishes" for the many changes during that period. The Late Devonian extinctions played a crucial role in shaping the evolution of fish, or vertebrates in general. [1]
One small clade of fish, showing how venom has evolved multiple times. [10]Taxonomy is the identification, naming, and classification of organisms. Compared to systemization, classification emphasizes whether a species has characteristics of a taxonomic group. [6]
Tiktaalik, a lobe-finned fish with some anatomical features similar to early tetrapods. It has been suggested to be a transitional species between fish and tetrapods. [81] 365 Ma Acanthostega is one of the earliest vertebrates capable of walking. [82] 363 Ma By the start of the Carboniferous Period, the Earth begins to resemble its present state.
Jawless fish belong to the superclass Agnatha in the phylum Chordata, subphylum Vertebrata. Agnatha means 'un-jawed, without jaws' (from Ancient Greek). [11] It excludes all vertebrates with jaws, known as gnathostomes. Although a minor element of modern marine fauna, jawless fish were prominent among the early fish in the early Paleozoic.
The scientific work of deciding how to define species has been called microtaxonomy. [26] [27] [20] By extension, macrotaxonomy is the study of groups at the higher taxonomic ranks subgenus and above, [20] or simply in clades that include more than one taxon considered a species, expressed in terms of phylogenetic nomenclature. [28]
Most oceanic species (78 per cent, or 44 per cent of all fish species), live near the shoreline. These coastal fish live on or above the relatively shallow continental shelf. Only 13 per cent of all fish species live in the open ocean, off the shelf. Of these, 1 per cent are epipelagic, 5 per cent are pelagic, and 7 per cent are deep water. [16]
Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship (shared descent), progenitor-descendant relationship (serial descent), and degree of evolutionary change.