Euteleostei

Euteleostei is a major clade within the Teleostei, the large and diverse group of ray-finned fishes. The name means “true teleosts,” and the group comprises most living teleost species, while the most primitive teleost lineages, such as Osteoglossiformes, lie outside Euteleostei. The clade is defined by evolutionary relationships rather than a single easily observed trait, and its monophyly is supported by molecular studies in combination with selected morphological characters.

Euteleostei likely originated in the Mesozoic era, with estimates placing its emergence in the Jurassic. The lineage subsequently underwent substantial diversification through the Cretaceous and into the Cenozoic, leading to the vast array of forms found in oceans, rivers, and lakes today. Members of this clade inhabit a wide range of ecological niches, including marine and freshwater environments, and they display extensive variation in feeding modes, reproduction, and life history strategies.

Taxonomically, Euteleostei sits within Teleostei and is commonly considered a sister group (in many phylogenetic trees) to Osteoglossiformes, though the exact relationships among early teleost lineages have been refined by molecular data and remain a subject of ongoing research. Within Euteleostei, the diversity is immense, giving rise to many familiar and economically important fishes, from small forage species to large pelagic predators and numerous freshwater lineages. The clade represents a dominant component of modern fish biodiversity.