Pseudoextinction (or phyletic extinction) of a species occurs when all members of the species are extinct, but members of a daughter species remain alive. As all species must have an ancestor of a previous species, much of evolution is believed to occur through pseudoextinction. However, it is difficult to prove that any particular fossil species is pseudoextinct unless genetic information has been preserved. For example, it is sometimes claimed that the extinct Hyracotherium (an ancient horse-like animal commonly known as an eohippus) is pseudoextinct, rather than extinct, because several species of horse, including the zebra and the donkey, are extant today. However, it is not known, and probably cannot be known, whether modern horses actually descend from members of the genus Hyracotherium, or whether they simply share a common ancestor.
Pseudotermination is an extreme form of pseudoextinction, when a lineage continues as a new species; phylogeny is often difficult to determine in such cases.
Extirpation or regional disappearance can be a stage in pseudoextinction when progressive diachronous range contraction leads to final extinction by the elimination of the last refuge or population growth from this temporal bottleneck.
The notion of pseudoextinction is sometimes applied to wider taxa than species. For instance, the entire superorder Dinosauria, as traditionally conceived, would have to be considered as pseudoextinct, because feathered dinosaurs are considered by the majority of modern palaeontologists as the ancestors of modern day birds. Pseudoextinction for such higher taxa higher appears to be easier to prove. However, pseudoextinct higher taxa are paraphyletic groups, which are rejected as formal taxa in phylogenetic nomenclature; either all dinosaurs are stem-group birds, or birds are derived dinosaurs, but there is no taxon Dinosauria, acceptable in cladistic taxonomy, that excludes the taxon Aves.