Pseudoextinction (or phyletic extinction) of a species occurs when all members of the species are extinct, but members of a daughter species remain alive. The term pseudoextinction refers to the evolution of a species into a new form, with the resultant disappearance of the ancestral form. Pseudoextinction results in the relationship between ancestor and descendant still existing even though the ancestor species no longer exists. The classic example is that of the non-avian dinosaurs. While the non-avian dinosaurs of the Mesozoic died out, their descendants, birds, live on today. Many other families of bird-like dinosaurs also died out as the heirs of the dinosaurs continued to evolve, but because birds continue to thrive in the world today their ancestors are only pseudoextinct. Overview From a taxonomic perspective, pseudoextinction is "within an evolutionary lineage, the disappearance of one taxon caused by the appearance of the next." The pseudoextinction of a species can be arbitrary, simply resulting from a change in the naming of a species as it evolves from its ancestral form to its descendant form. Taxonomic pseudoextinction has to do with the disappearance of taxa that are categorized together by taxonomists. As they are just grouped together, their extinction is not reflected through lineage; therefore, unlike evolutionary pseudoextinction, taxonomic pseudoextinction does not alter the evolution of daughter species. From an evolutionary perspective, pseudoextinction entails the loss of a species as a result of the creation of a new one. As the primordial species evolves into its daughter species, either by anagenesis or cladogenesis, the ancestral species can be subject to extinction. Throughout the process of evolution, a taxon can disappear; in this case, pseudoextinction is considered an evolutionary event. From a genetic perspective, pseudoextinction is the "disappearance of a taxon by virtue of its being evolved by anagenesis into another taxon." 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