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. The classic example is that of the dinosaurs. From a taxonomic perspective, pseudoextinction is "within an evolutionary lineage, the disappearance of one taxon caused by the appearance of the next." 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 whether they simply share a common ancestor.
One proposed mechanism of pseudoextinction is endocrine disruption (changing hormone levels). Additionally, when the primary sex-ratio (male to female ratio of a population) is male-biased, predicted levels of pseudoextinction increase.  Because the variance of the population size increases with time, the probability of pseudoextinction increases with the length of the time horizon used
Mammal systematist/paleobiologist David Achibald has estimated that as many as 25% of the extinctions recorded in three different early Puercan mammal lineages are pseudoextinctions.  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.
Basically, pseudoextinction is different from true extinction because true extinction is measured at a higher level of taxon, for example an entire clade becoming extinct rather than pseudoextinction which happens at a lower level of taxon. Therefore, true extinction is more useful in measuring extinction because it better reflects what diversity was lost. Thus, pseudoextinction is problematic when studying extinction.
- Newman, Mark. "Modeling Extincition". p. 102.
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- Westermann, Gerd E.G. (2001). "Modes of extinction, pseudo-extinction and distribution in Middle Jurassic ammonites: terminology". Canadian Journal of Earth Sciences 38: 187. Bibcode:2001CaJES..38..187W. doi:10.1139/cjes-38-2-187.
- Cascales-Miñana, B.; Cleal, C.J.; Diez, J.B. (September 2013). "What is the best way to measure extinction? A reflection from the palaeobotanical record". Earth-Science Reviews 124: 126 Extra
|at=(help). doi:10.1016/j.earscirev.2013.05.013. Retrieved 23 October 2014.
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