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 and paleobiologist David Archibald 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 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. Pseudoextinction cannot be applied to the genus or family levels as, “when a species evolves to a new form, causing the pseudoextinction of the ancestral form, the new species is normally assigned the same higher taxa as the ancestor.” When a family or genus goes extinct it must be true extinction, because pseudoextinction would mean that at least one member of the family or genus is still extant. Pseudoextinction is an event that occurs much more frequently under the assumption of a Phyletic gradualism model of evolution, under which speciation is slow, uniform and gradual. The majority of speciation would occur through anagenesis under this model, resulting in a majority of species undergoing Pseudoextinction. However, the model of punctuated equilibrium is more widely accepted, with the proposal that most species remain in stasis, a state of very little evolutionary change, for a large proportion of the species' lifespan. This would result in increased cases of speciation through cladogenesis and