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New data from the middle and late Miocene Siwalik deposits of Pakistan provide accurate estimates of real temporal durations of extinct species of rhizomyid rodents. Most early rhizomyid species survive on the order of millions of years, with at least two spanning about five million years, and display apparent stasis in most characters. Average species duration for all Rhizomyidae of the Potwar Plateau is about 1.2 million years, a figure in line with other estimates for all Mammalia. Three closely related species show sharp differences in hypsodonty, while other traits remain static in each species or change slowly within the clade, on a scale above the species level. Evolution of this lineage shows at least one step in a staircase pattern, with descendants replacing ancestors, and entails an abrupt morphological change that provides a nonarbitrary definition for species boundaries. One ancestral morphotype appears to survive for a short time with its daughter species. Whereas early nonburrowing Rhizomyidae display longterm stasis, later species, some with burrowing adaptations, are shorter lived and at least one rhizomyine shows rapid, perhaps continuous phyletic change.

“The time at which an evolving population became different enough from its ancestry to be called a different species cannot, even in theory, be a precise, naturally defined date unless the new, descendant species arose in a single, abrupt step.”

G. G. Simpson, 1953, p. 35

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