Long-period astronomical forcing of mammal turnover.

Mammals are among the fastest-radiating groups, being characterized by a mean species lifespan of the order of 2.5 million years (Myr). The basis for this characteristic timescale of origination, extinction and turnover is not well understood. Various studies have invoked climate change to explain mammalian species turnover, but other studies have either challenged or only partly confirmed the climate-turnover hypothesis. Here we use an exceptionally long (24.5-2.5 Myr ago), dense, and well-dated terrestrial record of rodent lineages from central Spain, and show the existence of turnover cycles with periods of 2.4-2.5 and 1.0 Myr. We link these cycles to low-frequency modulations of Milankovitch oscillations, and show that pulses of turnover occur at minima of the 2.37-Myr eccentricity cycle and nodes of the 1.2-Myr obliquity cycle. Because obliquity nodes and eccentricity minima are associated with ice sheet expansion and cooling and affect regional precipitation, we infer that long-period astronomical climate forcing is a major determinant of species turnover in small mammals and probably other groups as well.

Age structure, residents, and transients of Miocene rodent communities.

The age structures of two successive rodent communities are studied on the basis of a rich record from well-dated Miocene sections (17-10 Ma) in north-central Spain. Community age is defined as the mean of the residence times of the community members at the time of the locality age. Community ages are negatively correlated with the numbers of community members. These members are divided into residents (with continuous membership times > or =1.54 million years) and transients (with membership times <1.54 million years). During episodes of species loss, there is a preferential disappearance of transients while residents are retained, a pattern referred to as the "seniority rule." The residents define the studied communities. They are associated with early successional stages of vegetation, and transients are associated with later stages. Under stable conditions, early arrivals in succession are "transient" and replaced by competitive later arrivals. The reversed roles of transients and residents in the studied fossil record are explained by assuming high degrees of disturbance. We view the system within the context of nonequilibrium metapopulation theory, in which competitively superior species become transients because of their dependence on ephemeral late successional habitats.