Albert Jan van der Meulen

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.

Evolution of early-middle miocene rodent faunas in relation to long-term palaeoenvironmental changes

Abstract Cluster and principal components analyses have been applied to the composite sequence of 59 rodent assemblages from Ramblian to Lower Vallesian deposits in the Daroca-Calamocha area in the Calatayud-Teruel Basin (North Central Spain). The studied record is thought to represent the interval from the late Aquitanian to the end of the Serravallian (i.e. Early to Middle Miocene, from ca. 22-10.5 Ma ago). The quantitative results are interpreted by using individual rodent taxa as environmental indicators, and by extrapolating the predominant adaptive strategies of recent taxonomic groups, which also seem to form natural groups on a demographic scheme (French et al., 1975). A general warming trend starts in the Rambian (Early Miocene) and continues into the Middle Aragonisn (early Middle Miocene). The cooling in the Middle to Late Aragonian boundary interval is correlated to the middle Miocene cooling phase. Another cooling occurred around the Aragonian-Vallesian boundary (late Middle Miocene). We relate a major increase of aridity in the beginning of the Middle Aragonian (ca. 16.5 Ma) to the spread of low biomass vegetation due to intensification of the subtropical belt (Wolfe, 1985). A notable feature around the Early-Middle Aragonian boundary, indicating a major shift of climatic belts, is the reversal of the relationship between our relative temperature and humidity curves: before the reversal we find a positive correlation, after it a negative correlation. Around the Middle to Late Aragonian boundary humidity increased again, and a further increase of humidity is indicated by the earliest Vallesian faunas. Our data suggest that the nature of favoured adaptive strategy amongst rodents is related, in the end, to the degree of climatic humidity.

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 ≥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.

Residents and Transients in the Fossil Record

Ranges of fossil taxa have received great attention in the context of biostratigraphy and of evolutionary studies dealing with species longevity, but relatively little in terms of community membership. Recently, however, recognition of residence-time components has produced results of interest for the dynamics of communities, and for the comparison of community change on ecological and evolutionary scales. Residents and transients, as used here, refer to taxa with long and, respectively, short membership times in fossil metacommunities over evolutionary timescales. The distinction of the two components is based on the durations over which rodent taxa occur in a composite Miocene section in a restricted area. Residence times allow calculation of community age (CA), defined as the mean of the residence times of the community members at locality age. CAs are negatively correlated with the numbers of members, due to preferential loss of transients while retaining residents during episodes of decreasing community richness.

Micromamíferos miocenos de Jebel Zelten, Libia

Restos de micromamiferos de siete localidades han sido recogidas durante dos campanas de campo geologicas/paleontologicas (1983 y 1997). Las muestras son pequenas pero se han reconocido y descrito doce especies que representan siete familias de roedores, una de lagomorfos y una de quiropteros. La fauna de mamiferos (macromamiferos) de Jebel Zelten ha sido considerada, en la mayoria de las publicaciones, como un unico lapso temporal, aunque su interpretacion temporal ha sido diversa. En base al estado evolutivo de las especies de micromamiferos, la composicion faunistica y la secuencia estratigrafica concluimos que las asociaciones de Jebel Zelten representan tres periodos temporales differentes que cubren aproximadamente 4 millones de anos. Tres asociaciones pueden asignarse al Mioceno inferior medio (18-19 Ma), una al Mioceno inferior tardio (16-17 Ma) y dos al Mioceno medio (14-15 Ma).