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.