INTRODUCTION

The distribution and evolution of mammals over the past 145 million years show an interesting diversity of patterns. The two main physical parameters that appear to have controlled these patterns are the presence or absence of land bridges between land masses and climate change. A catastrophic event, possibly resulting from either volcanism or extraterrestrial impact, may also have played a part. Emergence of a land bridge may be the result of local tectonic uplift, but more widespread land corridors can be created by eustatic lowering of sea level. Climate change is a critical factor in the latitudinal movement of vegetation zones and in shortening or lengthening the annual period of food availability. Mammals have responded to these parameters of global change by evolving new ecological strategies and/or by shifting their geographical ranges. Long-term climate changes affecting annual ranges of temperature and thus determining seasonality vs. equability may be an important cause of community evolution (Janis, 1989; Collinson & Hooker, 1987, 1991). The absence of land connections may prevent or inhibit geographical range extensions and cause widespread endemism (vicariance). An example of this is the independent evolution of several now extinct mammalian orders in the Tertiary of South America, which until the Late Pliocene was an island continent (Pascual et al., 1985). The effects of tectonics and climate are thus closely linked, but their variable interrelationships may be expected to produce different faunal patterns in the geological record, which are capable of interpretation.