Meeting Abstract
Unlike early bursts, late bursting radiations have an initial shift to a new adaptive zone followed by a long quiescent period before diversity or disparity begin to increase. This late burst pattern challenges the notion that ecological opportunity giving access to the adaptive zone makes the entirety of zone available in one stroke. In the squirrel subfamily Xerinae, one lineage remained arboreal and retained the arboreal diet and two lineages shifted from arboreal to terrestrial habits, adopting novel diets that required less gnawing and more mastication; all three invaded new continents. We examine the influence of these ecological and geographic transitions on the evolution of mandibular size and shape. The arboreal one exhibits an early burst in size and shape, but neither terrestrial lineage follows this pattern. Both terrestrial lineages diverge from tree squirrels in shape preceding their invasion of new continents. One, invading Africa, undergoes little additional change in size or shape. The other, invading North America, undergoes a large initial divergence (separating chipmunk and ground squirrel lineages) but no further change until ground squirrels undergo late bursts of size and shape more than 10 Ma later. The timing of morphological divergence in North American ground squirrels corresponds closely to the timing of grassland expansion on that continent. These results suggest that subdividing the ground squirrel adaptive zone may not have been possible until the grasslands, themselves, expanded and diversified. When ecological opportunity arises from new trophic relationships, the evolutionary pattern of the consumer may depend on that of its new dietary resources.
