Abstract

Many studies have examined temporal changes in insect feeding on angiosperm leaves, but none have considered variability within a single stratigraphic level. If spatial variability within a level is high, a single sample will not adequately represent the level and may either mask true temporal changes or create spurious ones. In order to measure the spatial variability in fossil insect feeding damage, I collected 12 replicate samples from two laterally extensive carbonaceous shale beds (55.2 and 52.6 Ma) from the early Eocene of the Bighorn Basin, Wyoming. Over 2800 fossil angiosperm leaves were scored for presence or absence of 50 insect damage morphotypes. Damage frequency, diversity, and composition were computed for both the bulk flora and individual plant species in each sample, and variation within a bed was compared with differences between the two beds. Differences in diversity and composition between beds were significantly greater than variations within a bed, and intra-bed variation was primarily due to differing floral composition. Damage frequency within a bed, however, was more variable than diversity. Damage diversity and composition reflect the number of insect species present, whereas damage frequency also depends on the number of insects present, which may be much more variable over small distances.

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