NICHE BREADTH OF LATE CRETACEOUS PLANTS INFERRED FROM PATTERNS OF DISTRIBUTION AND ABUNDANCE IN THE EARLY MAASTRICHTIAN BIG CEDAR RIDGE TUFF, WYOMING

Ecological theory predicts that early-successional plants exposed to variable environments should occupy broad ('generalized') fundamental niches. Consequently, they are expected to have a wider distribution among microhabitats than late-successional, 'specialized' plants, although they may not display a high abundance in all habitats. Hypotheses about ecological strategy of taxa can be tested using abundance distribution data along relevant environmental resource gradients, but such data are generally not available for fossil plants.

The Big Cedar Ridge ashfall flora from the Late Cretaceous Meeteetse Formation, Wyoming, is an exception. Plants are preserved in situ at the base of a tuff along a 4 km long outcrop. The flora is dominated by ferns, palms, and to a lesser degree conifers, but also preserves many rare dicotyledonous angiosperms. It has been hypothesized that the dicotyledons were early-successional generalists that were only abundant in physically disturbed sites with low levels of soil carbon. We tested this hypothesis by examining the abundance distribution of individual Big Cedar Ridge plant taxa along a gradient of soil organic content. The data suggest that many plants, whether widely or narrowly distributed, appear to have a wide tolerance for variation in soil carbon content. However, there are differences in the relative abundance distribution along this gradient among represented plant groups. Many ferns, conifers, cycads, and monocotyledons show a relatively even abundance distribution along their tolerance interval or a higher abundance at higher levels of soil carbon (>40%). Whereas some dicotyledons are similarly evenly distributed along their tolerance interval, others are very rare in soils with >10% soil carbon. This pattern indicates that there may have been variation in fundamental niche breadth in terms of soil organic content among different plant taxa (including among different dicotyledons), that the distribution of plants across microhabitats was influenced by factors other than soil organic carbon, or a combination of both.