Floral and environmental gradients on a Late Cretaceous landscape

Abstract

We describe an in situ fossil flora of Late Cretaceous age (similar to 73 Ma [megaannum or million years]) from Big Cedar Ridge in central Wyoming, USA, which we sampled using a modified line-intercept method to quantify the relative abundances of 122 taxa at 100 sites across 4 km of exposed sedimentary deposits. We also measured three physical variables at each site: paleotopographic level, grain size, and total organic content. Paleoenvironmental conditions and paleolloral composition at Big Cedar Ridge covary strongly and are highly heterogeneous on small spatial scales. The reconstructed vegetation has some similarities with extant topogenous fens, but also important differences. Non-monocot angiosperms were abundant only on wet, mineral substrates that had been disturbed shortly before preservation, consistent with the weedy life histories that are inferred for their Early Cretaceous ancestors. Many non-monocot angiosperms grew in small, dispersed populations, consistent with the hypothesis that they were biotically pollinated. Overall, non-monocot angiosperm abundance was low compared with many modern wetlands. A single species of coryphoid palm was the dominant on moist, stable, moderately organic-rich sites, a pattern seen in some subtropical to tropical wetlands in the present day. Fern thickets at Big Cedar Ridge occupied highly organic, possibly low-nutrient substrates, and were dominated by Dipteridaceae, Gleicheniaceae, Schizaeaceae, and Matoniaceae. The overall high diversity and abundance of pteridophytes is unusual in the context of modern vegetation, regardless of climate zone, and probably represents a late occurrence of pieridophyte-dominated vegetation that was common earlier in the Mesozoic. Plant distributions at Big Cedar Ridge combine aspects of pre-angiosperm and modern vegetation in a way that suggests both niche conservatism and niche evolution on geological time scales.