Vegetation structure in Late Cretaceous ecosystems in North America and the role of angiosperms

What was the ecological role(s) of angiosperms before they became ecosystem dominants? Mid-Late Cretaceous angiosperms were highly diverse but had not reached ecological importance in all ecosystems, particularly not at mid-high latitudes of North America (e.g., Crane and Lidgard 1989). Here, many angiosperms appear to have been herbaceous and to have primarily occupied habitats interpreted as disturbed (e.g., river margins). Paleobotanists interpret this as evidence for a ruderal life strategy, whereby fast growth and a rapid life cycle allowed them to excel in local patches where frequent disturbance ensured that competition from other plants (conifers, cycads, ferns) was low and resources (light, nutrients) plentiful (Doyle and Hickey 1976, Wing and Boucher 1998).

However, testing these hypotheses has been difficult, because most Late Cretaceous leaf floras are too mixed to allow for reconstruction of
microhabitats and relative abundances of individual plant

Scott Wing pointing to the ash (event) bed that overlies the Big Cedar Ridge paleosol (dark horizon). Photo courtesy of F.A. Smith.

taxa across the landscape. They also rarely preserve leaves of herbaceous plants. An exception is the Big Cedar Ridge flora and six other, contemporaneous Late Cretaceous floras that were preserved as vegetated surfaces at the base of tuffs in the Meeteetse and Almond Formation, Wyoming (Wing et al. 1993). These landscape-scale deposits preserve snapshots of Late Cretaceous plant communities in their sedimentological setting, allowing for analysis of relative abundance and diversity.

With Scott Wing and others, we investigate Late Cretaceous vegetation structure in these floras. Scott and his collaborators collected census data from 100 sites along the 4-km long outcrop at Big Cedar Ridge that we use to reconstruct vegetation and the ecological role(s) of individual angiosperm taxa. Work in progress includes redescriptions of the ~160 leaf morphotypes at Big Cedar Ridge, characterization of vegetation associations across the landscape, and niche breadth analysis of individual plant taxa (Wing et al. 2012).
Exquisite preservation of Cercidiphyllum-like leaves on an axis, Big Cedar Ridge flora. © Smithsonian Institution.

Collaborators: Scott Wing, Kay Behrensmeyer (USNMNH, Smithsonian Institution), Leo Hickey (Yale University), Robyn Burnham (University of Michigan), Fleur Tiver (University of South Australia), and Brian Willis (Texas A&M University).