RELATIVE DISTRIBUTION OF CERCIDIPHYLLUM- AND ZIZYPHOIDES-LIKE PLANTS IN THE PALEOGENE: A CASE FOR ECOLOGICAL NICHE MODELING?

Ecological niche modeling (ENM) has become an established tool in paleontology and is widely employed to estimate the geographic ranges of species in the fossil record. In essence, ENM methods use occurrence points and principally the abiotic environmental factors at those locations to reconstruct the niche of a species. Recently, the evolutionary and population demographic histories of the endemic Asian plant Cercidiphyllum have been assessed using ENM and molecular data (Qi et al, 2012). It appears that genetic adaptation into new environments greatly influenced the current distribution of Cercidiphyllum in East Asia. The long absence of Cercidiphyllum in Europe and North America suggests that other factors (e.g. migration ability) may have contributed to its extinction there. Using the occurrence data and morphology of “Cercidiphyllum-like" plants (including Joffrea) at Late Paleocene localities (e.g., Almont, North Dakota; Joffre Bridge, Alberta) and early-middle Eocene localities of the Okanogan Highlands (e.g., Republic, Washington; One Mile Creek, British Columbia), we find an interesting trend in that either Cercidiphyllum-like plants or Zizyphoides/Nordenskioldia plants are dominant at a given locality and the corresponding taxon is rare or apparently missing. Based on morphological features and depositional environments at these localities, we suggest that these taxa may have occupied such similar niches that they directly competed with one another. ENM has a high potential for providing insights to detecting changes of distribution of fossil plants related to paleoclimate. However, the challenge will be recognizing suites of morphological, physiological and reproductive traits that define the tolerances potentially determining the distributional ranges of fossil plant taxa.