FOOD WEB AND BIODIVERSITY ARCHIVES IN EARLY CENOZOIC FOSSIL LEAVES FROM THE WESTERN INTERIOR USA AND PATAGONIA

WILF PETER; LABANDEIRA CONRAD; JOHNSON KIRK; IGLESIAS ARI; CÚNEO RUBEN

Bilbao

Congreso; Internacional Meeting Climate and Biota of the Early Paleogene; 2006

Internacional Meeting Climate and Biota of the Early Paleogene

Fossil angiosperm leaves occur extensively in the rock record. They are often well preserved at high local abundance and in fine stratigraphic context, making them suitable for unbiased, quantitative field collections that provide standardized data on local plant diversity through time. Moreover, fossil leaves typically preserve a high diversity of insect damage morphotypes (over 120 recognized to date) that represent the full ecological spectrum of insect folivory, independent of the taxonomically and taphonomically dissimilar, temporally patchy record of fossil insects. Uniquely in the fossil record, fossil leaves preserve significant quantifiable diversity from two or more levels of a food web in the same organ. We have compiled plant and insect-damage diversity from similarly collected and analyzed fossil floras of latest Cretaceous through middle Eocene age throughout the Western Interior USA, including the Hell Creek, Fort Union, Wasatch, Green River, and Klondike Mountain formations. We are also building a comparable data set of Patagonian floras from similar absolute latitudes, so far including the early-middle Paleocene Salamanca Formation and the early Eocene Laguna del Hunco floras of Chubut Province, Argentina. The current total from both continents is 22,273 identified and inventoried leaf specimens. Several major results have emerged from this standardized approach. (1) Cretaceous-Paleogene boundary (K-T) floras from North Dakota exhibit a near-extirpation of specialized damage morphotypes, such as mines and galls, coincident with a ~57% megafloral extinction. (2) Although most of the subsequent Paleocene in the Western USA is an ecological dead zone with low diversity of plants and insect damage, there are two major exceptions. A low-diversity 64.4 Ma flora from Mexican Hat, southeastern Montana, has extremely high feeding richness, especially of leaf mining, whereas the anomalously diverse Castle Rock flora from the Denver Basin of Colorado, 63.8 Ma, has little damage richness and virtually no specialized feeding. These findings reveal transient, severely unbalanced food webs 1-2 m.y. after ecosystem collapse and at least 8 m.y. before sustained recovery. (3) Fossil floras in southwestern Wyoming record an increase in insect-feeding diversity between the latest Paleocene and the early Eocene climatic optimum coincident with long-term global and regional warming. (4) Subsequent Green River floras show a marked bimodality in feeding diversity that reflects ecologically distinct herbivore strategies related to leaf-economic adaptations in the seasonally dry middle Eocene climate. (5) In Patagonia, Salamanca Formation floras have significantly greater plant diversity than typical Paleocene sites in the US as well as the highest insect feeding richness we have observed in the Cenozoic, showing diversity in food webs that is lacking in the Paleocene of the US. (6) Plant and insect-feeding diversity at Laguna del Hunco are both extremely high, making this early Eocene site one of the most biodiverse fossil ecosystems known. Two, possibly related implications from the Patagonian results are that elevated terrestrial diversity has an ancient history in South America, and that the ecological effects of the K-T event were less severe in Patagonia, at considerably greater distance from Chicxulub than the western US. We are now investigating suitable latest Cretaceous floras from Patagonia to test and refine these hypotheses.