Paleoclimate and paleoecology of the latest Eocene Florissant flora (Central Colorado, USA)

Abstract The uppermost Eocene lacustrine Florissant Formation in central Colorado preserves a diverse flora and fauna at a key time in Earth history immediately preceding the Eocene-Oligocene boundary. Laminated shales in the Florissant Formation record impression fossils of woody non-monocot angiosperm leaves that were used to estimate paleoecological and paleoclimatic parameters using leaf physiognomic methods (leaf mass per area (MA), digital leaf physiognomy (DiLP), leaf margin analysis (LMA), and leaf area analysis (LAA)). The majority (58%) of the morphotypes analyzed for MA suggested a semi-evergreen leaf lifespan, whereas another 27% indicated a deciduous habit and just 15% an evergreen habit. There was no significant relationship between MA and insect damage based on a small subset of Florissant's leaves. Higher MA values (~73% of leaves ≥ one-year lifespan), coupled with a tendency toward long and narrow leaf shapes and small leaf areas, support the presence of sclerophyllous vegetation at Florissant. Using the global regression for mean annual temperature (MAT), the DiLP estimate of MAT was anomalously cold: 5.5 ± 4 °C. However, using a ‘Northern Hemisphere’ regression the DiLP MAT estimate of 11.6 ± 3.3 °C was more plausible. Using DiLP, mean annual precipitation (MAP) for Florissant was estimated at 740 + 608/−334 mm∙yr−1, which supports dry conditions. Estimates for MAT and MAP using the univariate LMA and LAA methods overlapped within uncertainty of the DiLP results. In addition, Florissant taxa classified as growing in wet areas (riparian) had significantly more teeth than non-riparian taxa. These paleoclimatic and paleoecological results suggest that outside the riparian forest, the Florissant flora sampled a seasonally dry temperate sclerophyllous shrubland to woodland, perhaps similar to modern chaparral forests, in the Western Interior of the USA just before the transition into the cooler Oligocene.