Paleoclimate and paleoecology of the latest Eocene Florissant flora of Central Colorado, U.S.A.

Abstract

The uppermost Eocene Florissant Formation of central Colorado, U.S.A. contains a diverse flora and fauna preserved in lacustrine facies and represents a key episode in Earth history immediately preceding the Eocene-Oligocene boundary. Laminated shales contain impressions of non-monocot angiosperm leaves that were used to estimate paleoecological and paleoclimatic parameters using leaf physiognomic methods including: 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 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, indicate the existence of sclerophyllous vegetation. 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) 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, those 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 U.S.A. just before the transition into the cooler Oligocene.

Leaf‐economic strategies across the Eocene–Oligocene transition correlate with dry season precipitation and paleoelevation

“…Species were relatively evenly distributed across this range, suggesting that no one leaf‐economic strategy was strongly favored at Florissant. These results are consistent with independent LMA estimates of fossils from Florissant excavation site 9 (Allen et al, 2020), a quarry in the same middle shale layer as most of the fossils in our study, which includes 152 leaves from excavation site 9 itself. Compared to Florissant’s two taxonomic ancestors, its higher site LMA and wide range of species‐mean LMA most closely resemble the warm and dry Bonanza flora (113 g/m 2 ; 70–157 g/m 2 ) rather than the cold and wet Republic flora (77 g/m 2 ; 57–87 g/m 2 ; Table 2).…”

“…Located in the high‐elevation Rocky Mountains of Colorado (Meyer, 2001), Florissant (34.07 Ma; 40 Ar/ 39 Ar dating; Evanoff et al, 2001) is our best‐represented pre‐EOT highland flora. The flora contains a mix of subtropical evergreen and temperate deciduous species (MacGinitie, 1953; Leopold et al, 2008; Allen et al, 2020) that lived in a temperate climate, approaching microthermal, with winter temperatures that occasionally dipped below freezing (Wolfe, 1994; Gregory and McIntosh, 1996; Meyer, 2003; Boyle et al, 2008). Rainfall was limited, especially during the pronounced dry season (Gregory‐Wodzicki, 2001; Leopold and Clay‐Poole, 2001; Table 2).…”

Wheeler’s dilemma revisited: first Oecophylla–Lasius syninclusion and other ants syninclusions in the Bitterfeld amber (late Eocene)

“…MAT in the middle latitudes of the North America apparently decreased by 5-9 °C from the end of the Ypresian to the end of the Priabonian (Allen et al, 2020), while that of the latest Ypresian at Messel differs by 5 °C from that of Priabonian at Kučhlin (where Oecophylla is known) (Grein et al, 2011;Kvaček, Teodoridis, 2011), and differs by 9 °C from the Bembridge Marls (latest Priabonian).…”

Linking host plants to damage types in the fossil record of insect herbivory

“…For component communities, identification of the secondary consumers associated with the herbivores on a host plant is challenging with fossils (Greenwood 1991;Martínez-Delclòs and Martinell 1993;Smith and Moe-Hoffman 2007). Many of the iconic insect Lagerstätten also contain abundant plant fossils (from oldest to youngest : Carpenter 1997;Wittry 2006;Novokshonov 1997;Ponomareva et al 1998;Cairncross and Anderson 1995;Anderson 1999;Dobruskina 1995;Shcherbakov 2008;Huang et al 2016;Ren et al 2019;Huang 2016;Xiao et al 2022;Ribeiro et al 2021;Wappler et al 2009;MacGinitie 1969;Wilson 1978;Grande 1984;Dayvault et al 1995;Wappler et al 2012;Dunne et al 2014;Douglas and Stockey 1996;Labandeira 2002;Constenius et al 1989;Greenwalt and Labandeira 2013;Wilde and Frankenhäuser 1998;Lutz et al 2010;Wappler et al 2012;Wilson 1978;Meyer 2003;Allen et al 2020). When the relevant plants and insects do co-occur, it is nearly impossible to link a particular insect taxon (within a given feeding guild) to a particular damage type (within a given functional feeding group).…”