Paleobiology, 2019, pp. 1–19 DOI: 10.1017/pab.2019.18 Endemism in Wyoming plant and insect herbivore communities during the early Eocene hothouse Ellen D. Currano , Esther R. S. Pinheiro, Robert Buchwaldt, William C. Clyde, and Ian M. Miller Abstract.—The warm, equable, and ice-free early Eocene Epoch permits investigation of ecosystem func- tion and macro-ecological patterns during a very different climate regime than exists today. It also pro- vides insight into what the future may entail, as anthropogenic CO2 release drives Earth toward a comparable hothouse condition. Studying plant–insect herbivore food webs during hothouse intervals is warranted, because these account for the majority of nonmicrobial terrestrial biodiversity. Here, we report new plant and insect herbivore damage census data from two floodplain sites in the Wind River Basin of central Wyoming, one in the Aycross Formation (50–48.25 Ma) at the basin edge (WRE) and the second in the Wind River Formation in the interior of the basin (WRI). The WRI site is in stratigraphic proximity to a volcanic ash that is newly dated to 52.416 ± 0.016/0.028/0.063 (2σ). We compare the Wind River Basin assemblages to published data from a 52.65 Ma floodplain flora in the neighboring Bighorn (BH) Basin and find that only 5.6% of plant taxa occur at all three sites and approximately 10% occur in both basins. The dissimilar floras support distinct suites of insect herbivores, as recorded by leaf damage. The relatively low-diversity BH flora has the highest diversity of insect damage, contrary to hypotheses that insect herbivore diversity tracks floral diversity. The distinctiveness of the WRE flora is likely due to its younger age and cooler reconstructed paleotemperature, but these factors are nearly identical for the WRI and BH floras. Site-specific microenvironmental factors that cannot be measured easily in deep time may account for these differences. Alternatively, the Owl Creek Mountains between the two basins may have provided a formidable barrier to the thermophilic organisms that inhabited the basin interiors, supporting Janzen’s hypothesis that mountain passes appear higher in tropical environments. Ellen D. Currano. Departments of Botany and Geology & Geophysics, University of Wyoming, Laramie, Wyoming 82071, U.S.A. E-mail: [email protected] Esther R. S. Pinheiro. Department of Botany, University of Wyoming, Laramie, Wyoming 82071, U.S.A. E-mail: [email protected] Robert Buchwaldt. Department of Earth and the Environment, Boston University, Boston, Massachusetts 02215, U.S.A. E-mail: [email protected] William C. Clyde. Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, U.S.A. E-mail: [email protected] Ian M. Miller, Department of Earth Sciences, Denver Museum of Nature & Science, Denver, Colorado 80205, U.S.A. E-mail: [email protected] Accepted: 18 May 2019 Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.fb821k5 Introduction biodiversity distribution has greatly increased Documenting the organization and host spe- (Engemann et al. 2016; Pärtel et al. 2016), per- cificities of plant–herbivore food webs is of haps in association with increased concern great importance for understanding biodiver- about the future of biodiversity. Predicting the sity patterns and the processes that maintain response of biodiversity to global changes has these patterns (e.g., Lewinsohn et al. 2005; been identified as a research priority because Novotny et al. 2010; Nyman 2010; de Araújo of its utility in identifying the most effective et al. 2015). Today, biodiversity is distributed scheme for diversity conservation actions (Gas- heterogeneously across the Earth, and deter- ton 2000). mining why differences occur has long been a The modern world is characterized by a core objective for scientists. In the last decade, strong latitudinal diversity gradient in which the number of studies on patterns of the number of species and higher taxa peaks © 2019 The Paleontological Society. All rights reserved. 0094-8373/19 !( !#!%%"$ (((#!#!# '#$%)!("$###)! & % $&%%!%#!#%#$!&$ '%%%"$ (((#!#!#%#$%%"$ !!# " 2 ELLEN D. CURRANO ET AL. in equatorial regions and declines toward the et al. 2015), when mean annual surface air tem- poles (Willig et al. 2003; Hillebrand 2004). The peratures were more than 10°C warmer than tropics and subtropics, characterized by high during the preindustrial period (Caballero temperature and precipitation and relative and Huber 2013; Loptson et al. 2014). Our environmental stability, have the highest plant new floodplain paleofloral sites in the Wind and insect biodiversity and strong biotic inter- River Basin, central Wyoming, capture the actions (Erwin 1982; Coley and Barone 1996; basin interior (WRI) and the basin edge Pennings and Silliman 2005; Dyer et al. 2007; (WRE) ecosystems and can be compared with Adams et al. 2011). These regions also display published data from an EECO paleoflora in high plant and insect beta diversity and consid- the neighboring Bighorn (BH) Basin that occurs erable changes in species composition over in the same depositional environment as the short distances (Lewinsohn and Roslin 2008; WRI flora (Davies-Vollum and Wing 1998; Cur- Jankowski et al. 2009). Janzen (1967) hypothe- rano 2009). Our main goals are to: (1) analyze sized that the more predictable the environ- whether there is a uniform distribution and ment, both in terms of the range and diversity of plant species across the study area regularity of changes, the smaller the change and 2) determine whether there are similar required to serve as a barrier to dispersal. suites and diversities of insect herbivore dam- Organisms that live in less stable environments, age on the paleofloras in the study area. By like the present-day highly seasonal temperate looking for endemism in both plant and insect zone, may be better adapted to a wider range herbivore communities during the EECO, we of conditions than organisms that live in stable can test Janzen’s ideas on the importance of environments, like the wet tropics. Thus, mountain passes in driving macro-ecological “mountain passes appear higher in the tropics” patterns and investigate which modern macro- than in the temperate zone (Janzen 1967), a ecological patterns also occur during green- result that has been supported by a variety of house intervals. Additionally, we can clarify modern studies (Ghalambor et al. 2006; Calosi the extent to which studies in the Bighorn et al. 2010). Basin, one of the best-cited records for ecosys- Archibald and colleagues (2013) tested Jan- tem response to climate change in deep time zen’s hypothesis using high paleoelevation (e.g., Currano et al. 2010, 2016; Wing and Cur- midlatitude insect faunas from the warm rano 2013), can be generalized. early Eocene. They compared insect fossils at five sites spanning ∼1000 km in the Okanagan Materials and Methods Highlands, Canada, and found that less than 5% of insect species occurred at more than Geological Setting.—Tectonic activity asso- one site (Archibald et al. 2013). Although ciated with the Sevier Orogeny created a large these faunas occur in temperate latitudes, the foreland basin in the Late Jurassic that extended climate in which they lived more closely from northern Canada to the Gulf of Mexico. resembled that of the modern subtropics due During the Late Cretaceous through the to a shallower latitudinal temperature gradient Eocene, Laramide deformation dissected this and globally low temperature seasonality dur- large basin and divided it into a series of smal- ing the early Eocene (Bijl et al. 2009; Huber ler basins separated by basement-cored high- and Caballero 2011; Eberle and Greenwood lands (Dickinson et al. 1988). In this study, we 2012; Archibald et al. 2013). Archibald and col- compare paleobotanical assemblages from leagues’ work suggests that a rewrite of Jan- neighboring Laramide basins: the Wind River zen’s original hypothesis to “mountain passes and Bighorn Basins of central and northwestern appear higher in equable, low-seasonality Wyoming, respectively. Today, the Wind River regions” is warranted. Basin is surrounded on all sides by mountains Here, we present new plant and insect herbi- (Fig. 1), with the Washakie Range and Owl vore damage census data from the warmest Creek Mountains forming the northern bound- interval of the last 66 Myr, the early Eocene cli- ary and topographic barrier from the Bighorn matic optimum (EECO, 52.6–50.3 Ma; Payros Basin. Sedimentology, sandstone petrography, !( !#!%%"$ (((#!#!# '#$%)!("$###)! & % $&%%!%#!#%#$!&$ '%%%"$ (((#!#!#%#$%%"$ !!# " EOCENE HOTHOUSE PLANTS AND INSECTS FROM WYOMING 3 FIGURE 1. Studied early Eocene floras (BH, WRI, WRE) plotted on (A) a base map of the basins and uplifts of Wyoming, after Heller and Lui (2016), and (B) a satellite image. Nearby towns (Dubois, Shoshoni, and Worland) are shown to provide geographic context for each fossil flora within the state of Wyoming. Basins (light gray): BHB, Bighorn Basin; DJB, Denver- Julesburg Basin; GDB, Great Divide Basin; GRB, Green River Basin; HB, Hanna Basin; PRB, Powder River Basin; WRB, Wind River Basin. Uplifts (dark gray): BM, Bighorn Mountains; BlH, Black Hills; CA, Casper Arch; BWA, Beartooth– Washakie–Absaroka Ranges; FS, Ferris, Green, Seminoe, Shirley Mountains; LR, Laramie Range; OC, Owl Creek Moun- tains; PR, Park Range; R, Rock Springs uplift; WR, Wind River Mountains. !( !#!%%"$ (((#!#!# '#$%)!("$###)! & % $&%%!%#!#%#$!&$ '%%%"$ (((#!#!#%#$%%"$