An extreme climate gradient-induced ecological regionalization in the Upper Cretaceous Western Interior Basin of North America
The Upper Cretaceous Western Interior Basin of North America provides a unique laboratory for constraining the effects of spatial climate patterns on the macroevolution and spatiotemporal distribution of biological communities across geologic timescales. Previous studies suggested that Western Interior Basin terrestrial ecosystems were divided into distinct southern and northern communities, and that this provincialism was maintained by a putative climate barrier at ∼50°N paleolatitude; however, this climate barrier hypothesis has yet to be tested. We present mean annual temperature (MAT) spatial interpolations for the Western Interior Basin that confirm the presence of a distinct terrestrial climate barrier in the form of a MAT transition zone between 48°N and 58°N paleolatitude during the final 15 m.y. of the Cretaceous. This transition zone was characterized by steep latitudinal temperature gradients and divided the Western Interior Basin into warm southern and cool northern biomes. Similarity analyses of new compilations of fossil pollen and leaf records from the Western Interior Basin suggest that the biogeographical distribution of primary producers in the Western Interior Basin was heavily influenced by the presence of this temperature transition zone, which in turn may have impacted the distribution of the entire trophic system across western North America. INTRODUCTION A primary objective of evolutionary biology is linking the macroevolution and spatiotemporal distribution of organisms with abiotic factors across geologic timescales (e.g., Mayr, 1982; Wright, 1982, Jablonski, 2007). Indeed, studies on the evolution of life often invoke spatial and temporal environmental patterns to explain biogeographic trends such as expansion and contraction of biomes (e.g., Lieberman, 2005; Saupe et  al., 2014; Arbour et  al., 2016), endemism versus cosmopolitism (e.g., Lamoreux et  al., 2006; Jansson, 2003; Sidor et al., 2005), and the extinction or radiation of major lineages (e.g., Pardo et al., 2019; Alroy, 2010; Anderson et al., 2011). Most studies on the relationship between climate and biotic evolution focus on global constraints and drivers that can be tackled with a coarse approach; however, trends observed at the macroscale do not universally translate to local patterns across short time intervals, which may be heavily influenced by local events (Chiarenza et al., 2019; Barrett, 2014). Such data sets are challenging to assemble due to the unavailability of proportional data on environmental parameters and inherent biases in the preservation of geographically widespread fossil assemblages within geologically short time intervals (Chiarenza et al., 2019; Dean et al., 2020). The vertebrate fossil record of the Upper Cretaceous Western Interior Basin of North America is an exception, as it represents one of the most spatiotemporally contiguous records of Mesozoic vertebrates globally (Gates et al., 2010), providing an opportunity to study fine-scale interactions between climate, tectonic, and biologic systems. Much research has attempted to link the tectonic and environmental evolution of the Western Interior Basin with perceived paleobiogeographic patterns of terrestrial ecosystems. The hypothesis that Campanian (83.6–72.1 Ma) and Maastrichtian (72.1–66 Ma) vertebrate assemblages of the Western Interior Basin (especially dinosaurs) were endemic to distinct northern and southern communities originated in the 1970s (Sloan, 1970, 1976; Lehman, 1987; Wolfe and Upchurch, 1987). Attempts to explain the potential presence of distinct biomes within the Western Interior Basin have been the subject of intensifying research since that time (e.g., Lehman, 2001; Sampson and Loewen, 2005; Lehman et al., 2006; Gates et al., 2010, 2012; Sampson et al., 2010; Loewen et al., 2013; Thomson et al., 2013; Longrich, 2014; Leslie et al., 2018; Sankey, 2008; Nydam et al., 2013). These studies suggest that the evolving physiogeography of the Western Interior Basin, coupled with changing climate during the Late Cretaceous, resulted in periodic physical or ecological barriers to vertebrate movement. Other studies (e.g., Vavrek and Larsson, 2010; Wick and Lehman, 2013; Lucas et al., 2016; Berry, 2018) dispute a hypothesis of Western Interior Basin biomes altogether, typically over concerns regarding sampling biases and/or cladotaxonomic issues (e.g., Williamson, 2000; Longrich., 2014; Lucas et al., 2016; Berry, 2018), asynchronous temporal comparisons (Lucas et al., 2006; Lucas et al., 2016; Fowler, 2017), or lack of perceived evidence for a geologic or climatic barrier (e.g., Wolfe and Upchurch, 1987; Amiot et al., 2004; Longrich, 2014; Upchurch et al., 2015, Lucas et al., 2016; Berry, 2018). Multiple studies have presented compelling evidence that the movement of vertebrates between northern and southern regions of the Western Interior Basin was not impeded by true physical barriers (e.g., Gates et al., 2012; but see Fowler and Freedman Fowler, 2020, for a contrasting view). However, the hypothesis that the Western Interior Basin was divided into distinct climatic zones divided by an ecological boundary zone centered roughly at the paleolatitude of †lkburgen@ncsu.edu. Landon Burgener http://orcid.org/0000-00022542-2985 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35904.1/5226175/b35904.pdf by Landon Kelly Burgener on 04 February 2021