Flat latitudinal gradient in Paleocene mammal richness suggests decoupling of climate and biodiversity

Late Cenozoic onset of the latitudinal diversity gradient of North American mammals

“…The fossil slope is indistinguishable from the modern (negative) LDG during only three intervals: the earliest Paleocene (63-65 Ma), the late Eocene (36)(37)(38), and the Pliocene-Pleistocene (2-4 Ma). The modern-like LDG slope during the earliest Paleocene interval is surprising given the strong support for a lack of a LDG during the mid-Paleocene shown in this and our previous study (21), which used different approaches than we do here. Our result for the earliest Paleocene is puzzling in terms of the temporal and spatial proximity to the K/Pg impact event and associated mass extinction, and the Paleocene as a whole is not an interval of major global or regional climate change.…”

“…A recent review of studies of the LDG using the fossil record (6) summarizes the general influence of mean global temperature, showing that richness declines toward the poles only during intervals with relatively low global temperature (i.e., "icehouse worlds"), whereas globally warm intervals (i.e., "greenhouse worlds") typically are characterized by a flat LDG or peak in taxonomic richness at temperate latitudes. Although the latitudinal gradient of temperature in western North America during the Paleocene was similar to today (21), mean global temperature was higher during the Paleocene than today (25), which might, in part, explain the lack of LDG. Mean global temperature has varied substantially since the Paleocene (25), but its relationship to the strength of the LDG over that interval remains unknown.…”

“…[16][17][18][19][20]. However, we have shown previously (21) that the LDG for terrestrial North American mammals was absent during the mid-Paleocene Epoch (∼63-58 Ma). However, the latitudinal gradient in δ 18 O of Paleocene mammalian tooth enamel was similar to the modern latitudinal gradients in the oxygen isotopic composition of surface waters (precipitation, rivers, and lakes), which suggests that hydrological and climatic controls on δ 18 O gradients of surface waters were broadly similar during the Paleocene and today.…”

Is Rapoport's rule a recent phenomenon? A deep time perspective on potential causal mechanisms

“…The fossil record evinces latitudinal gradients in diversity since the Palaeozoic, though mostly for marine fauna, with the gradient strengthening during the Caenozoic [13]. North American terrestrial mammals in the Palaeocene, however, do not increase in richness at the equator [14]. The temporal ubiquity of latitudinal diversity gradients must be better ascertained before firm conclusions regarding the interspecific competition hypothesis can be drawn, though we might still expect greater adherence to Rapoport's rule through deep time than observed.…”

“…to be ubiquitous for mammals through deep time [14]; (ii) in the Oligocene and/or Late Miocene onwards, if seasonality drives Rapoport's rule, as those epochs mark increasing seasonality [15,16] [17,18]. We also assess Rapoport's rule throughout the Caenozoic using North American land mammal age (NALMA) time bins [19].…”