Impacts of spatial and environmental differentiation on early Palaeozoic marine biodiversity

Abstract

The unprecedented diversifications in the fossil record of the early Palaeozoic (541-419 Ma) increased both α (within-sample) and γ (global) diversity, generating considerable ecological complexity. Faunal difference (β diversity), including spatial heterogeneity, is thought to have played a major role in early Palaeozoic marine diversification, though α diversity is the major determinant of γ diversity through the Phanerozoic. Drivers for this Phanerozoic shift from β to α diversity are not yet resolved. Here, we evaluate the impacts of environmental and faunal heterogeneity on diversity patterns using a global spatial grid. We present early Palaeozoic genus-level α, β and γ diversity curves of molluscs, brachiopods, trilobites and echinoderms and compare them with measures of spatial lithological heterogeneity, which is our proxy for environmental heterogeneity. We find that α and β diversity are associated with increased lithological heterogeneity, and that β diversity declines over time while α increases. We suggest that the enhanced dispersal of marine taxa from the Middle Ordovician onwards facilitated increases in α diversity by encouraging the occupation of narrow niches and increasing the prevalence of transient species, simultaneously reducing spatial β diversity. This may have contributed to a shift from β to α diversity as the major determinant of γ diversity increase over this critical evolutionary interval.

Were all trilobites fully marine? Trilobite expansion into brackish water during the early Palaeozoic

“…Ordovician shallow-marine ecosystems are characterized by more complex food webs than those of the Cambrian and increased competition between taxa for specific resources [77]. A common ecological outcome of the Ordovician radiation was increased niche partitioning within already established habitats [75,78] and expansion to new ones in shallow-marine settings (e.g. reefs, hardgrounds).…”

Spatial scaling of beta diversity in the shallow-marine fossil record

“…Principally, we are concerned with variation in area. Previous studies have focused on measuring spatial variation using the area of the convex-hull, great-circle distances, summed MST length (Alroy 2010; Close et al 2017, 2020a,b), or a grid-based system (Crampton et al 2011; Penny and Kröger 2019). Our data represent shelfal fossil communities; ideally, estimates of spatial area should represent shelfal area.…”

“…We therefore adopt a grid system as a proxy for spatial area, based on an equal-area penta-hexagonal grid generated using the R (R Development Core Team 2020) package icosa (Kocsis 2020). Studies of beta diversity in the fossil record at the global scale have opted for a grid of side 111 km (Penny and Kröger 2019). We adopt a smaller grid of side 23 km, because our data are at the regional scale.…”

“…The variation in species composition between the two sites is due to differences in species richness, not species replacement, and forms the core concept of nestedness (Ulrich and Almeida-Neto 2012). Subdividing turnover into species replacement and nestedness components can help reveal underlying drivers of assemblage difference that may be obscured when they are combined (Wright et al 1997; Penny and Kröger 2019). We are interested in both the overall variation in beta diversity and the turnover across all shelfal gradients, and therefore both concepts are useful and applicable.…”

“…Previous studies of beta diversity in the fossil record at large spatiotemporal scales have allowed for spatial variability in sampling by combining collections based on either a grid system (Crampton et al 2011; Brocklehurst et al 2018; Penny and Kröger 2019) or a specified distance (He et al 2018). However, these approaches do not necessarily account for temporal variation in spatial distribution or area, and time series of beta diversity may be affected by this.…”