Whether mass extinctions consistently drive global homogenization or instead produce phase-dependent restructuring remains debated. Here, we evaluate the impact of the Late Ordovician mass extinction on global biogeographic structure and its relationship to biodiversity dynamics across the Ordovician/Silurian transition. Using 26,532 taxonomically vetted occurrences of 2474 genera, we quantify diversity partitioning and bipartite network topology using formation-based sampling standardization and equal-area geographic grid cells. We find that biogeographic homogenization peaks across the Hirnantian/Rhuddanian transition, when reduced spatial turnover and increased genus occupancy indicate the expansion of surviving taxa. This interval is characterized by declines in beta diversity driven by turnover loss, accompanied by a collapse in network modularity and path length, indicating the breakdown of provinciality. Geographic occupancy increased markedly after the extinction, driven by the persistence and range expansion of tolerant survivors. We interpret this pattern as crisis cosmopolitanism, whereby environmental disruption and the reduction of dispersal barriers facilitated transient global range expansion. Although taxonomic diversity rebounded in the Aeronian, biogeographic structure recovered more slowly, indicating a lag between taxonomic and spatial reorganization. The reestablishment of provinciality likely required the rebuilding of ecological barriers, such as increased competition and predation, during the recovery phase. Similarities with other mass extinction intervals suggest that biotic homogenization is a recurrent consequence of global crises.
Abstract Coastal tropical wetlands have seen substantial change over the course of Earth’s history. Examining what commonalities lie between ancient tropical wetlands and their modern counterparts allows us to better understand the ecological building blocks foundational to their functioning. We evaluate the similarity of modern and Pennsylvanian tetrapod community structures from coastal tropical wetlands through the lens of body-mass distributions. To obtain fossil body-mass distributions, we expanded upon an existing technique of body-mass estimation in fossil quadrupeds based on the surface areas of the centra of trunk vertebrae. Using this technique, we estimated the body masses of 21 tetrapods from the Pennsylvanian of Joggins and Point Aconi, Nova Scotia, the two assemblages against which we compared modern coastal wetland body-mass distributions. Our results indicate that taphonomic size biases can explain some of the differentiation between the body-mass distributions of Paleozoic terrestrial tetrapod communities in tropical coastal wetlands and their modern counterparts. However, our results also indicate that limitations in the low dietary diversity of Carboniferous tetrapods may have had an impact on body-mass distributions that would have caused them to differ from those of modern coastal tropical wetland ecosystems.
Lakes can serve as cradles of biodiversity by facilitating adaptive radiation linked to distinct environmental conditions and ecological opportunities. On San Salvador Island, the Bahamas, karstification during Holocene transgressive flooding created anchialine lakes that have supported rapid evolutionary diversification, most notably among endemic pupfishes with specialized feeding strategies. To assess whether similar phenotypic divergence occurs in other members of these lake communities, we investigated morphological variation in shells of the chemosymbiotic infaunal bivalve Ctena sp. (Lucinidae). Live and dead Ctena specimens were sampled from muddy sediment filling karst pockets along the shorelines of three anchialine lakes, a habitat type from which chemosymbiotic bivalves have not previously been reported. Using two-dimensional geometric landmarks of shell interiors and valve margins, we found significant differences in shell shape, size, and allometric relationships among living populations from each lake. Death assemblages preserved similar among-lake patterns, suggesting that morphological divergence predates the window of time averaging represented by death assemblages. Living and dead-shell assemblages within each lake also differed significantly in shell shape, and temporal shape-change vectors from the dead to living included both shared and distinct trends among lakes.Ongoing genomic work suggests that lake-dwelling Ctena on San Salvador represent a single endemic species. Thus, spatiotemporal patterns among lake populations likely reflect ecophenotypic responses to distinct sedimentological and geochemical conditions that influence their chemosymbiotic lifestyle. These results demonstrate the potential of combined life and death assemblage analyses to detect patterns of phenotypic divergence over geologically instantaneous timescales.
Abstract By linking form and function within a particular ecological context, ecomorphology provides a method for reconstructing unknown aspects of the ecology of fossil species. To test how differences in morphology can separate among ecological categories, canonical variates analysis (CVA) is typically employed. However, whether CVA is the most appropriate method for ecomorphology is questionable, as its underlying assumptions, particularly regarding sample size, are often unmet in high-dimensional biological datasets. Moreover, CVA cannot account for the influence of phylogeny. Therefore, alternative statistical methods, which can appropriately handle limited sample sizes and phylogenetic covariance are needed to improve ecomorphological inferences. To achieve this, we employ Bayesian ordinal regression models that discriminate among habitat categories using 3D landmark data of extant African Bovidae astragali, both with and without a phylogenetic parameter. Without a phylogenetic parameter, classification success rates of each model either meet or exceed those of CVAs using the same variables. Including a phylogenetic grouping variable generates nearly perfect predictions of habitat category when species identity and phylogenetic attributions are known, but poor predictions when unknown. Although consideration of similarity due to phylogenetic covariation is critical in evolutionary studies, our results indicate that separating the effect of phylogeny from morphological variables can be detrimental to group membership prediction for taxonomically indeterminate specimens. Therefore, whether to include a phylogenetic effect is contingent upon the research question and the study system. Our work indicates that a Bayesian modeling framework is a robust tool for investigating ecomorphology, with promising future applications for paleoecological studies.
