GLOBAL BIOGEOGRAPHY OF ALBIAN AMMONOIDS: A NETWORK-BASED APPROACH

The science of networks has brought significant advances to our understanding of biogeographic patterns. Here we present a case study using Albian ammonoid data from the PBDB. Biogeographic regions of Cretaceous ammonoids have been delimited using qualitative assessments. This approach has deep historical roots and highlights the lack of quantitative assessments. Here we test the following biogeographic scenario (Lehmann et al. 2015): Boreal (subdivided into Atlantic, Pacific and Arctic Subrealms), Tethyan, and Austral Realms. First, we construct a bipartite occurrence network based on 1795 occurrences of fully identified species. Then, we generate a projected one mode network from the bipartite network onto the primary set P using a weighted approach. Finally, we apply the Infomap algorithm to detect community structure within the projected network. The bipartite network we created (G) has n=P+S=540 and m= 859. The weighted projection of G onto the set P determines a network G_P with n=78, m=433, density 0.14 and typical power-law degree distribution. This geographic network G_P covers most epicontinental basins and gives an indication of how tightly different regions were connected. The partitioning at the network-topological level produces a partitioning of geographic space comparable to the scenario under investigation. The most obvious difference between the two models is that Infomap fails to delineate the Boreal-Atlantic Subrealm, either reflecting limitations of our data or inaccuracies of the biogeographic model. The modularity score of this division of G_p (Q=0.3) indicates significant community structure. To find out how stable those communities are, we implemented a number of modularity-based partitioning algorithms. The results indicate some variation along the Subrealms and provide finer subdivisions of the Tethyan Realm. Remarkably, we always find communities that closely match to the Realms. The analysis of Gp as a topological entity was performed with no geographical constraints, but successfully delineated regions in space consistent with large-scale boundaries and latitude-related factors. This analysis provides a quantitative validation of the Albian biogeographic realms and reinforces the value of qualitative models developed by ammonoid experts.