The biogeography of Central America is considered a classical case study in understanding the impact of vicariant events on patterns of biotic dispersal. While many biogeographers have focused on community composition and geographic limits of species at broad scales across Central America, much less work has focused on post-colonization diversification patterns at finer scales. The livebearing freshwater fish Xenophallus umbratilis presents an ideal system for determining the impact of recent earth history events on biodiversity in northern Costa Rica. Here, we test the hypothesis that marine inundation of the San Carlos and Northern Limón basins during the Miocene has caused genetic fragmentation among X. umbratilis populations, despite contemporary freshwater connections across this region. To test this idea, we collected mitochondrial (cytb) and nuclear (Xmrk-2) DNA sequence data from up to 162 individuals taken from 27 localities across northern Costa Rica. We employed a variety of analytical approaches, including: maximum parsimony (MP) and maximum likelihood (ML), analysis of molecular variance (AMOVA), nested clade phylogeographic analysis (NCPA), and demographic analysis of population size through time. We found four major clades within X. umbratilis, each geographically isolated with no shared haplotypes across drainages. Oddly, clades that occupy adjacent drainages are not always sister taxa in the phylogeny, suggesting that colonization in this species is more complex than a simple model of isolation by distance. All our results are consistent with the hypothesis that changes in sea level associated with glacial eustatic cycles have had an important effect in shaping diversification patterns in this species.

Isotopic assays, including stable carbon, stable oxygen, and radiogenic strontium were measured for 37 individuals from the Talgua cave ossuaries to understand human movement and mortuary practice during Formative Period Honduras. Likelihood assignment models demonstrated that the individuals had diverse childhood geographic origins within the surrounding valleys. This shows that different kin or ethnic groups from diverse geographic origins were utilizing the ossuaries. Five possible ‘non-local’ individuals were identified from the radiogenic strontium and stable oxygen isotope datasets, suggesting minimal human movement into northeast Honduras from outside Lower Central America. The low number of ‘non-local’ individuals at Talgua Caves also suggests that trade items were acquired by down-the-line exchange processes rather than through a long-distance trade connection. This type of trade network and bioarchaeological evidence of limited ‘non-local’ individuals at Talgua Caves suggests the surrounding region was culturally distinct from Mesoamerica during the Formative Period.