PHYLOGENETIC DIVERSITY OF THE LUCINIDAE FROM THE PLIOCENE TO MODERN IN THE WESTERN ATLANTIC

The Pliocene to Modern transition is important because the Pliocene represents the last geologic interval with CO₂ concentrations as high as today. Environmental changes associated with this interval are linked with mass turnover in the Western Atlantic, ultimately resulting in the elimination of approximately 70% of molluscan species. Here we measured phylogenetic diversity (PD) of lucinds, a family of chemosymbiotic bivalves, to determine how evolutionary history was lost across this interval.

We constructed a phylogeny similar to that of Huang et al. (2015) using modern molecular data calibrated with fossils. We used 18s rRNA sequences of 86 modern lucinid species from Genbank and integrated 205 fossil species from the Paleobiology Database by assigning topological constraints based on genus identities and temporal constraints based on fossil last occurrences. We ran four Markov Chain Monte Carlo analyses for 60 million generations in BEAST 2.3.0, discarded the first 10 million generations of each run, and sampled those trees every 200,000 iterations resulting in 1000 final trees. We culled those trees so that they only contained lucinids that occur from the Pliocene to Modern to create a phylogeny with 41 species, 18 of which are extirpated or extinct in the Modern Western Atlantic.

Over the entire Western Atlantic there is more PD among lucinids than would be expected by chance (PD = 1465.66 obs; 1356.68 exp). This indicates that extinction/extirpation was relatively over-dispersed and that many lineages have at least one surviving species. We also divided data into latitudinal bins to determine the effects of biogeography on PD. In the most southern bin (25N to 30N) there is slightly more PD then would be expected by chance indicating that the extinction is over-dispersed across the tree in Florida (PD = 1446.24 obs; 1406.33 exp). However, in the two more northern bins there is more PD lost then would be expected by chance, suggesting that extinction is clumped (30N to 35N, PD = 732.05 obs; 835.91 exp; 35N to 40N, PD = 664.83 obs; 781.04 exp). Overall, this suggests that lucinid evolutionary history has a refugium in Florida. This is in contrast to the Pliocene, where many lucinid species ranged farther north. We therefore predict that lineages will migrate north as the globe transitions to a more Pliocene-like climate.