Recent projections of anthropogenically-influenced climate change have emphasized the importance of understanding how organisms and biotic communities respond to perturbations on both the long and short term. Data from the fossil record can be instrumental in such studies, offering the potential to study the response of ecosystems to protracted climate change impossible to observe on human time scales of only a few years. PIs plan to evaluate the influence of climate on the ecology and evolution of marine mollusk faunas from the U.S. Gulf Coastal Plain during the Paleogene (~65-25 million years ago). Their approach allows them to integrate records of mean annual temperature and seasonality with shifts in diversity, community composition and structure, morphology (shape), and evolution. They are particularly interested in learning whether there is a correlation between the magnitude and direction of climate change and the amount and nature of evolutionary or ecological change, including whether or not there are thresholds of tolerance below which an ecosystem and its component taxa do not respond. PIs will generate the paleoclimate record from the chemistry of fossil mollusks and fish otoliths (ear stones), and paleoecological patterns will arise from a combination of existing museum collections, published literature, and new field sampling of mollusk faunas. Lastly, they will reconstruct evolutionary relationships and trends within two dominant mollusk groups - venericard bivalves and turritelline gastropods - that are characteristic of this region and time, and overlay this information on the temperature and ecologic records for a comprehensive picture of faunal response to climate change. Their research will contribute substantially to knowledge of how biodiversity in sub-tropical settings responds to climate change of varying types and magnitudes.
The primary goal of this project was to improve our understanding of the patterns and processes of evolution in marine mollusks that lived in the shallow sea that covered what is now the Gulf Coastal Plain of the U.S. between around 65 million and 35 million years ago (the Paleocene and Eocene epochs). Our team developed a new body of data on the extremely abundant and well-preserved fossil mollusks of the Coastal Plain and the environmental conditions under which they lived. This data included a comprehensive list (available online) of the species of marine fossil clams and snails from this time interval, new information about changes in shell shape in several evolving families of these animals, and new estimates of water temperature during this time. This interval was a time of much higher global temperatures than today, and we were specifically interested in investigating what the effects of changes in temperature were on the evolution of these species. Preliminary findings include: 1) One group of snails (turritellines) was very abundant and diverse throughout the Coastal Plain during this interval, showing an almost total extinction at the boundary between the Paleocene and Eocene epochs (ca 55 million years ago). This extinction eliminated all of the very large (> 3 cm) species of this group that had been prominent in marine mollusk communities for millions of years, leaving only one or two very small species. By analysis of the growth of these shells (using oxygen isotopes), we have found that the large Paleocene and much smaller Eocene species grew to approximately the same age, meaning that the larger ones grew much more rapidly than the later species. This is consistent with the interpretation that available nutrients may have been less in the Early Eocene seas than in the Late Paleocene. Other mollusks, however, do not show a similar pattern, which leads us to conclude that the turritelline extinction may have been due to some other environmental factor, specific to this group. Turritellines also suffer major species extinction across the Eocene-Oligocene boundary, but the causes of this event have never been investigated. We have found that at least one major evolutionary lineage that survives across this boundary is one of the few lineages that survives in the Western Atlantic today. 2) Turritelline gastropods are one of the major marine molluscan groups of the Coastal Plain during the Paleocene and Eocene, but analyzing their evolutionary relationships has always been challenging because of the simplicity of their shells. This grant provided support for a Cornell PhD student who developed a new method of shell shape analysis that allows for a much more complete analysis of turritelline shells. She applied this technique to New Zealand Neogene turritellines, and we are now collaborating on applying it to a reanalysis of a more traditional 1996 analysis of Paleocene and Eocene Coastal Plain species. 3) Three separate lineages of Coastal Plain marine snails (Ancillopsis altilis, Pseudoliva vetusta, and Athleta tuomeyi) have long been known to have independently evolved the same heavily callused shell shape in the Early Eocene, but the causes of this striking convergence have remained unresearched. For her undergraduate thesis, a Cornell undergraduate found that these three lineages form similar shells using the same basic shell layers, but by different growth mechanisms. This result provides a striking example of the independent evolution of similar features for no apparent adaptive reason, which may be relevant to other studies of convergent evolution. 4) The Paleocene-Eocene boundary was marked by a relatively abrupt warming event (less than 10,000 years in duration), coincident with extinctions of land mammals and deep-sea single-celled organisms. There have been relatively few studies of the effects of this event on shallow marine ecosystems. Our analysis is the first comprehensive analysis of one of the world's most well-known and best-preserved fossil sequences across this boundary, and – to our surprise – we found a notable lack of major evolutionary change at this time. The Late Paleocene was a globally warm time. A warming episode in shallow water during such an interval would be expected to have less impact than at times of globally cooler temperatures. Although the Paleocene-Eocene warming event is generally viewed as a geologically large and rapid change, probably caused by an increase in atmospheric carbon dioxide, it is likely that it was slower and not significantly larger than current projections of the increase in carbon dioxide over the next few centuries. The event is known to have substantially disrupted terrestrial and deep-sea biotic communities. Our results might to suggest that, because shallow marine communities were spared such change, modern communities would be also. However, we would argue that our results suggest that the larger and faster changes expected in the near future mean that these habitats are also at risk of major disruption.
