FAILURES OF THE MODERN ANALOG MENTALITY: EXAMPLES RELATING TO THERMAL TOLERANCE OF MOLLUSKS (Invited Presentation)

Environmental temperature plays a key role in determining where an organism can live, with each organism having a unique thermal niche, or range of temperatures in which it can survive. Thermal niches are often considered to be conserved through time in related species to the extent that occurrence of a given taxa in a fossil deposit has been used to infer paleoenvironment (aka the “faunal analog” method). The validity of this assumption, that thermal niche is static through time, can be directly tested by comparing quantitative paleotemperature estimates on fossil species to their modern observed thermal niche. In this talk, we present two examples where this thinking outright fails and two more illustrating that nuance is needed to correctly interpret paleoenvironment.

We present paleoenvironmental reconstructions derived from clumped isotope analysis of fossil mollusks from Last Interglacial coastlines of Bermuda (gastropod Cittarium pica) and Late Cretaceous rivers of the Western US (Unionid bivalves). In both cases, reconstructed temperatures are outside of the modern observed thermal range, directly contradicting the idea of a conserved thermal niche. Next, we present subannual-resolution isotope data from specimens of the bivalve Mercenaria sp. dating from the warmer MIS 5e and cooler MIS 5a intervals of the Last Interglacial. We find that shell growth occurs across a similar range in temperatures despite apparent differences in mean climate, illustrating the difference between preferred temperatures and true thermal niche. Our final example compares inferred habitat of co-occurring Late Cretaceous bivalves and ammonites (Sphenodiscus sp., Discoscaphites sp., and Baculites sp.) from the Gulf Coastal Plain based on two different paleothermometry methods (oxygen and clumped isotopes) and raises the issues of biologically derived isotopic fractionations affecting paleoenvironmental interpretations in extinct taxa. In total, these findings remind us that more factors than just temperature influence where an organism is found today and how and when it grows its shell, and that modern observed habitats do not always reflect the entire niche an organism is possible of inhabiting.