"Can changes in the Larval Developmental Modes of Olivella in the Caribbean Neogene be used as a reliable productivity proxy?"

John H. Wilbur III, Thesis 2016

Introduction: We do not understand if it is environmental or ecological change that drives evolution. (Vermeij 1986, Jablonski and Sepkoski 1996, Jablonski 2003, Jackson and Erwin 2006). For a long time paleontologists have been limited in their ability to make broad, ecological inferences about the fossil record because of the lack of ecological context; ecological context here meaning both the biotic and physical properties of the system. By accurately reconstructing the past environment and examining the fossil record, we can attempt to determine quantitatively which has a greater effect on macroevoltionary trends.

The Closure of the Isthmus of Panama functions as an excellent natural experiment because of our knowledge of the geologic record and the changing environment that influence the temperature, salinity, nutrients, and productivity of each area.  The modern day Caribbean resembled the Pacific before the Closure of the Isthmus of Panama. The present day Pacific can serve as an analogue for the ancient Caribbean environ(Erwin et. al 2011).  Extending this thought, life modes that are common within the modern Pacific should be present in the ancient Caribbean (Fortunato 2004).  This provides an opportunity to examine how evolution was influenced by the changing productivity within each environment. The Closure of the Isthmus of Panama provides a clear study area in which to test whether environmental or ecological change drives evolution.  The two environments generated by the closure provide an excellent opportunity to see how organisms evolve in response to their environments.  In this paper, I will be focusing primarily on the physical changes in the environment, specifically planktonic productivity.  The fossilized remains of gastropods, bivalves and bryozoans provide an opportunity to study and track these changes.

The closure of the Isthmus of Panama, and the effects of that closure, are recorded in the geologic record (Allmon 2001, Henderson et al. 2002, O’Dea et al. 2007).  Deep-water circulation between the Caribbean and the Pacific halted no later than 3.6Ma and shallow water circulation no later than 3.0Ma (Allmon 2001).  The blockage of water flow had a major impact on the Caribbean.  The Caribbean lacks upwelling, is relatively nutrient poor, and has much smaller seasonal variability in sea surface temperature (Allmon 2001, Todd et al. 2002, O’Dea et. al 2007).  The Pacific possesses inter-annual and seasonal variations along with changes in temperatures and upwelling that promotes planktonic feeders (Allmon 2001, Todd et al. 2002, O’Dea et. al 2007). The western direction of the trade winds enable the cooler nutrient-rich water to cycle to the surface and replace the warmer and nutrient-poor water in the Pacific.  Environmental changes such as the lack of nutrients have been known to drive distinct evolutionary events (Fortunato 2004, Nutzel et al. 2006, O’dea and Jackson 2009, Leonard-Pingel et. al 2012).

Fossils are used to better understand the past environment by comparing the fossils to their living analogues.  This allows assumptions to be made about the past environment.  The newly formed Caribbean changed benthic communities and promoted expansion of coral reefs (Todd et al. 2002, Travis 2007, Smith and Jackson 2009, Leonard-Pingel et al. 2012).  The increase in reefs and sea grasses in the early Pleistocene due to the decline in productivity in the Caribbean was a major factor in the turnover of bivalve assemblages (Leonard-Pingel et al. 2012).   In particular, gastropods, bivalves and bryozoans can help better construct past environments by comparing the past organisms to their modern analogues in the western Caribbean.  Currently bryozoans (O’Dea and Jackson et. al 2009) and bivalves (Travis 2007) are the best understood within the Panama system. (Todd et al. 2002, Nutzel et al. 2006, Smith 2007, O’Dea and Jackson et. al 2009, Smith and Jackson 2009). In particular, the extreme turnover of the American scallop shows a dramatic change in scallop diversity in the southwest Caribbean over the past 12 Myr but not in the eastern Pacific (Travis 2007, Smith and Jackson 2009).  The species that did survive in the Caribbean had larger eggs and shorter larval durations than those that went extinct (Smith et al. 2003, Travis 2007, Smith and Jackson 2009).  These changes can be directly tied to a decrease in production and a lack of nutrients necessary for planktotrophic organisms to survive (Allmon 2001, Nutzel et al. 2006, Kirby and Jackson 2009, Leonard-Pingell et al. 2012).  These traits in bivalves are extremely similar to those in gastropods.  Thus we can infer how gastropods were developing at the same time.  Due to their similarity in larval development, gastropods make an ideal candidate to further reinforce the changing environment during the closure of the Isthmus. 

By examining the gastropods before, during, and after the closure of the Isthmus of Panama, I hope to provide support for using gastropods as another reliable proxy for environment.  Gastropods are numerous, present in many types of environments and a common fossil found in oceanic environments (Allmon 1988).  Gastropods keep their larval stage (protoconchs) throughout their adult lives and therefore the larval modes are preserved in the fossil record. 

Gastropods are divided into two distinct larval categories: 1) Planktonic or suspension feeders and 2) non-planktonic, which includes both lecithotrophic and direct developers (Allmon 1988, Fortunato 2004, Nutzel 2014).  In gastropods, the type of development can be directly measured in the size of the protoconch, or larval shell. Adult gastropods retain this development in the size and shape of their protoconchs.  The diameter of the embryotic protoconch and number of whorls depends on the egg yolk and egg size (Allmon 1988, Van Osselear 1999, Fortunato 2004, Nutzel et al. 2006, Nutzel 2014).  The number of whorls within the protoconch denotes the amount of time spent within the water column: the longer the time, the more whorls the protoconch is likely to have.  Planktonic gastropods survive by preying upon plankton and other primary developers in the photic zone.  This allows them to possess a smaller egg size because they are able to feed within the water column.  Lecithotrophic gastropods have larger egg size because they are only able to survive for a limited time through suspension feeding.  They spend shorter amounts of time in the embryotic stage and quickly metamorphose into their juvenile forms.  Direct developers cannot feed within the water column and require a larger egg yolk than lecithotrophic and planktotrophic developers.  They must metamorphose into their juvenile forms before they can begin feeding (Fotunato 2004, Nutzel et al. 2006).

This study seeks to look at fossil data of the gastropod Olivella in order to understand the macro-evolutionary trends since the closure of the Isthmus of Panama.  The number of modern non-planktonic neogastropods in the Caribbean is significantly higher than in the Eastern Pacific (Fortunato 2004)  due to the decrease in productivity in the Caribbean after the closure of the Isthmus of Panama, we predict that non-planktotrophic feeders will survive preferentially because of the changing environment.  This change may appear after a ‘lag time’ of an estimated 1-2Ma.  This lag time is predicted because the environment would not change suddenly when the Isthmus closed.  Instead, the Caribbean would adjust and reach its new equilibrium slowly over several million years.  During this lag time, planktotrophic and non-planktotrophic gastropods could coexist in the Caribbean (O’Dea et al 2007).  It is after this time the effects of the closure were seen throughout the Caribbean.  Thus there is an expected gap before planktonic gastropods are selected against in the Caribbean.  I expect to see that if the size of the protoconchs increases with time in the Caribbean then evolution in response to the changing environment is responsible for this change.

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