Body Size Evolution of Ostracods
Ostracods, commonly called seed shrimp, are a large and diverse group of crustaceans characterized by their small size and having a carapace composed of two shells. Two groups of History of Life Interns studied the evolution body size in Ostracoda in 2013.
Intern Research Projects Presented at the 2014 AGU Meeting
Surface Area to Volume Ratio over Temperature and Time for Ostracods
by Christopher Jackson and Samuel Zaroff
In 1877 Joseph Allen proposed that endothermic terrestrial organisms would have lower surface area to volume ratios (SAVR) in colder climates and higher SAVRs in warmer climates. With a smaller surface area compared to volume, organisms can retain more heat in cold climates. We tested to see if this principle applied to ostracods, a type of ectothermic marine invertebrate. We hypothesized that Allen's rule applies to ostracods, as Allen's rule has been demonstrated in frogs (Alho 2011), which are also ectotherms . We used the linear dimensions of the three major carapace axes of ostracod holotypes to estimate the SAVR. We compared ostracod SAVRs with paleotemperatures from Royer et al. (2004). We found that there was a correlation between surface area and temperature; it is a small, but statistically significant correlation (adj. R2=0.0167). This means that as temperature increased, the SAVR also increased. We also found a negative correlation between ostracod SAVR to geologic time(adj. R2=0.0114), which shows us that as time has gone on, ostracod SAVR has decreased. We then plotted the correlation coefficient of SAVR to temperature over geologic time to explore trends in the strength of Allen's rule. For most of time there was no relationship but during the Devonian, Allen's Rule did explain the trend. In short, temperature does explain some of the correlation between the SAVR and temperature, but it is likely there were other environmental factors affecting this relationship.
Comparison of Genus and Species-level Compilations of Metabolic Rate through Time
by Divya Sundararajan
Metabolism is the basis of fundamental principles of biology and sustains life through vital processes such as growth and reproduction. Brown et al. (2004) showed that metabolism is central to our understanding of patterns and dynamics at all levels of biological organization. Often, paleontologists use the holotypes of type species to represent genera in global analyses, but they rarely test how representative the type species are of the genus of a whole. Through my analyses, I compared genus and species-level compilations through time by comparing the mean metabolic rate of each genus to the metabolic rate of the type species to see if using this representative provided effective data when conducting genus-level analyses. To achieve these objectives, I used sizes collected from Catalogue of Ostracoda and Treatise on Invertebrate Paleontology. The range of the type species' metabolic rate varied, but there is no systematic bias towards higher or lower metabolic rates. Therefore, using type species in genus-level analyses is effective when looking for general trends, but the absolute values based on the holotype of type species have some bias to them and are not as accurate.
Testing for Bergmann's Rule in the Evolution of Ostracods
by Jackelyn Gonzalez and Jennifer Rascon
Bergmann's Rule predicts that body sizes of species in cold regions are larger than body sizes of species in warm regions. Using Bergmann's Rule, we have used data on Ostracods (microscopic crustaceans) that includes their stratigraphic ranges and body size to create graphs to help us identify whether this hypothesis is true. We started by plotting the mean size throughout geologic time, from the Cambrian period to the Holocene. We also plotted the temperature changes within this time range. Both mean size and temperature gradually decreased through time. When we compare size and temperature directly, we notice a positive correlation of the mean body size versus the average temperature of the Earth over the past 540 million years; however, this graph contradicts Bergmann's Rule. Evidently, as environmental temperature decreased over time, the mean size of Ostracods also decreased. Lastly, we created our last plot on volume and paleolatitude. The latitude is used as a proxy for temperature because it is difficult to decipher an exact degree of temperature throughout time. Our results demonstrated that as latitude increased, the volume decreased. We discovered a weak, but significant relationship between volume and latitude with a correlation coefficient of -0.144 (p-value << 0.001). This proves that a genus that occurs in multiple locations or time intervals has the same size in our data due to the limitation that we only have one measurement per genus. In conclusion, using stratigraphic range data, our project exemplified a correlation with the mean size and mean global sea surface temperatures since the Cambrian. This relationship, though, is not as obvious when you look at size vs. paleolatitude. We have found that Bergmann's Rule, which was initially described for mammals, does not apply to Ostracods. This is likely due to distinct physiological consequences of environmental temperature for endotherms and ectotherms.
Body Size Change in Relation to Location and Time
by Lesly Ann Llarena, Lauren Nolen and Juliette Saux
Many factors drive evolution, although it is not always clear which factors are more influential. Miller et al. (2009) found that there is a change in geographic disparity in diversity in marine biotas over time. We tested if there was also geographic disparity in body size during different epochs.
We used marine ostracods, which are tiny crustaceans, as a study group for this analysis. We also studied which factor is more influential in body size change: distance or time. We compared the mean body size from different geologic time intervals as well as the mean body size from different locations for each epoch. We grouped ostracod occurrences from the Paleobiology Database into 10º x 10º grid cells on a paleogeographic map. Then we calculated the difference in mean size and the distance between the grid cells containing specimens. Our size data came from the Ellis & Messina“Catalogue of Ostracod” as well as the“Treatise on Invertebrate Paleontology”. Sizes were calculated by applying the formula for the volume of an ellipsoid to three linear dimensions of the ostracod carapace (anteroposterior, dorsoventral, and right-left lengths).
Throughout this analysis we have come to the realization that there is a trend in ostracods towards smaller size over time. Therefore there is also a trend through time of decreasing difference in size between occurrences in different grid cells. However, if time is not taken into account, there is no correlation between size and geographic distance. This may be attributed to the fact that one might not expect a big size difference between locations that are far apart but still at a similar latitude (for example, at the equator). This analysis suggests that distance alone is not the main factor in driving changes in ostracod size over time.
Size Evolution and Stochastic Models: Explaining Ostracod Size through Probabilistic Distributions
by Max Krawczyk and Sean Decker
The biovolume of animals has functioned as an important benchmark for measuring evolution throughout geologic time. In our project, we examined the observed average body size of ostracods over time in order to understand the mechanism of size evolution in these marine organisms. The body size of ostracods has varied since the beginning of the Ordovician, where the first true ostracods appeared. We created a stochastic branching model to create possible evolutionary trees of ostracod size. Using stratigraphic ranges for ostracods compiled from over 750 genera in the Treatise on Invertebrate Paleontology, we calculated overall speciation and extinction rates for our model. At each timestep in our model, new lineages can evolve or existing lineages can become extinct. Newly evolved lineages are assigned sizes based on their parent genera. We parameterized our model to generate neutral and directional changes in ostracod size to compare with the observed data. New sizes were chosen via a normal distribution, and the neutral model selected new sizes differentials centered on zero, allowing for an equal chance of larger or smaller ostracods at each speciation. Conversely, the directional model centered the distribution on a negative value, giving a larger chance of smaller ostracods. Our data strongly suggests that the overall direction of ostracod evolution has been following a model that directionally pushes mean ostracod size down, shying away from a neutral model. Our model was able to match the magnitude of size decrease. Our models had a constant linear decrease while the actual data had a much more rapid initial rate followed by a constant size. The nuance of the observed trends ultimately suggests a more complex method of size evolution. In conclusion, probabilistic methods can provide valuable insight into possible evolutionary mechanisms determining size evolution in ostracods.
The Influence of Oxygen, Temperature, and Salinity on Ostracod Body Size in the Gulf of California and the Pacific Coast of North America
by Nicole Wong and Melody Weber
The study of body size and its evolution is important to paleontological understanding of ecological niches that different organisms have occupied. Ostracods, with frequent occurrences throughout the geological record beginning in the Ordovician, are useful in analyzing how body size has changed. Therefore, understanding the impact of environmental variables is crucial to understanding patterns of body size evolution in extinct ostracods. In order to further investigate this question, ostracod populations in the Gulf of California and the Pacific Coast of North America were studied. By compiling data from Ostracoda from the Gulf of California (Swain, 1967), Marine Holocene Ostracoda from the Pacific Coast of North and Central America (Swain and Gilby, 1974), and the National Oceanographic Data Center, we compare environmental factors, including oxygen levels, temperature, salinity, and depth with anteroposterior length measurements of about 180 recent ostracod species in order to determine the impact of these variables. Using R, we constructed graphs of oxygen levels, temperature, and salinity versus average body size. Using a correlation test, the correlation coefficient for oxygen is -0.193, temperature is -0.398, and salinity is -0.322, with the corresponding p-values of 0.067, 9.196 x 10-5, and 0.002; only the latter two p-values are significant at the alpha = 0.05 level. The correlation test for depth was calculated but showed no trends. The statistically significant correlation coefficient between temperature and body size suggests a strong negative correlation. Because oxygen levels and salinity levels are, to some extent, dependent upon temperature, this may explain the smaller, yet still statistically significant, correlation between body size and salinity. Relationships between ostracod body size and temperature may be relevant to our understanding of the impacts on ecological structure as the ocean temperatures fluctuate in the future.
Ostracod Body Size: Locality in Accordance with Cope's and Bergmann's Rules
by Rufhiline Tolosa and Tram Vo
A wide range of climates exists on planet Earth, and the different kinds of life inhabiting each area vary greatly in accordance with its topography and weather conditions. The nature of each climate is in part determined by its latitude—latitudes closer to 90º suggest a colder climate while latitudes closer to 0º suggest a warmer, more tropical climate. The evolution of organisms is expected to differ in different parts of the world because environment plays such a significant role in it. In our study, we focus on the relationship between location and the extent to which the evolution of ostracod body size follows Cope's Rule (i.e., the tendency for body size to increase over time) and Bergmann's Rule (i.e., body size decreases with temperature) from the Ordovician to the Holocene. Using body sizes of ostracod occurrences, we explored the relationships among size, latitude and time. Modern ecosystems near the poles are more sensitive to environmental and climate change than those near the equator, we hypothesized that ostracods with latitudes closer to the poles will follow Cope's Rule more closely. To test this hypothesis, we compared body size and latitude as well as trends of body size evolution over time in tropical, temperate and polar regions. The graphs produced showed that over different latitudes, there was a decreasing trend in the mean size of ostracods over time. This means that the evolution of ostracod body size does not follow Cope's Rule any more in polar and temperate regions than it does in tropical regions. In fact, our data suggests that ostracods do not necessarily follow Cope's Rule or Bergmann's Rule at all, which concurs with a notion that has been previously brought up—the possibility that ectothermic marine organisms are exceptions to Bergmann's Rule.
Intern Research Projects Presented at the 2013 AGU Meeting
Ostracod body size trends do not follow either Bergmann's rule or Cope's rule during periods of constant temperature increase
by Catherina Xu, Purnima Seshadri and Vaishali Amin
Over time, organisms have adapted to changing environments by evolving to be larger or smaller. Scientists have described body-size trends using two generalized theories. Bergmann's rule states that body size is inversely related to temperature, and Cope's rule establishes an increase over time. Cope's rule has been hypothesized as a temporal manifestation of Bergmann's rule, as the temperature of the Earth has consistently decreased over time and mean body size has increased. However, during times of constant temperature increase, Bergmann's rule and Cope's rule predict opposite effects on body size. Our goal was to clarify this relationship using both accessible proxies of historic temperature – atmospheric CO2 levels and paleo-latitude. We measured ostracod lengths throughout the Paleozoic and Mesozoic eras (using the Catalogue of Ostracoda) and utilized ostracod latitudinal information from the Paleobiology Database. By closely studying body-size trends during four time periods of constant CO2 increase across spectrums of time and latitude, we were able to compare the effects of Cope's and Bergmann's rule. The correlation, p-values, and slopes of each of our graphs showed that there is no clear relationship between body size and each of these rules in times of temperature increase, both latitudinally and temporally. Therefore, both Cope's and Bergmann's rule act on marine ostracods and no rule is dominant, though our results more strongly disprove the latitudinal variation in ostracod size.
Ostracod Body Size as a Variable of Biomass
by Amogh Pathi, Daniel Guo and Aditya Sriram
When one looks throughout the rock layers, they aim to find either fossils or petroleum. Our project encompasses both. We are looking at ostracod fossils throughout the continental United States in an attempt to link environmental factors to a larger body size. In our research the environmental factor we studied was biomass. In essence our project seeks to find a trend towards larger body size in locations where, prehistorically, there was a lot of food. You may wonder where petroleum comes into this. Our indicators of biomass are the many oil wells throughout the United States. We used location data provided to us and found all fossils within the same layer and a 100 kilometer radius of our oil wells. We compared the size of those ostracods to those that were still within the radius (in an effort to avoid data from other unknown oil wells within the same oil basin) but not from the same time period. Our results indicated a clear size difference between the ostracods within the oil difference. The ostracods were larger by approximately 1 millimeter on our log scaled graphs. Our hypothesis was clearly supported by this data through time.