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Research

Current Research

Using extant and fossil mammals and snails to understand how habitat affects body size evolution

My overall goal is to improve our understanding of the effect of major evolutionary environmental transitions on the sizes of organisms. Using phylogenetic comparative methods, I plan to analyze various mammalian and gastropod groups that inhabit marine, freshwater, and terrestrial environments for significant differences in body size between group members that inhabit those different habitats. Then, taking into account fossil taxa and geologic time, I will analyze the reaction of body sizes of marine and non-marine taxa to oxygen concentration fluctuations. Controlling for systematics for analyses such as these removes any bias that may occur due to species sorting and takes into account the non-independence of related taxa. Therefore, this project will require the integration of a new body size and habitat dataset with gastropod and mammal systematics.



Resolving the Relationships of the Squamate Tree of Life: An Assessment of New Approaches and Problems

Since the division of The Deep Scaly Project into separate morphological and molecular teams, a truly integrated and wide scoped project has not been attempted. Much more can be done to understand how the members of Squamata are related to one another through an approach that combines the importance of both morphological and molecular evolution. Here we have developed a novel three-step methodological approach to squamate phylogenetics that incorporates the newest phylogeny-creating techniques and data from previous morphological and genetic analyses. First, we analyze a large squamate morphological dataset using Lewis's Mkv model under both a Bayesian and maximum likelihood framework. Second, we incorporate a previously constructed squamate DNA dataset and analyze the combined data within a "total evidence" framework. Finally, we adopt a methodology that treats genes, rather than nucleotides, as the character of interest.
We find that the separate analyses of the morphological and molecular datasets, even under Bayesian and maximum likelihood frameworks, still result in drastically different relationships between higher-order clades within Squamata. Additionally, we find that the combination of these two datasets results in a phylogeny with limited support for either topology, although it definitively leans in the direction of the molecular results. Finally, by reducing the molecular dataset to gene characters, we find significantly lower support for the higher-order relationships that are strongly supported in previous analyses. By combining this data with our morphological dataset, we discover that we have inversed the effect of the power in numbers problem.
We conclude that combining datasets, although possibly detrimental to results, should be treated as a source of understanding how the datasets may differ and how they may reflect different evolutionary histories.
[Senior Thesis, Geology and Geophysics, Yale University 2014; currently being adapted for press]
 


Using Lazarus Intervals and Simulations to Infer the Simultaneity of Extinctions

Lazarus intervals, or periods of "missingness", can be extremely informative when exploring the origination and extinction of taxa. This research project uses both real data for fossil ammonoids from Seymour Island, Antarctica, and simulated data under various variables and assumptions to attempt to produce a new, more thorough method for using these intervals to extrapolate to extinction events. With this new method, I hope to allow for corrections based on fossil record completeness and/or preservation potential. Eventually, this method will be used to infer the simultaneity of certain extinctions such as those of the K-Pg and P-T boundaries.


Past Research

Understanding the Past through the Present: An Investigation of Nesodon imbricatus, A South American Notoungulate

The notoungulates of South America evolved in isolation from the rest of the world over the course of the Tertiary and part of the Quaternary periods. However, they have become extinct due to multiple factors that may have included climate change, changes in vegetation, and the fairly recent connection of the North and South American continents that could have made it possible for North American ungulates to invade the niches that the notoungulates inhabited and out-compete them. Due to the similar niches in North and South America, the notoungulates tended to evolve convergently with the modern ungulates; however, their true diets and habits remain unknown because no descendants of the group exist. The focus of this study is a Miocene notoungulate, Nesodon imbricatus. The cranial and dental anatomies of Nesodon are compared with other notoungulates and contemporary and recent groups of ungulates. The most similarities exist between the specimens within the notoungulate group, but similarities between Nesodon and the other mammals produce lead to conclusions of Nesodon following a broad herbivorous diet, most likely dominated by grazing of abrasive foods. Its tusks may have acted as weapons in fighting or as symbols of superiority in mating rituals.


Angiosperm Origin and Evolution: Separating Fact from Fiction

Modern angiosperms are extremely diverse, occupy a broad range of habitats, and display extreme morphology varieties. Flowering plants are presently the dominant group of terrestrial plants, spanning niches from those of large trees to those of smaller shrubs, vines, and herbs. Most of the world’s major biomes, excluding those of high-latitude and montane conifer forests, are overwhelmingly dominated by flowering plants both in diversity and biomass. However, the angiosperms have not always been the dominant plant group. While land plants have inhabited the Earth for approximately 450 to 500 million years, this subdivision of plants did not originate until sometime between the Late Triassic and early Cretaceous periods; the oldest confirmed fossilized angiosperm has been dated to about 135 million years old. The very recent origin of this branch begs the question of how it has become so dominant in such a short period of time over the other branches that had already been ecologically stable for at least 100 million years. Although many scientists have sought to advance a single, unique reason for the taxon’s success, there may well have been multiple reasons for the increased fitness of the angiosperm line.
This research spans the breadth of knowledge from current and past research and literature to attempt to summarize and analyze the origin and early evolution of the flowering plants. I reviewed multiple hypotheses of the origin and early diversification of the angiosperm lineage, indicating the evidence for, or against, them. This includes considerations of the ancestry of the angiosperm lineage and diversification of the plant group preceding and following the Cretaceous-Tertiary boundary. The research briefly interprets the impacts that this event has had on early ecosystems and later ecological impacts after the diversification of the angiosperms, partially including modern day domination. Following this, a number of reasons that may or may not help explain this abnormal rapid diversification were analyzed by the impacts they would have had on the plant species.


Silicification, Bias through Time

I worked with Susan Butts, Ph.D., the Senior Collections Manager of Invertebrate Paleontology at the Peabody Museum, and Richard Krause, Ph.D., a Post-Doctoral Associate of the Geology and Geophysics Department of Yale University, on a research project that revolved around the analysis of prevalence of silicification across the Paleozoic Era. Silicification is the process by which the original shell material of a fossil dissolves in the environmental acid and is replaced by the precipitation of silica into the fossil cavity. In a similar manner to all forms of taphonomy, silicification is affected by different factors of the environment in which it is taking place, such as climate and global ocean chemistry. Also, the formation of silica fossils is mediated by the composition and structure of the original shell material and depositional factors, such as permeability, porosity, lithology, and stratigraphy. The occurrence of silicified fossils is much more prevalent in the Paleozoic, disappearing in the fossil record as you move through the younger time periods. This may have something to do with either the abundance of carbonate shelly fossils during the Paleozoic or the abundance of available silica mineral during the era. However, the exact reason(s) for the large quantity of silicified fossils during the Paleozoic, but not in time periods afterwards, is not agreed upon.
The stratigraphic collections of the Peabody Museum represent the invertebrate species of the Paleozoic, organized by age. These specimens have been acquired over a period of at least 150 years, including irreplaceable collections from lost localities and modern inaccessibility. In testing the fossils for silicification, acid was used to determine whether the fossils still contained the original calcium carbonate material or had been silicified. Also, silica is harder than metal, so the fossils could be tested by scratching them with metal. Finally, they are also more resistant to weathering and often stand out in relief. When the testing of all of the specimens was completed, I was able to record my data across multiple individual time periods, producing a display of the changes in prevalence of silicification across the Paleozoic.
Additionally, I performed a literature search across articles published in the Journal of Paleontology. This literature search had the same function as that of the physical testing, but was extremely valuable to increase the number of data points and to widen our field of study. Each paper represented an additional specimen and the literature search as a whole produced very similar data to the physical specimens.
By the end of the summer, patterns began to form in the data. Our results showed that silicification was most prevalent in the Middle to Late Ordovician, the Late Silurian, and the Middle Permian. Possible explanations for these ranges include global climate and mass extinctions. In fact, modern hypotheses of prehistoric global climate seem to correlate very well with our data. Additionally, there is a drastic decrease in silicification near the end of the Permian period, correlating with the largest mass extinction of Earth’s history, and other additional possible correlations. However, the true reasons for these changes are still unknown.