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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Organismic and Evolutionary Biology

Year Degree Awarded

2017

Month Degree Awarded

May

First Advisor

Elizabeth Dumont

Second Advisor

R. Craig Albertson

Third Advisor

Jason Kamilar

Fourth Advisor

R. Mark Leckie

Subject Categories

Biodiversity | Evolution | Integrative Biology | Zoology

Abstract

Here I investigate how two major components of the mammalian feeding system, teeth and jaws, are influenced by functional, environment, and developmental factors. First, I build physical models of the molars from two early mammals, Morganucodon and Kuehneotherium, and compare their ability to process a proxy food item. Early mammals were under strong selection to reduce metabolic costs, so any savings in energy during feeding would be advantageous. I tested the ability of both mammals to process a hard and soft food item with material properties similar to that of the insects they would have likely consumed. Morganucodon was better able to process hard food while Kuehneotherium was better able to process soft foods. Perhaps more importantly, Kuehneotherium molars inflicted significantly more damage on food items regardless of their material properties. These results suggest that changes in dental morphology in some early mammals was driven primarily by selection for maximizing damage, and secondarily for maximizing biomechanical efficiency for a given food material property. Second, I assess the association between climatic variability and phenotypic integration (the degree of covariation among suites of traits). Taxa in climatically variable regions experience fluctuating selective pressures which may influence phenotypic integration, and by extension, disparity (morphological diversity) and the rate of morphological evolution. I extracted climatic data from range maps, and gathered landmark data from the jaws of three large families of carnivorans, Canidae, Felidae, and Mustelidae. I found that canids and felids from climatically variable regions exhibited low levels of jaw integration, low disparity, and fast rates of morphological evolution. This suggests climatic variability predicts phenotypic integration in canids and felids and may be a consequence of, or a response to, the relative magnitude of long and short-term environmental changes (e.g., glaciations, seasonality). Climate may therefore have a profound influence on the evolvability of species. Finally, I investigate how differences in the mode of placental and marsupial reproduction can influence the pattern and magnitude of phenotypic integration. Marsupial neonates are born without a fully developed oral apparatus but immediately begin suckling in the pouch. Due to suckling at such an early age, the mechanical environment in the developing jaw is vastly different to placental mammals, which exhibit simultaneous development of jaw muscles and bones that only function after they are fully developed. The marsupial mode of reproduction may therefore limit morphological evolution of the jaw in marsupials. I investigate the association among phenotypic integration, disparity, and rates of morphological evolution using the placental Carnivora (made up of the Feliformia and Caniformia) and the marsupial Dasyuromorphia. I found dasyuromorphs exhibit fewer modules and higher integration between the angular and coronoid processes, the primary attachment sites for the jaw closing muscles. This pattern of integration may result from the overwhelming influence of muscles on the developing jaw. Carnivorans are free from this constraint and each process on the jaw can evolve independently. Despite differences in integration, dasyuromorphs and carnivorans display similar levels of disparity. Rates of morphological evolution are also similar in dasyuromorphs and caniforms, and rates are 3x slower in feliforms. Taken together, this suggests that the mode of reproduction has not affected dasyuriomroph morphological evolution and developing a carnivore-like jaw is a developmentally straightforward task given its similarity to the ancestral state. Overall, my work has demonstrated how the mammalian feeding system is influenced by phenotypic integration, the mode of reproduction, phylogeny, and the environment. Together, these factors appear to influence the disparity, rate of morphological evolution, and ultimately, the evolvability of mammalian teeth and jaws.

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