Dembitzer, Jacob (2024) Rates and predictors of ecomorphological diversification in tetrapods through deep time. [Tesi di dottorato]

	Documento PDF Jacob_Dembitzer_37_Full.pdf Visibile a [TBR] Amministratori dell'archivio Download (3MB) | Richiedi una copia
	Documento PDF Jacob_Dembitzer_37_Partial.pdf Visibile a [TBR] Amministratori dell'archivio Download (1MB) | Richiedi una copia

Abstract

The study of macroevolution is one of the most important additions to evolutionary theory since Darwin. This field has been the focus of great minds such as Stephen Jay Gould and George Gaylord Simpson and emphasizes the importance of evolution above the “species level”. Some important topics explored through the lens of macroevolution are speciation, extinction, and evolution through deep time. Many studies in recent years have focused on the “tempo and mode” of evolution of clades or traits as well as the role of mass extinctions. However, it is still unclear what traits might predict macroevolution through deep time, as well as which species might survive or diversify after mass extinctions. In my dissertation, I explore the predictors of macroevolution, speciation, and extinction spanning from the Permian and until the present day. I have chosen the evolution of brain size in mammals and their ancestors (the group known as synapsids), and its role in speciation and extinction, as a focus of my doctorate alongside the evolution of morphology in lepidosaurs (lizards, snakes, amphisbaenians, and mosasaurs). The evolution of mammalian brain size has captivated the natural sciences for decades. The first chapter focuses on the role of brain size as a predictor of survival and extinction during the Late Quaternary extinction event. The Late Quaternary extinctions stand out amongst the history of extinctions as they were quite recent (126k – 500 years before present), the main victims were “large” mammals (typically defined as >45kg), and anatomically modern humans were a part of the environment. I found that the species that survived the Late Quaternary extinctions until the present day possessed significantly larger brains than their relatives who went extinct, and that this played a role in the extinction regime. For the second chapter, I expanded the previous study by collecting all available brain and body size data for living and extinct mammals and their ancestors. I aimed to evaluate when and why brains got larger in mammals through deep time, and if this phenomenon played a role in the overall diversification of mammals. This study included species from the group Therapsida as earlier pelycosaurs did not have fully ossified skulls capable of preserving an intact brain endocast. The subsequent dataset spans from the Permian and until the present day. I also collected life history data such as diet (e.g. carnivore, herbivore, etc.) and substrate (e.g. aquatic, terrestrial, etc.) to see how these traits may have played a role in mammalian evolution alongside or separately to brain size evolution. I found bursts in the rates of brain size evolution that occur in the aftermath of mass extinction events, as well as the Eocene-Oligocene transition otherwise known as The Grand Coupure. Moreover, rates of brain size evolution were found to be the trait most correlated with mammalian diversification, and relative brain size was found to be negatively correlated with extinction rates. In other words, I found that the evolution of brain size in all directions (towards larger and smaller brains) drove the diversification of mammals in post-mass-extinction environments, and that over time, species with small brains went extinct more often than those with large brains, leading to an overall increase in mammalian brain size through deep time. Lastly, the role of relative brain size and brain size evolution was found to be more important in mammalian diversification than body mass, rates of body mass evolution, substrate, or diet. For the last chapter, I explored which traits promote evolutionary potential, or evolvability, in Lepidosauria. The field of evolvability has gained attention in recent years as an important yet previously unexplored part of macroevolutionary science. However, few studies have explored practically which traits promote evolutionary potential in clades. Using lepidosaurs as a model clade, I collected morphological data from the literature in the form of limb lengths, head length, head width, body length, and tail length and calculated body mass with existing allometric equations. I found that overall evolutionary rates of lepidosaurs were below what was expected under Brownian Motion and declined through deep time. Fossorial species exhibited the most derived morphologies, followed by semifossorial, and then terrestrial ones. Scansorial, generalist, and aquatic species all converged on similar overall body plans. This goes well with our results that show that fossorial and semifossorial ecologies were correlated with positive evolutionary rates, alongside viviparity (birth of live young) and carnivory for body shape but herbivory for body mass. I performed ancestral state reconstructions of the species in the dataset and found that terrestriality was the most common ancestral state, and the ancestral state of all lepidosaurs, squamates, and rhynchocephalians. Moreover, terrestriality was also found to transition to and from the most other states and fossoriality the least. Lastly, I analyzed morphological disparity as a function of different traits to see which traits are most correlated with evolution towards disparate forms. Viviparous species were overall much more disparate than oviparous ones. Carnivorous species were the most disparate followed by omnivores and then herbivores. In lizards, fossorial species were the most disparate followed by semifossorial and then terrestrial ones. Generalist, scansorial, and aquatic species once again converged at low levels of disparity. In summary, I suggest that terrestrial species possessed the most evolutionary potential to evolve to and from different states. However, once terrestrial species adopted viviparity, semifossoriality, or fossoriality they evolved quickly towards disparate morphologies. However, this became an “evolutionary dead-end” as the evolution of fossoriality appears to be a one-way street as displayed in this study and others. These studies shed new light on the rates and predictors of tetrapod macroevolution through deep time. I take advantage of the latest phylogenetic comparative methods, as well as having developed new ones, in conjunction with new and large datasets. Lastly, it exhibits an early, concrete, step in the study of evolvability which has so far has been concentrated on theory as opposed to practical studies.

Downloads

Actions (login required)

Modifica documento