Tetrapods water-to-land transition in the Devonian was accompanied by an array of morphological modifications aiding in locomotion and food acquisition, which included diversification in teeth morphology. Different teeth morphology allowed tetrapods to take advantage of different ecological niches through food specialization. As useful as teeth are, we can see the repeated development of edentulous (=toothless) system throughout the fossil record, most frequently in Archosauria. Archosauria, represented today by living crocodylians and birds and includes extinct non-avian dinosaurs and pseudosuchians, first appeared in the early Mesozoic Era, during the Middle Triassic. Archosauria continue to diversify through the rest of the Mesozoic Era and during that time, we see a plethora of modifications made to the feeding apparatus in this group, such as dental batteries in hadrosaurids, bone crushing teeth in tyrannosaurids, or edentulous jaws covered in a rhamphotheca (=beak) in oviraptors. In the fossil record, morphological modifications can be seen in fossilized skeletal remains, but this is an incomplete picture of a living organism. The skeletal system of an organism is the housing and support, and it is powered by the muscles and ligaments and ultimately controlled by the nervous system. Without the soft tissues, we recognize that there are missing gaps in the anatomy, and modern organisms have been studied as analogs to fill these gaps. Traces of soft tissue are not completely undetectable in the fossil record. In exceptional preservation sites, materials such as keratinous integuments and gut materials have been found, but more commonly, we utilize osteological correlates such as muscle attachment scars that are derived from studying modern homologs to make inference about the presence of soft tissues. One advantage of using modern analogs to study soft tissue morphology is that we are able to incorporate how the targeted morphology grows through the observable developmental timescale. Ontogeny, or prenatal development and postnatal growth, has been utilized as an approach to understand how millions of years of natural selection affected the phenotypic expression in an organism. Through improvement of technology and laboratory techniques such as CT scanning and contrast-enhanced staining, in situ anatomical studies have revealed more information and details about the soft-tissue morphology in modern organisms to improve our interpretation of fossil organisms and address broader morphological macroevolution questions. This dissertation focuses on the construction and ontogenetic changes in the soft tissue (i.e., jaw muscles and keratinous sheath or rhamphotheca) and skeletal morphology of the avian edentulous feeding system and apply it to extinct edentulous feeding system across reptiles. My first chapter describes the ontogenetic changes in the musculoskeletal system of the jaws of emus (Dromaius novaehollandiae) to make inferences about potential influences of feeding function on the feeding apparatus during development. I combined microCT scanning, including contrast-stained CT scanning, and 3D geometric morphometric analyses to explore how the feeding apparatus changes through ontogeny and highlight intraspecific complexity within skeletally immature individuals. The second chapter explores the keratin layers making up the simple rhamphotheca of the chicken (Gallus gallus domesticus) and documents the varying mechanical properties within a single rhamphothecal sheath. This chapter establishes that biomechanical functions such as food and object manipulation affect the keratinous sheathing that covers the avian jaw bones by potentially selecting for specific regions of the rhamphotheca to be more mechanically resistant than others. In the third chapter, I review osteological correlates for rhamphotheca in modern edentulous taxa, birds and turtles, and in the extinct taxon, Trilophosaurus buettneri, a Late Triassic archosauromorph that was proposed to have both a beak and transversely-oriented teeth, to determine whether T. buettneri had a rhamphotheca and if so to what extent. This chapter reveals that one of the osteological correlates, foramina patterns, will benefit from future study that incorporates more turtle species and establishes that lack of wear on the oral/occlusal edge might be a valid osteological correlate to use for future fossil examination. These chapters showed a possible underlying influence of the feeding biomechanical function onto the anatomical construction and ontogeny in both the modern edentulous feeding system, providing an avenue for further exploration to address the repeated development of the edentulous feeding system. / Doctor of Philosophy / How an animal obtained and processed food has been changing since animals first started to come onto land in the Devonian. One major development in how animals feed was the development of teeth, which function as versatile tools that can help crush and tear food items up and give animals the ability to eat a variety of food items. As useful as teeth are, however, we can find many animal lineages repeatedly lose some or all of their teeth permanently and gained a beak. Many of these lineages are found within Archosauria, which are represented by living crocodylians and birds and extinct lineages like pterosaurs and non-avian dinosaurs, have existed since the Middle Triassic (252 to 201.5 million years ago). Archosaurs saw a major rise and fall in the number of lineages through the Mesozoic Era. These archosaurian lineages were diverse in how they obtained and processed food, which included bone crushing teeth in tyrannosaurs, dental batteries in hadrosaurs, and beaks (toothless jaws covered in keratin sheathing) in oviraptors. The fossil record can tell us about the changes to the bony structure of these extinct archosaurs, but an organism is made of more than just their bones. In vertebrates, movements are powered by the muscles and ligaments attached to them and controlled by the nervous system. In the fossil record, that soft-tissue information is often lost except in exceptionally preserved fossils. However, we can use bony correlates, indicators on the surface of the bones for the presence of a particular soft tissue, to interpret the missing soft tissues of fossils. In order for us to study soft tissue information in an extinct species, we can utilize living organisms as a model. One advantage in studying modern organisms is that we can watch the soft and hard tissues grow through their lifetime and document changes in shape and size of particular features. Growth and development, or ontogeny, is one way to see how millions of years of evolution can affect the how an animal look like. With new technology such as CT scanning, studying the soft and hard tissues in tandem has revealed new information and allowed us to improve our understanding of how different parts of an animal function, but also make better inferences of how extinct animals were built. This dissertation focuses on exploring the anatomy and growth of features making up the jaws of beaked animals, the jaw muscles, the bones to which they attach, and the keratin sheaths covering them. Chapter one focused on a growth series of emu skulls to look at how bones and jaw muscles develop and how they are linked together. This chapter documents changes in shape and size of the bones and muscles to understand factors that can affect their growth. Chapter one also highlights more complexity in younger specimens that should be explored in future research. The second chapter looked at the various keratin layers that make up the rhamphothecae, or keratin sheaths, that cover the jaw bones in a beak. This chapter tested whether functions like grabbing food and other objects affects the hardness of the rhamphotheca in chickens. In chapter three, I examined three bony correlates that are used to infer the presence of a rhamphotheca in modern archosaurs (birds and turtles) and a stem archosaur called Trilophosaurus buettneri to determine if T. buettneri had a rhamphotheca. This chapter helped establish the validity of old and new bony correlates and determine that at least one of the bony correlates needs more sampling in modern taxa for stronger comparison and linkage of morphology between the modern and extinct taxa. These three chapters showed that the mechanical function of feeding can influence the anatomy toothless feeding system and how the system develops through growth, which can be a way for us to address how the toothless feeding system continuously developed in the fossil record.

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