Exploring the Soft Tissue of the Archosaurian Feeding System through Evolutionary and Developmental Temporal Space

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Virginia Tech

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.

Comparative anatomy, rhamphotheca, feeding apparatus, Aves, Trilophosauridae, development