Graduation Year

2023

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Ryan Carney, Ph.D., M.P.H., M.B.A.

Committee Member

Stephen Deban, Ph.D.

Committee Member

Ashley Heers, Ph.D.

Keywords

bird, bone, model, muscle, paleognath, wing

Abstract

To understand the evolutionary gain, and loss, of avian powered flight, attention must be paid to the wing morphology of species across these transitions. However, paleognaths, while being the most well-known flightless birds, have greatly understudied forelimbs. Additionally, current literature regarding their forelimb anatomy generally lacks a three-dimensional (3D), biomechanical approach. Among the flightless paleognaths, the rhea is notable in having the least reduced forelimb. Though not used for flight, the wings of rheas are used in other behaviors such as courtship and threat display, as well as nest defense. In this study, I investigate the forelimb anatomy of an adult Greater Rhea (Rhea americana) by creating a 3D musculoskeletal model of the wing and pectoral girdle. This utilized contrast-enhanced staining with 5% phosphomolybdic acid, computed tomographic scanning, and segmentation of the bones and muscles, supplemented by cadaveric dissections. Articular joint surfaces were analyzed to create topological coordinate systems (TCSs). TCSs were used for calculating values of inter- and intra-bone joint orientations (e.g., torsion), which in turn were compared to those of other extant and extinct archosaurs. Due to their significance in the flight stroke of volant birds, shoulder muscles including the pectoralis thoracica, supracoracoideus, deltoideus major, and scapulohumeralis caudalis of the Greater Rhea were modeled separately to test functional hypotheses from the literature. Muscle moment arms were then calculated in four standardized poses that represent the wing's range of motion. The 3D modeling and examination of anatomical characters at the rhea wing and shoulder emphasized the unique anatomy of this flightless species and revealed novel, functional hypotheses regarding their musculature. These results aid in understanding the functional morphology of forelimbs in the understudied paleognaths and across locomotor transitions in extinct organisms, as well as provide an open-access 3D digital atlas of the Greater Rhea forelimb for future studies involving this key taxon.

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