Mammals and their closest fossil relatives use their shoulders and forelimbs for many functions, which is reflected by the great range of mammalian forelimb shapes. Little work has been done to quantify this diversity as it relates to deep mammalian evolutionary history. Using geometric morphometric techniques on the humerus and ulna, I sought to quantify morphometric disparity, functional diversity, and the phylogenetic influence of the two across the 300-million-year evolution of this clade. I found that forelimb shape diversity in the early mammalian lineage (Synapsida) began to increase about 270 million years ago, with the emergence of a group called Therapsida, and is accompanied by new forelimb functions. The functional diversification of therapsid forelimbs was curtailed by the Permo-Triassic mass extinction, but eventually continued as more mammal-like therapsids evolved new ecologies. The analyses presented in this dissertation characterize the deep time origin of a quintessential part of the mammalian body plan: evolutionarily labile forelimbs that can be deployed in a wide range of functional and ecological roles. Continuing this research on the origins and diversification of synapsid forelimb structure, I undertook a critical assessment of the ecological comparability of mammals to their fossil forerunners. Three interrelated goals are addressed: (1) to estimate when in synapsid evolutionary history modern mammal morphologies become effective for predicting fossil ecologies; (2) to investigate examples of morphological convergence within our geometric morphometric framework; and (3) to compare the functional solutions of distinct synapsid radiations in light of their shared phylogenetic history. I found that mammal limb shapes are not analogous to fossil synapsids until very close to the origin of crown Mammalia. These results suggest that phylogenetic placement strongly influences how an organism can respond to functional pressures, emphasizing that each synapsid radiation explored distinct areas of morphospace and arrived at functional solutions that reflected their separate ancestral morphologies. Building upon this work, I quantified the influence of shared ancestry upon the macroevolution of synapsid forelimbs. Using a composite phylogenetic tree including 218 genera across 320 million years of synapsid evolution, I compared the phylogenetic signal and phenotypic rate change among forelimb metrics between phylogenetic groups and across anatomical forelimb elements. This work points to a critical but previously under appreciated feature of the synapsid forelimb: that individual forelimb elements are undergoing independent evolutionary pressures and responding to those pressures at different rates. The work presented here challenges the traditional narrative of synapsid forelimb evolution as clear progression towards increasingly mammalian morphologies, and instead reveals a broad diversification of forelimb shapes early in synapsid history. This mosaic evolution of the synapsid forelimb reveals the complexity of forelimb evolutionary history, highlights the importance of forelimb morphometry to functional interpretation, and presents an increasingly dynamic picture for the forelimb evolution of Mammalia’s deepest fossil ancestors.