Upon his arrival in Chicago, Bryan assumed a position as vertebrate preparator at the Field Museum of Natural History.
He worked under the direction of Elmer S. Riggs, who was at that time engaged in studies of South American Tertiary mammals.
As a recipient of two Guggenheim Fellowships he spent the years of 1952–1954 in Argentina studying the great collections amassed by the Ameghino brothers.
In 1958 he returned to Argentina with Alfred S. Romer, but this time for field work in the Triassic formations in search of mammal-like reptiles.
These were exhibited at a small museum, featuring the complete skeleton of a mastodon and duly named Museo de Paleontologia Bryan Patterson.