They are called Lesser celandine (Ficaria verna). The root of the plant resembles bunches of small figs—hence one common name for them is Fig buttercup or Fig-root buttercup. Another common name is Marsh Marigolds. They are an invasive species, native to Europe and western Asia, and are fatal when eaten by grazers.
Thanks to Marilyn Bottjer, Alice Clague, Doris Eder, and Mary Alice Walker for sending us this information.