  We report Shubnikov-de Haas oscillations measurements revealing experimental
signatures of an annular Fermi sea that develops near the energy band edge of
the excited subband of two-dimensional holes confined in a wide GaAs quantum
well. As we increase the hole density, when the Fermi level reaches the excited
subband edge, the low-field magnetoresistance traces show a sudden emergence of
new oscillations at an unexpectedly large frequency whose value does
$\textit{not}$ correspond to the (negligible) density of holes in the excited
subband. There is also a sharp and significant increase in zero-field
resistance near this onset of subband occupation. Guided by numerical energy
dispersion calculations, we associate these observations with the unusual shape
of the excited subband dispersion which results in a "ring of extrema" at
finite wavevectors and an annular Fermi sea. Such a dispersion and Fermi sea
have long been expected from energy band calculations in systems with strong
spin-orbit interaction but their experimental signatures have been elusive.
