One night this week I camped beside an isolated cenote. During the night I could feel and smell the balmy, musty humidity issuing from deep inside the sinkhole. That humidity nurtured certain plants at the pit's rim not typically found elsewhere in the drier forest. One such plant was the fern shown below:
I think that that's the genus Microgramma, and since there's only one Microgramma, MICROGRAMMA NITIDA, listed for the adjoining state of Quintana Roo (no flora for Yucatan state), I'm guessing that it's that.
Northerners will regard this as a curious fern not only because its fronds consist of single, willow- leaf-like blades lacking the frilliness typical of most fronds, but also because its fronds grow widely spaced along somewhat woody stems that creep along supporting branches.
The dark, round spots on the undersurface of each frond are "fruit dots" or sori, which are clusters of tiny capsules that open to release spores. Such round sori are bound to remind northern fern fanciers of the Polypodies often seen up there gracing mossy rocks in the north and tree trunks in the Deep South. Microgramma is closely related to the Polypodies.