Lip Fern, HEMIONITIS MICROPHYLLA var FIMBRIATA

from the November 22, 2009 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO; limestone bedrock, elevation ~39m (~128ft), ~N20.676°, ~W88.569°
LIP FERN
On old stone walls here built of limestone -- many of those stones surely once incorporated in ancient Maya temples -- several interesting fern species have found homes, such as the foot-high one shown above. A close-up showing the underside of a pinna is shown below:

UPDATE: In 2019, with much more information on the Internet, I see that our Yucatan ferns match those named by CICY (The Yucatan's Center for Scientific Investigation) as Cheilanthes fimbriata. However, the authoritative online Tropicos database of Flora Mesoamericana considers that name as an outdated synonym for Cheilanthes microphylla var fimbriata. In other words, our Yucatan plants can be considered a variety of Cheilanthes microphylla, in the US known as the Southern Lip Fern.
SECOND UPDATE: In 2022 The World Flora Online indicates that this species has been shifted to a different genus. It is now HEMIONITIS MICROPHYLLA.

The manner in which the pinna's margins curl under partially enclosing lines of spore-containing, sandgrain-like sporangia helps us identify this as a "lip fern" of the genus Cheilanthes. Lip ferns are typical of such dry, sunny spots as our stone walls and often are invested with stiff hairs just like ours. In The Flora of North America our wall-fern keys out to the Southern Lip Fern, CHEILANTHES MICROPHYLLA {see sidebar at right}. Southern Lip Ferns are listed for the Yucatan, and our plants match drawings on the Internet, but I can't find good pictures on the Internet to cinch the ID. Word descriptions fit it perfectly, however, especially the way the black stem color extends a good way up the center of the pinna, as shown in the last photo. So, I'm about 95% sure of the ID, but there might be a local species very similar to the Southern Lip Fern I don't know about.

The Flora of North America describes the Southern Lip Fern's habitat as "calcareous rock outcrops and shell mounds," and our limestone walls are perfect calcareous rock outcrops. The species is distributed from the US Southeast south to South America, including the Caribbean, where it is most common.