This fern belongs to the successful fern family Hymenophyllaceae, which is comprised of over 600 species and has been around since at least the Upper Triassic Period some 200 million years ago. Filmy Ferns (or Bristle Ferns, as they are sometimes known) have a cosmopolitan distribution, occurring over much of the world but nearly always in constantly damp or misty sites. The fern featured here is no exception - this small sandstone recess is liberally wetted by constant seeps percolating from the rocks.
NOTE: I have come to dislike flash photography for plants, in many cases. The previous photos that I have made of this species were with flash, and I never much liked the look. But flash, or a VERY long shutter speed is required given the darkness of the site. So this time, I lugged my tripod along and tried something different. These images were made sans flash. The photo above was created with the Canon 5D Mark III and Tamron 70-200 f/2.8 lens, at 200mm. Settings were f/14, ISO 100, and 20 (twenty!) second exposure. Not a very conventional plant photography setup but it worked okay - much better than with flash in my opinion.
Flora North America, here is a map of the distribution of the Appalachian Filmy Fern. It would be endemic to eastern North America were it not for one bizarre outlier population in the state of Chihuahua, Mexico. Its core distribution is compact, and the populations are few and far between. The fern has been found in 12 states, and it is listed as endangered in eight of them, threatened in two, and as "S3" (rare but not highly imperiled) in one. Only in the Commonwealth of Kentucky is Appalachian Filmy Fern not listed. It's known from 21 counties there, but I bet the casual botanist would still have to work to find a population.
There are two Ohio sites. The other is about three miles west and ever so slightly north of the population shown in this article. These are the most northerly Trichomanes boschianum populations in the world. One of them is diligently protected, the other is just sort of hidden in an area of little interest to passersby. Saxicolous (rock-dwelling) plants often suffer greatly due to trampling and other abuses inflicted by rock climbers and others who can't resist playing on the rocks. Hopefully the two Ohio populations will be spared the excesses of man.