Thelypteris yaucoensis is a rare species of fern

Fern

A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

 known by the common name Puerto Rico maiden fern. It is endemic to Puerto Rico

Puerto Rico

Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...

, where it is known from only three localities. It is a federally listed endangered species

Endangered species

An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

 of the United States.

This fern was placed on the endangered species list in 1993. At that time it was known to exist at three locations: two in Yauco

Yauco, Puerto Rico

Yauco is a city and municipality in southwestern Puerto Rico located in the southern coast of the island by the Caribbean, south of Maricao, Lares and Adjuntas; east of Sabana Grande and Guánica; and west of Guayanilla. Yauco is spread over 20 wards and Yauco Pueblo...

 and one in Ciales

Ciales, Puerto Rico

Ciales is a municipality of Puerto Rico, located on the Central Mountain Range, northwest of Orocovis; south of Florida and Manatí; east of Utuado and Jayuya; and west of Morovis. Ciales is spread over eight wards and Ciales Pueblo...

in Puerto Rico. There are a total of about 65 plants known at all three combined.

This terrestrial fern has fronds up to 52 centimeters long with the blades divided into 13 to 15 pairs of segments.