dryopterus

Latin -us is the commonest ending on a plant genus by a wide margin, so the ear reaches for it by default. Dryopteris does not take it. The ending here belongs to pteris, the Greek word for fern, which arrives whole and keeps its own final syllable. Change the i to a u and the fern part of the fern's name stops being a word.

An unstressed vowel, flattened until it gives the ear no information.