The first vowel is an o
schwaCoriaceous carries its stress on the third syllable. The two in front of it are unstressed and their vowels are barely present.
heard asceriaceous
spelledcoriaceous
saidko-ree-AY-shus
coriaceous.
Two unstressed syllables run past before the word reaches its stress, and the first of them barely registers. An e is what you write when the vowel has faded that far. The o is fixed by corium.
Also on this leaf
coriaseus corriaceous coriaseous coriaceus coreaceous
All six are on the same page. All 6 spellings of coriaceous.