The ending is -ate
schwaOnly the SIN of laciniate is stressed. Everything on either side of it is reduced, and reduced vowels get written however they land.
heard aslaciniute
spelledlaciniate
saidluh-SIN-ee-ut
laciniate, like crenate and pinnate.
Descriptive adjectives of this kind almost all end in the Latin -atus, and English lands that as -ate whether or not you can hear it. Here you cannot: by the fourth syllable the vowel has faded to nothing.
More of the same word
lacciniate laciniat luciniate leciniate lasiniate lacineate
Every one of them has an entry. All 9 spellings of laciniate.