When did the mammalian outer ear evolve?

2018-03-13 20:20:09

All Eutheria and Metatheria have outer ears, and as far as I found out, monotremes once also had them, so they seem to be universal for mammals. Did other synapsids have them, too?

I know that soft tissue doesn't fossilize well, but isn't there any other way to find out, like muscle attachment? Has it something to do with mammalian ear bones?