^ Good point.The script doesn't address it explicitly, simply refering to him as "science officer", but Sonak does have the stripes, so it's not unreasonable to assume he might have been in the running for XO under Decker:
KIRK: Commander Sonak, you received your appointment as Enterprise science officer?
SONAK: Based, I am told, on your recommendation, Admiral. Thank you.
KIRK: Why aren't you on board?
SONAK: Captain Decker requested I complete final science briefing here before we leave on our mission.
That's too bad that a transporter malfunction turned him into something... that fortunately didn't live long...
I thought transporters were safe!!!