Who ate my Marigold seedlings?

I planted some marigold seeds so I could put seedlings in my mum's small garden in front of her unit. About a week after I planted them they were sprouting and looking healthy. The next morning all but 3 were gone. I thought marigolds kept pests away but, obviously, I was wrong. Something ate them during the night. Can someone please tell me who the culprit might be? I live in Queensland, Australia.