That's an interesting question, Lotus, and I must admit I've never seen or used the term 'ma' before, but it does exist: 'mega-annum' (usually abbreviated 'Ma'), which means million of years before present. However, judging by the scientific discussion on this page (see link below), the use of 'Ma' to describe geological time periods is quite contentious, and the more traditional use of m.y. (millions of years), or even the phrase '146 million years ago' is sometimes preferred. http://www.geosociety.org/TimeUnits/viewComments.asp

Lotus, please stop studying Chinglish. Don't read anything else in English that is written by Chinese people. Use authentic materials produced by native-speakers. This rubbish is not going to help you at all. The internet is full of texts in good English. Please use them.

Um... Millennia? Pass.