Mya or mya : A common abbreviation (in English speaking countries) for "million years ago." The form "Mya" is recommended, since the capital M, taken from the metric prefix mega- (M-), is the appropriate symbol for a million
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}