The abbreviation myr, "million years", is a unit of a quantity of 7006100000000000000♠1,000,000 (i.e. 7006100000000000000♠1×106) years, or 31.6 teraseconds.
1 Usage 2 Debate 3 See also 4 References
The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr.
The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having myr for a duration and Mya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a milliyear (a thousandth of a year, or 8 hours). On this side of the debate, one avoids myr and simply adds ago explicitly (or adds BP), as in
The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma.
In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago". See also
Megaannum (Ma) Byr kyr Symbols y and yr
^ Mozley, Peter. "Discussion of GSA Time Unit Conventions". web page. Geological Society of America. Retrieved