> I'd say that the modern Fortran is almost completely backwards compatible with legacy FORTRAN, as far back as FORTRAN IV (predecessor to FORTRAN 77).
That's very impressive! I assumed the compatibility difference was akin to what Perl 6 is to Perl 5- -- that is, not compatible at all.
The rules for the lines formatting are different, but lucky you can write code in the "intersection format" that can be read by both http://fortranwiki.org/fortran/show/Continuation+lines
If useful if your coworkers are stuck to a old semi unknown machine and want to mix the new code with the old code and run it there too.
New standards mark obsolescent or deprecate a few ancient features as they come out, however compilers maintain the support for these features. I'd say that the modern Fortran is almost completely backwards compatible with legacy FORTRAN, as far back as FORTRAN IV (predecessor to FORTRAN 77).