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Penn, Cro-Cop ride into the sunset

This is a period of great transition for mixed martial arts. Out of the shadows and into the light, as it were. Part and parcel of that is the reality that a generation of fighters who rose to prominence during this era of increased visibility and money has reached the end of the line.

To a list that features Chuck Liddell and Randy Couture, which will soon include Matt Hughes and Fedor Emelianenko, we can add Mirko Filipovic and B.J. Penn. Like Liddell and Couture, Filipovic, 37, and Penn, 32, called it quits the hard way. Battered and bruised Saturday night in Las Vegas, they both recognize their physical capabilities no longer match their will to fight. They know what it is to put family ahead of personal glory. And they know that, as empowering as it can be to assert dominance over another competitor, there are things ahead of them that can be as equally fulfilling.

There's no business less forgiving than the fight game. For the men and women who choose to participate come accepted risks and potential glory. The common thread, the one thing they'll all experience, is the end. Some more gracefully than others, but nonetheless, fighting time is finite. Filipovic essentially said that his bout against Roy Nelson was going to be his last. Penn, while hinting at retirement for a couple of years now, was less obvious. His departure is both surprising and expected, if an event can be both at the same time.

The game rolls on, and we have fights and fighters to discuss. That won't change even as the players do. Not included in the grades this week are Ben Askren and Jay Hieron, who put on a competitive five- round effort in Kansas City for the Bellator welterweight championship. Askren remained unbeaten thanks to a razor-thin split decision. It was a quality win, one some might contest, and it leaves Askren in a position in which he can continue to make some noise. Other fighters of note left off the grading rolls were Brandon Vera and Eliot Marshall, who went to a somewhat controversial decision that Vera won. I had it a draw. If it had been conducted under the old Pride rules, Marshall clearly would have earned the decision. As it was, Vera likely cashed in karmic points earned after the fiasco with Thiago Silva, and things went his way -- save for a badly mangled arm.

From A to F, here's a look at fighters who competed at UFC 137:

Josh Gross covers MMA for ESPN.com. Follow him on Twitter at JoshGrossESPN.