Mercer is an iron-chinned juggernaut who took shots flush from ATG bangers like Morrison and Lewis and smirked. Holyfield had a good chin but his ability to roll and his better defensive skills compensated. He was shook up by Dokes, Cooper, & Bowe, among others... Mercer's neck and shoulders and wide jaw was designed to absorb shots. Holyfield's ability was more artificial -shall we say, and based on his superb will.
I'll also shoot the question in your direction, SH. What do you think of the Briggs and Wladimir fights?
Mercer was in his early to mid 40s for both of these bouts. I don't think that Briggs' finishing shots were finishers at all -they were point shots with no leverage. Mercer just had enough, was tired and he was fat. He laid down. Klitschko was cracking him almost at will with those right hands toward the end, the ref saw that he had more or less surrendered on his feet and stopped it. "Chin strength" is not merely a matter of jaw structure, neck width, and trapezius development. Some of it is will. And some of it is conditioning. And some of it is youthful reflexes. Hell, some of it is blind luck! Mercer's chin, unlike Holyfield's and Hagler's was never as much due to will and conditioning as it was structural. Again, he was built to take shots. You'll notice however that Lamotta and Cobb's chins gave out in their 30s. I believe that the reason for that is not only continuous testing, but also body deterioration. The mind's eye if you will reacts almost microscopically with a shot --it's a matter of reflexes for young men. A man in his 40s loses this and so takes shots more flush. Watch Duran against Castro and Joppy -he wasn't taking shots well at all because he wasn't reacting with even the slightest youthful reflexes. The shots were landing flush. No man can eat those. Hagler got out when he was 32 and was always in condition, always determined, and had great structure. I give the "chin crown" to him without the slightest hesitation, but had he fought on, his chin would have began to give way... not because he'd get lazy like Mercer, but because of natural physical deterioration that would mean he would be taking big shots without that slight reflexive reaction that is the advantage of youth.
Stonehands. Check out Wayne McCullough sometime. His chin is right on par with Hagler's. I've never seen him awknowledge punches, far less hurt or in the early stages of looking like being floored. And combine that with his come forward style, never say die attitude, and poor defense and you obviously have a fighter who took a few to land one himself. The punchers McCullough faced weren't just chilling hitters for their weights, they also had physical advantages over him. McCullough was a natural bantamweight who had no business sharing a ring with Hamed when they met 10 years ago. That was at featherweight. The decision to move up was purely based on money and the challenge for McCullough. When it comes to power Hamed is easily among the hardest hitting featherweights of the last 20 years, arguably of all-time. McCullough stood firm and never looked like going down over the 12 rounds. And the same goes for the Morales fight at super-bantamweight. Morales' power was at it's most lethal in this division even though he carried his power up with him as moved out of the division. McCullough walked through Morales' uppercuts and right hands so convincingly that he got inside to make it a war with Morales later claiming he was thinking about quitting during the late stages. I also recall McCullough doing the Ali shuffle after being hit with a three punch combo to the head and chin. McCullough was made of steel, just like Hagler. Maybe advantage McCullough as he fought naturally bigger opponents and moved out of his depth. Hagler on the otherhand fought other naturals at the weight or punchers moving up like Hearns and Mugabi.