Exactly! Well, maybe not…
Seriously, though: consider Kentrosaurus. He was a stegosaur, but he had only half the amount of plates, with the rest turning into spines around the middle of the back. If stegosaurs really needed those plates as heat exchangers, then why would “Ken” have evolved only half the “normal” compliment, and turned the rest into spines?
Might have something to do with a pending Ice age, or a general cooling in temperatures which contributed to the extinction of dinosaurs in general. Less heat, less need for cooling plates. Just MHO.
Heh. Kind of funny - I have a tiny little brain somewhere down there, too. huh-huh! huh-huh! huh-huh!
One potential evolutionary advantage of having a long neck on a big body (but not using it to reach the treetops) is that the sauropods could just sweep those long necks back and forth and cover more area to get at food instead of having to move their big bulky bodies to and fro. The less energy expended to get at food, the better.
I like Robert Bakker’s idea that Brontosaurus might have had moose-like lips instead of lizard lips. While it can’t be proved, and I don’t think it’s been met with much acceptance, it is an interesting image to consider.
Then I must be doing great! slowly dials for pizza delivery
And this is why humans have come to dominate the planet.
The Straight Dope on the brains in butts idea, Og I wish it were true.
Speed dial, you evolutionary dead end! 
Different solution to the same problem? After all, we could turn one kidney into a solution for something else. The chances seem pretty good that we would have fulfilled our species survival duty to have offspring before something happened to that one kidney in enough cases that the species would survive. Maybe"Ken" and all the others needed only half the plates for cooling. “Ken” could have mutated, i.e. blundered, onto a solution for a different problem by using those superfluous plates.
Actually my smartass crack came from my surprise at: “It didn’t make sense that they would need them, while dozens of other, similarly shaped and sized species wouldn’t.”
—shuff shuff shuff—
“Ceratops cradle!”
—shuff shuff shuff—
“Jobaria’s ladder!”
—shuff shuff shuff—
“Apatosaur’s door!”
Nah. Stegosaurs were Jurassic (meaning they existed long before the great extinction at the end of the Cretaceous), and Kentrosaurus was comtemporaneous with more typical stegosaurs.
Funniest thing on the 'Dope this week.
Different solutions are fine and well, but certain critters, such as the later hadrosaurs, didn’t have anything really resembling heat exchange structures. Nor did the massive sauropods, nor did the heavily armored ankylosaurs, etc. It seems rather ad hoc to me to posit that stegosaurs, uniquely among dinosaur groups, required them.
They wouldn’t require them, they’d have an advantage with them.
Giraffes don’t require long necks. Of course, then they wouldn’t be giraffes, but I will not be led into digression concerning such a serious point.
Well, sure they would have been advantageous., I just think that display provides a better explanation for the advantage conferred: bigger, better plates than your fellows gets the chicks (or the studs, as the case may be).
If stegosaurs were ectothermic, and thus in a position to benefit from such thermal regulation, than other dinosaurs would necessarily also have been ectothermic, and would likewise have benefitted from such. Given the wide array of horns, hooks, knobs, rugosities, and other accoutrements which did evolve within the Dinosauria, surely it would have been shown that heat exchangers are a good thing, and would therefore likely have evolved in numerous groups (since those horns, hooks, knobs, rugosities, etc., would all make good starting points upon which to build an efficient heat exchanger; indeed, it is speculated that the plates of Stegosaurus evolved from the spines possessed by the likes of Kentrosaurus). In other words, it would not have been “difficult” for other groups to evolve similar thermoregulatory structures. But they didn’t. Why?
One possible answer is, of course, because the necessary mutations never arose. That isn’t a satisfying answer, though, because the bump, hook or knob was already there, and a bumpier, hookier or knobier version would certainly provide more surface area for heat exchange. Another possible answer - and the more reasonable, in my opinion - is that the bump, hook, or knob didn’t evolve for heat exchange, but for display. Heck, we think Stegosaurus looked cool with its plates and tail spines - why couldn’t a female Stegosaurus be similarly impressed with those manly plates? Or a male think to himself, with his walnut-sized brain, “Would you look at the size of the plates on that one! Grawwr!”
That’s easy.
When Kentosaurus would go into the phone booth to change into Supersaurus, the extra plates would get in the way.
I dunno.
Why did carp get scales, temperate catfish didn’t and South American catfish get armoured plate? Why do some fish eat meat, some veggies and guppies are omnivorous? Why can goldfish live under ice, Koi can’t and corydoras die at 50 F?
It’s not all as beautifull as your namesake’s orchid and moth.
Why do fools fall in love?
I find this about as interesting (and as significant) as theological arguments about whether Christ owned his clothes. What possible interest can this be? The theories (about why the Stegosaurus had plates) cannot be proven(or disproven), because there aren’t any alive today.
So, can somebody write a Ph.D. thesis on this?
I’m still wondering how many angels fit on a pin head.