e-GMAT wrote:Great discussion. Even though the original has been clarified by @Ilana, I am tempted to respond to the PM, since this sentence exemplifies the importance of understanding the meaning appropriately.

For the Stegosaurus, a dinosaur from the late Jurassic period, the seventeen bony plates embedded in its back were necessary elements for survival, to regulate its temperature throughout its bus-sized body and to protect it from much larger carnivores.

The sentence states that 17 bony plates embedded in the back of the specific dinosaur were necessary elements for survival. This is the core message.
Then the sentence goes on to explain how these bony plates helped with the survival
1: these plates helped with regulation of temperature of its body
2: these plates helped by protecting it from larger carnivores

So as you can see "to regulate" and "to protect" really are the two elements that explains how these 17 bony plates were essential for survival. So in a way you can say that these two elements explain the preceding clause and hence I am suspecting the correct construction here is regulating...and protecting...- so comma + verb-ing construction. (please post the complete sentence here)

Now from meaning perspective, why is "to regulate" and "to protect" incorrect.
Lets remove the fluff from the sentence and review it:

17 bony plates were necessary elements for survival to regulate the body temperature and to protect from larger animals. - This sentence implies that the purpose of survival was "to regulate the temperature...". This is not correct. The logical meaning here is as explained above - regulating and protecting explain how these parts were necessary for survival.

17 bony plates were required to regulate the body temperature and to protect from larger animals. - This sentence is correct. It implies that the purpose of these 17 bony plates was two fold - to regulate the temperature and to protect from larger animals.

Thanks,
Payal