The Simplest Solutions Are Often The Best

What were tall theropod neural spines for? They may have served a number of secondary functions, but I'm going to place my bet that they were deception aids. Mammals do not usually have tall neural spines (obviously tallish neural spines are present in some artiodactyls, but they are much smaller than those in some theropods, and serve as fat and/or muscle attachment points), but they, too, exhibit size deception. Extended neural spines seem like an easy way for theropods to look 3-5 times heavier than they really are, at a reasonable cost. This does imply that these theropods were unable to arch their back as certain mammals do (and had no integument large enough to raise to assist with the deception). This could be due to the structure of their spine, or their extremely lengthy history as bipeds, I honestly don't know enough about theropod anatomy to say.