Re: Why didn't some dinosaurs evolve a primate-like body form?

You have an interesting question that cuts across both evolution and ecology. The path that an evolutionary lineage, such as dinosaurs, takes is never predictable, a primate-like body form can in no way be suggested to be the end-point of a search for evolutionary perfection. This body- form was hit on by chance and by chance humans evolved from primate ancestors.

There are a number of cases of convergent evolution (the evolution of similar external shape (morphology) in organisms which are unrelated, except through distant ancestors). For example sharks (fish), dolphins (mammals) and ichthyosaurs (extinct reptiles) have all converged on body plans which are suited to an aquatic habitat. The aquatic habitat has conditions which were fairly similar across all periods when these organisms evolved and has large constraints to the way of life you can pursue there.

Ecological niches comprise 1.the habitat in which the organism lives, 2. the periods of time during which it occurs and is active there, and 3. the resources it obtains there. Dinosaurs evolved in a very different world at a different time to primates - the vegetation was completely different, and competing animals were very different so it might be that the "primate niche" was non-existant then.

So becoming a primate-like form was not a "goal" of evolution, dinosaurs were constrained in what they could evolve to become due to their evolutionary history and the "primate niche" was probably very different or non-existant to what it is today or when primates evolved.

I hope this answers your question.
Yvonne Buckley

References:
I used The Concise Oxford Dictionary of Zoology (ed. Michael Allaby) for definitions of convergent evolution and evolutionary niche.