Why Do a lot of Jurassic Bone Frags Seem Indeterminate?

[Kurufossils]

Since getting into fossils something has been bugging me alot about material from the Jurassic and especially material from such places as colorado, utah, etc. Alot of bone material I see online for sale, mostly fragments in various sizes are usually unlabeled and generalized as being from any dinosaur from the Jurrasic. I wonder if there is at least a way to differentiate at least herbivore bone from theropod bone (like allosaurus from camarsaurus)? Is it similar to the method used for cretaceous bone of the honey comb pattern or not? I'm curious why it seems harder to identify Jurassic bone piece, I'd appreciate any input or discussion, thank you.

 

For example would the first two pictures be considered theropod and the last herbivore?

[Miocene_Mason]

Speaking from no experience whatsoever, I would think the scientists get the more complete ones, as they are the only ones with acces to most sites. I Don't think bone structure alone can tell, because of how closely related they are, but if you get one complete enough to show thin walls, it's most likely therapod. Again take this with a pinch (or a generous sprinkling) of salt.