ornithopodans vs ornithopods

[Jay <sappororaptor@yahoo.com>]

This question regards use of non-formal names for groups/clades, which I hope 
can be clarified.

When discussing a natural group, such as Sauropodomorpha, the typical reference 
to it in most contexts seems to be simply by appending an 's' to the end. 

E.g., "there are several sauropods known from the Morrison Formation"
"..the sauropodomorphs Plateosaurus and Unaysaurus are sister taxa"
"Unlike the known bipedal ornithopods from DPF, the pachycephalosaurs are...

However, I've very occasionally come across in the published literature a few 
cases where an '-ans' is suffixed to the formal group name to derive the casual 
use - i.e., sauropodans, ornithopodans, sauropodomorphans.


Does anyone know 1, why one style if any might be incorrect; 2, and if both are 
acceptable, then what if any are the circumstances where one style must only be 
used over the other.

I can see a use of the '-ans' suffix specifically as an adverb when referring 
to phylogenetic outgroups -- e.g., "...Lesothosaurus is a non-ornithopodan 
ornithischian" -- but don't see why this must be so.


Note that in regards to Dinosauria, I'm not too concerned about choice use of 
'dinosaurs' vs 'dinosaurians'