[Doctor Stochastic]

Saturation may also be a contributory factor. There are only so many useful shapes. Were most already used, not many more would be developed. (I don't know if there are any results along this line. It's just speculation.) This is saturation of the "creature" space.

Another type of saturation would be if some creature(s) filled a niche and thus didn't allow other forms to enter. This is saturation of the environment. There is evidence of this happening with female bears. They have turf-wars. Again it's speculation with respect to microbes (or anyone eles'e crobes.)

[Alamo-Girl]

Thank you so much for your reply and insight! I wish there were some way we could know if all (or most) body plans were tried and whether the environmental niches were filled at the various extinctions.