Trilobite evolution - Miniaturization

Ptychopariida
Acanthopleurella

We learn from Sam Gon III page on evolutionary trends that the size of some trilobites got smaller.
"Reduction in size is seen in several trilobites, such as Acanthopleurella (just about 1 millimeter at maturity)."

I am not able to argue about the chronological sequences from big Acanthopleurella to small ones. I fully trust the biologists who have studied the evidence that some did get smaller by time, let us say between Ordovici and Permian, and are not just variations of the same animal during Cambrian.


Why, Sam , why?
"The general argument for evolution of small size typically evokes the numerous microhabitats of complex marine systems, coupled with a correlation of size to rapid maturity (early maturation means smaller size at adulthood)."

Hmmm...
Thank you, I need to digest that...


AND...
"When this reduction is due to progenesis (arrested development) the trilobites may also display a reduction in the number of thoracic segments."

Okay, that is great to hear that biologists can detect progenesis...


AND...

Agnostida

Pagetia 

"The entire order Agnostida is composed of small species that may have originated as a miniaturized and specialized early offbranch of the Redlichiida or Ptychopariida."

Hmm...
Now I am a bit confused.

If so, where is the Darvinian natural selection pressure?  You say that Agnostida appear in Cambrian period and originated as mini-trilobites...