Trilobite evolution - effacement

According to the classic theory of Darwin trilobite itself and the variations are participants in the fight for survival of the fittest. But it is, of course, possible that several variations of the basic type, more or less mutated children of the Grand daddy Redclichia, exist in the same period of life on Earth all doing well in those changing oceans that once existed.

One such example of change by adaptation into the environment is what is called the effacement of trilobite types meaning the loss of detail and complexity as in these examples (see the full article for more examples of the same)

Corynexochida   Bumastus	Agnostida   Lejopyge

I very much like the way Sam describes effacement in exact scientific language. As a simple student of his magnificent page I try to break the rather compact and hard nut into smaller and hopefully easier to digest bits and pieces:

What is effacement?
Loss of surface detail.

"In several trilobite orders, but perhaps most notably among the Agnostida, Corynexochida (Suborder Illaenina), and Asaphida, effacement of cephalic, pygidial, and even thoracic furrows is not uncommon."

Causes problem with taxonomy
Because diagnostic details disappear the established classification of types can get confused because of the missing signs.

"This loss of surface detail can be confounding to systematics, since effaced features (for example loss of glabellar details) can mask evidence of relationships."


Why this happens?
Some palaeobiologists suggest that some trilobites began to dig for food. The suggested mechanism is, I guess, that those mutants who got rid of too much surface detail were able to eat more and reproduce more efficiently and so we have this evolutionary thing.

"Some workers suggest that effacement is an adaptation related to a burrowing lifestyle, especially in Illaenina, but such effacement might also play a role in streamlining of pelagic Asaphida, and is also seen in many Agnostida (which may have been planktonic)."

Not so sure, however
Sam is an honest seeker of truth and critical as good scientists should be.

"No single selection pressure seems to have been responsible for the effaced morphotype."