Some librostomes may not even be within the group, with Whittington (2003) placing some genera (Hemibarrandia, Lakaspis, Peraspis and Symphysurina) of the asaphid family Nileidae in the order Corynexochida.

The first Harpetida would appear in the late cambrian with the primitive genera Baikadamaspis and Entomaspis of the family Entomaspididae.

Since the earliest Phacopida (which are most likely librostomes) appeared during the early ordovician, it is assumed that a late cambrian sister taxon was present.

The End-Ordovician extinction event would drastically reduce trilobite diversity, with librostomes included.

[11] Many trilobites in the group had some ability to stay above the sea floor, such as asaphids in the superfamilies Cyclopygoidea and Remopleuroidea,[12][13] proetids in the family Telephinidae, and perhaps some Cheirurid phacopids such as Deiphon and Crotalocephalus.