see text for genera Trinucleidae is a family of small to average size asaphid trilobites that first occurred at the start of the Ordovician and became extinct at the end of that period.

To the front and sides of this tri-nucleate centre is a wide seam (or fringe), that often inclinates towards the outer margin, and is perforated by funnel shaped pits.

At first, the pits in the fringe were considered to have a sensory function, but the suggestion that they may play a role in filtering food particles was introduced however now this has also been disproved with no specimen ever having a clear hole shown in the pit, which is that would be expected if filter feeding was its purpose.

The family started diversifying during the Floian, had a major diversification burst in the Darriwilian, and remained important until its demise at the end over the Ordovician.

During the Lower Ordovician the family was present in Gondwana and associated areas, but in the Dapingian it spread to Baltica and Laurentia, and it remained cosmopolitan after.