Brachiopods aren't an evolutionary dead-end which got wiped out in a mass extinction. Well, there was a mass extinction, but it didn't wipe out the Brachiopods. Many were buried in Noah's flood but many alive today.
Brachiopods
Graptiloids, Nautiloids, Cystoids in Maximum Abundance
If a sedimentary rock is full of the remains of Graptiloids does it means that the Graptiloids were in their maximum abundance? No, it means that the place where the Graptiloids lived got buried in sediment.