Why are lamp shells much less common than they were in the fossil record?

Lamp shells, or brachiopods, were once among the most abundant forms of life in the ocean, particularly during the Paleozoic Era. Their decline in relative commonness is attributed to several factors:

• Mass Extinction: Many successful Paleozoic articulate brachiopod superfamilies went extinct at the end of the Permian Period, significantly reducing their overall diversity and presence.1
• Competitive Pressure: Following the Permian-Triassic crisis, brachiopods never regained the dominance they held among the benthos (bottom-dwelling organisms) of the Paleozoic. It is believed they may have suffered competitively from the adaptive radiation of bivalves during the Mesozoic Era.1

While they remain present in all oceans today, they are no longer as numerous as they were during their peak in the fossil record.2