The relationship between taxonomic diversity and extinction risk is fundamental to understanding the impact of biotic crises in Earth's history. Here, we challenge the assumption that high diversity provides a buffer against extinction by analyzing the similar to 100-million-year history of the Atrypida, a major Paleozoic brachiopod clade that underwent significant shifts in biodiversity and shell morphology over its evolutionary history. This group survived the Late Ordovician mass extinction early in its evolutionary history when its generic richness was low, yet went extinct during the Late Devonian crisis following an interval of high diversity earlier in the Devonian. By integrating a newly compiled dataset of phylogenetic, morphological, and spatiotemporal data, we uncover a three-phase macroevolutionary trajectory. A Late Ordovician-early Silurian phase of high morphological innovation was followed by a prolonged mid-Silurian interval where high taxonomic richness was decoupled from morphological disparity. This culminated in a phase of specialization during the Devonian, which eroded the evolutionary plasticity of the clade. Our results demonstrate that a high genus count masked a critical loss of functional adaptability and morphological diversity, creating an intrinsic vulnerability. This study reveals a previously unrecognized macroevolutionary mechanism for extinction selectivity, suggesting that the internal evolutionary dynamics of a clade, not just its taxonomic richness, are a key factor in survival of major ecological upheavals.