Dense monospecific assemblages are common in certain living and fossil molluscan taxa such as turritellid gastropods, pectinid bivalves, and corbulid bivalves. These occurrences can be attributed to physical or biological factors. Physical factors leading to monospecific accumulations are generally post-mortem and include winnowing, dissolution, and transport. Biological factors leading to monospecific aggregations generally relate to recruitment and development. Detailed field and laboratory examination of the fossils can refute hypotheses related to physical factors, but often are equivocal for hypotheses related to biological factors, particularly for taxa with features suggestive of indirect development. Biological factors are better tested with paleobiology and extant phylogenetic bracketing. Bracketing is particularly useful in clades that vary in their tendency to form monospecific aggregations, such as corbulid bivalves. Living corbulids such as Varicorbula disparilis form large byssally-attached masses, and their close fossil relatives (e.g. Varicorbula caloosae) tend to be found in monospecific aggregations. Other shallow-burrowing corbulids in which the byssus plays a less important role, such as Corbula contracta, are rarely found as aggregations. Once the biological origin for an aggregation has been established, we can begin to address related questions including why it occurs in a particular place and/or time.