Recognition of diverse ecologic associations of marine benthic invertebrates is essential in the study of organic evolution as a systematic means of recognizing and measuring the ecologic niches and changes in ecologic niches of evolving species. Precision in this endeavor may be related to three general kinds of “communities” which have been recognized (and confused) in ecology and paleoecology. The Peterson animal community (PAC) is a local, descriptive unit of recurring combinations of species which can be reproduced in the fossil record. Parallel communities are groups of PACs related by common ancestry of contained taxa, and commonly are characterized by genera or families. Marine benthic life zones are groupings of PACs by occupation of broad-scale environmental areas, and may be characterized at the level of broad morphologic grades. Each faunal region or province of the world possesses the same life zones, but each of these is occupied by PACs unique to the region or province. Life zones of different provinces may or may not share parallel communities in common. Because of their different taxonomic, biogeographic, and historical scales, the three kinds of communities cited should not be confused in comparison with and interpretation of paleoecologic data.