Acar Gray, 1857.
Geological range: Paleogene (Paleocene) to Recent (source: Cox et al., 1969, Treatise N250). The Paleobiology Database reports the genus extending into the Late Cretaceous.
Geographic distribution: A distributional map for modern Acar may be accessed from OBIS. A distributional map for ancient Acar may be accessed from the Paleobiology Database (Primary data contributors: Hendy, A. J. W., W. Kiessling, and A. I. Miller.).
Diversity: The WoRMS database recognizes 25 species of extant Acar (unvetted). The Paleobiology Database recognizes 26 species from the fossil record (unvetted) (Primary data contributors: Hendy, A. J. W., W. Kiessling, and A. I. Miller.); WoRMS recognizes an additional 4 species (30 total).
Paleoecology: All species of Acar are epifaunal, sedentary (can reposition), bysally attached suspension feeders. Source: Todd (2015).
Phylogenetic status: Monophyletic. A recent molecular phylogenetic analysis by Combosch and Giribet (2016) supports the monophyly of Acar.