Clupeiformes are physostomes, which means that their gas bladder has a pneumatic duct connecting it to the gut.

[5] Their sister group were the extinct Ellimmichthyiformes, which were dominant throughout much of the Cretaceous and into the Paleogene,[6] and often coexisted with clupeiforms at many known localities.

Both groups closely resembled each other morphologically, although the ellimmichthyiformes evolved some highly divergent body plans later in the Cretaceous.

Several fossil clupeiforms are known from the Early Cretaceous of South America that appear to be more closely allied with Clupeioidei over the Denticipitidae.

[7][8] Denticipitidae Coiliinae Engraulinae Spratelloidinae Pristigasteridae Dussumieriidae s.s. Chirocentridae Clupeinae Dorosomatinae The order includes about 405 species in ten families:[10][11][12]