Tarphycerida

The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as and Estonioceras, are loosely coiled and gyroconic; others, such as Campbelloceras, Tarphyceras, and Trocholites, are tightly coiled, but evolute with all whorls showing. The body chamber of tarphycerids is typically long and tubular, as much as half the length of the containing whorl in most, greater than in the Silurian Ophidioceratidae. The Tarphycerida evolved from the elongated, compressed, exogastric Bassleroceratidae, probably Bassleroceras, around the end of the Gasconadian through forms like Aphetoceras. Close coiling developed rather quickly, and both gyroconic and evolute forms are found in the early middle Canadian.

Tarphycerids tend to uncoil in the late mature stage of their growth, indicating they settled into a benthic lifestyle as they became older. Younger, wholly coiled forms were probably more active, nektobenthic, certainly more maneuverable

The Tarphycerida comprise three phylogenetically related groups of families. They are: the tarphyceratid group consisting of the Estonioceratidae, Tarphyceratidae, Trocholitidae, and Ophidioceratidae; the barradeoceratid group, derived from Centrotarphyceras, consisting of the Barrandeoceratidae, Bickmoritidae, Nephriticeratidae, and Uranoceratidae; and the plectoceratid group, derived from Campbelloceras, consisting of the , Lechritrochoceratidae, and Apsidoceratidae.