Tarphycerida

Systematic Paleontology
 * Phylum Mollusca
 * Class Cephalopoda
 * Subclass Nautiloidea
 * Order Tarphycerida

Tarphycerida is the earliest and most primitive of the coiled cephalopoda, with a range extending from the lower/Early Ordovician to the upper/Late Silurian. Coiling is typically discoidal and evolute, with all whorls exposed. Siphuncles (the tube that interconnects the chambers) have layered connecting rings in all but one family.

Derivation is from the Ellesmerocerida, through the Bassleroceritidae, the species of which are upwardly curved, exogastric, like a rocking chair rocker. This same tightening of the curvature, leading  ultimately to true coiling can be seen in the evolution of ammonoids in the early Devonian.

The Tarphycerida includes five families, the Tarphyceratidae, Estonioceratidae, Trocholitidae, Lituitidae, and Ophidioceratidae. Most primitive are the loosely coiled Estonioceratidae in which whorls are either not in contact with each other, or barely touching. The Tarphyceratidae consists of species in which whorls are strongly in contact subsequent whorls overlapping previous. In both the Estonioceratidae and Tarphyceratidae, coiling is planar and siphuncles are subcentral to ventral in location. Both are confined to the lower or Early Ordovician.