Xainzanoceras, defined by Chen, 1987, is a nautiloid cephalopod from the Upper Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 of China, assigned to the tarphyceratid

Tarphycerida

The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...

 family Lituitidae

Lituitidae

The Lituitidae is a family of evolved tarphycerids characterized by a long orthoconic section that follows a coiled juvenile portion at the apex, along with a generally tubular siphuncle, which like that of the barrandeocerids is composed of thin connecting rings.-Taxonomic Position:Flower and...

. The type, Xainzanoceras xizangense Chen resembles the ophidioceratid

Ophidioceratidae

The Ophidioceratidae is a family of closely coiled tarphycerids, represented solely by the Upper Silurian genus, Ophioceras, characterized by an evolute shell with narrow, subrounded, annulated whorls and a subcentral siphuncle composed of thin connecting rings that show no evidence of layering...

 genus Ophioceras. The shell is evolute, discoidal, serpenticonic; whorl section rounded, covered with thick, straight, close spaced, radial ribs that curve slightly forward at the ventral margin where they become quickly reduced or lost.