Mollusca		Nautiloidea
CYRTOSOMA		Tarphycerida

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Tarphycerida

Cephalopoda
|--NAUTILOIDEA
|  |--TARPHYCERIDA
|  |  |--Tarphycerina  
|  |  `--BARRANDEOCERINA
|  `--ONCOCERIDA 
|     |--Oncoceratidae  
|     `--NAUTILIDA    
|        |--NAUTILIDA i.s.
|        `--NAUTILINA
`--Neocephalopoda
   |--Ammonoidea
   `--Coleoidea

Discoceras roemeri (Strand, 1934)
suborder Tarphycerina, family Trocholitidae
Late Ordovician (Katian or Hirnantian)
Zawidowice, Poland
graphic from Jerzy Dzik, Phylogeny of the Nautiloidea, p.41

Introduction

A small group of loosely to tightly coiled cephalopods, these were the most primitive members of the Infraclass Nautiloidea ss.  They evolved from the Bassleroceratidae, but did not seem to leave any descendants. There are two suborders (originally considered distinct orders), the Tarphycerina and the Barrandeocerana.