Mollusca		Neocephalopoda
CYRTOSOMA		Bactritida

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Bactritida

Cephalopoda
|--Nautiloidea
`--NEOCEPHALOPODA  
   |--ORTHOCERIDA  
   `--+--ASCOCERIDA  
      `--BACTRITIDA  
         |--AMMONOIDEA
         `--Coleoidea

Introduction

Devonian to Permian

The Bactritida are intermediate forms transitional between Orthocerid nautiloids and more advanced cephalopods like the Ammonoidea and Coleoidea.  Primitive forms had long mostly straight very slender shells.  From this basic ancestor one lineage evolved into coiled shells, these became the Ammonoidea. Another lineage, represented by genera like Lobobactrites from the Early Devonian Wissenbacher Schiefer (Hünsrückschiefer) of Germany, had an internal shell, and these were ancestral to the Coleoidea

sketch of Bactrites shell, and cross-section family Bactritidae image from Moore, Lalicker and Fischer, Invertebrate Fossils

sketch of Lobactrites shell (internal straight shell - ancestral to Coleoidea) family Bactritidae

sketch of two species of Cyrtobactrites (external coiled shell - ancestral to Ammonoidea) family Cyrtobactritidae

The Position of the Bactritida - and the Problem of Grade Groups