Mollusca		CONCHIFERA
Basal Mollusca		Helcionellidae

Helcionellidae

Basal Mollusca  
|--Polyplacophora
`--CONCHIFERA (Helcionelloida)  
   |--SCENELLIDAE  
   `--+--HELCIONELLIDAE  
      |?-YOCHELCIONELLIDAE  
      |?-STENOTHECIDAE  
      |?-COREOSPIRIDAE
      |--Monoplacophora
      `--+--Diasoma
         |  |--Rostroconchia  
         |  `--Bivalvia
         `--Cyrtosoma

Introduction

Earliest Cambrian (Fortunian) to Middle Cambrian (Amgan or Guzhangian)

The Helcionellidae are among the very first shelly animals to appear. They are the earliest and most generalized of the Helcionelloida, and represent the ancestral lineage from which, ultimately, most, if not all, higher conchifera developed. Hence they are a paraphyletic grade or stem-group.

Helcionellids probably spent much of their time motionless and half-buried in the sediment, feeding on microscopic organisms and organic detritus, which they processed by means of a docoglossan radula. They could however crawl, probably slowly, by means of a large foot, similar to that of chitons, Tryblidiida (= "monoplacophora"), and gastropods. The head, like Tryblidiida, was primitive, and the senses may have consisted of specially modified touch and light sensitive cells. Most were tiny, from a few millimeters long in the earliest Cambrian (Fortunian and Tommotian) to 1.5 cm later, although Dzik (1991) reports specimens with shells about 2 cm in length, although perhaps these belonged to the large but otherwise very similar-shelled Halkeriids

Muscle scars are unknown for all members of the family, although the Early Ordovician species Nyuelia bjalyi (Rosov, 1975) is based on internal moulds that are similar in shape to some species of Helcionella. These have well-preserved paired pedal muscle insertions.  Runnegar & Jell (1976: 117).  It is not certain however whether Nyuelia actually does belong to this family, or whether it represents a separate, even unrelated lineage of early mollusk.

These simple and unspecialized mollusks did not survive even to the Furongian; possibly they were out-competed by their descendents, or feel victim to one of the periodic fluctuations and extinction events to which the fragile Cambrian ecosystem was vulnerable.

Diagnosis

The technical following diagnosis is based on the Treatise:

Family Helcionellidae Wenz 1938