Stenothecidae

From Palaeos

o Eutrochozoa
`--o MOLLUSCA
   `-+--Solenogastres
     `--+--Caudofoveata
        `--o Testaria 
           `--+--o Polyplacophora 
              `--o Conchifera 
                 |--Tryblidiida
                 |?-- Stenothecoida
                 `--+?--Bivalvia (if molecular phylogeny)
                    `==Helcionelloida (paraphyletic)
                       |  |--+-- †Yochelcionellidae 
                       |  |  `?-- Cephalopoda
                       |  |? -- †Criconarida  
                       |  `-+?--Bivalvia (if transitional fossils)
                       |    `-- †Rostroconchia
                       |         `-- Scaphopoda 
                       `--+--†Paragastropoda
                          `-- Gastropoda

Introduction

The Stenothecoida are a small group of enigmatic Early to Middle Cambrian conchiferan mollusks. They do not appear closely related to any other group. Previously included among the "Monoplacophora", they were placed by Aksarina (1968) and Yochelson (1969) in a separate class, the Stenothecoida or Probivalvia. However, Runnegar & Pojeta (1974) return them to the "Monoplacophora"

In view of the tremendous diversity of Cambrian mollusks, I would not be surprised if the Stenothecoida are indeed a distinct and short-lived experimental lineage, which, I suggest, developed a mineralized shell independently of other groups, as did the Polyplacophora, Tryblidiida and Helcionelloida.

Stenothecoids have a bivalved shell, each valve resembling a Tryblidiidan "monoplacophoran" in shape, except for a slightly asymmetry. In this the two valves resemble the right and left valves of some bivalves, and, like brachiopods and some bivalves, are slightly inequivalved. Externally, the shells range from relatively smooth shells to elongate oyster-like forms ornamented by divergent angular folds, which interlock at the valve margins. The shells are normally found disarticulated.

Yochelson (1969) interprets Stenothecoides as a brachiopod-like mollusk. Runnegar & Pojeta (1974) suggest it may have been a bivalved monoplacophoran, with the lower or smaller valve formed by the sole of the foot, as a few living limpets form a second valve in this way, although in these cases the lower valve is cemented to rocks.

Regardless of the interpretation, inasmuch as the "Monoplacophora" can no longer be considered a valid class, it is not unlikely that the stenothecoids represent a quite distinct lineage of early molluscan evolution.

Credits

MAK030101