Palaeos		Rostroconchia
Mollusca		Rostroconchia

Rostroconchia

MOLLUSCA
|--ACULIFERA
`--CONCHIFERA
   |==TRYBLIDIIDA (Monoplacophora)
   |   `--CEPHALOPODA
   `==HELCIONELLOIDA
       |--ROSTROCONCHIA
       |  |--+--Conocardiida
       |  |   `--+==Ribeirioida
       |  |      `--SCAPHOPODA
       |  `--BIVALVIA
       `--GASTROPODA

Apotocardium sp, (Rostroconchia - Conocardiida - Bransoniidae) Graham Formation (Late Carboniferous), Jack co., Tx. . Photo © Lance L. Hall, North Texas Fossils, Rostroconchia.

The Rostroconchia are a group of exclusively Paleozoic molluscs which combine the features of scaphopods and bivalves and may or may not (depending on your preferred phylogeny) be related to both. Alrthough typical rostroconches only come after the first bivalves, earlier stem forms such as Heraultipegma and Watsonella may well be ancestral to both groups. Alternatively, rostroconches may simply be stem group scaphopods, and bivalves may have evolved from separate conchiferan ancestors. MAK120614