Mollusca		Eogastropoda
GASTROPODA		Eogastropoda

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Eogastropoda

GASTROPODA (stem group)
|--PARAGASTROPODA
`--+--TERGOMYA (paraphyletic)
   `--Gastropoda (crown group)  
      |--EOGASTROPODA  
      |  |-?Ophiletoidea
      |  `--+--Macluritoidea
      |     `--+--Euomphalida  
      |        |--Platyceratoidea  
      |        |--Neomphalida  
      |        `--PATELLOGASTROPODA
      `--ORTHOGASTROPODA  
         |--NERITOPSINA
         |  |--Neritoida
         |  `--VETIGASTROPODA
         `--+--Apogastropoda
            `--Pulmonata

Introduction

Cellana tramoserica (Holten, 1802)
Patellogastropoda - Nacellina - Nacelloidea - Nacellidae

Euomphalus pernodosus Euomphalida - Euomphaloidea - Euomphalidae diameter about 4 cm Carboniferous (Late Moscovian) of western Euramerica image © McGraw Hill Book Company, from Moore, et al. 1952, p.306

The clade Eogastropoda was defined by Ponder and Lindberg 1997 to distinguish the most primitive gastropods from their more derived cousins. This was based on the argument that the limpets (Patellacea/Patelloidea/Patellina), originally classified as "archaeogastropods" because of soft-part anatomy [Moore, et al. 1952, p.309 ], and considered specialized descendants of 

Pleurotomariaceans, are actually a group of much more primitive survivors from the original gastropod radiation. There are a number of reasons for this. Limpets retain the same docoglossa-type radula as Polyplacophora and Scaphopoda, whereas other "archaeogastropods" have a rhipidoglossa type of radula [Golikov and Starobogatov 1975). They also have much more primitive eyes and other characteristics. Lindberg 1988, Haszprunar 1988, Ponder and Lindberg 1997, Sasaki 1998 and others all argue that the patelloids - now called Patellogastropoda or Doccoglossa - are the sister taxon of all other extant gastropods. Phylogenetic analyses of partial 18S rDNA sequences indicate that the Patellogastropoda comprises a robust clade with high statistical support [M. G. Harasewych, A. G. McArthur,  A molecular phylogeny of the Patellogastropoda (Mollusca: Gastropoda)]

Because the Eogastropoda includes not only extant species but also many extinct (mostly Palaeozoic forms, e.g. Euomphalus, shown left) as well, most of which look nothing at all like modern limpets, and probably had very different habits as well, it can better be defined cladistically as a "stem group" taxon - i.e. all gastropods more similar to (say) Patella than to (say) Helix, rather than a "crown group" definition (the common ancestor of all extant limpets and its descendants). For more on cladistics and how this works see this page.

In the Linnean framework, the Eogastropoda is a subclass, the counterpart of the Orthogastropoda (which includes all other gastropods). Possibly other extinct groups, like the Bellerophontida, might belong under their own subclass

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