Evolution and systematics

The origin of the Vetigastropoda in the Paleozoic era (544-248 million years ago) is difficult to interpret because the fossil record contains such large and diverse gastropod clades (groups of organisms with features reflecting a common ancestor) as the Bellerophonta and Euomphalina. These clades may or may not be related to living vetigastropods. Morphological analysis of the fossil shells suggests that their coiling parameters (i.e., the way the shells were built) are different from those of extant vetigastropods. In addition, there is no consensus as to whether the Bellerophonta were gastropods at all or whether they were untorted (without twists) Monoplacophora. Traditionally, shells with slits or emar-ginations (notched margins) are common features of Palaeozoic gastropods and are often regarded as diagnostic of the most "primitive" vetigastropods. The earliest putative gastropod (Aldanella) completely lacks these structures, however, and it is possible that slits and emarginations were derived more than once from ancestors without slits. In addition, living representatives of some of the most basal veti-gastropods have poorly developed shell elaborations or lack them altogether.

The vetigastropods represent a large subset of the snails that were once commonly known as the Archaeogastropoda. Around 1950 some researchers began to recognize that Ar-chaeogastropoda was actually a collection of several distinct lineages, many of which were subsequently removed and given a higher taxonomic rank. Ultimately, most of the remaining "archaeogastropods" were found to possess a small sensory organ called a busicle at the base of their gills, and were grouped together as vetigastropods on the basis of this character. After the reclassification, researchers found that the species in this group shared other characters, many of them sensory structures.

Today the vetigastropods form a well demarcated group placed near the base of the gastropod tree. There are several major branches within the Vetigastropoda. One branch includes the Lepetelloidea or Pseudococculina, which were formerly groped together with the Cocculoidea in the Coccu-liniformia. Another branch includes such vetigastropods with shell slits and emarginations as the Pleurotomarioidea, Fis-surelloidea, and Haliotoidea. A third branch includes groups that have shells without secondary shell openings, such as the Trochoidea. The Sequenzoidea are also members of the Vetigastropoda. Neomphalida, a group of organisms that lives near deep-sea hot vents and has a unique combination of characters, may also belong to the Vetigastropoda.

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