Thursday 27 February 2014

Three new species of Vetigastropod Snails from the Plio-Pliestocene of The Philippines.

Vetigastropods are considered to be the most ancient, and primative, group of Snails. They are exclusively marine, but found in almost all marine environments, from the intertidal zones to the deepest abysal depths. Vetigastropods have nacrous (mother of pearl) shells, made from layers of aragonite platelets connected by protein fibres. This is thought to be the earliest form of shell to have appeared in Molluscs, with many groups of Molluscs separately evolving more durable solid calcite shells. Many Vetigastrolpods also retain breathing holes in their shells separate to the appatures from which the body extends, another trait considered to be primative in Gastropods (i.e. a trait seen in the earliest memers of the group, and subsequently lost in most members). Keyhole Limpets, Abalones, Top Shells and Turban Snails are Vetigastropods.

In a paper published in the journal Zootaxa on 21 January 2014, Renate Helwerda and Frank Wesselingh of the Naturalis Bidiversity Center in Leiden, The Netherlands, and Suzanne Williams of The Natural History Museum in London descibe three new species of fossil Vetigastropod Snail from the Plio-Pliestocene Santa Cruz Formation on Cabarruyan (or Anda) Island  in the and around Tiep on neighbouring Luzon Island in the Philippines, as part of a wider study into Vetigastropod fossils at the area.

Map showing the locations on Cabarruyan where the majority of the specimens were obtained. Hewlwerda et al. (2014).
The first new species described is placed in the genus Halystina (a genus of small deepwater Snails known from the Pacific around the Philippines and New Caladonia), and given the specific name conoidea, meaning conical. Halystina conoidea is a 2.1 mm high, 1.7 mm wide Snail, with a ribbed shell, described from 34 specimens from Cabarruyan Island and one from Luzon.

Halystina conoidea, (13) specimen in (a) rear view, (b) apertural view, (c) side view, (d) basal view, (e) apical view, and (14) second specimen in (a) rear view, (b) apertural view, (c) side view, (d) basal view, (e) apical view. Helwerda et al. (2014).

The second new species described is placed in the genus Calliotropis (small to mediume sized marine Snails, found globally and typically in deep waters, and given the specific name arenosa, meaning 'sediment covered'. Calliotropis arenosa has a 3.4 mm by 2.7 mm conical shell, ribbed and sediment covered; this sediment covering is considered to be an adaptation of the species rather than a preservational feature, as it is present in all discovered specimens (175, all from Cabbaruyan), all of which are covered by a layter of sediment of even thickness. Furthermore the apatures of the shells are not filled with sediment, which would be expected if the covering were a result of preservational artifact.

Calliotropis arenosa in (a) rear view, (b) apertural view, (c) basal view, (d) apical view. Helwerda et al. (2012).

The fthird new species described is placed in the genus Ethminolia (a genus of large Top Shells currently found in the waters around Australia and Tasmania) and given the specific name wareni, Anders Warén of the Swedish Museum of Natural History, a noted deepwater malacologist. Ethminolia wareni is described from 310 specimens recovered from 14 different sites on Anda Island. It has a flattened conical shell, 3.0 mm high by 4.4 mm wide, weekly ribed and with brown blotches and a nacrous shine.

Ethminolia wareni in (50)(a) rear view, (b)apertural view, (c) basal view, (d) apical view. (51)(a)rear view, (b) apertural view, (c) basal view, (d) apical view. (52) SEM detail of protoconch. Helwerda et al. (2014). 

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