Tryblidiida
Tryblidiida is a taxon of monoplacophoran molluscans containing the only extant representatives: 37 species are still alive today, inhabiting the ocean at depths of between 175 and 6,400 metres (574 and 21,000 ft).[2]
The first captured living monoplacophoran was Veleropilina zografi in 1896, but at that time it was described as if it were an archaeogastropod, a true limpet, mainly because of its patelliform (limpet-like) shell. This species was finally revealed to be monoplacophoran 87 years later, in 1983.
In April 1952, a living specimen was collected from deep depths in the Middle America Trench off Costa Rica's Pacific coast.[3] In 1957 that species was described and named Neopilina galatheae by its discoverer, Danish biologist Henning Mourier Lemche (1904–1977).[4] An expert in the field has called this discovery "one of the greatest sensations in the [twentieth] century."[5] As of 2008, there were 31 living species known, discovered in waters from 200 meters in depth to hadal depths, or more than 6,000 meters in the deepest ocean trenches.[5][6]
The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977.[7] Scientists believe that the taxon Monoplacophora is probably polyphyletic and have proposed including all the living members in the order Tryblidiida.[5]
In 1989, fossils in Italy from the middle Pleistocene were described which appear to be identical with the living species Micropilina minuta.[8]
Little is known about monoplacophorans. They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres (in recent species).[citation needed] The apex of the shell is at the anterior end. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism; indeed, with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation, which suggested a "missing link" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary.[citation needed]
