Monoplacophora

Living representatives of this Class were not discovered until 1952, although Paleozoic fossil monoplacophorans had been known for some time. At present, 11 species are known. Most live at great depths and all are marine. Monoplacophorans are small and have a single, caplike shell, giving them a limpet-like appearance. A number of their organs (nephridia, heart, etc.) are repeated serially, making them resemble metamerically-arranged species such as annelids and arthropods. Whether this resemblance indicates a close relationship between monoplacophorans and phyla exhibiting true metamerism is an open question.












Neopilina galatheae, top (dorsal) view. Neopilina is peculiar because of the replication of various of its organs and organ systems, reminiscent of metameric animals. The class Monoplacophora is well known as fossils, and until 1952 all of its members were believed to have been extinct since the Devonian period, about 350 million years ago. [This shell, relatively thin, was damaged while being dredged.]












Neopilina galatheae, bottom (ventral) view. The oval, flat foot is characteristic of the classes Monoplacophora, Polyplacophora and Gastropoda. The foot is bordered on each (left and right) sides by five gills.












Neopilina galatheae, side (lateral) view. The anterior end, denoted by the slightly coiled apex, is on the right.


One reason for having evaded detection for so long is that they are generally found in the deep ocean. Finding them has been quite a boon to malacologists however, as monoplacophorans are often thought to be among the most primitive of molluscs. Indeed, many researchers believe that monoplacophoran-like ancestors gave rise to the rest of Mollusca.

Modern systematic research has borne out the idea of Monoplacophora being the basal member of the Mollusca clade. Their morphology then, proves to be remarkably important in understanding what the first molluscs may have looked like, as well as how the other major groups such as bivalves and gastropods may have evolved.

As only a few species of living monoplacophorans are known, and all being somewhat similar, much of our knowledge of the group comes from fossils.

Fossil record
Monoplacophorans are the first undoubted molluscs, being found in rocks from the earliest Cambrian. The fossil record indicates that the group was quite diverse during the Paleozoic.

Recent monoplacophorans form a distinct clade, and their similarities and differences with the other extant molluscan groups are easily recognized. There is little question that some Paleozoic taxa are also members of this clade. However, the characters that distinguish some Paleozoic monoplacophorans from the torted gastropods and vice versa are open to alternative interpretations and the relationships of several major groups of early-shelled molluscs have therefore been the subject of much debate.

Life history & ecology
Monoplacophorans are found on both soft bottoms and hard substrates on the continental shelf and seamounts, generally in the very deep sea. However, some Paleozoic taxa are associated with relatively shallow water faunas (greater than 100 m), indicating that their relegation to the deep sea is a more recent phenomenon.

Unfortunately, there have thus far been no developmental studies done on monoplacophorans. Indeed, most of our knowledge about Monoplacophora comes from the first description of Neopilina galathaea by Lemche and Wingstrand in 1959.

More on morphology
Monoplacophorans are small and limpet-like, having a single, cap-like shell. Some organs (kidneys, heart, gills) are repeated serially, giving rise to the now falsified hypothesis that they may have a close relationship with segmented organisms such as annelids and arthropods.

In fossil monoplacophorans, the aperture (shell opening) varies in shape from almost circular to pear-shaped. Shell height is also variable and ranges from relatively flat to tall. The monoplacophoran animal has a poorly defined head with an elaborate mouth structure on the ventral surface. The mouth is typically surround by a V-shaped, thickened anterior lip and post-oral tentacles; post-oral tentacles come in a variety of morphologies and configurations. Below the head lies the semi-circular foot. In the pallial groove, between the lateral sides of the foot and the ventral mantle edge, are found five or six pairs of gills (there are fewer in very tiny taxa).







































You can download many scinece journals of Monoplacophora from this link:

http://www.zmuc.dk/inverweb/Galathea/Pdf_filer/Volume_16/galathea-vol.16-pp_007-094.pdf

http://www.mapress.com/zootaxa/2008/f/zt01866p222.pdf

http://www.zmuc.dk/InverWeb/Galathea/Pdf_filer/Volume_16/galathea-vol.16-pp_095-098.pdf

http://www.pfeil-verlag.de/04biol/pdf/spix29_3_21.pdf

http://www.auburn.edu/academic/science_math/cosam/departments/biology/faculty/webpages/zzhalanych/Pub.pdfs/Passamaneck2004a.pdf