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Lophotrochozoa : Mollusca

The Monoplacophora
Living fossils from the ocean deep

No. of described species: 20
First appearance: earliest Cambrian
Habitats: deep sea
Shapes: limpet-like
Feeding types: detritivores

Once known only from Paleozoic fossils, living monoplacophorans were discovered in 1952 in one of the most important discoveries in modern biology. Since that first recent species (Neopilina galathaea) was discovered, around 20 other species have been identified.

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).

Monoplacophoran anatomy

In Recent and fossil limpet-like monoplacophoran shells the apex is typically positioned at the anterior end of the shell, and in some species the apex actually overhangs the anterior edge of the shell.

Internally the monoplacophoran is organized with a long, looped alimentary system, two pairs of gonads, and multiple paired excretory organs (four of which also serve as gonoducts). A bilobed ventricle lies on either side of the rectum and is connected via a long aorta to a complex plumbing of multiple paired atria that in turn are connected to the excretory organs. The nervous system is ladder-like and has weakly developed anterior ganglia. Paired muscle bundles enclose the visceral mass. Large, dorsal paired cavities are extensions of glands associated with the pharynx. The monoplacophoran radula is docoglossate, i.e., each row having a central tooth, three pairs of lateral teeth, and two pairs of marginal teeth.
 

Original text by Paul Bunje, UCMP. Monoplacophoran anatomy by Ivy Livingstone, © BIODIDAC.