Evolution of the Molluscs
Interest lay in bridging the gap between studies of the zoologist and the palaeontologist - one interested in present day life, the latter in the fossil record, yet the present day molluscs could trace their ancestry back some 570 million years. Even in those times a basic body plan had evolved, a head, a foot, gills, and a feeding organ enclosed in a mantle and often protected by a shell.
One very primitive type of mollusc from the Silurian suggested development from worms, which have pairs of organs, like 2 hearts, 2 kidneys, etc, then in 1957 one of these was dredged up from 3,000 feet deep in the ocean, a living fossil which had not been seen since the Silurian, 400 million years ago. But this did not have pairs of organs but an odd number of hearts, kidneys, etc. Since then several more have been found to provide a new modern group.
Back in Cambrian times the molluscan pattern was there and the members spread out to find different ways of living in a real burst of evolutionary activity and which are now found as fossils. There seemed to have been plenty of room to explore and no predators present. This is known radiating evolution. But by the end of the Silurian many had died out.
The three main groups of the mollusca are Gastropods, Bivalves and Cephalopods. The first developed eyes, head, a foot and were mobile, while the Bivalves, like the oyster, became sedentary wih no head, eyes or feet, but the Cephalopods became highly, eg: octopus, squids, and nautiloids and became free living, fast swimmers with eyes and an advanced brain structure, like the fish. 90% have shells with a buoyancy device to keep them upright in the water, and a jet mechanism to enable them to move backward and forward. The present day nautilus has 94 tentacles and it had been assumed extinct ammonites had the same number but recent x-ray work has shown they had only 8.
In the gastopods like the modern whelk, the shell has a dextral opening through which protrudes a siphon for testing the purity of the water through which the animal is moving. The internal structure is under torsion, all twisted round a central support. In the Cretaceous the pleurotomaria, with a problem of keeping dirty and clean water seperate, developed a band of slits near the edge of the shell. As fossils they have been found in shallow water deposits, yet today they have found at great depths, still surviving.
Although cowries are gastropods they have developed a different way of life. the mantle, instead of withdrawing inside the shell, comes out and covers the shell which keeps it smooth and shiny in comparison with others - rough and turreted. The cone gastropods have lost all but one tooth, which has developed a poison with which to inject its prey, often small fishes which it then proceeds to swallow and digest.
This poison is fatal to men. Sea slugs - gastropods with no shells - have made themselves so unpleasant to eat that they are left alone as food for other predators.
These marine gastropods have gills which do not work on land, so land snails, found as far back as Coal Measures, have adapted accordingly.
By comparison the Bivalvia have been very staid in their evolution. The present day scallops are identical with ancient pecten, after developing two valves which they clap together for swimming and eyes on the edge of the mantle that design has not changed for millions of years.
The trigonia bivalve was the first living fossil to be noticed in Australian waters, back in 1820. One was caught but jumped back into the sea before capture, however, the sight of it was enough to upset the accepted ideas prevailing at that time. In Mesozoic times the bivalvia developed tubes from the siphons and this enabled them to burrow into soft sediments and hard rock so finding more living space. Some species made an acid for dissolving hard rock, while others made hard rasping shells to bore out holes to live in.
In Jurassic and Cretaceous times one bivalve developed the same shape as a single coral and was so successful that it built up large reefs, but it disappeared at the end of the Cretaceous along with the ammonites and dinosaurs.
The Cephalopoda were swimming carnivores, with eventually, curled chambered shells kept upright in the water by a siphuncle. They began with Orthoceras, a straight form with the back chambers filled in, but this would have ‘weighed’ them down and they were bottom feeders, so they coiled the shell up. First came the nautiloids, then goniatites, then ceratites and finally ammonites, in all, from Ordovician to Cretaceous. The ammonites developed so quickly and being free swimmers were world wide, so their fossil remains have proved invaluable as zone index fossils for correlating Jurassic and Cretaceous strata.
Modern X-ray research has shown that ammonites had only 8 tentacles, the same as modern octopus, and not the nautilus with its 94, while the belemnites had 10 which equates them with the modern squids. Often large and small ammonites are found together, which were thought to be mature and baby ones side by side, but now the larger ones are identified as female and the small ones as male.