This is a Nautilus shell that I photographed at the Carnegie Museum of Natural History in Pittsburgh last October when I was there for the OVUM meeting. Nautiluses are cephalopod mollusks with external shells. The cross section of the shell in the next picture shows the unusual internal structure that leads to the common name "chambered nautilus". The animal occupies the very last chamber that opens to the outside. As its body grows, it secretes more shell and moves forward within its shell; a portion of the shell space it used to occupy then gets partially sealed and filled with gases from its own body. These gas-filled chambers inside its shell provide a nautilus with neutral buoyancy.
Nautiluses have been around for about 500 millions years. The fossil record indicates that there were many more nautilus species in the past than there are now and the extant species don’t seem to have changed much compared to their fossil relatives. Therefore, the common notion is that nautiluses are living fossils, perhaps near the end of their evolutionary journey.
In an article in this week’s New Scientist, Peter Ward of the University of Washington challenges those ideas.
The idea of nautiluses as living fossils now has to be rejected...What we are witnessing is not the final flickering of an ancient group, but a vigorous radiation of new species.This new assessment derives from recent research that has shown that there are actually more extant species of nautiluses than were formerly realized. In fact, some of the species that were previously lumped in the genus Nautilus are now in the relatively new Allonautilus. According to the article, the present day nautilus lineage may have originated around New Guinea as recently as 2 million years ago and the splitting of the Nautilus and Allonautilus lineages may have taken place even more recently.
Stay tuned for more new species.
Unfortunately, because of overharvesting, nautilus populations are becoming increasingly threatened. There is a short review of their conservation status here.