09 April 2008

Nautilus: still evolving after 500 million years

Nautilus1

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.

Nautilus2

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.


3 comments:

Dave Coulter said...

Thanks for the description. I have always thought they were pretty cool.

Frank Anderson said...

I just (well, o.k., last fall) wrote up a phylogenetic definition for Nautilus, and I was unconvinced by Ward's arguments. See Harvey et al.'s response to Ward and Saunder's original paper erecting Allonautilus (cited below) for some additional insight. I think that there are two extant Nautilus species -- pompilius and scrobiculatus (which Ward puts in Allonautilus).

If people want to have two monospecific genera of extant nautiloid (Nautilus and Allonautilus) to highlight the distinctiveness of the two species, I guess I don't really care, but there really isn't much known genetic diversity out there among extant nautiluses other than that seen between scrobiculatus and pompilius.

But they are certainly still evolving! No "living fossil" has stopped evolving, even if superficially it looks more like 300-million-year old forms than, say, we do. One of my pet peeves...

Harvey, A. W., R. Mooi, and T. M. Gosliner. 1999. Phylogenetic taxonomy and the status of Allonautilus Ward and Saunders, 1997. Journal of Paleontology 73:1214-1217.

Andrew said...

In fact Peter Ward
has had second thoughts. It appears that all forms but N. stenomphalus, N. pompilius and N. scrobiculatus do not differ significantly in molecular trems, though it is obviuos that N. marcomphalus is distinct in shell characters, too much so to be anything but a good species. Allonautilus id dead. Nautilus only goes back 36 million or so years, to the late Eocene; however there is a late Triassic species that appears extremely close to N. scrobiculatus and is currently being investigated by Peter and myself as the earliest known species of the genus.
Andrew Grebneff
Dunedin
New Zealand