When you think of a mollusk, you probably have something shelled, slimy, and possibly stalk-eyed in mind. But mollusks include creatures that are none of these things.
In fact, there are mollusks that are wormy, be-spined, and eyeless. They are called aplacophorans, and scientists have long puzzled over their place in the mollusk family tree.
For molluks, shells are practically de rigeur: snails, conchs, and chitons all sport shells with varying amounts of bling (the bling kings may be these guys). But aplacophorans lack shells secreted by a mantle like other mollusks. Instead, they possess calcareous spines or spicules that give them a bristly appearance, like this. Others are more colorful, or more sparsely spined.
How do we know they are mollusks, then? They often have two major features unique to the group. Some have a distinctive food-harvesting structure called a radula that only mollusks make (if you've ever watched an aquarium snail scour algae from the inside of an aquarium, you've seen a radula in action). Some possess characteristic molluskan gills called ctenidia. But -- no shell.
So are aplacophorans super-primitive molluscs that first appeared hundreds of millions of years ago and never had shells? Or are they descended from shelled ancestors who for some reason or another, decided that shells were SO 50 million years ago?
But Scientists reported in Nature in October the discovery of a 425 million-year-old fossil in the United Kingdom that seems to indicate that it's the latter. Kulindroplax perissokomos has the worm-like body and be-spined appearance of an aplacophoran, but valves closely resembling those of the related shield-shaped mollusk called a chiton (albeit it has one less valve than the chiton standard-issue eight). Its discovery supports previous molecular evidence pointing in this evolutionary direction.
Here is a scanned reconstruction of the worm-like fossil they found and its seven shelled valves:
And here are the valves of a modern chiton:
The fossil also clearly showed signs of tiny spicules and a cuticle -- a feature that aplacophorans possess today in the form of a chitinous covering. Chitin, for those of you keeping track, is also the protein that constitutes mollusk radulas, squid and octopus beaks, fungal cell walls, and the armor of crustaceans and insects.
As for the spicules, aplacophorans aren't the only modern mollusks to bear them. Chitons, too, often possess mineral spines or ornaments on the fleshy girdle ringing their valves.
Kulindroplax thus supports the hypothesis that chitons and aplacophorans share a common ancestor, given the similarity of their spicules and valves, and are together ancestral to the rest of the modern mollusks. In other words, they're among the oldest extant members of the group.
There are two major groups of modern aplocophorans. The solenogasters eat or suck the juices from corals, comb jellies, and other cnidarians. The caudofoveates burrow in sediment. Incredibly, the evidence seems to imply the two aplacophoran groups lost their shells separately -- and as recently as 40 million years ago.
Far from being primitive, aplacophorans seem to be the recently evolved, highly modified -- and only recently de-shelled -- descendants of ancient armored mollusks.