Mark Wilson October 16th, 2011
OK, it’s true: our Fossil of the Week is not actually a fossil. (The “Recent” in the title was a clue.) I bought this shell at the Wayne County Fair and it was so beautiful it just had to make the blog. (I paid $4 for it, which I think was quite a deal.)
What we have above is Xenophora pallidula (Reeve, 1842), commonly known as the Pallid Carrier Shell. It is a remarkable gastropod (snail) that ornaments its shell with “foreign” objects, usually other shells. (The genus name Xenophora means “foreign-bearing”.) They provide a nice sampling of the shelly debris surrounding their seafloor home.
The snail cements the items to the periphery of its shell as it grows, embedding the objects with its mantle into its aragonite. It selects dead shells and carefully rotates them with its foot and proboscis so that the concave side is upwards and the smaller end is attached. Attached snail shells thus have the aperture facing up, and clams have the inner side upwards as well. It takes almost two hours for a single object to be added to the shell, and up to 10 hours for the xenophorid snail to be confident enough to resume its normal life.
Why do xenophorids decorate their shells in this way? Apparently it is a kind of camouflage on a gravelly substrate. The long shells at the periphery of the shell also lift the shell above the substrate so that the snail’s body can extend inside a protective cage. The xenophorids can then peacefully feed on algae, diatoms and foraminiferans on the sediment. A curious habit they have which is rare among invertebrates: they dig holes in the sediment and bury their feces!
The genus Xenophora was named and described by the natural historian Johann Gotthelf Fischer von Waldheim (1771-1853). He was a German who specialized in marine invertebrates, insects, and fossils. von Waldheim studied under the famous Georges Cuvier in Paris, had a professorship in Germany, and then moved to Moscow in 1804 to become Director of the Natural History Museum at the University of Moscow. His work in Russia included the description of many new fossils, so we ultimately come back to paleontology!
Kreipl, K. and Alf, A. 1999. Recent Xenophoridae. ConchBooks: Hackenheim, Germany.
Ponder, W.F. 1983. A revision of the recent Xenophoridae of the world and of the Australian fossil species (Mollusca: Gastropoda). Memoirs of the Australian Museum 17: 1-126.