Thursday, November 18, 2010
Fossil of the Week
11/18/10 – Ultraviolet Cone
This week’s fossil is a cone snail from the Plio-Pleistocene of Florida (5.3 to 1.8 million years ago). It is shown here under ultraviolet light, which reveals the original color pattern of the bleached shell that looks chalky white under normal light. The PRI collections are particularly rich in shells such as this from the Pliocene and Pleistocene epochs of the Gulf and Atlantic Coastal Plains.
There are approximately 500 living species of cone snails recognized today. Another 1,000 fossil species have also been named, dating back to the Eocene (approx. 55 million years ago). They are fascinating animals – active predators that capture prey by stinging them with a poisoned harpoon-like tooth before engulfing them for internal digestion. Prey ranges from worms to fish, and those cones that specialize on fish use very powerful neurotoxins (called conotoxins – needed to quickly subdue a fish), so powerful that they can be deadly to humans too. Although all cone snails can sting, the really dangerous ones are restricted to the Indo-Pacific region of the globe.
Cone snails are also among the most beautiful of gastropods, and their color patterns are varied and intricate. These color patterns make them popular with shell collectors and are important in identifying cone species. Yet cone snails have poor eyesight themselves, and spend most of their days hiding under rocks or below the surface of the sand (called “infaunal”). Some also hide their color patterns under a thick, dark organic layer called periostracum (note: all snails have periostracum – critical in producing the shell – but it is thin and nonpersistent in many species).
So of what use are these colors? That’s a difficult question to answer. Some researchers have proposed that the patterns are the result of the excretion of waste products, stored in the shell – but there are no proven examples to support this conclusion. Another explanation says that pigment strengthens the shell, so the color patterns might serve a structural function. Color, aligned as it often is with spiral or axial sculpture, can strengthen the shell against crushing predators such as crabs. Thicker shells might serve the same purpose, but color might be more energy efficient to produce. The same explanation is often used to explain the colorful interiors of clam shells.
This image appeared on the cover of American Paleontologist, May 2003.
Text by Paula Mikkelsen