Wednesday, December 8, 2010

Fossil of the Week

12/8/10 – Bubble Snail with Borehole

This is a fossil “bubble snail,” so named because of its “involute” spire – the normally pointed tip of the snail shell is sunken into the top of the shell, so that all you see is the last whorl (the “body whorl”), creating a bubble-like shape with a dimple at the top. Bubble snails are members of several families of the gastropod order Cephalaspidea, which are very closely related to the shell-less sea slugs or nudibranchs. About 20 families of bubble snails are alive today. The word Cephalaspidea refers to the head shield – a shovel-shaped head that protects the opening of the shell and allows these snails to plow their way through soft sand.

This bubble snail is Abderospira leblanci, described by Katherine Palmer* in 1946 in the family Cylichnidae. It is from the Late Eocene Epoch (Jackson Group, Yazoo Formation) of Danville Landing, Louisiana. It is a paratype** specimen, described in Bulletins of American Paleontology no. 117, in a 560-page monograph entitled “The Mollusca of the Jackson Eocene of the Mississippi Embayment (Sabine River To the Alabama River).” The shell is only 4.5 mm in height (that’s 3/16ths of an inch).

The shell here is still filled with sand and tiny pebbles from the excavation, but another thing is really interesting. The round hole in the shell tells us how the snail died! This hole was created by another snail – probably a moon snail (family Naticidae) or a rock snail (family Muricidae) – that attacked, bored, and ate the bubble snail. The beveled sides of the hole reveal this kind predator, as opposed to other mollusks that drill (for instance, octopuses drill shells in this way, but the holes that they leave behind are straight-sided). The hole was made by a combination of acidic chemicals that softened the shell’s calcium carbonate and the teeth or radula of the predator that physically scraped it away. Once the hole was made, the predatory snail could squirt digestive enzymes into the bubble to turn it into a liquid lunch! So, traces like these left on fossils can often tell us a lot about the ecology of the animal, including how it lived and died.

*For more about Katherine Palmer, see Fossil of the Week 2/3/10 – A Rib-less Wentletrap.

**For a definition of paratype specimens, see Fossil of the Week 10/14/09 - Atrypa aperanta Crickmay.

Text by Paula Mikkelsen

No comments: