Fossil of the Week

4/21/10 – Planktonic Snail

Typical snails, at least in the sea, are usually thought of as rather thick-shelled and lumbering, crawling slowly across the seafloor, grazing on algae or the occasional worm. This snail is absolutely none of that! This is a “heteropod” snail from the Eocene – it was (like its close relatives today) entirely planktonic (drifting mid-water in the ocean currents throughout its life, never settling on the bottom), with an extremely fragile, light-weight shell and a body modified for catching the currents. These and other holoplanktonic snails are sometimes called Sea Butterflies, referring to their wing-like appendages that flap in delicate swimming motions. Notice also how tiny the shell is – only about a half-centimeter across (that’s approximately 3/16 of an inch) – keeping it again very lightweight. In life, the shell and animal were probably almost transparent, as are many members of the zooplankton, to avoid being seen by visual predators such as fish or squid. Assuming that it lived like its congeners today, it was itself a veracious predator, with large eyes to spot likewise delicate prey in dim light and a huge snout equipped with a ribbon of chitinous fang-like teeth. That’s another reason to be transparent – to avoid being seen as one approaches its prey! Good images and descriptions of the remarkable image-forming eyes and sharp teeth of this and other heteropods are found on a Tree of Life webpage (http://tolweb.org/Carinarioidea).

This is Atlanta eocenica, named by PRI’s second director, Katherine Van Winkle Palmer*, in 1937 in her massive, 730-page monograph, “The Claibornian Scaphopoda, Gastropoda and Dibranchiate Cephalopoda of the Southern United States.” It is the holotype** (PRI 3383) from the Lower Claiborne Formation (Eocene) at Moseley’s Ferry, Texas. At the time of its description, it was the only Atlanta species known earlier than the Miocene (therefore the oldest Atlanta in the fossil record), and was the only one known with sculptured early whorls; the significance of this last character is not really known. Most species of Atlanta today are cosmopolitan in tropical and subtropical seas, that is, they are pretty much found worldwide in warmer waters. The Tree of Life website has a very complete description, with beautiful images, of all of the living species of Atlanta (see http://tolweb.org/Atlanta/28752), presented by one of the world’s experts on this group, Dr. Roger Seapy at California State University, Fullerton.

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

**See Fossil of the Week 8/19/09 - Cerithium gainesensis for a definition of "holotype."

Text by Paula Mikkelsen