Describing Half a Shell

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TOP: Sthenorytis turbina (Dall, 1908)  [+ Epitonium (Sthenorhytis) turbinum, toroense Dall, 1912]  [Epitoniidae] dredged off Santa Cruz Island, Galapagos in 200-300 meters of water.  Size: 41mm height X 41mm diameter.  (Note the spelling of the genus Sthenorytis Conrad, 1862 and its spelling in the original description as subgenus Sthenorhytis.) BOTTOM: The Holotype of Sthenorytis turbina, a fragment of a whole intact shell.

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By Richard L. Goldberg

 

Though thought to be endemic to the Galapagos Islands, the known range for the rare and beautiful Sthenorytis turbina is much wider in the Eastern Pacific.  A specimen in the Hancock Foundation collection was dredged sometime between 1931 and 1941 off Nolasco Island, Mexico half way into the Gulf of California, 2450 miles (3885 km.) north of the type locality. Two other specimens in the Hancock Foundation collection are from off Cabo San Lucas, Mexico (DuShane, 1965). 

 

William Healy Dall originally described this remarkable and unmistakable species from a fragment collected in 1904 that consisted of only the bottom half of the shell and lacked the long costae or fronds (Holotype figure bottom left and photograph bottom right).  The specimen was dredged by the U.S. Fish Commission steamer U.S.S. Albatross in the Galapagos Islands, 4 miles southeast of Hood (Española) Island in 300 meters of water on a substrate of broken shells.  Interestingly, the data includes a temperature reading of 48° F at this extreme depth, water that would seem rather chilly for a tropical molluscan species.                                                           

 

Based on the fragment he described Dall believed that S. turbinum was most closely related to a fossil species he described in 1912, Epitonium stearnsii from the Pliocene of San Diego, California. More than 60 years later in her book Seashells of Tropical West America Myra Keen, who most likely studied better quality specimens of S. turbina, stated that it was most closely related to the Pliocene fossil Epitonium toroense from Panama.  Keen, in fact, was spot on; so much so, that E. toroense is now considered a synonym for S. turbina.

Given the fact that he only had half of a very worn shell missing the upper whorls from which to formulate his description Dall was remarkably accurate in describing Sthenorytis turbina.  © worldwideconchology.com (This article and illustration may not be copied or reproduced in full or in part without the express written consent of the author).