Fossil of the Week

4/28/10 – Freshwater Clam

Freshwater pearl mussels are a large and varied group, today consisting of six families within the taxonomic order Unionoida. Collectively, they are sometimes called naiads or freshwater clams. They are important ecologically (filtering water, providing food for muskrats and raccoons) and economically (as a source of natural and cultured pearls, and pearl buttons and inlay), and are today one of the most threatened invertebrate groups on Earth, having been reduced severely by pollution, dams, and the introduction of non-native species such as zebra mussels. Freshwater pearl mussels have proliferated most especially here, in eastern North America, but approximately 70% of their original 300 species are either extinct or classified as endangered or threatened today.

Unionoida has a long fossil history, back to the Triassic Period, more than 200 million years ago. Here is one relatively new example. This paratype* of Diplodon liddlei, described by Katherine Van Winkle Palmer** in 1941, is from the Pliocene of Ecuador. This fossil is actually a conglomerate of many fossils still embedded in rock. There are several pieces and several more impressions of a high-spired snail (you can see three whorls of one good one just below center on the rock). The Diplodon specimen that we are discussing is the light-yellow crescent-shaped piece along the bottom edge. Only half of the shell is present, but you can clearly see the most important feature of the species (and in fact for its entire family, the Hyriidae) – the wavy radial ribs that form a pretty, complex pattern on the shell. It was collected by, donated to PRI by, and is named for Ralph Alexander Liddle (1896-1963), who coauthored the geological description of this material with Katherine Palmer. Liddle was a geologist from Fort Worth, Texas, who worked for Standard Oil Company.*** He received his AB degree from Cornell University in 1918, having studied under Gilbert Harris. Liddle is perhaps best known for his 1928 book The Geology of Venezuela and Trinidad (J. P. MacGowan, Forth Worth), which was dedicated to Harris.

*The holotype and three additional paratype specimens are also in PRI’s collection. See 10/14/09 - Atrypa aperanta Crickmay for a definition of paratype specimens.

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

***See 4/14/10 – Scallop Bed and 3/10/10 – The Hodson Collection for more about paleontologists working for commercial oil companies.

Text by Paula Mikkelsen