Wednesday, October 20, 2010

Fossil of the Week


10/20/10 – Boring Bivalve

This week’s Fossil of the Week is another specimen from our type collection. It’s a boring bivalve, not in the sense of being uninteresting, but in the sense of a clam that burrows itself into solid rock. This is Lithophaga gainesensis [the species name was misspelled on the hand-written label in the photo], a member of the mussel family, Mytilidae. It was, as the label indicates, originally called Lithodomus, but that name is now (for technical nomenclatural reasons) considered a synonym of the genus Lithophaga, which still has members alive today. Lithophaga gainesensis was named from the Middle Eocene (Mcbean Formation), of Ft. Gaines, Georgia.

Lithophaga species are called Date Mussels because of their smooth cylindrical body form. The name literally means “rock eater,” an epithet that is partly true. The soft tissue lining the shells in life contains a gland that secretes an acid-like mucus that dissolves calcium carbonate or limestone (the soft “rock” that most date mussels inhabit). The Date Mussels do not literally consume the rock for nutrition, as the name might imply, but the rock is “eaten away” resulting in a smooth, blind-ended tube within which the mussel can safely protrude its siphons from the open end to access seawater for oxygen and food particles. The empty burrows of date mussels and other boring bivalves in the fossil record are given the trace fossil name Gastrochaenolites.

This species was named and described by PRI’s founder, Gilbert Harris, in the fourth issue of Bulletins of American Paleontology in 1896. The title of the monograph was simply “The Midway Stage,” referring to the basal-most subdivision of the Eocene Epoch in the southern United States. Ft. Gaines, Georgia, is on the Chattahoochee River at the border of Georgia and Alabama. Harris wrote, “[the deposits here] consist of light gray or yellowish impure limestone…frequently eroded in a peculiar rough and irregular manner. … Fossils are of frequent occurrence in this formation, but here… they consist only of impressions, moulds and casts … the shelly matter of the specimens inclosed [sic] is entirely removed, and hence one is left to work out the details of the various specific forms by means of gutta-percha moulds.” [“Gutta-percha” is an interesting phrase here – it’s a rubbery latex-like material produced by a tropical tree, and was one of the first natural plastics to be used by humans; it is waterproof, and has been used to insulate underwater cables or in dentistry for temporary fillings.] Indeed the syntype* here (PRI 58 – a really early catalog number!) is a shell-less internal mould (with its siphon end pointing toward the right).

*See Fossil of the Week 11/11/09 - Bellerophon calcifer for a definition of "syntype."

Text by Paula Mikkelsen

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