Monday, April 25, 2011

Beecher Trilobite Beds

My previous post of a lecture by Derek Briggs on Extraordinary Fossils mentioned the Beecher trilobite bed.  Since this is located in New York, and it's an interesting deposit, I thought I'd write more about this locality.

The Beecher bed is what paleontologists label a konservat-lagerstätte - a term from German meaning "conservation storage place." Lagerstätten are mother-lode fossil assemblages typically noted for exquisite soft-bodied preservation of organisms. This particular lagerstätte dates from the Late Ordovician Period (about 450 million years ago).

The Beecher bed is a 4-9 cm thick bed within the Frankfort Shale from a small quarry just outside of Rome in Oneinda County (43° 15.2' N, 75° 24.5' W).  Here's a photo of the quarry from the Brigg's Lab website.

The site was excavated between 1893 to 1895 by Charles Emerson Beecher, a paleontologist at Yale's Peabody Museum. Beecher died shortly thereafter and the location was lost and thought to be mined out until it was rediscovered in 1982.  The site has been periodically worked by paleontologists from Yale (and other places) since then for research.  Don't even think of collecting there - it's closed to the general public (I'd drive there tomorrow with shovel and pick if I could get in!).

The amazing thing about the Beecher bed fossils are the fact that they're pyritized - replaced by the golden iron sulfide mineral pyrite (FeS2) - and often show soft-tissure preservation.  Some 85% of the fossil specimens found are of one trilobite - Triarthus eatoni - although additional trilobites, graptolites, and a few other organisms are present as well.

Not only are the trilobites fossilized, but often antennae, appendages, gill structures, and even musculature is preserved in exceptional detail which is quite rare.

The neat images below are from Amherst College professor Whitey Hagadorn's research on Beecher bed trilobites.  On left is a pyritized Triarthus eatoni trilobite.  On right is an x-ray image of the same fossil showing antennae and appendages not visible on the surface of the specimen.  Go to the website above to see the image rotate around for a 3-D view!

Why are these trilobites so exceptionally well preserved?  They were entombed in turbidity currents - underwater flows of water and sediment that quickly buried and sealed them off from oxygen.  This prevented decomposition of their soft tissues.

Geochemical conditions were such that pore water in the sediments were enriched in iron and sulfur leading to replacement of the tissues by the mineral pyrite (FeS2) - the exact details of this process are still being argued about.

Why were there turbidity currents here?  Because during the Ordovician Period, a chain of volcanic islands (the Taconic island arc) was approaching and colliding with proto-North America.  Between was a deep trench and subduction zone where large earthquakes and would have periodically shaken the seafloor.

Here's what North America looked like during the Late Ordovician Period.  Notice that large parts of New York were under shallow sea water (the Franklin Shale was forming in this environment) and those landmasses to the east are the Taconic arc.

More recently, pyritized trilobites have been found in several other localities in the Ordovician Lorraine Group in Lewis County, NY.  Poke around, you might get lucky.

1 comment:

  1. stunning designs. there will be very interesting if they are on sale? And is it possible to buy them in Russia?