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jdhooker

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Hey all! I'm new to the forum so I thought I'd introduce myself with a find! I found this one while searching for artifacts in my historical geology class. 

The rock was found beside a dock located on a river right off the ocean (wilmington, nc). The location this rock was found in had several varieties of rocks including clay, siltstone, and scorria. This rock appears to be chert. It has glassy cleavage and banding along its side. It also reacts slightly with HCl. The small bore holes on the top surface come in pairs. My guess is that they were formed by Polychaeda? I am really having trouble identifying the marking on the front, however. They appear to look almost like tire tracks with several ridges along the edges is them. Can anybody help me ID this one? 

IMG_20170831_231724.jpg

Here is a view from the side. 

IMG_20170831_231933.jpg

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It looks to be a shard of a weathered bivalve. As bivalve shells are calcite, it would explain the reaction with HCl.

Regards, Jason

 

"Trilobites survived for a total of three hundred million years, almost the whole duration of the Palaeozoic era: who are we johnny-come-latelies to label them as either ‘primitive’ or ‘unsuccessful’? Men have so far survived half a per cent as long."  - Richard Fortey, Trilobite: Eyewitness to Evolution.

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Oh cool! I had origionally suspected that this was a shell of sorts but the thickness of it really threw me off. Also, the banding on the side threw me off (really hard to see in the picture but there are uniform layers of banding along the broken edges with slightly different colors in each layer.)

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Welcome to the forum from Central Maryland!

“...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved.” ~ Charles Darwin

Happy hunting,

Mason

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18 hours ago, jdhooker said:

The small bore holes on the top surface come in pairs. My guess is that they were formed by Polychaeda?

You are correct. The borings visible on the shell surface are made by polychaete worms.

 


" Bromley & D’Alessandro (1983) discussed the taxonomy of Caulostrepsis taeniola. This trace fossil is produced mainly by spionid polychaetes of the genus Polydora Bosc (Boekschoten 1966), particularly Polydora ciliata (Johnston), which is common on mid-latitude Atlantic coasts (Radwan´ ski 1969, and references therein), mostly no deeper than 25m (Boekschoten 1966). This polychaete can live in loose sand, form tubes from mud, or can bore chemically and mechanically in mollusc shells, limestones, mudstones, peat and wood, and use sand-sized particles to line its borings and build low chimneys extending from the limb outlets above the substrate surface (Douville´ 1908; So¨ derstro¨ m 1923; Prell 1926; Hempel 1957; Dorsett 1961; Boekschoten 1966; Blake & Evans 1973). Polydora ciliata feeds on fine detritus, small animals and diatoms (e.g. Daro & Polok 1973). It also lives in brackish water, for example in the Baltic Sea (D’Andrea et al. 1996) and the Black Sea (Murina 1997). The eunicid polychaete Lysidice ninetta Audouin & Milne-Edwards can also produce this trace fossil (Bromley 1978, 2004). This polychaete occurs in the Atlantic and Indo-Pacific, but its occurrence in Svalbard waters is unknown.
Caulostrepsis occurs mostly in infralittoral Plio- Pleistocene and recent Mediterranean environments (Bromley & D’Alessandro 1990). Ekdale et al. (1984: p. 127) placed the acme of the bathymetric range of Caulostrepsis in the lower intertidal and subtidal zones, decreasing in abundance on the shelf and with very rare occurrences on the continental slope and the abyssal zone. Caulostrepsis ranges from Devonian to Recent (Clarke 1908; Bromley 2004). " - as it was stated in Hanken et al, 2012

 

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