File:Brachiopod geopetal structure in fossiliferous limestone (Lower Mercer Limestone, Middle Pennsylvanian; Rt. 16 roadcut near Trinway, Ohio, USA) (33213419236).jpg
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Geopetal structure inside a productid brachiopod in the Pennsylvanian of Ohio, USA. Brachiopods are sessile, benthic, filter-feeding, marine invertebrates. They first appear in Cambrian rocks and were abundant in Earth's oceans throughout the Paleozoic. They were also common in Mesozoic oceans, but are scarce in modern oceanic biotas. Brachiopods have two shells, called valves, that are usually calcareous (made of calcite - CaCO3 - calcium carbonate). Each shell of a brachiopod is bilaterally symmetrical, unlike each shell of a bivalve (clam). Geopetal structures are geologic features that indicate the original orientation of gravity-defined “up” and “down” at the time of rock formation. The fossil shown above is a productid brachiopod in fossiliferous limestone, as seen in cross-section on a crack surface. The brachiopod contains syndepositional and post-depositional material that infilled the shell after decay of soft tissues. The medium-gray material in the lower part of the shell is lime mud (sedimentary micrite). The grayish-white crystals in the rest of the shell are calcite. Note that the calcite crystals have not completely filled up the available empty space. So, which way was up? At the time of final deposition, the shell was oriented with the micrite at the bottom. The calcite crystals are partially filling what was originally water-filled empty space in the shell. Not enough lime mud entered the shell interior to fill it up, so the mud obviously accumulated in the “down” portions of the shell. The “top” portions of the shell were left empty, until diagenetic or post-diagenetic fluids that were rich in dissolved calcium carbonate precipitated the calcite crystals. Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Strophomenida, Productidae Stratigraphy: Lower Mercer Limestone, Pottsville Group, lower Atokan Stage, lower Middle Pennsylvanian Locality: roadcut along the northern side Rt. 16, southern margin of Irish Ridge, west of the Rt. 16-Rt. 60 intersection, northwest of the town of Trinway, northwestern Muskingum County, eastern Ohio, USA (40° 09’ 12.95” North latitude, 82° 02’ 43.27” West longitude) |
| Date | |
| Source | Brachiopod geopetal structure in fossiliferous limestone (Lower Mercer Limestone, Middle Pennsylvanian; Rt. 16 roadcut near Trinway, Ohio, USA) |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/33213419236. It was reviewed on 24 June 2017 by FlickreviewR and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
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| current | 17:35, 24 June 2017 | | 3,044 × 1,612 (3.87 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via Flickr2Commons |
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