File:Agnostoid trilobite packstone (Alum Shale Formation, Upper Cambrian; southern Sweden) 3.jpg
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English: Agnostoid trilobites in limestone from the Cambrian of Sweden. (field of view ~7.7 centimeters across)
Trilobites are extinct marine arthropods. They first appear in Lower Cambrian rocks and the entire group went extinct at the end of the Permian. Trilobites had a calcitic exoskeleton and nonmineralizing parts underneath (legs, gills, gut, etc.). The calcite skeleton is most commonly preserved in the fossil record, although soft-part preservation is known in some trilobites (Ex: Burgess Shale and Hunsruck Slate). Trilobites had a head (cephalon), a body of many segments (thorax), and a tail (pygidium). Molts and carcasses usually fell apart quickly - most trilobite fossils are isolated parts of the head (cranidium and free cheeks), individual thoracic segments, or isolated pygidia. The name "trilobite" was introduced in 1771 by Johann Ernst Immanuel Walch and refers to the tripartite division of the trilobite body - it has a central axial lobe that runs longitudinally from the head to the tail, plus two side lobes (pleural lobes). Well-preserved trilobites occur in Sweden’s Alum Shale Formation (Middle to Upper Cambrian). The limestone beds in this unit are often trilobite packstones. Seen here is one such fossiliferous limestone rich in disarticulated agnostoid sclerites. The limestones are slightly petroliferous and release a distinctive oily smell when broken. Early Swedish workers called these rocks “stinkstones”. Agnostoids differ from "ordinary" trilobites (= polymeroids) by having the following characeristics: two-segmented thorax; blind (no eyes); isopygous (the heads & tails are equal-sized). Agnostoids are usually quite small. Many modern arthropod phylogenetic studies conclude that agnostoids should be excluded from the trilobites. Classification: Animalia, Arthropoda, Trilobita, Agnostida Stratigraphy: Alum Shale Formation, Upper Cambrian Locality: locality 1 on 29 August 1998 of the IV International Subcommission on Cambrian Stratigraphy Field Conference, southern Sweden |
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| Source | https://www.flickr.com/photos/47445767@N05/49053039822/ |
| 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@N05/49053039822. It was reviewed on 13 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 01:54, 13 October 2020 | | 3,795 × 2,726 (6.24 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/49053039822/ with UploadWizard |
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