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Homotelus bromidensis fossil trilobites (Bromide Formation, Middle Ordovician; Criner Hills, southeastern Carter County, southern Oklahoma, USA) 3 | by James St. John
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Homotelus bromidensis fossil trilobites (Bromide Formation, Middle Ordovician; Criner Hills, southeastern Carter County, southern Oklahoma, USA) 3

Homotelus bromidensis Esker, 1964 fossil trilobites in fossiliferous limestone from the Ordovician of Oklahoma, USA (public display, Nebraska State Museum of Natural History, Lincoln, Nebraska, USA).

 

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).

 

Oklahoma's Bromide Formation has produced some amazing concentrations of complete asaphid trilobites. This is a slab of complete Homotelus bromidensis trllobites, preserved in a fossiliferous limestone. Densities have been measured up to 170 individuals per square meter. At the Dunn Quarry, five Homotelus horizons are present that appear relatively laterally consistent. Convex-up exoskeletons dominate these surfaces. The exoskeletons have a narrow size range (all are holaspids - adults) and do not show a preferred directional orientation, so there's been little to no hydrodynamic movement. A very small number of specimens are enrolled.

 

Interpretations of these monospecific assemblages range from mating congregations (as seen in modern horseshoe crabs) to mass mortality surfaces to mass molting surfaces. Some clusters are likely carcasses. Other clusters have been interpreted as mixtures of molts and carcasses.

 

There's been disagreement over the generic assignment of this species. "bromidensis" has been variously assigned to Homotelus, Isotelus, Vogdesia, and Anataphrus.

 

Classification: Animalia, Arthropoda, Trilobita, Polymerida, Asaphidae

 

Stratigraphy: Pooleville Member, Bromide Formation, upper Simpson Group, lower Blackriveran Stage, lower Mohawkian Series, middle Middle Ordovician (sensu traditio) (= lower Upper Ordovician, sensu Gradstein et al., 2004)

 

Locality: Criner Hills, southwest of the city of Ardmore, southeastern Carter County, southern Oklahoma, USA

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Mostly synthesized from info. provided by:

 

Esker, G.C. 1964. New species of trilobites from the Bromide Formation (Pooleville Member) of Oklahoma. Oklahoma Geology Notes 24: 195-209.

 

Gradstein, F., J. Ogg & A. Smith. 2004. A Geologic Time Scale 2004. Cambridge, England. Cambridge University Press. 589 pp. 1 poster.

 

Ross, R.J. 1970. Ordovician brachiopods, trilobites, and stratigraphy in eastern and central Nevada. United States Geological Survey Professional Paper 639. 103 pp. 19 pls.

 

Danita Brandt (pers. comm.)

 

Talia Karim (pers. comm.)

 

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Taken on July 18, 2011