Geniculograptus pygmaeus fossil graptolites (Kope Formation, Upper Ordovician; Kenton County, Kentucky, USA) : 無料・フリー素材/写真
Geniculograptus pygmaeus fossil graptolites (Kope Formation, Upper Ordovician; Kenton County, Kentucky, USA) / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Geniculograptus pygmaeus (Ruedemann, 1925) - fossil graptolites from the Ordovician of Kentucky, USA. (Rich Fuchs collection)Graptolites are an extinct group of hemichordates that are most commonly preserved as carbonized compressions on shale bedding planes. They are typically not glamorous fossils, but they are critically important guide fossils and are widely used in biostratigraphy and for international correlation. The most abundant group of graptolites in the fossil record is the graptoloids (seen above). Graptoloid graptolites typically resemble small hacksaw blades. Each “tooth” of the hacksaw blades housed a tentaculate, filter-feeding organism. The entire hacksaw blade is the graptolite skeleton, known as a rhabdosome - a nonmineralized colonial skeleton. Most graptolites were planktonic.The second most abundant group of graptolites is the dendroids. Dendroid graptolites attached to substrates and had colonial skeletons (rhabdosomes) that are generally broadly branching (conical to fan-shaped to shrub-like to flat spirals).Other graptolite groups are very rare: the crustoids, tuboids, camaroids, and stolonoids.The fossils shown above are carbonized compressions of Geniculograptus pygmaeus from the Upper Ordovician Cincinnatian Series of the Cincinnati, Ohio area. Geniculograptus is the most common graptoloid graptolite found in Cincinnatian rocks.--------------The following is a synthesis of information about Geniculograptus that was provided by Rich Fuchs during presentations at Dry Dredgers meetings in November 1997 and May 2010:Geniculograptus typicalis typicalis - the first identified Cincinnatian graptolite, the most common, and most well studied Cincinnatian graptolite is the graptoloid Geniculograptus typicalis typicalis (there are other subspecies as well). It’s common in the Southgate Member of the Kope Formation; it's upper range is not really known, but it goes down into the Middle Ordovician. James Hall’s 1800s list of New York graptolites listed Climacograptus typicalis but no description accompanied the name. Hall named it and figured it. This was the first reference to that graptolite. Rudolf Ruedemann’s description gave species credit to Hall, though he never originally described Climacograptus typicalis. Ruedemann claims he described it in 1908. Joseph James apparently described it in 1892. This species is biserial. John Riva recognized that typicalis isn't a species of Climacograptus. Charles Mitchell in 1987, using sicular morphology, put typicalis in a new genus Geniculograptus - it is no longer in Climacograptus. Also in 1987, Riva proposed the new genus name Uticagraptus for typicalis. In northern Kentucky, troughs/trough fillings of Geniculograptus have been found with the graptolites all lined up (Ron Fine found these). Geniculograptus typicalis typicalis is 2 to 5 cm long (3/4 inches to 2 inches). Its thecae have a slightly tapering square appearance. These alternate on either side of the virgula. Geniculograptus typicalis magnificus is a big biserial form in the Maysville Stage (middle Cincinnatian Series, Upper Ordovician). typicalis typicalis has lengths up to 3.5 cm (1 3/8"), but typicalis magnificus can get to 7 cm long (almost 3"). It looks like it has smaller thecae, but they are just spread out more, because the rhabdosome is wider between thecae. Geniculograptus typicalis posterus is a smaller-than-magnificus form in the Fairview Formation (lower Maysvillian Stage, middle Cincinnatian Series, Upper Ordovician). Geniculograptus pygmaeus is also a now-well studied form. It has been tossed around from time to time, as was typicalis. It can be confused with typicalis, but it’s a lot smaller than typicalis. pygmaeus is younger - it occurs in the upper Kope Formation and the lower Fairview Formation. pygmaeus and typicalis do overlap, but pygmaeus is younger & typicalis is older. Geniculograptus pygmaeus is usually <1 cm long and ~1 mm or so wide (small & narrow). Geniculograptus typicalis is ~1.25 inches (2 to 3 cm long). Geniculograptus pygmaeus has square thecae that taper more than in Geniculograptus typicalis. An intermediate form between Geniculograptus typicalis typicalis and Geniculograptus pygmaeus is known - it is found in the top of the Southgate Member of the Kope Formation in Kentucky. This species is not named yet, apparently - refer to it as Geniculograptus sp. It is longer and skinnier than pygmaeus, but still smaller than typicalis.Classification: Animalia, Hemichordata, Graptolithina, Graptoloidea, OrthograptidaeClassification: Kope Formation, Edenian Stage, lower Cincinnatian Series, Upper OrdovicianLocality: Kenton County, Kentucky, USA |
| 撮影日 | 2015-05-02 10:56:43 |
| 撮影者 | James St. John |
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