| 词条 | Geragnostus |
| 释义 |
| fossil_range = {{Geological range|485|442|Ordovician (Tremadocian to Ashgill)}} | image = Geragnostus mediterraneus CRF.jpg | image_caption = Geragnostus mediterraneus, 6mm, St.Chinian Formation, France, Middle Ordovician | taxon = Geragnostus | authority = Howell, 1935 | synonyms = Geragnostella, Geratrinodus, Neptunagnostella | subdivision_ranks = Species | subdivision =
}} Geragnostus is a genus of very small agnostid trilobites whose fossils are found Ordovician-aged marine strata from Eurasia, North America and Argentina. Etymology
TaxonomyGeragnostus is very similar to Trinodus and may in future be included in that genus. In Trinodus the posterior lobe (M3) of the pygidial axis (or rhachis) is clearly shorter than the anterior and middle lobes (M1+M2) combined, while in Geragnostus it is equal or greater in length. Geragnostus is not closely related to Micragnostus, because its glabella is structurally different.[4]Species previously assigned to Geragnostus
Distribution
Unidentified species in the genus Geragnostus have among others been found in Colombia (Guaviare River, Meta).[7] DescriptionLike all Agnostida, Geragnostus is diminutive, with the headshield (or cephalon) and tailshield (or pygidium) of approximately the same size (or isopygous) and outline. Like all Agnostina, Geragnostus has only two thorax segments, where known. The cephalon is subquadrate in outline, and almost as long as wide. The highest point is the node in the centre of the centrally raised section of the cephalon (or glabella). The glabella is about {{frac|2|3}} times the length of the cephalon, and its sides are very gently tapered forward, with a broadly rounded front, and is well defined by shallow furrows and an abrupt change in exoskeletal slope. It is 2 times as long as wide. The elongated glabellar node is prominent. The rear of the glabella is straight. Just in front of the node, a depression curves forward- and outward. The basal lobes are wider than they are long and almost touch. The border is relatively narrow, gently convex, and defined by a wide, shallow border furrow, widest at the front. The genal angles are pointed without spines. The pygidial axis (or rhachis) is {{frac|1|2}}–{{frac|2|3}} times as long as the pygidium, almost parallel-sided, very slightly constricted at the middle lobe (M2), broadly rounded, and {{frac|1|1|3}}–{{frac|1|2|3}} times as long as wide. It has three distinct pairs of lobes, and is defined by shallow furrows. The forward lobe of the rhachis (M1) is a bit shorter and wider than the middle lobe (M2) and separated from it by a furrow (F1) which is directed outward and slightly backward from the dorsal furrow, and then curved strongly forward adaxially. The rear lobe (M3) is {{frac|1|1|3}}–{{frac|1|2|3}} longer than both others together, and the furrow is directed outward and slightly forward from the dorsal furrow. The middle lobe has a prominent tubercle, which extends backwards above the anterior portion of rear lobe. It is the highest point of the pygidium. The border is moderately wide, gently convex, defined by a shallow furrow, and widest at the rear corners which carry a pair of strong, backwardly directed spines.[4] References{{Portal|Paleontology|Paleozoic}}1. ^1 {{cite journal|last= Karim|first=T.S.|year= 2009|title= Ordovician trilobites from Iran|journal= Earth and Environmental Science Transactions of the Royal Society of Edinburgh|issue= 99|volume= 02|pages= 101–124|doi=10.1017/s1755691009007063 }} {{Taxonbar|from=Q1009594}}2. ^1 Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997 3. ^1 2 3 4 5 6 7 {{cite journal|last= Turvey|first= S.T.|year= 2005|title= Agnostid trilobites from the Arenig-Llanvirn of South China|journal= Transactions of the Royal Society of Edinburgh: Earth Sciences|volume= 95|issue= 3–4|pages= 527–542|doi=10.1017/s026359330000119x}} 4. ^1 2 {{cite journal|last= Ahlberg|first= P.|year= 1989|title= The type species of the Ordovician agnostid trilobite Geragnostus, Howell, 1935|journal= Paläontologische Zeitschrift|volume= 63|issue= 3–4|pages= 309–317|url= https://link.springer.com/content/pdf/10.1007%2FBF02989517.pdf#page-1|doi=10.1007/bf02989517}} 5. ^Geragnostus fabius at Fossilworks.org 6. ^Geragnostus longicollis at Fossilworks.org 7. ^Guaviare River Ordovician at Fossilworks.org 30 : Agnostoidea|Agnostida genera|Ordovician trilobites|Ordovician trilobites of Africa|Fossils of Morocco|Fezouata Formation fossils|Ordovician trilobites of Asia|Fossils of Turkey|Fossils of Tajikistan|Fossils of Uzbekistan|Fossils of China|Fossils of Malaysia|Fossils of Iran|Ordovician trilobites of Europe|Fossils of Sweden|Fossils of Norway|Fossils of France|Fossils of the Czech Republic|Fossils of the United Kingdom|Ordovician trilobites of North America|Paleozoic life of Newfoundland and Labrador|Paleozoic life of the Northwest Territories|Fossils of Canada|Fossils of the United States|Ordovician trilobites of South America|Ordovician Argentina|Fossils of Argentina|Ordovician Colombia|Fossils of Colombia|Fossil taxa described in 1935 |
| 随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。