Tentaculites
| Tentaculites | ||||||||||||
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Tentaculitis from the Devonian of Maryland (USA). |
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| Temporal occurrence | ||||||||||||
| Cambrian to Upper Devonian | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Tentaculitoidea | ||||||||||||
| Lyashenko , 1957 |
The tentaculites (Tentaculitoidea, also Cricoconarida) are an extinct group of small invertebrates living in the sea with a calcareous shell. Their fossil record ranges from the middle Cambrian to the upper Devonian (possibly to the early Permian ). In the zoological system , they are placed as a class with three orders ( Tentaculitida , Homoctenida and Dacryoconarida ) to the molluscs (Mollusca).
Description of the morphology
The fossils of the tentaculites are among the microfossils ; the housings usually only reach lengths of a few millimeters. They are pointedly conical and only in a few groups are curved or rolled up spirally. The outside of the bowl is sculptured: it often has rings across the longitudinal direction of the cone, which in the largest species can be up to 10 centimeters long, but is usually in the millimeter range. Two stages of development can be recognized on the casing: The first is the juvenile stage, in some genera this part is divided into chambers. The adult stage follows, the living chamber of the housing in which the adult animal lived, clearly delimited. This part is not separated by chambers and eventually opens outwards with the mouth. An operculum , i.e. a lid like the one found in recent snails , has not yet been found. At the pointed end, the apex , you can usually see a drop-shaped enlargement, which is sometimes also referred to as the embryonic chamber, this is where the growth of the animal began.
Incidentally, the name of the taxon is based on a scientific error. The first tentaculites described in the literature happened to be embedded in a very fossil-rich rock close to trilobite head shields. This led Ernst Friedrich von Schlotheim to the erroneous assumption that - also because of the ringed structure - it would be the "antennae horns" (today antennae or antennae) of the trilobites, similar to the antennae of insects and other arthropods . The term “antennae” was not yet common at the beginning of the 19th century - at that time they were called “tentacles”, a term that is reserved for tentacles in today's zoological terminology. This change in meaning has subsequently caused too much confusion, as it apparently prompted various editors to search for tentaculites in "soft tissue preservation" with preserved tentacles. This has not yet been achieved, but given the widespread use of tentacles in various marine animal groups, it is quite possible that tentaculites actually had tentacles. However, such a find would not have any systematic significance; it would most likely be an example of a similarity (or convergence ).
Palecology and geological importance
Tentaculites are found exclusively in marine sedimentary rocks. The Tentaculitida had a thicker shell and lived benthic , the Homocteniden and Dacryoconariden, however, were planktonic organisms and as such achieved a mass and worldwide distribution in the Devonian . In the Devonian, the tentaculites had their peak of development: During this period they appeared, especially the order Dacryoconarida, forming rocks in some marine sedimentary rocks. For example, the so-called tentaculite slate was formed in the "Rhenish Facies ".
Due to their rapid evolutionary change in the course of the Devonian, the tentaculites are very useful for biostratigraphy , i.e. H. as guide fossils . They can also be used to determine the direction of flow of the water in individual geological regions, since their housings were often moved by currents and deposited according to their size .
Systematics
The Tentaculitoidea be the molluscs mostly as a class assigned, but by some authors in the vicinity of the wing screw (Pteropoda) to the class of snails found. The sculpting of the housing and the shape of the apex are important for the systematic subdivision.
They are usually divided into three orders:
- Tentaculitida (tentaculites in the sense, early Cambrian / Ordovician to Upper Devonian)
- Homoctenida (Middle to Upper Devonian)
- Dacryoconarida ( Silurian to Devonian)
The first order, the tentaculitida , are characterized by a relatively thick shell. All representatives of this order lived benthonic . About 30 genera are known. The inside of the case is smooth, while in representatives of the other tentaculite orders the pattern of the transverse rings, which determines the sculpture of the case outside, also exists on the inside. Tentaculites is a genus with one of the longest survival times of the tentaculites, which otherwise only appear over short periods of time; they can be found from the Ordovician to the Devonian.
Several genera from the Lower to the Upper Devonian are known of the Homoctenida . These include u. a. the genera Homoctenus (Lower to Upper Devonian ) and Polycylindrites (Upper Devonian ).
The Dacryoconarida have a thin shell and are usually smaller than the Tentaculitida. The housing of this order are never chambered and the apex is bubble-shaped. They occurred from the Silurian to the Devonian and probably lived in planktonic form, which can be recognized by their light shell, among other things. They first appeared in the Silurian. Its heyday was the Devonian, around 20 genera are known. These include, for example, the Nowakia, which occurs from the Lower Devonian to the Upper Devonian, and the early Dacryoconarida genus Styliolina . A characteristic of the Styliolina is a three-layer lime shell, which is structured prismatically on the outside and inside. The middle layer is formed in layers. This genus lived from the Upper Silurian to the Upper Devonian. It is still controversial whether the Dacryoconarida descend directly from the Tentaculitida or whether it is a convergent development.
On the question of extinction
In one of the five great mass extinctions in the history of the earth , a large part of the marine fauna died out in the course of the Famennium , the top tier of the Upper Devonian. Almost all groups of tentaculites also succumbed to mass extinction. Until recently it was believed that the class Tentaculitoidea was definitely extinct in the Upper Devonian. In 2000 Shuji Niko described fossil finds of putative tentaculites from Japanese limestone, for which a Carbon - to Permian age is proven. These findings suggest that the group of tentaculites survived the end of the Devonian for tens of millions of years.
literature
- Niko, S. (2000): Youngest record of tentaculitoids: Hidagaienites new genus from near the Carboniferous-Permian boundary in central Japan - Journal of Paleontology 74 (3): pp. 381-385
- Schindler, E. (2005): Tentaculites: Fossils of Paleozoic Seas. - Nature and Museum 135 (7/8): pp. 182–183
- Bernhard Ziegler: Introduction to Paleobiology, Part 2, Special Paleontology, Protists, Sponges and Coelenterates, Mollusks ; Stuttgart: Schweizerbartsche Verlagbuchhandlung, 2004, ISBN 3-510-65036-0
- Ulrich Lehmann and Gero Hillmer: Invertebrates of prehistoric times: Guide to the systematic paleontology of invertebrates ; Spektrum Akademischer Verlag, 1997, ISBN 3-432-90654-4