Tentaculita

Tentaculitiden from the Devonian of Maryland (USA).

• Worldwide

The Tentakuliten ( Tentaculitoidea, also Cricoconarida ) are an extinct group of small, living in the sea invertebrates with kalkschaligem housing. Your fossil record extends from the middle Cambrian to Upper Devonian (possibly to the early Permian). In the zoological hierarchy, they are provided as a class with three orders ( Tentaculitida, Homoctenida and Dacryoconarida ) to the molluscs ( Mollusca ).

Description of the morphology

The fossils of Tentakuliten among the microfossils; the housing usually only reach lengths of a few millimeters. You are spitzkonisch and curved in only a few groups or rolled up spirally. The outer side of the shell is sculpted: Often it has rings transversely to the longitudinal direction of the cone on which may be up to 10 centimeters long in the major species but usually lies in the millimeter range. The enclosures can be seen two stages: The first is the juvenile stage, for some species, this part is divided into chambers. Delineated clearly follows the adult stage, the living chamber of the housing, in which lived the adult animal. This part is not separated by chambers and finally opens with the mouth outwards. An operculum, that occurs as a cap he wore at the recent worm has not yet been found. At the sharp end, the apex, one usually sees a drop-shaped extension that is also sometimes referred to as Embryonalkammer, here the growth of the animal began.

The name of the taxon based, moreover, on a scientific error. The first Tentakuliten described in the literature were randomly embedded in a very fossil-rich rocks close to trilobite head shields. This prompted Ernst Friedrich von Schlotheim to the mistaken assumption that it is - would act to " feelers " (today sensors or antennas) of trilobites, similar to the Antennten of insects and other arthropods - also due to the ringed structure. The term "antenna " but was at the beginning of the 19th century not common - they were then called " tentacles ", a term which is reserved in today's zoological terminology tentacles. This change in meaning has taken care in the sequence too much confusion, as he had apparently led various workers to look after Tentakuliten in " soft-bodied " with preserved tentacles. This is still not yet succeeded, although, in view of the widespread use of tentacles in various marine animal groups quite possible that Tentakuliten actually also had tentacles. A systematic significance would such a finding does not, however, it would be most likely an example of a similarity ( or convergence).

Palaeoecology and geological significance

Tentakuliten 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 reached as such in Devon a massive and worldwide distribution. In Devon the Tentakuliten had their development highlight: During this period they performed, including particularly the order Dacryoconarida in some marine sedimentary rocks on rock-forming. Thus, the so-called Tentaculitenschiefer For example, in the " Rhenish facies " formed.

The Tentakuliten can be very well used for biostratigraphy, ie as index fossils because of their rapid evolutionary change in the course of the Devonian. Also for the determination of flow directions of water in individual geological regions they can be used, as their housing were often moved by currents and deposited adjusted according to their size.

System

The Tentaculitoidea are usually assigned to the molluscs as a class, but placed by some authors in the vicinity of the wing screw ( pteropods ) to the class of snails. For the systematic subdivision of the sculpturing of the housing and the shape of the apex is important.

They are usually divided into three orders:

• Tentaculitida ( ie Tentakuliten sense, early Cambrian / Ordovician to Upper Devonian )
• Homoctenida (Middle Devonian )
• Dacryoconarida ( Silurian - Devonian)

The first order, the Tentaculitida are characterized by a relatively thick shell. All representatives of this order lived benthic. There are about 30 known species. The inside of the case is smooth, while representatives of the other Tentakuliten - orders the pattern of cross rings, which determines the sculpture outside the housing, and on the inside there. Tentaculites is a genus with one of the highest survival time of Tentakuliten otherwise occur only over short periods of time, they can be found from the Ordovician to the Devonian.

Of the Homoctenida several classes from the Lower to the Upper Devonian are known. These include the genera Homoctenus ( Lower to Upper Devonian ) and Polycylindrites ( Upper Devonian ).

The Dacryoconarida have a thin skin and are usually smaller than the Tentaculitida. The case of this order are never chambered and the Apex is bubble -shaped. They came from the Silurian to Devonian before and probably lived planktonic what is at the light shell is recognizable among others. They went first to the Silurian. Their heyday was the Devonian, about 20 species are known. These include, for example, occurring from the Devonian to the Upper Devonian Nowakia and the counting of the early Dacryoconarida genus Styliolina. Characteristic of the Styliolina is a three-layered calcareous shell, which is externally and internally prismatic structured. The middle layer is formed by way of location. This genus lived from the Upper Silurian to the Upper Devonian. It is still controversial whether the Dacryoconarida directly descended from the Tentaculitida or whether it is a convergent development.

On the question of extinction

In one of the big five mass extinctions of Earth's history, died during the Famenniums, the uppermost stage of the Upper Devonian, a large part of the marine faunas from. The mass extinction also succumbed to almost all groups of Tentakuliten. Until recently it was believed that the class went out Tentaculitoidea definitive nor in the upper Devonian. In 2000 described Shuji Niko fossil finds alleged Tentakuliten from Japanese limestones for which a carbon - to - Perm age has been demonstrated. These findings suggest that the group of Tentakuliten even survived the end of the Devonian by several 10 million years.