Cephalopods

from Wikipedia, the free encyclopedia
Cephalopods
Type material of the Ellesmeroceraten Robsonceras robsonense from the Lower Ordovician of Canada

Type material of Ellesmeroceraten Robsonceras robsonense from the Unterordovizium of Canada

Systematics
without rank: Tissue animals (Eumetazoa)
without rank: Bilateria
without rank: Primordial mouths (protostomia)
Over trunk : Lophotrochozoa (Lophotrochozoa)
Trunk : Molluscs (mollusca)
Subclass : Cephalopods
Scientific name
Palcephalopoda
Lehmann & Hillmer , 1980
Superordinates

The cephalopods (Palcephalopoda) are an extremely diverse and species-rich, but almost completely extinct group of cephalopods (Cephalopoda), to which the recent pearl boats (genera Nautilus and Allonautilus ) with five or six species belong. A total of 11,000 fossil species and 1,800 genera have been described. However, the group's monophyly is controversial, and it may even be a polyphyletic grouping that must then be eliminated from the cephalopod system. It is regarded as the sister group of the new cephalopods (Neocephalopoda). The division of the cephalopods into cephalopods (Palcephalopoda) and new cephalopods (Neocephalopoda) reflects the phylogenetics of today's cephalopods, in which the many species of the inner-shelled cuttlefish (Coleoidea ) are compared to the few species of the pearl boats (genera Nautilus and Allonautilus ). H. Represent sister groups. The inclusion of the many fossil groups in these two groups would completely break the concept of the pearl boats (genera Nautilus and Allonautilus ) and the squid (Coleoidea). For this reason, Lehmann & Hillmer (1980) proposed the division of cephalopods into Palcephalopoda (“ Nautilus / Allonautilus line” = cephalopods) and Neocephalopoda (“Coleoidea line” = new cephalopods), each including the fossil groups. In other text books the cephalopods are also called Nautiloidea i. w. S. designated. The group of cephalopods includes the oldest orders of cephalopods as well as groups that are characterized by an extremely yolk-rich early development comparable to the recent pearl boats.

characterization

The cephalopods (Palcephalopoda) are characterized by an extremely yolk-rich development compared to most new cephalopods (Neocephalopoda). The eggs are relatively very large and the development time in the egg takes a very long time (with today's pearl boat up to over a year). Ready-made miniature adults then hatch from the egg; in today's pearl boat with a housing diameter of up to 3 cm. The propagation strategy is designed for a few, very well developed offspring. In contrast, most new cephalopods have eggs that are small (a few millimeters) and usually take roughly a month to develop. The propagation strategy is designed for many offspring (several hundred), which are already fully developed (miniature adults), but are still quite small. Because the cephalopods usually preserve their individual development in the housing, the above reproduction strategy can also be demonstrated in the many fossil forms with appropriate shell preservation. However, due to its special way of life and many presumably derived features (e.g. four gills), today's pearl boat is not a good example of the blueprints and way of life of most of the extinct groups. Only the functionality of the housing (phragmocone) should also be transferable to the fossil cephalopods.

The shape of the housing is extremely varied and ranges from elongated (orthoconical), curved (cyrtoconical) to loosely rolled (gyrokon) and completely rolled-up shapes. There are even forms that are rolled up like a snail and with a special design of the living chamber. The first cephalopods appear in the Upper Cambrian with a high degree of diversity. However, the highest diversity is achieved in the Ordovician. After several crises in evolution, more and more groups gradually became extinct until only the five or six species of today's pearl boats remain.

Systematics

A total of around 50 groups of fossil cephalopods have been proposed for the ranking. However, many orders are ill-founded and cannot be recognized. The relationships between the individual groups are also often poorly established or not yet researched, and many authors therefore simply prefer a list of the generally recognized orders, although some large groups (e.g. Actinocerida, Endocerida) certainly developed independently from the basic group the cephalopods that Ellesmerocerida have gone through. The system is currently still very "in flux", and new regulations are constantly being proposed.

Systematics of the cephalopods (as a list of orders)

The order Orthocerida, which is assigned to the cephalopods or the large group Nautiloidea in most text books, must, according to recent studies, be assigned to the new cephalopods (Neocephalopoda) (Kröger).

Alternatively, the cephalopods could also be divided into four groups (Shevyrev).

Systematics of the cephalopods (with superordinates)

However, the affiliation of several orders (e.g. Bajkalocerida, Dissidocerida, Intejocerida, Discosorida) to one of these superordinate orders is still highly uncertain. The recently established superorder Astrovioidea Zhuravleva & Doguzhaeva, 2004 and their only order Palliocerida Marek, 1999 cannot be accepted because they are based on a completely wrong interpretation of the origin of intracameral deposits.

literature

  • Flower, Rousseau H. & Bernhard Kummel 1950: A classification of the Nautiloidea. Journal of Paleontology, 24: 604-616.
  • King, Andrew H. 1993: Mollusca: Cephalopoda (Nautiloidea). In: Benton, MJ (ed.): The fossil record 2, pp. 169–188, Chapman & Hall, London, Glasgow etc.
  • Kröger, Björn 2004: Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia. Acta Palaeontologica Polonica, 49: 57-74.
  • Shevyrev, AA 2006: The cephalopod macrosystem: A historical review, the present state of knowledge, and unsolved problems: 2. Classification of nautiloid cephalopods. Paleontological Journal, 40: 46-54.
  • Teichert, Curt & Raymond C. Moore 1964: Classification and Stratigraphic Distribution. In: Teichert, C., Kummel, B., Sweet, WC, Stenzel, HB, Furnish, WM, Glenister, BF, Erben, HK, Moore, RC & Zeller, DEN (eds.), Treatise on Invertebrate Paleontology, Part K, Mollusca 3, Cephalopoda - General Features Endoceratoidea - Actinoceratoidea - Nautiloidea - Bactritoidea, K94-K106, Geological Society of America & The University of Kansas Press, Boulder, Colorado.
  • Teichert, Curt 1986: Times of crises in the evolution of the Cephalopoda. Paläontologische Zeitschrift, 60: 227–243, Stuttgart.