Merostomata

from Wikipedia, the free encyclopedia

The division of living beings into systematics is a continuous subject of research. Different systematic classifications exist side by side and one after the other. The taxon treated here has become obsolete due to new research or is not part of the group systematics presented in the German-language Wikipedia.

The Merostomata are a controversial taxon of the arachnids, which summarizes the Xiphosura or swordtails (with the recent family of horseshoe crabs ) and the extinct Eurypterida or sea scorpions (rarely also broad-fin fish). The scientific name is derived from ancient Greek meros , hip and stoma , mouth, opening, therefore very rarely referred to in German as " hip mouths ", it refers to the shape of the front limbs, the basal segments of which are used for food intake, while the outer (distal ) are used for locomotion. The name Merostomata was originally introduced for horseshoe crabs by James Dwight Dana in 1852 . Henry Woodward redefined it in 1866 so that it also included the sea scorpions (both groups were still considered to belong to the crustaceans at that time ). In the classical system the Merostomata are one of the two classes of the "real" arachnids or Euchelicerata, with the two orders Xiphosura and Eurypterida. Later the group of Chasmataspidida was separated from the Xiphosura and raised to an independent order. The relationship between these groups, and the monophyly of the Xiphosura and the Chasmataspidida, have long been disputed. Most systematics today assume that the sea scorpions are more closely related to the arachnids than to the swordtails. A taxon Merostomata would therefore not exist.

anatomy

All organisms summarized here have a body that consists of segments that appear to be grouped into two sections ( tagmata ), a front body (prosoma or cephalothorax) and an abdomen (opisthosoma), which can be divided into a different number of subsections and one at the end long tail spine ( telson ). The front section is covered from above (dorsally) by a uniform shield, which often reveals compound eyes on the upper side . It carries six pairs of extremities, of which the foremost pair is designed as scissor- bearing chelicerae , the rest are leg-like or the rear ones are shaped like swimming legs. There are plate-like gills on the inside of the legs as breathing organs. Special, button-like pedipalps are not recognizable. The first leg links, called the hips or coxa, have toothed appendages on the inside that serve as mouth parts when eating. Of great importance for the system is the presence or absence of a large, unpaired plate that covers the hips of different pairs of legs. This is called a metastoma. A metastoma is present in the Eurypterids and some Chasmataspidida. According to one hypothesis, it is homologous to the abdominal plate (sternum) of the arachnida. Some also see them as homologous to the Chilaria of the Xiphosura.

The segments of the opisthosoma can partially support extremities or be completely devoid of them. If there are extremity-bearing segments, these are always the anterior, the corresponding section of the abdomen is then called the mesosoma. The second, limb-free section is called the metasoma. Often the first six or seven abdominal segments have limbs, the first pair of which is often reduced to small plates called chilaria. The opisthosoma is always noticeably narrowed compared to the prosoma. Usually all segments of the opisthosoma are free and can move against each other, but some can be fused together in sections.

Way of life

Since all groups included in the Merostomata with the exception of four species of horseshoe crabs are extinct and only exist in fossil form, little is known about their way of life. It is assumed that all assigned groups were aquatic (aquatic). There are finds from both marine sediments and freshwater sediments. Some trace fossils leave the possibility open that some representatives were able to leave the water at least for short shore walks. Like all relatives, they were presumably predatory.

Phylogeny and Kinship

Traditionally, within the (Eu-) Cheliceraten two groups were distinguished in the rank of classes, the Merostomata with primarily water-living (aquatic) way of life and the Arachnida with primarily land-living (terrestrial) way of life. This relationship has been increasingly disputed since the 1990s. The common characteristics of the taxa grouped together to form the Merostomata are therefore only common trunk group characteristics ( Symplesiomorphien ), which often resulted from the common aquatic way of life (which was primary within the Chelicerata).

Since monophyly has been contested for a long time, a taxon Merostomata is hardly considered in the more recent literature. In a more recent study from 2014, a possible relationship between the Xiphosura sensu stricto, Eurypterida and Chasmataspidida was brought into play again as a possibility without formally recreating a taxon. Many newer authors use Merostomata again in the original meaning after Dana, therefore the name would be a synonym of Xiphosurida.

Literature and Sources

  • Jason A. Dunlop (2010): Geological history and phylogeny of Chelicerata. Arthropod Structure & Development 39: 124-142. doi: 10.1016 / j.asd.2010.01.003
  • PA Seldon & JA Dunlop (1998) Fossil taxa and relationships of chelicerates. Chapter 7 in Gregory D. Edgecombe (editor): Arthropod Fossils and Phylogeny. Columbia University Press, New York. ISBN 978-0-231-09654-6 .
  • Jason A. Dunlop & David Penney: Fossil Arachnids. Siri Scientific Press, Manchester 2012. ISBN 978-0-9567795-4-0 . Evolutionary relationships, p.18.
  • Jeffrey W. Schulz (2007): A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150: 221-265.

Web links

  • Merostomata . Lexicon of Biology, www.spektrum.de.
  • Chelicerata Free University of Berlin, Department of Geosciences.

Individual evidence

  1. ^ Theodor CH Cole: Dictionary of Biology / Dictionary of Biology: German / English, English / German. Springer Verlag, 4th edition 2014. ISBN 978-3-642-55328-8 , on page 128.
  2. ↑ Wolfgang Oschmann: Life of the prehistoric times: Basics of the general and special paleontology. Haupt Verlag, UTB Taschenbücher 4893, 2018. ISBN 978-3-8252-4893-2 .
  3. ^ Paul A. Selden & Derek J. Siveter (1987): The origin of the limuloids. Lethaia 20: 383-392.
  4. ↑ HE Gruner, M.Moritz, W. fertilizer: Textbook of special Zoology (founded by Alfred Kaestner), Volume 1 invertebrates, the fourth part of arthropods (without Insecta). Gustav Fischer Verlag Jena etc. 1993. ISBN 3-334-60404-7 .
  5. ^ Jason A. Dunlop (1997): Palaeozoic arachnids and their significance for arachnid phylogeny. Proceedings of the 16th European Colioquium on Arachnology: 65-82.
  6. ^ Russell J. Garwood. & Jason A. Dunlop (2014): Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders. PeerJ. 2: e641. doi: 10.7717 / peerj.641