Round mouths

from Wikipedia, the free encyclopedia
Round mouths
The mouth of the sea lamprey (Petromyzon marinus) attached to an aquarium pane.

The mouth of the sea lamprey ( Petromyzon marinus ) attached to an aquarium pane.

Systematics
without rank: Tissue animals (Eumetazoa)
without rank: Bilateria
Over trunk : Neumünder (Deuterostomia)
Trunk : Chordates (chordata)
Sub-stem : Vertebrates (vertebrata)
Superclass : Round mouths
Scientific name
Cyclostomata
Duméril , 1806

The round mouths (Cyclostomata, from Greek κύκλος kýklos "circle" + στόμα stóma "mouth") are a superclass of chordates (Chordata), in which the jawless vertebrates still alive today, the hagfish (Myxinoidea) and the lampreys (Petromyzonta), are united become. There are still about 131 round-mouthed species, 82 of them hagfish and about 47 to 49 lampreys. The taxon was established in 1806, but has since been discarded in the 1970s - based on morphological studies, the view prevailed that the lampreys are more closely related to the other vertebrates (the jaw-mouths ) than to the hagfish. However, this view was again refuted by molecular-biological findings, which proved the relationship between the two groups and thus also confirmed the round-mouths as clade .

Hagfish live in all oceans worldwide with the exception of the Red Sea and the polar seas . In tropical seas they are exclusively inhabitants of the deep sea. Lampreys inhabit marginal seas and fresh water in temperate zones; the majority of the species in the northern hemisphere, five more in southeast Australia , New Zealand and southern South America .

features

All round mouths have an eel-like, elongated and scale-free body with a cartilaginous inner skeleton and have no paired fins and the associated belt formations ( shoulder girdle and pelvic girdle ). The ability to form bones is lacking. The most important element of the axial skeleton is the notochord , which remains flexible for life. The spinal cord is flattened, but less strong in the lampreys. The neurocranium ends with the labyrinth and has no occipital lobe , the nervous system has no myelin sheaths .

A gill basket is only formed in the lampreys. In both groups, the gill pockets lie within the gill arches and the gill leaves arise from the endoderm , in the jaw mouths (Gnathostomata) from the ectoderm and the gill pockets lie outside the gill arches.

The name Rundmäuler (Cyclostomata) was given because of the round, jawless mouths of both taxa. The mouths have horny teeth and are possibly an adaptation to the parasitic way of life. A so-called tongue apparatus takes over the rasping of food particles in both taxa. Whether this Raspelapparat in both groups homologous is, is still unclear.

Systematics

The superclass of the round mouths (Cyclostomata) was established in 1806 by the French zoologist André Duméril . With the extinct, armored Ostracodermi they were united in the group of the jawless (Agnatha). With the advent of the principles of cladistics in the late 1970s, the view prevailed that the round- mouthed taxon must be a paraphyletic taxon and that the lampreys are more closely related to the jaw- mouths (Gnathostomata) than to the hagfish.

Molecular biological studies in recent years have shown, however, that the round- mouthed species are monophyletic , that is, lampreys and hagfish have a recent common ancestral form, from which no other group emerges. They share four unique microRNA families and 15 unique paralogies between primitive microRNA families.

The round mouths are said to have developed around 500 million years ago in the Cambrian from a last common ancestor of all vertebrates, which was, however, much more complex than the round mouths. The round mouths then went through a degeneration and lost many of the characteristics typical of vertebrates. 360 million year old lamprey fossils and 300 million year old hagfish fossils are quite similar to the modern forms.

literature

  • Gunde Rieger, Wolfgang Maier: "Agnatha", jawless. Page 175–176 in Wilfried Westheide & Reinhard Rieger: Special Zoology Part 2: Vertebrae and Skull Animals , 1st edition, Spectrum Academic Publishing House Heidelberg • Berlin, 2004, ISBN 3-8274-0307-3 .
  • Alfred Romer : Vertebrate Paleontology. The University of Chicago Press, 1955, ISBN 0-2267-2488-3 .

Individual evidence

  1. ↑ a b c Rieger & Maier (2006).
  2. ↑ Volker Storch, Ulrich Welsch: Systematic Zoologie , Fischer, 1997, pages 544-548, ISBN 3-437-25160-0 .
  3. ↑ a b c Philippe Janvier: microRNAs revive old views about jawless vertebrate divergence and evolution. PNAS November 9, 2010 Volume 107 No. 45, doi: 10.1073 / pnas.1014583107
  4. ↑ Christiane Delarbre, Cyril Gallut, Veronique Barriel, Philippe Janvier, Gabriel Gachelin (2002): Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. Molecular phylogenetics and evolution 22 (2): 184-192. doi : 10.1006 / mpev.2001.1045
  5. ↑ Shigehiro Kuraku, Kinya G. Ota, & Shigeru Kuratani, S. Blair Hedges (2009b): Jawless fishes (Cyclostomata). In SB Hedges & S. Kumar. Timetree of Life. Oxford University Press. pp. 317-319. ISBN 978-0-19-953503-3
  6. ↑ Jon Mallatt, Christopher J. Winchell: Ribosomal RNA genes and deuterostome phylogeny revisited: More cyclostomes, elasmobranchs, reptiles, and a brittle star. Molecular Phylogenetics and Evolution, Volume 43, Issue 3, June 2007, Pages 1005-1022 doi : 10.1016 / j.ympev.2006.11.023
  7. ↑ Jon Mallatt, J. Sullivan: 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes. Mol Biol Evol. 1998 Dec; 15 (12): 1706-18. PDF
  8. ↑ Alysha M. Heimberg, Richard Cowper-Sal·lari, Marie Sémon, Philip CJ Donoghue & Kevin J. Peterson: microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. PNAS November 9, 2010 vol. 107 no. 45 19379-19383 doi : 10.1073 / pnas.1010350107