Thelodonti
| Thelodonti | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Lanarkia (top left), Loganellia (top right and middle), Furcacauda (bottom) |
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| Temporal occurrence | ||||||||||||
| Upper Ordovician to Upper Devonian | ||||||||||||
| 440 to 370 million years | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Thelodonti | ||||||||||||
| Lambe , 1915 |
The thelodonti are a group of extinct, jawless , fish-like vertebrates that lived in shallow marine regions around the world from the Upper Ordovician to the early Upper Devonian. They disappeared from the northern hemisphere in the early Central Devon and have since been restricted to the coasts of the southern major continent of Gondwana . They are best known for their isolated small scales, but fossils of almost complete bodies have also been found.
features
The animals had a mostly flattened body, small eyes and a hypocerkic caudal fin. Dorsal and anal fins were present in some forms. There could be pectoral fins or fin-like skin folds over the row of mostly eight gill openings. The Furcacaudiformes ("fork-tailed melodonts") had a tall body, larger eyes and a large, fork-like, symmetrical caudal fin. The scales did not overlap.
The internal anatomy of animals is largely unknown, some had paired nostrils, which Furcacaudiformes had a big stomach, a feature it with the pine mouths share (Gnathostomata).
Systematics
According to the histology and morphology of their scales, the thelodonti are subdivided into the sub-taxa Achanolepida, Loganiida, Turiniida and Katoporida. It has been shown, however, that different forms of scale can occur in a single individual.
The monophyly and the external systematics of the thelodonti is controversial. As a monophylum, they are often seen as a sister group of the Pteraspidomorphi or the jaw- mouthed. The most important common feature ( synapomorphism ) of a monophyletic thelodontic caste would be the scales that did not contain any bone cells and only grew at their base. As a paraphylum , the thelodonti are considered to be a collection of primitive forms of the other Palaeozoic jaw-less animals (Pteraspidomorphi, Anaspida , Osteostraci and Galeaspida ). The Furcacaudiformes share the shape of the tail with the Heterostraci . Some may also be more closely related to the jaw mouths. Like the jaws, the katoporida had a special form of dentine in the scales called mesodentine.
Individual evidence
- ^ A b M. VH Wilson & MW Caldwell: New Silurian and Devonian fork-tailed 'thelodonts' are jawless vertebrates with stomachs and deep bodies. Nature 361: 442-444 (February 4, 1993); doi : 10.1038 / 361442a0
literature
- Hans-Peter Schultze: Thelodonti , page 194 in Wilfried Westheide & Reinhard Rieger : Special Zoology Part 2: Vertebrae and cranial animals , 1st edition, Spectrum Academic Publishing House.
- Joseph S. Nelson : Fishes of the World . John Wiley & Sons, 2006, ISBN 0-471-25031-7 .
- Robert L. Carroll: Paleontology and Evolution of the Vertebrates , Thieme, Stuttgart (1993), ISBN 3-13774-401-6 .
- John A. Long: The Rise of Fishes . The Johns Hopkins University Press, 1995, ISBN 0801849926 .
Web links
- Philippe Janvier: Thelodonti . Version 01 January 1997. In: The Tree of Life Web Project