Thelodonti

Lanarkia ( top left), Loganellia ( above right and center ), Furcacauda ( below)

The Thelodonti are a group of extinct, jawless, fishlike vertebrates that lived from the Upper Ordovician to the early Devonian worldwide in shallow sea regions. In the early Middle Devonian they disappeared from the northern hemisphere and have since been limited to the southern shores of the Great Gondwana. They are known mainly by isolated small scales, but it also nearly complete fossil specimens were found.

Features

The animals had a generally flattened body, small eyes and a hypocerke tail fin. Dorsal and anal fin were present in some forms. Pectoral fins or fin- skin folds were found over the range of usually eight gill openings. The Furcacaudiformes ( " Gabelschwanzthelodonten " ) had a high body, larger eyes and a large, fork-like, symmetrical tail fin. The scales did not overlap itself.

The internal anatomy of animals is largely unknown, some had paired nostrils, which Furcacaudiformes had a big stomach, a feature that they share with the jaw mouths ( gnathostomata ).

System

After the histology and morphology of their scales in the Thelodonti the senior non Untertaxa Achanolepida, Loganiida, Turiniida and Katoporida be divided. But it has been shown that different forms of dandruff can occur in a single individual.

The monophyly and the outer classification of Thelodonti is controversial. You will, regarded as a monophyletic group, often as a sister group of the Pteraspidomorphi or gnathostomes. The most important common feature ( synapomorphy ) of a monophyletic Thelodontenklade were the scales that did not contain bone cells and grew only at their base. As Paraphylum the Thelodonti be viewed as a collection of primitive forms of other Paleozoic pine losers ( Pteraspidomorphi, Anaspida, Osteostraci and Galeaspida ). The Furcacaudiformes share with the Heterostraci the tail shape. Some may be more closely related to the jaw mouths. The Katoporida had as well as the gnathostomes, a special Dent Inform in the shed, the Mesodentin is called.