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Thelodontigroup is extinct

Philippe Janvier
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Containing group: Vertebrata

Introduction

Thelodonts are an ensemble of fossil, presumably jawless vertebrates, distinguished from other jawless vertebrate taxa by the structure of their exoskeleton, which is entirely made up by minute scales. These scales are hollowed by a pulp cavity and superficially resemble the placoid scales of sharks. They are known from the Lowermost Silurian (and possibly the Late Ordovician) to the Late Devonian (430 to 370 million years ago).

Thelodonts lived in shallow-water marine environments. They seem to have survived longer in the Gondwana than in Euramerica, where they disappear from the fossil record after the early Middle Devonian.

Characteristics

There are two competing theories regarding the affinities of the thelodonts. Some consider them as clade, characterized by:

Others consider them as paraphyletic; that is, an ensemble of primitive forms respectively related to other major vertebrate groups. Some thelodonts would thus be relatives of the pteraspidomorphs, others of anaspids, galeaspids, osteostracans, and the gnathostomes.

The overall morphology of the thelodonts is only known from a few forms but seems to be relatively homogeneous. Most of them have a dorsoventrally flattened head with lateral flaps (or paired fins?), small eyes, and eight, ventrally placed gill slits. There is one dorsal fin, one anal fin, and the caudal fin is slightly hypocercal, with a large dorsal web strengthened by radials. Other thelodonts are deep-bodied, with larger eyes, laterally placed gill slits, and a large fork-shaped tail.

There are very few information on the internal anatomy of thelodonts, although some distinctly show imprints of paired olfactory organs. Some thelodonts have minute, internal denticles which cover the pharynx and possibly a large, median, inhalent duct comparable to the median dorsal opening of the Galeaspida. The presence of such denticles is unique among jawless vertebrates and recalls the condition in the gnathostomes. Well preserved thelodonts from Canada show the impression of a large, barrel-shaped stomach, a characteristic which is shared with the gnathostomes.

Discussion of Phylogenetic Relationships

When thelodonts are regarded as clade, they are placed as the sister-group of either the Pteraspidomorphi, or the Gnathostomata (see Vertebrata page). When considered as being paraphyletic, they may include stem forms of all "ostracoderm" groups (pteraspidomorphs, galeaspids, anaspids, osteostracans, and pituriaspids) and the gnathostomes. In particular, one group of thelodonts, the Katoporida, shares with the osteostracans, the presence of a special type of dentine, called mesodentine, in the scales. It is probable that the fork-tailed thelodonts (unamed to date) are more closely related to the Heterostraci (or all the Pteraspidomorphi) than to any other vertebrate taxon, as they share the same tail structure. Thelodonts with internal denticles, such as Loganellia, may be more closely related to the gnathostomes than to any group of jawless vertebrate.

Thelodont taxa have been defined on the basis of scale histology and morphology, but recent studies on articulated thelodonts show that scale morphology can be misleading, as a wide range of scale morphologies can occur in the same individual. Thelodonts are currently classified into four groups: Achanolepida, Loganiida, Turiniida, and Katoporida, on the basis of scale histology and morphology, but this is merely a phenetic classification.

References

Janvier, P. (1996). Early vertebrates. Oxford Monographs in Geology and Geophysics, 33, Oxford University Press, Oxford.

Karatayute-Talimaa, V. N. (1978). (Silurian and Devonian thelodonts of the USSR and Spitsbergen). Mosklas, Vilnius. (In Russian.)

Märss, T. (1986). Squamation of the thelodont agnathan Phlebolepis. Journal of Vertebrate Paleontology, 6, 1-11.

Wilson, M. V. H., and Caldwell, M. W. (1993). New Silurian and Devonien fork-tailed 'thelodonts' are jawless vertebrates with stomachs and deep bodies. Nature, 361, 442-444.

Stensiö, E.A. (1964). Les Cyclostomes fossiles ou Ostracodermes. In: Traité de paléontologie (ed. J. Piveteau), Vol. 4(1), pp. 96-383, Masson, Paris

Traquair, R. H. (1899). Report of fossil fishes collected by the Geological Survey of Scotland in the Silurian rocks of South Scotland. Transactions of the Royal Society of Edinburgh, 39, 827-864.

Turner, S. (1991). Monophyly and interrelationships of the Thelodonti.In Early vertebrates and related problems of evolutionary biology (ed. M. M. Chang, Y. H. Liu, and G. R. Zhang), pp. 87-119. Science Press, Beijing.

Turner, S. (1992). Thelodont lifestyles. In Fossil fishes as living animals (ed. E. Mark-Kurik), Akademia, 1, 21-40.

Van der Brugghen, W., and Janvier, P. (1993). Denticles in thelodonts. Nature, 364, 107.

Title Illustrations
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Scientific Name Lanarkia, Loganellia
Comments Thelodonts are a still poorly known group of Silurian and Devonian jawless vertebrates, whose dermal skeleton consists of minute scales. Most thelodonts have a dorsoventrally depressed body and head, the gill openings being situated on the ventral side. It is assumed that the snout was more or less as in primitive galeaspids; that is, with a broad, inhalent median opening and a slightly ventral mouth. Among the flat-bodied forms are Lanarkia (top left), provided with long, spine-shaped scales, and Loganellia (top right and middle). Other thelodonts, such as an unnamed form from the Devonian of Canada (bottom) are deep-bodied, with lateral gill openings and a very large, forked tail.
Reference Based on Turner, S. 1991. Monophyly and interrelationships of the Thelodonti.In Early vertebrates and related problems of evolutionary biology (ed. M. M. Chang, Y. H. Liu, and G. R. Zhang), pp. 87-119. Science Press, Beijing. AND Turner, S. 1992. Thelodont lifestyles. In Fossil fishes as living animals (ed. E. Mark-Kurik), Akademia, 1:21-40. AND Wilson & Cadwell 1993
Specimen Condition Fossil
Image Use creative commons This media file is licensed under the Creative Commons Attribution License - Version 3.0.
Copyright © 1997
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Muséum National d'Histoire Naturelle Paris, France

Page: Tree of Life Thelodonti. Authored by Philippe Janvier. The TEXT of this page is licensed under the Creative Commons Attribution License - Version 3.0. Note that images and other media featured on this page are each governed by their own license, and they may or may not be available for reuse. Click on an image or a media link to access the media data window, which provides the relevant licensing information. For the general terms and conditions of ToL material reuse and redistribution, please see the Tree of Life Copyright Policies.

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Janvier, Philippe. 1997. Thelodonti. Version 01 January 1997 (under construction). http://tolweb.org/Thelodonti/14835/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/

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