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Jawed Vertebrates

Philippe Janvier
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Hippopotamus with mouth wide open
taxon links [up-->]Actinopterygii [up-->]Sarcopterygii extinct icon extinct icon [down<--]Vertebrata Interpreting the tree
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This tree diagram shows the relationships between several groups of organisms.

The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.

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Interrelationships of the Gnathostomata:

The main characteristics supporting the nodes of this phylogeny are:

Containing group: Vertebrata


The Gnathostomata, or gnathostomes, are the majority of the Middle Devonian (-380 million years ago) to Recent vertebrates. They differ from all other craniates or vertebrates in having a vertically biting device, the jaws, which consist of an endoskeletal mandibular arch and a variety of exoskeletal grasping, crushing, or shearing organs, i.e. the teeth, and jaw bones. Among Recent vertebrates, the gnathostomes include sharks, rays, chimaeras, ray-finned fishes, lobe-finned fishes and land vertebrates.

Extant gnathostomes fall into two major clades, the Chondrichthyes and Osteichthyes. In addition, there are two extinct major gnathostome clades, the Placodermi (Early Silurian-Late Devonian) and the Acanthodii (Latest Ordovician or Earliest Silurian - Early Permian). There may be other fossil gnathostome taxa which fall outside of these four taxa. This could be the case for the Mongolepida, only known from isolated scales from the Early Silurian, and which are provisionally assigned to the chondrichthyans, yet with great reservations.

The Chondrichthyes are characterized by a special type of hard tissue lining the cartilages of the endoskeleton: the prismatic calcified cartilage. Another chondrichthyan characteristic is the pelvic clasper, as special copulatory organ derived from the metapterygium, i.e. the posterior part of the pelvic fin. A pelvic clasper may, however, be present in the fossil Placodermi. Chondrichthyans include two major extant clades, the Elasmobranchii and the Holocephali, and a number of fossil clades (Cladoselachidae, Symmoriida, Xenacanthiformes, Iniopterygia, Eugeneodontida) which may fall outside these two clades.

The Osteichthyes are characterized by endochondral ("spongy") bone in the endoskeleton, dermal fin rays made up by lepidotrichiae (modified, tile-shaped scales), and three pairs of tooth-bearing dermal bones lining the jaws (dentary, premaxillary and maxillary). The Osteichthyes include two major clades, the Actinopterygii and the Sarcopterygii.

The Placodermi are characterized by a dermal armor consisting of a head armor and a thoracic armor. In the thoracic armor, the foremost dermal plates form a complete "ring" around the body and always include at least one median dorsal plate.

The Acanthodii are characterized by dermal spines inserted in front of all fins but the caudal one. They also possess minute, growing scales which have a special onion-like structure, i.e. the crown consists of overlying layers of dentine or mesodentine.


Gnathostomes are characterized by:

There are numerous other characteristics of the soft anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to the gnathostomes among extant craniates, but cannot by observed in fossils.

Discussion of Phylogenetic Relationships

Gnathostomes have long been placed as the sister-group of the ensemble of the Agnatha, or jawless vertebrates, regarded as a clade. Current phylogenies, however, suggest that the Agnatha are not a clade, and that, among Recent craniates, the gnathostomes are the sister-group of the Hyperoartia (lampreys) only. Among fossil craniates, the Osteostraci share the largest number of uniquely shared derived characteristics with the gnathostomes.

The controversies about gnathostome interrelationships mainly bear on the position of the two major fossil groups, the Placodermi and Acanthodii. Placoderms have long been regarded as the sister-group of chondrichthyans, with which they share an eye-stalk (a small cartilage linking the eyeball to the braincase) and possibly a pelvic clasper. It has also been suggested that they could be the sister-group of the osteichthyans, because they share with them large dermal plates and a median dermal bone in the palate, the parasphenoid. The current consensus is that placoderms are the sister-group to all other known gnathostomes. All other gnathostomes share a special arrangement of the eye muscles (the superior oblique eye muscle being attached anteriorly in the orbit), whereas that of placoderms is similar to that in the Osteostraci (the superior oblique eye muscle being attached posteriorly in the orbit). Also, the braincase of placoderms shows a separate ossification containing the nasal capsules, whereas the latter are always included in the braincase in all other gnathostomes.

The Acanthodii have long been regarded as closely related to either chondrichthyans or placoderms, but they are now almost unanimously regarded as the sister-group of the Osteichthyes. They share with them otoliths in the inner ear, lepidotrich-like scales on fins, and a narrow-based braincase with a transverse ventrale fissure. Acanthodians, however, are rarely known from articulated specimens, and their internal anatomy remains poorly known. It is possible that Acanthodii is paraphyletic.

Other Names for Gnathostomata


Carroll, R. H. (1988). Vertebrate paleontology and evolution. W. H. Freeman & Co. New York.

Forey, P. L. (1980). Latimeria: a paradoxical fish. Proceedings of the Royal Society of London, B, 208, 369-384.

Gardiner, B. G. (1984). The relationships of placoderms. Journal of Vertebrate Paleontology, 4, 379-395.

Goujet, D. (1984). Placoderm interrelationships: a new interpretation, with a short review of placoderm classification. Proceedings of the Linnean Society of New South Wales. 107, 211-241.

Jarvik, E. (1977).The systematic position of acanthodian fishes. In Problems in vertebrate evolution (ed. S. M. Andrews, R. S. Miles and A. D. Walker), pp. 199-225. Academic Press, London.

Mallatt, J. (1984). Early vertebrate evolution: pharyngeal structure and the origin of gnathostomes. Journal of Zoology, 204, 169-183.

Miles, R. S. (1973). Relationships of acanthodians. In Interrelationships of fishes (ed. P. H. Greenwood, R. S. Miles, and C. Patterson). Zoological Journal of the Linnean Society, Supplement 1, 53, 63-103.

Schultze, H. P. (1993). Patterns of diversity in the skull of jawed fishes. In The skull (ed. J. Hanken and B. K. Hall), Vol. 2, pp. 189-254. University of Chicago Press.

Young, G. C. (1986). The relationships of placoderm fishes. Zoological Journal of the Linnean Society, 88, 1-57.

Information on the Internet

Deep homologies in the pharyngeal arches. Pharyngula.
Title Illustrations
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Hippopotamus with mouth wide open
Scientific Name Hippopotamus amphibius
Location Okavango Delta of Botswana
Sex Male
Body Part head
Copyright © 1997 Greg and Marybeth Dimijian
About This Page

Muséum National d'Histoire Naturelle Paris, France

Page: Tree of Life Gnathostomata. Jawed Vertebrates. 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.

Citing this page:

Janvier, Philippe. 1997. Gnathostomata. Jawed Vertebrates. Version 01 January 1997 (under construction). http://tolweb.org/Gnathostomata/14843/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/

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