8
1.2.2 Gnathostomes
As reviewed by Janvier ( 1997b ), the Gnathostomata, or gnathostomes, comprise the
majority of vertebrates from the Middle Devonian (about 380 million years ago) to
Recent. These animals differ morphologically from all other vertebrates by having
a vertically biting device, the jaws, which consist of and a variety of exoskeletal grasping, crushing, or shearing organs, and endoskeletal mandibular arch. Gnathostomes
possess the teeth, and jaw bones (Mallatt 1984 ). This group of recent vertebrates
include chimaeras, sharks, rays, ray-fi nned and lobe-fi nned fi shes and terrestrial
vertebrates (see for review Schultze 2010 ).
The Chondrichthyes and Osteichthyes are two major clades of extant gnathostomes.
In addition, the Placodermi (Early Silurian-Late Devonian) and the Acanthodii
(Latest Ordovician or Earliest Silurian – Early Permian) are two extinct major
gnathostome clades. Outside of the taxa listed above, there may be other fossil
gnathostome groups, too.
The Chondrichthyes are characterized by the prismatic calcifi ed cartilage, a
special type of hard tissue lining the cartilages of their endoskeleton. The pelvic
clasper is another chondrichthyan characteristic. This special copulatory organ is
derived from the posterior part of the pelvic fi n (metapterygium). However, a pelvic
clasper may be present also in the fossil Placodermi. The Elasmobranchii and
the Holocephali represent two major extant clades of Chondrichthyans. Several
fossil clades like Cladoselachidae, Symmoriida, Xenacanthiformes, Iniopterygia,
Eugeneodontida may fall outside these two clades.
The Osteichthyes possess such specifi c characteristics in the endoskeleton as
endochondral (“spongy”) bone, dermal fi n rays made up by modifi ed, tile-shaped
scales (lepidotrichiae), and three pairs of tooth-bearing dermal bones lining the jaws
(premaxillary, maxillary and dentary). The Actinopterygii and the Sarcopterygii are
two major clades of the Osteichthyes.
Fig. 1.1 Sea Lamprey teeth are located on the oral disc of lampreys (Photograph by Brian
W. Coad, image manipulation by Noel Alfonso. Canadian Museum of Nature www.briancoad.
com . Reprinted with permission.)
1 Introduction
1.2.2 Gnathostomes
As reviewed by Janvier ( 1997b ), the Gnathostomata, or gnathostomes, comprise the
majority of vertebrates from the Middle Devonian (about 380 million years ago) to
Recent. These animals differ morphologically from all other vertebrates by having
a vertically biting device, the jaws, which consist of and a variety of exoskeletal grasping, crushing, or shearing organs, and endoskeletal mandibular arch. Gnathostomes
possess the teeth, and jaw bones (Mallatt 1984 ). This group of recent vertebrates
include chimaeras, sharks, rays, ray-fi nned and lobe-fi nned fi shes and terrestrial
vertebrates (see for review Schultze 2010 ).
The Chondrichthyes and Osteichthyes are two major clades of extant gnathostomes.
In addition, the Placodermi (Early Silurian-Late Devonian) and the Acanthodii
(Latest Ordovician or Earliest Silurian – Early Permian) are two extinct major
gnathostome clades. Outside of the taxa listed above, there may be other fossil
gnathostome groups, too.
The Chondrichthyes are characterized by the prismatic calcifi ed cartilage, a
special type of hard tissue lining the cartilages of their endoskeleton. The pelvic
clasper is another chondrichthyan characteristic. This special copulatory organ is
derived from the posterior part of the pelvic fi n (metapterygium). However, a pelvic
clasper may be present also in the fossil Placodermi. The Elasmobranchii and
the Holocephali represent two major extant clades of Chondrichthyans. Several
fossil clades like Cladoselachidae, Symmoriida, Xenacanthiformes, Iniopterygia,
Eugeneodontida may fall outside these two clades.
The Osteichthyes possess such specifi c characteristics in the endoskeleton as
endochondral (“spongy”) bone, dermal fi n rays made up by modifi ed, tile-shaped
scales (lepidotrichiae), and three pairs of tooth-bearing dermal bones lining the jaws
(premaxillary, maxillary and dentary). The Actinopterygii and the Sarcopterygii are
two major clades of the Osteichthyes.
Fig. 1.1 Sea Lamprey teeth are located on the oral disc of lampreys (Photograph by Brian
W. Coad, image manipulation by Noel Alfonso. Canadian Museum of Nature www.briancoad.
com . Reprinted with permission.)
1 Introduction