This is not surprising since different paralogs of effector genes are deployed at different times and places among different gnathostome models, likely conferring some species-specific properties.
As a basal vertebrate, lamprey possesses a bona fide neural crest and many neural crest-derived structures found in gnathostomes, though they lack several important neural crest derivatives, including sympathetic chain ganglia (Johnels, 1956; Butler and Hodos, 1996).
Support for the forebrain in gnathostomes is provided by the paired polar bodies and anterior trabeculae.
The two structures of primary importance in the transition from agnathan (jawless) fish to gnathostomes were the mandibular arch (gill arch 2) and the hyoid arch (gill arch 3).
Other paleontologists, however, argue that gnathostomes may not have evolved that early.
He asks why, if gnathostomes existed in the Ordovician, scientists haven't found complete fossils of these animals.
In 1975 a palaeontological expedition from Gottingen and Cologne universities (Germany) visited Arctic Canada (Langenstrassen & Schultze 1996) and inter alia collected agnathans and gnathostomes
in Lower Devonian localities on the northern part of Prince of Wales Island (Fig.
When the first gnathostomes
evolved during the Silurian, not only were teeth already formed, but the ability to form denticles like those found over the external surface of cartilaginous fishes had evolved and was found to extend throughout the entire oral mucosa (Kemp, 1999).
Exoskeletal elements such as scales, tesserae and bony plates of Palaeozoic jawless vertebrates (agnathans) and fishes with jaws (gnathostomes
) have been shown to exhibit characteristic morphology, sculpture and microstructure, all of which have been useful for taxonomic identification, systematics and phylogenetic analyses of these fossils (Gross 1967; Karatajute-Talimaa 1978; Marss & Ritchie 1998; Wilson & Caldwell 1998; Wilson & Marss 2004, 2009; Marss et al.
The myelinated nervous system arose in a common ancestor of all modern-day gnathostomes
Remarks about the cladistic analysis in connection with myopterygian hypothesis and the problem of the origin of gnathostomes
. In Morfologiya i evolyutsiya zhivotnykh [Morphology and evolution of animals] (Vorobyeva, E.