Discussion of Phylogenetic Relationships
The main characteristics supporting the nodes of this phylogeny are:
- Node 1: Superior oblique muscle of eye attached anteriorly to eyeball. Braincase including nasal capsules. Jaw muscle external to mandibular arch.
- Teleostomi: Mouth terminal in position, narrow-based braincase, three otoliths in ear.
- Osteichthyes: Endochondral bone, lepidotrichs on fins, jaws lined by dentary, premaxillary and maxillary.
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.
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