Palaeos:		Gnathostomata
The Vertebrates		Gnathostomata

Gnathostomata

Vertebrata
|--Conodonta
`--+--Pteraspidomorphi
   `--Thelodonti
      `--+--Loganiidae or Osteostraci
         `--Gnathostomata
            |--+--Stensioella
            |   `--Ramirosuarezia
            |--Pseudopetalichthyida
            |--Placodermi
            `--Acanthodii
               |--Chondrichthyes
               `--Teleostomi
                  |--Acanthodii (in part)
                  `--Osteichthyes
                     |--Actinopterygii
                     `--Sarcopterygii

A choice of phylogenies

There is no consensus on where gnathostomes came from. Here are three overlapping hyypotheses, each emphasising a different group of organisms. The first is from a study of the soft-bodied jawless fish Jamoytius (Sansometal2010). A succesion of early and transitiobnal forms culminates in higher agnaths and finally jawed vertebrates. Here the Gnathostomes are shown as the sister taxon of a type of armoured jawless fish called Osteostraci

Craniata
|--Myxinoidea
|--Haikouichthys
`--Vertebrata
   |--Conodonta
   `--+--Hyperoartia (= Petromyzontiformes)
      `--+--Jamoytiiformes
         `--+--Anaspida
            `--+--+--Loganellia   
               |   `-- Turinia
               |     |--Heterostraci
               |     `--+--Arandaspida
               |        `--Astraspis
               `--+---Galeaspida
                  `--+--Osteostraci
                      `--Gnathostomata

Palaeos co-author ATW derived the gnathostomes from thelodonts, a group of unarmoured jawless fish that sometimes converged on jawed fishes in appearance. (represented by Loganellia and Turinia in the previous dendrogram, and as can be seen are quitre low on the phylogenetic scale).

Vertebrata
`--+--Pteraspidomorphi
   `--Thelodonti
      |--+--Furcacaudiformes
      |  `--Thelodontida
      `--+--+--Katoporida  
         |  `--Cephalaspidomorphi 
         |     |--Galeaspida
         |     `--+--Pituriaspida
         |        `--Osteostraci
         |           |--Ateleaspis
         |           `--Cornuata
         `--+--Loganiidae            
            `--Gnathostomata
                 |--Placodermi
                 `--Acanthodii
                    |--Chondrichthyes
                    `--Teleostomi
                       |--Derived Acanthodii
                       `--Osteichthyes

Because of their many highly specialised features, the placoderms are thought to be only distantly related to all other gnathosomes, called eugnathostomes, are considered to have diverged from then at an early date; not sharing an unknown common ancestor of which there are no known fossil remains. However a new cladistic analysis of the braincase and jaws of a primitive Devonian acanthodian fish totally revolutises the picture of gnathostome evolution, showing that Acanthodii paraphyletic, but placoderms possiblty were as well (Brazeau 2009). This is shown in the following dendrogram:

Galeaspida
|--Osteostraci
`--+--+--Bothriolepis
   |  `--Pterichthyodes
   `--+--Brindabellaspis
      |--+--Macropetalichthys
      |  `--+--Campbellodus
      |     `--Rhamphodopsis
      `--+--+--Dicksonosteus
         |  `--+--Buchanosteus
         |     `--Coccosteus
         `==Paraphyletic Climatiformes
            |--Chondrichthyes
            `--o Teleostomi
               |--+--Ischnacanthiformes
               |   `--Acanthodiformes
               `--Osteichthyes

At the rate science is progressing, there is little doubt that further discoveries and hypotheses are just around the corner! MAK111024