A Silurian placoderm with osteichthyan-like marginal jaw bones

Area of Science:

• Paleontology
• Evolutionary Biology
• Vertebrate Zoology

Background:

• Gnathostomes (jawed vertebrates) consist of Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish and tetrapods).
• Key differences include the presence of large dermal bones in Osteichthyes, absent in Chondrichthyes.
• The evolutionary origin of these dermal bones and the ancestral gnathostome morphology are debated.

Purpose of the Study:

• To describe a newly discovered stem gnathostome fossil.
• To investigate the phylogenetic position of this fossil within gnathostome evolution.
• To infer the characteristics of the last common ancestor of crown gnathostomes.

Main Methods:

• Description of a 3D-preserved 419-million-year-old placoderm fish from the Silurian of China.
• Phylogenetic analysis incorporating morphological data.

Main Results:

• The fossil represents the first stem gnathostome with dermal marginal jaw bones (premaxilla, maxilla, dentary).
• Phylogenetic analysis places the new form high in the gnathostome stem group, near the divergence of major clades.
• Acanthodians are assigned to the chondrichthyan stem group.

Conclusions:

• The last common ancestor of Chondrichthyes and Osteichthyes likely possessed a macromeric dermal skeleton.
• This discovery provides a new framework for understanding the divergence of jawed vertebrates.
• It suggests dermal jaw bones evolved prior to the split between cartilaginous and bony fish lineages.