By examining a 325 million year old, Palaeozoic shark-like fossil (Ozarcus mapesae), the researchers found many similarities to the visceral branchial arch arrangement of osteichthyans (bony fish). This has thrown into question the existing idea that cartilaginous fish are responsible for the origin of the vertebrate jaw.
All vertebrate embryos develop pharyngeal arches - in fish these develop into the gill (or branchial) arches and in humans they ultimately become part of the larynx and trachea.
Cartilaginous fish (sharks, skates, rays and chimeras) are often described as "primitive fish", and are thought not to have changed much since their original evolution. Bony fish (or Osteichthyes) are considered to be the derived state and to have a more "modern" physiology. However, this new shark-like skeleton suggests that far from being unchanged since their evolutionary origin - around 420 million years ago - sharks may have undergone several evolutionary adaptations along the way and that our own mammalian jaw may have a closer evolutionary history with that of bony fish rather than cartilaginous fish.
Pradel, A., Maisey, J. G., Tafforeau, P., Mapes, R. H. and Mallatt, J. (2014) A Palaeozoic shark with osteichthyan-like branchial arches. Nature (16th April 2014)
