The taxonomic group gnathostomes – or more commonly known as jawed vertebrate – is divided into two main groups: bony fishes (osteichthyans) and cartilaginous fishes (chondrichthyans). Chondrichthyans, a group that includes rays and modern sharks, used to be thought of as primitive and evolutionarily ancient creatures that haven’t faced the pressure to adapt and evolve all that much through time. It was believed by many a paleontologist that cartilaginous fishes were more primitive than bony fishes.
However, research lrecently published in Nature, led by Alan Pradel from the American Museum of Natural History in New York, says otherwise. John Maisey, the curator of the American Museum of Natural History and a co-author of the paper expressed that: “standard anatomical textbooks say that the shark is a model of a primitive jawed vertebrate, [but] that’s all wrong.” He and the other researchers reached this conclusion by examining a foot long shark fossil found in Arkansas.
The fossil, belonging to the Paleozoic species Ozarcus mapesae, was inspected first through a CT scan and then with a Synchrotron (a particle accelerator that uses extremely high energy X-rays). The scans showed that the 325-million-year-old shark’s gill arches of the ancient shark were complete and more closely resembled the osteicthyan structure. In addition, the jaw structure of O. mapesae was more like bony fishes than sharks. Modern sharks have their jaws attached to their cartilaginous skulls through pliable ligaments. The fossil displays a jaw that was fused to the cranium – the same set up as bony fishes.
This similarity is a link between modern sharks and ancient bony fishes – not the other way around. Because of this connection, the fossil gives evolutionary evidence against the common belief that sharks are the ancestors of modern bony fishes. Instead, the fossil suggests that ancient bony fishes gave rise to modern sharks.
Source:
Pradel, A., Maisey, J. G., Tafforeau, P., Mapes, R. H., & Mallatt, J. (2014). A Palaeozoic shark with osteichthyan-like branchial arches. Nature. doi:10.1038/nature13195
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