Sandrine Debecker July 14, 2016

A new study led by scientists at the American Museum of Natural History shows that living sharks are actually quite advanced in evolutionary terms, despite having retained their basic “sharkiness” over millions of years. This new study is based on an extremely well-preserved shark fossil named Ozarcus mapesae and a 3D reconstruction of it. The research was published in the journal Nature.

A 3D reconstruction of the skull of the Ozarcus mapesae. The braincase is shown in yellow, the jaw is shown in blue, the hyoid arch and the gill arches are shown in orange
A 3D reconstruction of the skull of the Ozarcus mapesae. The braincase is shown in yellow, the jaw is shown in blue, the hyoid arch and the gill arches are shown in orange.

The skull of a newly discovered 325-million-year-old Palaeozoic shark-like species suggests that early cartilaginous and bony fishes have more to tell us about the early evolution of jawed vertebrates—including humans—than modern sharks do. The heads of all fishes—sharks included—are segmented into the jaws and a series of arches that support the jaws and the gills. These arches are thought to have given rise to jaws early in the tree of life.

Due to the fact that shark skeletons are made of cartilage, not bone, so their fossils are very fragile and are usually found in flattened fragments, making it impossible to study the shape of these internal structures. However, the Ozarcus mapesae specimen was preserved in a nearly three-dimensional state, giving researchers a rare glimpse at the organization of the arches in a prehistoric animal. “We discovered that the arrangement of the arches is not like anything you’d see in a modern shark or shark-like fish,” said Dr. Alain Pradel, a postdoctoral researcher at the Museum and the lead author of the study. “Instead, the arrangement is fundamentally the same as bony fishes.”

Transparent 3D model of the fossil showing the internal bony structures.
Transparent 3D model of the fossil showing the internal bony structures.

Working with scientists at the European Synchrotron Radiation Facility, Dr. Pradel imaged the specimen with a high-resolution CT scanner to get a detailed view of each individual arch shape and organization. The 3D image stack was imported in Mimics, where the different parts of the skull were segmented to obtain highly accurate virtual 3D models.

3D model of the outer surface of the fossil
3D model of the outer surface of the fossil

New fossil discoveries can profoundly affect our understanding of evolutionary history by revealing thus far unseen combinations of primitive and derived characters.

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