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CNR: Alamanacco della Scienza

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N. 8 - 14 mag 2014
ISSN 2037-4801

International info   a cura di Cecilia Migali

Cultura

New fossil casts light on shark evolution

The skull of a newly discovered 325-million-year-old shark-like species suggests that early cartilaginous and bony fish have more to tell us about the early evolution of humans and other jawed vertebrates than modern sharks, as had previously been thought. Using X-rays at the European Synchrotron (Esrf) to image the ancient fossil, scientists have shown that living sharks are actually quite advanced in evolutionary terms - even though they have retained their basic 'sharkiness’ over millions of years. The research led by the American Museum of Natural History is published in Nature.

Sharks are traditionally thought to be one of the most primitive surviving jawed vertebrates. Most textbooks in schools today say that the internal jaw structures of modern sharks should look very similar to those in primitive shark-like fishes”, said Alan Pradel, a postdoctoral researcher at the American Museum of Natural History and the lead author of the study. But we’ve found that’s not the case. The modern shark condition is very specialised, very derived, and not primitive”.

The new study is based on an extremely well-preserved shark fossil collected by Ohio University professors Royal Mapes and Gene Mapes in Arkansas, where an ocean basin once was home to a diverse marine ecosystem. The fossilised skull of the new species, named Ozarcus mapesae was preserved in a nearly three-dimensional state, giving researchers a rare glimpse at the organisation of the arches in a pre-historic animal. The heads of all fishes—sharks included—are segmented into the jaws and a series of arches that support the jaw and the gills. These arches are thought to have given rise to jaws early in the tree of life, however, because shark skeletons are made of cartilage, not bone, their fossils are very fragile and are usually found in flattened fragments, making it impossible to study the shape of these internal structures. 

This beautiful fossil offers one of the first complete looks at all of the gill arches and associated structures in an early shark. There are other shark fossils like this in existence, but this is the oldest one in which you can see everything”, said John Maisey, a curator in the American Museum of Natural History’s Division of Paleontology and one of the authors on the study. “There’s enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton”.

The specimen was imaged using the high-resolution X-rays produced at the Esrf. The team worked with Paul Tafforeau, a beamline scientist at the Esrf and another author on the paper, to use the ID19 beamline at the Esrf and get a detailed view of each individual arch shape and organisation.

Lead author Alan Pradel said: “ID19 at the Esrf offers the most accurate X-ray scanning techniques for the study of ancient and highly mineralised specimens, meaning the final resolution is much better than a conventional X-ray CT Scan. We discovered that the arrangement of the arches is not like anything you’d see in a modern shark or shark-like fish. Instead, the arrangement is fundamentally the same as bony fishes”.

The authors say it’s not unexpected that sharks—which have existed for about 420 million years—would undergo evolution of these structures. But the new work, especially when considered alongside other recent developments about early jawed vertebrates, has significant implications for the future of evolutionary studies of this group.  “Bony fishes might have more to tell us about our first jawed ancestors than do living sharks”, Maisey said.

Per saperne di più: - www.esrf.eu/home/news/general/content-news/general/325-million-year-old-shark-fossils-reveal-new-insight-into-jaw-evolution.html