New Fossil Provides Insight Into Shark Evolution

A 3D reconstruction of the skull of Ozarcus mapesae. The braincase in light grey, the jaw is in red, hyoid arch is blue, and gill arches are shown in yellow / AMNH/A. Pradel
Janet Fang 17/04/2014, 17:10
 
Watch any Jaws movie and you’ll understand why scientists have presumed that these eating machines have remained unchanged from their ancestral condition. But turns out, sharks are not “living fossils” that have retained their primitive anatomy and prehistoric “sharkiness” over millions of years. As simple and perfect as they seem, modern sharks have evolved many new adaptations over time.
 
Older theories about gill-structure evolution -- which provided the basis for understanding jaw evolution -- relied on modern cartilaginous fish (like sharks and rays) and bony fish (which includes today’s tunas and oarfishes). 
 
Now, a well-preserved, 325-million-year-old shark-like fossil from Paleozoic Arkansas shows how the skeletal structure that supports the gills of a very early shark resembles bony fishes much more than it does modern sharks. The fossil of the new species Ozarcus mapesae is the earliest identified cartilaginous fish where the entire gill skeleton is preserved intact in its natural position (in three dimensions, not flattened the way we usually find sharks fossils). 
 
Using high resolution CT scans of the early shark fossil, a team led by Alan Pradel and John Maisey from the American Museum of Natural History found that it combines characteristics from both cartilaginous and bony fish. 
 
“Sharks are traditionally thought to be one of the most primitive surviving jawed vertebrates. And 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,” Pradel says in a news release. “But we’ve found that’s not the case. The modern shark condition is very specialized, very derived, and not primitive.”
 
 
This is the fossil pictured from two different views (scale bar is 10 millimeters.) “There’s enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton,” Maisey says
 
Fish have these structures called visceral branchial arches, which are serially arranged, jointed endoskeletal supports that are internal to the gills. The scans revealed how the gill arches (yellow, above) of the fossil are arranged like bony fishes. The most recent common ancestor of jawed vertebrates must have possessed some elements that were more bony fish-like than shark-like. 
 
Additionally, the arches were separated by small bits of cartilage that are found in some species of bony fish and their relatives but previously unknown in any living or extinct cartilaginous fish. 
 
The basal shark’s blend of traits indicates that modern sharks have acquired features through evolutionary innovation. And if we want to learn more about our first jawed ancestors (yes, humans have jaws too), we might have to turn bony fishes rather than modern sharks.
 
The work was published in Nature this week. 
 
[AMNH via Science]
 
Images: AMNH/A. Pradel (top) & AMNH/F. Ippolito (middle)
 
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