These Prehistoric Sharks Had Jaws Shaped Like Circular Saws and Sawtoothed Scissors

Science
جمعه 13 فروردین 1400 ساعت 12:19
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Smith sonian mag
Imagine a great white shark with a set of sawtoothed scissors for a mouth. Ridiculous as that image might seem at first, such a creature once swam through Earth’s seas. More than 300 million years ago, Edestus giganteous bit through its fishy prey with a set of thin, blade like jaws with each serrated tooth set in line right behind the last. There’s nothing quite like this fish alive today, and paleontologists have only recently been able to piece together the relevant clues to understand Edestus and other strange shark relatives from the deep past.
The fossils were awaiting technologies capable of revealing the elusive structures of jaws. For more than a century after fish like Edestus were first described, experts could only look at the shape of fossils containing teeth and wonder about what was inside. CT scans and paleo visualization software can now see inside the fossils to capture cartilaginous anatomy that holds clues about what these animals were doing. The resulting research indicates that prehistoric sharks and their relatives bit down in ways that seem alien to scientists now. Shark relatives had buzzsaw arrangements in their lower jaws and jaws that could swing out side-to-side to impale prey. By understanding how these predators captured prey, scientists can better understand what role they played in ancient ecosystems.
In the tree of life, Edestus was a chondrichthyan. Today’s sharks, rays and deep-sea ratfish all belong to this group, defined by skeletons that are primarily made of flexible cartilage rather than hardened bone tissue. But despite the notion that sharks are “living fossils” that are little different from their ancestors of 400 million years ago, the fact of the matter is that sharks and their relatives have changed dramatically and entire groups of cartilaginous fish have come and gone through time.
Paleontologists have known about strange fossil chondrichthyans from almost the beginning of the discipline. Edestus was originally named from a fragmentary jaw described by naturalist Joseph Leidy in 1856. The teeth resembled those of sharks, but nothing else of the animal was known beyond the jaws. Experts could only guess at what all the missing parts—from the skull to the tail —looked like.
The story of Helicoprion is even stranger. In 1886, ichthyologist Henry Woodward named what he thought was a new species of Edestus from a circular whorl of teeth found in Western Australia. But in 1899 Russian geologist Alexander Karpinsky proposed that this fossil was different enough to belong to a new genus of fish— Helicoprion —that had an almost trunk-like appendage jutting up from its snout that supported the teeth. Other experts disagreed, however, starting more than a century of alternative interpretations. Scientist guessed the teeth of Helicoprion were defensive spines, the vortex-like jaws of a ray, a buzzsaw-like apparatus held in the throat and more. Through it all, no one found a better specimen that depicted where the whorl was located. Just as with Edestus , experts could only guess what the entire animal looked like. More complete fossils were needed—skeletons with cartilage of the skull, fins and the rest of the body intact —but such specimens proved invisible. All paleontologists had to work with were the inscrutable tooth whorls.
The nature of these fossil chondrichthyans didn’t help scientists much. “The very thing that sets chondrichthyans apart from bony fish, that their hard anatomy is mostly composed of cartilage, makes them difficult to preserve in most environmental settings,” says Idaho Museum of Natural History paleontologist Leif Tapanila. Outside of exceptional circumstances, non-mineralized cartilage usually decays away and leaves the harder parts behind.
The way fossil experts went about studying fossil chondrichthyans didn’t always help, either. “Funnily enough,” Tapanila says, “sometimes the beauty of the teeth overshadows the potential for associated body parts.” Experts of eras past would sometimes pick away rock to see the teeth and not realize that they were busting through remnants of cartilage. “As many as one in 15 Helicoprion have jaw material preserved, but have historically gone unnoticed because of overzealous preparation,” Tapanila says.
But in those lucky cases where the fossils have remained intact, experts have now been able to glean some crucial clues.
One of the best places in the world to find Helicoprion fossils is in Idaho, and the Idaho Museum of Natural History had a very impressive tooth whorl labeled IMNH 37899 in their collections. The fossil seemed like a good candidate for CT scans that might reveal new details of what the animal was like. What the researchers found were traces of cartilage around the jaw in the rock, remnants of the skull as well as the jaws that held the tooth whorl. This not only confirmed that the tooth whorl belonged in the lower jaw and was buttressed by cartilage, but that Helicoprion was more closely related to ratfish in the chondrichthyan family than to sharks. Paleontologists couldn’t arrive at this conclusion until CT scans revealed what was invisible to the naked eye.
A restoration of what the whorl-tooth ratfish Helicoprion looked like by artist Gary Staab. (Riley Black)
Using similar techniques, Tapanila and colleagues then turned their attention to Edestus , again finding remnants of prehistoric cartilage that could not be detected before, and reconstructed what this shark-like snipper looked like .
Other research teams have taken similar approaches. In 2020, paleontologist Linda Frey and colleagues described another ancient shark-like fish called Ferromirum . CT scans allowed the researchers to scan, visualize and manipulate the fish’s skeletal elements in a way that has never been possible before. The teeth of this carnivore’s jaws rotated to point outward when the fish opened its mouth and inward when it closed them, a technique that “presents a great number of teeth to prey through the bite-cycle,” as the researchers wrote.
Taken together, all these “saws, scissors and sharks” would seem to suggest that cartilaginous fish of more than 250 million years ago were far stranger than anything alive today. No living equivalents of Edestus or Ferromirum exist. But familiarity often results in indifference.
Paleoichthyologists have found that today’s sharks and rays have actually evolved a greater array of different jaw shapes —mouths suited to chomping, cutting, crushing, filter feeding and more— than were present during the deep past. “There are many different jaw and tooth types and ways of suspending the jaws from the skull in modern cartilaginous fish,” University of Alaska Anchorage ichthyologist Cheryl Wilga says. She notes the jaws of goblin sharks, cookie-cutter sharks, nurse sharks and more are all vastly different, suited to different prey and feeding, in a broader diversity of form than seen in the past.
Both Edestus and Helicoprion , Tapanila points out, evolved to cut through soft prey with teeth arranged along the midline of the jaw. This kind of biting strategy made sense when the seas were full of fossil squid and cuttlefish relatives—the way the jaws of Helicoprion work might have even effectively shucked the soft parts of ammonites from their coiled shells. But this type of biting didn’t persist. “One thing we see in both Edestus and Helicoprion is that they both appear to be deadends to their respective lineages,” Tapanila says, and the same type of predators never evolved again. These hunters were specialized, and they were very good at what they did. In the same deposits that paleontologists find Edestus jaws, for example, experts sometimes find fish tails that were lopped off the body, possible remnants of Edestus meals.

As much as paleontologists have learned about these mysterious fish, though, unanswered questions remain. Much of what scientist know of these fish comes from teeth, jaws and some skull cartilage. The rest of their appearance is often based on conjecture or rumored specimens. “I’m hoping one day to get a call from an East Idaho mine,” Tapanila says, with a report of a Helicoprion body associated with the beautiful tooth whorl. And the fossil record may very well offer such a surprise one day. At the present moment, the body of Helicoprion is often envisioned as streamlined and shark-like befitting a predatory fish. But the fossil record itself will provide the ultimate test. “As a researcher, I can’t let myself get too precious about my speculations,” Tapanila says. “The literature is filled with ‘I told you so’s.”
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