Extinct sea predator sliced prey with toothy spiral jaw
An ancient sea predator had a spiraling whorl of teeth that acted as a lethal slicing tool, according to new scans of a mysterious fossil.
Helicoprion听was a bizarre creature that went extinct some 225 million years ago. Like听,听贬别濒颈肠辞辫谤颈辞苍听had cartilaginous bones rather than calcified ones, so the only traces it left in the fossil record were weird, whorl-like spirals of teeth that look quite unlike anything sharks sport today.
The dearth of fossil evidence has led to multiple attempted reconstructions of what听Helicoprion听would have looked like. In some, the tooth whorl is placed on the upper jaw, curling outward like a spiky elephant trunk. In others, it's on the lower lip, giving the fish a fearsomely pouty expression. Researchers have also debated whether听Helicoprion听was more like a modern shark or another ancient group of cartilaginous fish, the chimaera. []
Now, a team of researchers from led by Leif Tapanila of Idaho State University has scanned a tooth whorl fossil from the Idaho Museum of Natural History using computed tomography (CT), the same type of technology used for disease screening in medicine. This technique provides a more detailed look than ever before at the tooth whorl, revealing the only way the whorl would've fit into the creature's mouth is if it took up听Helicoprion's entire lower jaw and grew continuously in a spiral, curling under itself like a conveyer belt of teeth. Previous reconstructions pictured the spiral as an appendage on the tip of the jaw, the researchers wrote Tuesday in the journal Biology Letters.
The scanned specimen, found in Idaho in 1950, dates back about 270 million years. It's about 9 inches (23 cm) in diameter, about half the size of the largest听听ever found. For comparison, the diameter of a regulation men's basketball is just over 9 inches.
奥丑别苍听Helicoprion听bit down on prey, the tooth whorl would have been forced backward, slicing and dicing the meal and moving it down toward the throat. Few听Helicoprion听fossils show signs of tooth breakage, suggesting that the fish likely ate soft-bodied animals such as听.
The anatomy of the jaw also confirms that听Helicoprion听belonged to a group called the听Euchondrocephali, a Greek word meaning "three cartilaginous heads," for the way their jaws fuse. These fish share characteristics of both cartilaginous sharks and bony fishes. That makes听Helicoprion听a distant relative of today's rabbitfish, ratfish and other chimaeras.
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