A new, more bizarre group of chimaeras called the Eugeneodontids seized the throne at the opening of the Carboniferous. Eugenodontids had what’s called symphyseal tooth whorls -- large, sometimes spiral assemblages of teeth with vertical orientations directly in the middle of their mouth cavity. Helicoprion was one such fish, with a lower tooth whorl that was much like a circular saw.
Another member of the Eugenodontid royal family was Edestus. Like Helicoprion, it also had a symphyseal tooth whorl, except instead of a circular saw Edestus had an arc of teeth on both the roof and floor of its mouth in what has been described by many as pinking shears. Because Edestus had a very odd tooth configuration with highly curved whorls, paleontologists have long puzzled over how it used its tooth whorls to subdue and eat its prey.
New research by Wayne M. Itano of the Natural History Museum of the University of Colorado suggests that instead of crushing its prey with its tooth whorls in scissor-like motions, Edestus used its outwardly projecting teeth in a vertical thrashing manner. With up and down movements of the front part of its body, Edestus could have slashed and traumatized its prey in mere split-seconds and then ingested it wholesale.
Itano analyzed the surface wear of a tooth from the late Carboniferous of Texas and discovered grooves perpendicular to the axis of the crown. This suggests that, as Edestus thrashed its prey, the hard parts of sea animals made tiny scratches on the surface of its teeth. But in animals like mammals, these tiny scratches tend to form an up-and-down pattern because our teeth occlude, meaning the upper and lower portions rest on each other. These parallel scratches are the opposite of what Itano observed in his Edestus teeth.
Itano also analyzed a tooth from the outermost part of the whorl that had a broken and abraded tip. If Edestus thrashed its prey, the tip could break off by hitting a hard object, exposing the softer inner part of the tooth that could then be worn down by prey with tough skin.
Edestus had a younger relative called Helicampodus with similar teeth but less of an arc. Itano analyzed a Helicampodus tooth whorl from Pakistan that, in contrast to Edestus, showed vertical wear facets on the teeth. These vertical wear facets are tell-tale signs that the fish’s teeth occluded in some way and its mode of predation involved a scissor-like movement. Itano’s Edestus had no such wear facets.
While this thrashing predation view is still not widely accepted, Itano finds it difficult to explain Edestus tooth wear through any other mode of predation. If Itano’s evidence holds up to scrutiny, Edestus will become the only known animal to have ever used this mode of predation to subdue its prey. Truly weird.
Special thanks to Wayne M. Itano for his guidance while writing this article and Julio Lacerda for his amazing reconstruction of Edestus minor. And to Serena Garland who suggested writing about this crazy ‘shark’.
You can read the original research paper on Academia.edu.
And an abstract of Edestus tooth wear is here.