Wikimedia Commons: Krugerr. Dimetrodon are usually portrayed as terrestrial, such as this one at Baltow Jurassic Park, but may have got most of their food from freshwater. (They also died out long before the Jurassic)

The most fearsome land predator of the early Permian Era 290 million years ago fed on sharks, according to new fossil evidence.

Dimetrodon was a terror of the pre-dinosaur era, the first common large predator on land. Dimetrodons were four-legged creatures, walking on land and sporting huge sails that form their most recognizable feature. The largest species grew to four meters long and boasted the first saw-like teeth. They also sported fangs, leading to their name, which translates to “two measures of teeth." However, it had a problem – a shortage of food. 

Paleontologist Dr. Robert Bakker of the Houston Museum of Natural Science, known for developing the theory that dinosaurs were warm blooded, told Live Science that at the time, “There was a meat deficit all over the world.”

With those jaws, Dimetrodon would have been unsuited to a plant-based diet. However, the era appears to have been marked by a shortage of terrestrial herbivores on which the various Dimetrodon species could prey. While digging at Craddock Ranch, in the Dimetrodon Texas heartland, Bakker told the Geological Society of America (GSA) that he found 8.5 of the predators for every one herbivore. They just didn't seem like a sustainable diet. 

Paleontologists have been puzzling over this for some time. Was there some prey that didn't fossilize? The answer, Bakker concluded, was right under their noses. Mixed in with the Dimetrodon remains were remnants of 60 freshwater Xenacanth sharks, which—unusually for creatures with cartilage rather than bone—have fossilized well.

Whatever benefits drove Dimetrodon onto land, Bakker proposes they continued to hunt in the water, accounting for most of their diet. He draws on the work of distinguished paleontologist Everett Olson who proposed that aquatic plants were still the major primary producers of the era, providing food for fish and amphibians.

The feeding frenzy didn't all run one way. Some Dimetrodon bones have bites in them that match the shape of shark teeth, and coprolites formed from shark dung contain fragments of Dimetrodon bone. “The long, deeply incised marks show that carnivores had twisted limb bones around as if employing a body roll to dismember the Dimetrodon carcass,” Bakker and his colleagues told the GSA.

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