"Where's the beef?"

There's a mystery paleontologists have been puzzling over for decades that's pretty well summed up by everyone's favorite Wendy's ad.

Dimetrodon vertebrae found at HMNS site; additional object present for scale.

The Permian Red Beds in North Texas have turned up hundreds of Dimetrodons - the top predator of the era - but only a few fossils of the plant eaters they would have munched on. Without evidence of enough meat for this species to survive, how did they flourish for so long?

Dimetrodon ribs found by HMNS team.

In other words, Where's the beef?

About 50 years ago, E.C. Ulson, UCLA, hypothesized that Dimetrodons didn't eat plant eaters very often - because they were eating sharks.

Xenacanthus sharks of the era were unlike anything we've seen today. While the front of this prehistoric shark resembles modern species, it's back was shaped like an eel, and it had a long, poisonous spine coming out of its head. Most importantly, these were freshwater sharks that lived in the shallow, swamp-like habitat that was also populated by Dimetrodon 290 million years ago.

Evidence found by the HMNS paleo dig team supports this theory. For example, the team has found lots of Xenacathus teeth and poison spines at the site - but comparatively few examples of Xenacanthus coprolite (or shark poop). If the teeth were lost in the act of eating something, there would be coprolite around to prove it. Instead, the team has found lots of chewed-up shark skull.

An example of coprolite found at the site.

Something was eating the Xenacanthus at this site. Dimetrodon is the only species in the same weight class.

So, mystery solved?