Some dinosaurs have a direct path to becoming taxonomically distinguished from their peers. Others are not so lucky, stomping from paper to paper until their true identity becomes clear. Medusaceratops is one of the latter dinosaurs.

The story of Medusaceratops - one of the newer horned dinosaurs added to the ceratopsid family tree - centers on a 79 million year old site in Montana’s Judith River Formation. Known as the Mansfield Bonebed by experts, this spot preserves bones from multiple horned dinosaurs – initially all thought to be Albertaceratops. But in 2010 paleontologist Michael Ryan and colleagues announced that there was a second variety of horned dinosaur present. Based on two parietal bones – or bowed bones that make up part of the frill jutting out of the back of the skull – Ryan and coauthors set Medusaceratops apart as a previously-unknown species of horned dinosaur.

So here’s where it gets a little bit tricky. From those two parietals, Medusaceratops seemed to be a chasmosaurine. That is, that it belonged to the family of horned dinosaurs that includes Triceratops and Chasmosaurus rather than Centrosaurus, which is a separate lineage called centrosaurines.

And now that’s changed.

Drawing from additional bones found in the Mansfield bonebed, Kentaro Chiba, Ryan, and colleagues have revised the look and relationships of the Medusa-horned face. While the original reconstruction of the dinosaur’s bone included only three epiparietals – or horns jutting out from the side of the parietal bone – the new specimens show that Medusaceratops had five. This dinosaur also had a row of bumps on the broad midline bar of its frill. And when the paleontologists coded the characters from these bones and compared them to what’s found in other horned dinosaurs, Medusaceratops switched families. It wasn’t an early relative of Triceratops. It was a centrosaurine, related to dinosaurs like Pachyrhinosaurus and Centrosaurus itself.

This goes beyond moving Medusaceratops to a new position in the dinosaur family tree, though. For one thing, it means that only centrosaurine dinosaurs are found in the Mansfield Bonebed. And more than that, it turns out that only Medusaceratops was present in this place – bones previously assigned as Albertaceratops from the site belonged to Medusaceratops instead. It’s easy to see the reason why this wasn't immediately clear. The two dinosaurs have turned out to be relatively close relatives, and if you were looking at them out on the Cretaceous lowlands you’d only really be able to tell them apart by the fact that Medusaceratops had two large hooks jutting from the sides of its frill while Albertaceratops just had one. 

And so suddenly paleontologists have gone from possessing just two skull bones to a whole bonebed of Medusaceratops. This lets Medusaceratops join the club of other centrosaurs who died en masse during this part of the Cretaceous – similar bonebeds of Wendiceratops and Xenoceratops have been found in rocks of about the same age in the same area. The question is why.

Perhaps these dinosaurs were bad at swimming. Torrential rains that likely whipped up the prehistoric coast in this area may have caused local flooding, and it’s hard to keep your head above water when your whole body is front-loaded to carry a hefty array of horns. Or maybe there’s no single explanation. But over and over again paleontologists have found centrosaurine bonebeds in the roughly 79 million year old rock of Montana and Alberta. Something peculiar was going on, leading to the death of dinosaurs like Medusaceratops who would one day turn to stone.

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