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Warty-Faced Wonder Reconfigures Horned Dinosaur Family Tree

Think of a “horned dinosaur,” and you probably think of the gigantic three-horned Triceratops, a staple in museum displays and pop cultural depictions worldwide. Yet, the group includes a whole array of animals from around the northern hemisphere, many of them radically different in appearance from Triceratops. Numerous discoveries have established the origins of the group as small (<2 meters long), bipedal, and fairly plain-looking critters. Although this overall pattern is relatively well understood, numerous details of the evolutionary story remain to be worked out. For instance, which species or group of species best exemplifies the “primitive” features for the group?

Until today, an animal called Yinlong seemed solidly in place as the most “primitive” horned dinosaur (see tree below; I put “primitive” in scare quotes because even early members of groups can show their own unique or “advanced” features). At around 160 millon years old, it was certainly the oldest, and many of the features in its skeleton showed a mix of those seen in other early horned dinosaurs as well as close cousins to horned dinosaurs. Analyses of the evolutionary relationships of horned dinosaurs (like the schematic below) showed Yinlong as its own lonely branch near the base of the tree.

Extremely simplified family tree of horned dinosaurs, from recent analyses.
Extremely simplified family tree of horned dinosaurs, from recent analyses. All silhouettes from PhyloPic; Psittacosaurus by Jaime Headden; Triceratops by Raven Amos; all others by A. Farke. CC-BY.

Of course, it only takes one new fossil to disrupt a tidy story. Fenglu Han and colleagues just announced a new horned dinosaur from the same rock layers in China as Yinlong, given the name of Hualianceratops wucaiwanensis (roughly, “ornamental horned face from Wucaiwan”). Hualianceratops is pretty clearly different from Yinlong, and the authors make a strong case that they’re not just different life stages of the same species (for instance, in showing individuals of roughly the same body size from each species with very different anatomical features). The full description for this new animal is published in PLOS ONE.

Hualianceratops Skull
Ain’t it cute? The partial skull of Hualianceratops (nose to right of image). Image provided by Han et al., CC-BY.

So, what is a Hualianceratops? The fossil includes a partial skull as well as the hind foot and a few vertebrae, more than enough to provide an overall image of the critter. It was a small-ish animal, probably around 2 meters in overall body length, with a pointy beak at the front of its face and a super beefy lower jaw (shown in the picture above). The skull bones had a gnarled and knobby surface texture, which undoubtedly was reflected in the skin when the animal was alive. The big jaws indicate a strong bite, so I envision it as an animal that was so ugly it was cute, and probably pretty hazardous to fingertips if you tried to keep it as a pet.

"New" family tree for horned dinosaurs.
“New” family tree for horned dinosaurs, simplified from that presented by Han et al. 2015. `Hualianceratops skull image from Han et al.; silhouettes all from PhyloPic. See above for full image credits.

Hualianceratops is cool for being a new species, but its real importance only becomes clear when viewed in the bigger picture. Han and colleagues reconstructed the overall evolutionary relationships of horned dinosaurs, using a computer algorithm to sort which species share the most anatomical features. Where Yinlong used to be its own lonely twig on the evolutionary tree, it now bunches up on a branch with several other early horned dinosaurs, including Hualianceratops. Most of these various early animals had previously been reconstructed as their own distinct twigs off the main trunk (compare the two trees in this post to see how–the new tree is much more tangled!). Now, rather than a whole bunch of isolated lineages, it looks like there was one big and unique radiation of horned dinosaurs early on. One group within this radiation–including HualianceratopsYinlong, and Chaoyangsaurus–is called Chaoyangsauridae. The other, known for nearly a century now from slightly younger rocks, is Psittacosauridae.

Although the horned dinosaur family tree is now a little messier, it’s also way more interesting. Assuming that the branching pattern found in the new analysis is correct, there are more horned dinosaurs yet to find in the Jurassic era. These undiscovered animals would include early members of the group that led to Triceratops and its kin. This is pretty exciting stuff, and it’s what keeps paleontologists going back to the rocks for more fossils!

Han F, Forster CA, Clark JM, Xu X (2015) A New Taxon of Basal Ceratopsian from China and the Early Evolution of Ceratopsia. PLoS ONE 10(12): e0143369. doi:10.1371/journal.pone.0143369

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