Xuanhanosaurus quilixiaensis: The Jaws that (Probably Didn’t Often) Bite, the Claws That Catch.

Size: 19.7 feet (6 meters) long.

Time Period: The Bathonian to Callovian Stages of the Middle Jurassic Period.
The Shaximiao Formation of China, a formation that is part of the larger Dashanpu Formation.

Name: The generic name refers to Xuanhan County in China, where it was found. The specific name aptly gets more specific, and honors Quilixia, where the animal was found near.

By far the most famous carnivorous dinosaur is Tyrannosaurus, a creature with a huge head and very tiny arms. However, theropods weren’t always small-armed and big-headed. In this post, I show you a strange evolutionary side-road that hails from all the way back in the Middle Jurassic. This animal is Xuanhanosaurus, an animal that was a far cry from the tyrannosaurs, which used their heads to take down their prey.

The animal’s remains consist of a partial skeleton that was found without a skull. The best components of its skeleton that have been found belong to the animal’s arms. But as it turns out, the arms were actually the most interesting component of the anatomy of Xuanhanosaurus.

You see, Xuanhanosaurus had unusually long arms for a predatory theropod. This is interesting, because in most theropods (carnosaurs, coelurosaurs, ceratosaurs), the limbs were proportionally short, presumably because they no longer had much of a role in prey-catching. Anyway, the forelimbs were very powerful and measured at over two feet long (over sixty centimeters long). The fourth metacarpal was also retained in the animal’s hand. The original description of this dinosaur went as far as to suggest that Xuanhanosaurus walked on all fours, a highly unusual gait for a theropod. Later paleontologists have disagreed, thinking that this animal would have been a biped like all other theropods. Though the quadruped hypothesis may have been plausible long ago, studies show that theropods didn’t have pronated hands, as pronation of the lower arm would have been impossible. In fact, this find also invalidates the early hypotheses that animals like Baryonyx and Spinosaurus, which also had long arms, may have been quadrupeds.

These arms were instead probably used to grab and catch the animal’s prey. Though the skull of Xuanhanosaurus hasn’t yet been found, it’s possible that it had a weaker bite than theropods that didn’t employ their arms, but instead used their heads and teeth to take their prey down. It’s possible that the animal was an ambush predator, or it may have been a pursuit predator. Of course, there really isn’t enough evidence to tell what niche Xuanhanosaurus occupied. You can be sure that it used its arms to catch prey.

In its original description, Xuanhanosaurus was assigned to the Megalosauridae. In the year 2009, it was found by Roger Benson to be a basal member of the Megalosauria. One analysis in 2012 that incorporated Xuanhanosaurus places it as the most basal member of the Metriacanthosauridae, a group that includes Sinraptor and Yangchuanosaurus. This isn’t a grouping of megalosaur, of course, as the metriocanthosaurids are a group of allosauroids. Of course, animals like Xuanhanosaurus and Poekilopleuron are known from bad material and are highly unstable taxa when it comes to phylogenetic analyses.

So, the long-armed Xuanhanosaurus is radically different from Tyrannosaurus, Carnotaurus, and other short armed theropods. It’s a very interesting creature that indicates just how diverse the theropods were. Though this animal isn’t known from many bones or even a whole skeleton, it shows that primitive theropods evolved down roads that we don’t know existed.