Guest post: Haplocheirus – the skilful one

Jonah Choiniere continues his guest spot on the Musings with the news of the fascinating Haplocheirus, a basal alvarezsaur from the Jurassic that tells us quite a bit about theropod diversification and alvarezsaur evolution. If you have missed out, my brief review of alvarezsaurs is here, with a more detailed one by Jonah here.

I started working with Dr. Clark in 2004 when I entered the Ph.D. program at George Washington University. Dr. Clark set me up with a project working on the systematic relationships of basal coelurosaurs, a big group of theropod dinosaurs that includes things like Tyrannosaurus, Velociraptor, and of course birds. I began work on this project in the summer of 2005, on my first visit to China. One of the first fossils I looked at was the theropod skeleton unearthed in 2004. On first glance, the skull looked like an ornithomimosaur, but Dr. Xu and Dr. Clark bade me to carefully consider the specimen. We began to notice similarities between the skeleton and that of alvarezsaurids, and a large phylogenetic analysis confirmed that many of these similarities were shared, derived characteristics – this was an alvarezsaur. The paper released today is a first look at these characteristics. After toying with some other names, we finally settled on Haplocheirus sollers as the scientific name for the new animal. The name means “simple-handed skilful one,” in reference to the fact that Haplocheirus has a simple (plesiomorphic) hand relative to the derived hand of other alvarezsaurids, and that it may have used this hand in ways that other alvarezsaurids couldn’t have (i.e., grasping prey rather than merely digging). Because Haplocheirus slots in at the base of Alvarezsauridae, we elected to call this larger group (i.e., Alvarezsauridae plus Haplocheirus) the Alvarezsauroidea, which is consistent with some other larger groups of theropods (eg., Spinosauroidea, Tyrannosauroidea) and reflects the increased diversity of this clade.

The specimen of Haplocheirus (less the skull). Image courtesy of Jonah Choiniere.

I present the details of why we think Haplocheirus is an alvarezsauroid in the paper, so I’ll let the reader look there, but below I’ll talk a bit about some of the neater features of the skull and the forelimb.

The most significant thing about the skull of Haplocheirus is that it’s there! As I mentioned previously, we have some wonderfully-preserved skulls for Shuvuuia, a little bit of Mononykus‘ skull, and a small skull of a very new taxon named Ceratonykus. All of these animals are derived alvarezsauroids and fairly closely related to each other. We know nothing about the skulls of Alvarezsaurus or Patagonykus, and it’s really these two South American taxa that began to provide evidence that alvarezsauroids weren’t birds. The skull of Haplocheirus is lightly-built, with a huge orbit (including a well-preserved sclerotic ring) and a long, narrow snout. Unlike the skull of Shuvuuia, however, the bones of the skull are well-connected and the snout was not flexible.

The skull of Haplocheirus. Image courtesy of Jonah Choiniere.

The teeth of Haplocheirus are especially interesting. Most theropods have less than 20 teeth in the maxilla, and these teeth are curved and serrated like steak knives. Some theropods have lost their teeth (e.g., most ornithomimosaurs, Limusaurus), and a select few theropods have numerous small teeth (Shuvuuia, an early ornithomimosaur named Pelecanimimus, some troodontids). Haplocheirus has relatively normal-sized front maxillary teeth that are curved and serrated, like most theropods. The back teeth, however, decrease rapidly in size and the roots of the teeth become circular in cross section. From what we can see of the tooth row, Haplocheirus had at least 30 teeth in the maxilla. Because teeth vary widely in Maniraptora, the larger theropod group to which the Alvarezsauroidea belong, it’s hard to say what Haplocheirus‘ teeth mean in the larger evolutionary picture, but it’s safe to say that early alvarezsauroids had a lot of small teeth that looked similar to those of more primitive theropods, and that over the course of the evolution of the Alvarezsauroidea, the teeth became smaller, more densely packed, and more numerous.

Part of the jaws of Haplocheirus. The tiny tooth sockets are makred with arrows. Image courtesy of Jonah Choiniere.

The arm and hand of Haplocheirus are also interesting. The humerus isn’t as bulky and short as those of other alvarezsauroids, but it has a large muscle attachment on the proximal end and a bulbous distal end with large muscle attaches where it meets the ulna. These latter two features are characteristic for alvarezsauroids, and they show that the foundations for a powerful front limb were already present in Haplocheirus. We don’t have a complete radius and ulna yet for Haplocheirus, but what’s preserved shows that the forearm was not as stocky as in Mononykus, although there is evidence that these two bones were tightly joined close to the humerus, which is typical for alvarezsauroids.

Alvarezsaur hands. Top, Allosaurus; middle, Haplocheirus; bottom, Shuvuuia. Image modified from Choiniere et al., 2010.

In most alvarezsauroids, the ulna shows that the triceps muscles must have been incredibly powerful, but in Haplocheirus it’s more likely that these muscles were about average. The hand of Haplocheirus is particularly intriguing. The first digit (meaning the one closest to the body when the hand is held in front, palm down; see our research group’s paper on this) is by far the most robust. It has a large claw and evidence for powerful musculature on the palmar surface, much like other alvarezsauroids. The second digit is longer than the first digit, unlike in Shuvuuia, where the second and third digits are reduced to short accessory structures. However, as in Shuvuuia, the second digit is very skinny, and the claw is much shorter and probably less powerful than the claw on the first digit. The third digit of the hand has a very short metacarpal (palm-bone), a feature that is unique to Haplocheirus. This finger is slender as well. In total, the hand of Haplocheirus is somewhere between the usual morphology of a Maniraptoran hand and the hand of a derived alvarezsauroid. We interpret this as a transitional morphology, where the evolution of the hand of derived alvarezsauroids proceeded by the thinning of the lateral two digits and reduction in length of the metacarpals starting with the third digit, while the first digit was becoming more robust. It’s very likely that Haplocheirus could still grasp things with its hands, as most theropods can do, but it’s also possible that it was beginning to show signs of adaptations in the hand for digging or scratching at substrates.

The reconstruction of Haplocheirus. Imgae modified from Choiniere et al., 2010.

In short, Haplocheirus is really nice as it fits into a big gap in the alvarezsaur fossil record. Since all other alvarezsaurs are known from the Late Cretaceous, then the presence of the Jurassic Haplocheirus extends their fossil record by 60 million years and confirms that they were present at a time when we would expect them to be around – this is when the maniraptorans are diversifying and before the evolution of the first birds. Further to this, the morphology of Haplocheirus is also interesting and important as it is transitional between the more derived alvarezsaurs like Mononykus and more generalized maniraptorans and other tetanurans.

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