On the Road to All Thumbs: Bannykus and Xiyunykus

Out of all the weird and wonderful forms that maniraptors have taken, it will probably surprise no one to hear that it is the alvarezsaurids that fascinate me the most. The most noticeable anatomical curiosities of these theropods are their forelimbs: the arms of alvarezsaurids were very short and their hands were composed almost entirely of an enlarged thumb, with the second and third fingers highly reduced or (in at least one species) lost.

Their limited reach, stiffened elbow and wrist joints, and possession of only a single functional finger would have made alvarezsaurid forelimbs little good for grasping. However, despite their small size, those arms almost certainly had a practical use. The humerus (upper arm bone) had a very large deltopectoral crest and the ulna (the larger bone in the lower arm) had a large olecranon process (the point projecting towards the elbow). These features, combined with the well-developed sternum (breastbone), are generally found in animals that need to make powerful forelimb motions, more specifically pulling the forelimbs towards the body and straightening the elbow joint. In the case of alvarezsaurids, such specializations most commonly interpreted as adaptations for breaking into insect nests or wood.

Reconstructed skeleton of the alvarezsaurid Patagonykus, photographed by "Kabacchi", under CC BY 2.0. Note that the vestigial second and third fingers are not included in this mount.

When alvarezsaurids were first recognized in the 1990s, they were commonly thought to be strange flightless avialans more closely related to modern birds than was Archaeopteryx. They did indeed have skeletal similarities to modern birds, particularly in the skull and limbs. However, later analyses have recovered alvarezsaurids as non-eumaniraptoran maniraptors, suggesting that their similarities to birds were the result of convergent evolution. (In addition, a few studies in the late 1990s and early 2000s argued that they were even more distant relatives of birds, being closer to ornithomimosaurs.)

In 2010, the description of Haplocheirus from the Late Jurassic Shishugou Formation of China (older than any alvarezsaurid) lent support to the non-avialan status of alvarezsaurs. Considered a non-alvarezsaurid alvarezsaur, Haplocheirus looked much like a "standard", non-paravian coelurosaurian theropod; it was an "alvarezsaur that looked normal". However, it shared with alvarezsaurids a slightly enlarged thumb relative to the other fingers, as well as details of the skull and hindlimbs. Subsequent studies have largely supported its alvarezsaurian affinities (though at least one analysis found it to be an early ornithomimiosaur instead).

Haplocheirus shed a good amount of light on alvarezsaurid origins, but there remained a noticeable gap in the picture. All known alvarezsaurids with their specialized forelimb morphology lived during the Late Cretaceous, whereas Haplocheirus with its incipient forelimb specializations lived in the Late Jurassic, leaving the Early Cretaceous history of alvarezsaurs unknown... until now. In a recently-published paper, Xu Xing and colleagues describe two new alvarezsaur species recovered from Early Cretaceous deposits in China.

First of all, it's about damn time. The last newly-named dinosaur to be described as an alvarezsaur was Alnashetri, which was published back in 2012. But more than simply increasing our knowledge of alvarezsaur diversity, the new species described by Xu et al. are significant in what they reveal about alvarezsaur evolutionary history.

One of the new alvarezsaurs, Xiyunykus pengi, was found in the Tugulu Group of northwestern China. The holotype specimen is fairly complete, though it lacks much of the skull, the pelvis, and a complete forelimb. Enough of the forelimb is preserved, however, to show that Xiyunykus had a typical coelurosaurian humerus while also having a well-developed olecranon process on the ulna. Thus, Xiyunykus exhibits an intermediate forelimb morphology between ancestral coelurosaurs and alvarezsaurids.

Skeletal and select elements of Xiyunykus, from Xu et al. (in press). Preserved bones are shown in gray on the skeletal.

Restoration of Xiynykus with human for scale, by Tom Parker, under CC BY-SA 4.0.

Possibly an even more dramatic intermediate condition is represented by the other new alvarezsaur, Bannykus wulatensis. Bannykus was found in the Bayin-Gobi Formation of northern China and was slightly larger than Xiyunykus. (Both new alvarezsaurs are known from roughly coyote- or rhea-sized subadult specimens, making them larger than most alvarezsaurids.) The holotype of Bannykus is again quite complete, also lacking much of the skull but this time including a nearly complete forelimb. Its arms were shorter relative to its body size than those of Haplocheirus, and its second and third fingers were also reduced. Its olecranon process was further developed still compared to Xiyunykus. However, none of these features reached the extremes seen in alvarezsaurids. If one were to try and infer how the forelimb of an ancestral coelurosaur was evolutionarily transformed into the very unusual alvarezsaurid forelimb, the forelimb anatomy of Bannykus would probably fit the halfway point of that transformation almost perfectly. Indeed, Bannykus translates to "half claw", a reference to its transitional characteristics.

Skeletal and select elements of Bannykus, from Xu et al. (in press). Preserved bones are shown in gray on the skeletal.

Restoration of Bannykus with human for scale, by Tom Parker, under CC BY-SA 4.0.

Interestingly, whereas the second and third fingers of alvarezsaurids were so highly reduced that they likely served no practical purpose, there are hints that the second finger of Bannykus had a specialized function. The joints between the bones of the second finger appear to have been exceptionally flexible. In particular, the claw of the second finger could not only curl toward and extend away from the palm in a wide arc, but may have also been capable of some side-to-side movements and even rotation. Xu et al. suggest that this level of flexibility is consistent with a probing function. Bannykus may have used its second finger to flush out prey from crevices, maybe after it had already broken into their hideout using its robust thumb!

This is not the first time that a probing function has been attributed to dinosaur fingers. A similar foraging method has also been suggested for scansoriopterygids (involving the use of their third finger) and the oviraptorosaur Chirostenotes (involving the use of its second finger). However, both of these previous ideas have been questioned. Being a pennaraptoran, Chirostenotes likely bore large feathers on its second finger, which would have prevented it from using its finger as a probing tool, and the discovery of Yi opens up the possibility that the third finger of scansoriopterygids was mainly used to support a membranous wing structure. Notably, the idea that these dinosaurs probed with their fingers was based primarily on the fact that their fingers were elongate, with no evidence that they were unusually flexible. In any case, regardless of how well the probing hypothesis for Bannykus holds up in the future, the contrast with the vestigial second finger of alvarezsaurids is a good illustration that organisms that we consider morphologically transitional in hindsight can simultaneously have their own unique specializations, and are not simply "waiting around to evolve into something else".

From top to bottom: the forelimbs of Guanlong (a tyrannosauroid, representing the ancestral coelurosaur condition), Haplocheirus (a Late Jurassic alvarezsaur), Bannykus (an Early Cretaceous alvarezsaur), and Shuvuuia (an alvarezsaurid), from Xu et al. (in press). All are scaled relative to respective femur length. Note the successive decrease in relative arm length, increase in prominence of projections on the humerus and ulna, increase in robusticity of the thumb, and reduction of the other fingers.

In addition to their unusual forelimbs, alvarezsaurids (especially the subgroup known as parvicursorines) also had quite specialized hindlimbs, with very long feet and short toes (likely adaptations for running). The hindlimbs of both Xiyunykus and Bannykus possessed some similarities to those of alvarezsaurids, but were otherwise fairly typical of coelurosaurs. Evidently, alvarezsaurs started doing whatever it is they were doing with their forelimbs before they were selected for specialized locomotion.

The phylogenetic analysis in Xu et al.'s study has some surprises. As expected, Haplocheirus, Xiyunykus, and Bannykus are found to be successively more closely related to alvarezsaurids. What is more curious is that Aorun and Tugulusaurus, previously considered basal coelurosaurs or basal maniraptoromorphs, are also found to be alvarezsaurs! Aorun (known from a partial juvenile specimen from the Shishugou Formation) is recovered as the oldest and basalmost known alvarezsaur, whereas Tugulusaurus (known from a fragmentary skeleton from the Tugulu Group) is recovered as a close relative of Xiyunykus. As has been a recurring element of our knowledge about alvarezsaur evolution, the thumb might tell the tale: the most obvious features that Aorun and Tugulusaurus shared with more definite alvarezsaurs are features of the thumb. It would be interesting to see if these results are supported by future studies.

The phylogeny of alvarezsaurs recovered by Xu et al. (in press), from their study.

With these new discoveries, alvarezsaurs have gone from being one of the most enigmatic groups of maniraptors to one of the most completely understood in terms of their evolutionary history. That six-year gap between now and the description of Alnashetri has been worth the wait.

Speaking of Alnashetri, it is potentially the oldest known alvarezsaurid proper. It sure would be nice to have some more complete specimens of it... ;)

Reference: Xu, X., J. Choiniere, Q. Tan, R.B.J. Benson, J. Clark, C. Sullivan, Q. Zhao, F. Han, Q. Ma, Y. He, S. Wang, H. Xing, and L. Tan. In press. Two Early Cretaceous fossils document transitional stages in alvarezsaurian dinosaur evolution. Current Biology in press. doi: 10.1016/j.cub.2018.07.057

Dinosaurs in the Wild

Having heard many good things about the "immersive experience" Dinosaurs in the Wild, I figured that I should pay it a visit before it closes in London on September 2. I had largely neglected photography during the trip (and the few pictures I took didn't turn out great), so this post will be illustrated with images borrowed from the Dinosaurs in the Wild website.

For those who are unfamiliar with the experience, the conceit of Dinosaurs in the Wild is that time travel has been developed by the fictional company Chronotex, who have set up a research station in the Maastrichtian of western North America that is now open to visitors. Upon arrival, visitors are "transported" into the past and then "driven" to the research station. Along the way, they get their first look at the fauna of the Late Cretaceous (portrayed by photorealistic animation on 3D screens). As has been discussed at length by scientific consultant Darren Naish, the depictions of the animals are heavily informed by science, incorporating elements of both recent research and plausible All-Yesterdays-style speculation.

At the research station itself, visitors are first given a tour of the labs. Others have rightly praised the attention to detail presented by Dinosaurs in the Wild, and it is particularly prominent during this segment of the experience. The tables and cabinets are filled with "biological samples" taken from the "field", far too many for a visitor to completely take in on a single trip, but all inspired by real-world scientific research. Highlights include an Alamosaurus heart (which can be stimulated to beat), dinosaur fecal matter, and a preserved specimen of a stem-primate in a jar. I paid particular attention to the tray of feathers on one of the tables, which included not only "standard" pennaceous feathers but also speculative monofilaments from Pachycephalosaurus, as well as samples for which the source species had not been confidently identified (adding to the authenticity of the laboratory setting).

Continuing on, we were treated to an autopsy of a recently deceased Pachycephalosaurus (again, incorporating many recent findings about dinosaur anatomy) and then to a dinosaur hatchery. The eggs of different dinosaur species are shown to require different incubation strategies (also based on fossil evidence), and we were given the opportunity to meet a newly-hatched Dakotaraptor (portrayed by an animatronic).

Among the maniraptors depicted at Dinosaurs in the Wild, the recently-described large dromaeosaurid Dakotaraptor was the most prominently featured.

The next couple of rooms showcased "captive specimens" of Cretaceous animals (portrayed by more animatronics) being kept in the research station. I especially liked the "night room" dedicated to (speculatively) nocturnal species such as Didelphodon, Acheroraptor, and Leptoceratops. A nice touch was the "revelation" that the Leptoceratops had color patterns only visible under ultraviolet light.

But the best was naturally saved for last: having seen the inner workings of the research station, visitors are taken to an observation deck where four giant "windows" allow them to observe what's going on in the ecosystem "outside". It is impossible to keep track of all four windows at once, so one will inevitably miss some sequences, but any amount of attention given is sure to be rewarded by the sight of interesting (and believable) behaviors exhibited by the animals. A Prognathodon launches an attack on an unsuspecting dinosaur on a riverbank. A breeding colony of Dakotaraptor doesn't take kindly to an Alamosaurus wandering through their nesting ground. A group of Thescelosaurus repels a marauding Dakotaraptor by pelting it with sand, but lose one of their young to some Quetzalcoatlus. (Keen-eyed visitors will likely notice that some of these events are foreshadowed early in the tour: for example, one report shown at the beginning of the experience mentions that the Thescelosaurus appear to be unaccustomed to dealing with attacks from the Quetzalcoatlus.)

Despite Chronotex's apparent success, one gets the increasing impression during the tour that they still face some challenges that need to be ironed out, from contagious diseases that they're unprepared for to implied dinosaur escapes. When the tour eventually comes to an end, it is because the visitors are forced to evacuate. This is a good time for me to give props to the on-site staff I encountered, all of whom did an excellent job at staying in-character throughout. All of their words and actions, down to their conversations among themselves, were presented as organic reactions to the simulated events around them, even when they weren't the centerpiece of a given segment.

Notice the attendant enantiornithines perched on this Triceratops (also present on the Tyrannosaurus in the first image of this post). Unfortunately, I didn't notice any enantiornithines rendered in animated form at the actual experience (though it's possible they were simply overshadowed by larger animals), but their existence was certainly alluded to: they are included on observatory signage and samples of their feathers are on display at the lab.

Having already read glowing reviews of Dinosaurs in the Wild by Mark Witton and Marc Vincent (not to mention the positive insider's perspective of Darren Naish), I didn't exactly need any convincing of its overall merit. Indeed, my review offers little beyond adding to the chorus of positive reactions to it. However, being able to confirm its quality for myself was certainly a worthwhile experience.

What's more, the attraction seems to have been well received by the general public as well. At a time when many paleontologists frequently lament how mainstream media rarely reflects their latest research, Dinosaurs in the Wild appears to be an effective and successful means of exhibiting our current paleontological understanding to a broader audience.

Is there anything about Dinosaurs in the Wild that I would modify if I had the power to do so? It's easy to desire even bigger and better things. (More paleoenvironments! More dinosaurs!) However, in its current iteration, there's probably not much I'd change, to be honest. Okay... I suppose it needed more references to alvarezsaurids. (I didn't see any.)

Dinosaurs in the Wild is open in London until September 2, with future locations yet to be announced at the time of writing. If you are a paleontology enthusiast who is in reach of southern England during the next week or so, I would recommend a visit!