Tuesday, November 21, 2023

Cincinnati Zoo

As mentioned previously, the organizers of this year's SVP had arranged free entry to the Cincinnati Zoo for conference attendees. I ended up going to the zoo twice during the conference period just so I could see (almost) everything. I was quite impressed; the Cincinnati Zoo isn't massive, but it boasts a large diversity of species (many of which I'd rarely or never seen in other zoos) housed in generally well-designed exhibits.

One of the highlights for me was this expansive grassy habitat in the African section of the zoo, which features multiple species of large African birds (as well as lesser kudu). Here are just a few of the inhabitants, starting with a flock of pink-backed pelicans.

A gray crowned crane.

A lappet-faced vulture.

Some Rüppell's vultures.

Some eastern crested guineafowl.

A good look at the two-toed feet of a common ostrich.

Something I noticed about several of the exhibits at the Cincinnati Zoo was that they had mirrors placed within them, presumably as enrichment for the animals.

Another exhibit nearby is home to African wild dogs.

As avian paleontologists, my labmates and I were naturally eager to check out the bird house. Just outside of it is a habitat for Cape Barren goose, an unusual-looking Australian waterfowl.

A salmon-crested cockatoo perched above.

Unfortunately, I did not get many satisfactory photos inside the bird house itself, as wire mesh, wet glass, and the fast movements of the birds themselves made it challenging to do so. However, I very much enjoyed what I saw. Highlights included two walk-in aviaries (one containing Neotropical birds, the other birds of Southeast Asia and Australia), as well as several seabird exhibits with underwater viewing.

It was quite surreal to see these Inca terns perched on a branch and exhibited in a tropical rainforest setup.

A tawny frogmouth. I will always stop for strisoreans.

A helmeted hornbill.

In addition to a nice bird collection, the Cincinnati Zoo also houses a diverse range of primates. Here are some Coquerel's sifakas, also known to North Americans of a certain age as Zoboomafoo.

The zoo has a building dedicated to wildlife from the wetlands of the Southeastern United States with Florida manatees as the centerpiece, one of the few places outside of Florida where America's afrotheres can be seen. The open-mouthed (probably prey-luring) pose of this alligator snapping turtle was quite striking.

Just outdoors are some exhibits for hoofed mammals, such as these Visayan warty pigs.

A pair of bongo antelope.

Several displays near the entrance of the zoo provided homes for large raptors, including this Andean condor.

The reptile (and amphibian) house also had a nice selection of species, including these Titicaca water frogs. They are able to breathe through their baggy skin in the cold water where they live.

A juvenile Komodo dragon, having mostly lost its hatchling coloration but still far from full size.

At the center of the reptile house is an open-topped habitat for this Chinese alligator.

One of my favorite exhibits at the zoo overall was probably the nocturnal house, but photographing the inhabitants in the dark was of course difficult. I had to try and get a picture of this aardwolf though, this being the first time I've seen this species. It is the smallest of all extant hyenas and feeds almost entirely on termites. Hmm, I wonder why that might be of interest to me...

Most of the outdoor exhibits next to the nocturnal house featured wild cats (including mountain lions, tigers, and snow leopards), which we got some excellent views of. However, I was unsurprisingly most taken by this Eurasian eagle owl.

As the site where the last passenger pigeon and Carolina parakeet passed away, the Cincinnati Zoo has dedicated a poignant memorial to these species.

The insect house is quite large and contains more than just arthropods. It also displays, for example, these Solomon Island leaf frogs (eyeing a feeder cricket).

A black-breasted leaf turtle. According to exhibit signage, this species can rotate each of its eyes independently, similar to a chameleon.

There is no lack of actual arthropod displays in the insect house though. Here is a Baja whipspider.

Some giant water bugs (including males carrying eggs).

A Peruvian firestick, apparently a poisonous stick insect that has evolved bright colors to warn predators of its toxicity.

Another building nearby is primarily dedicated to exhibiting lizards, such as this quince monitor.

The star of the show here is this adult Komodo dragon. It lives in perhaps one of the most unexpected mixed-species zoo habitats I've seen, sharing its exhibit with zebra finches. Presumably, they are too small and quick to be seen as worthwhile prey. They certainly didn't seem too bothered from what I saw of them, actively fluttering to the exhibit floor to collect nesting material.

Monday, November 6, 2023

SVP 2023

Despite those of us on international flights being made to go through security twice, SVP attendees received a warm welcome in Cincinnati, Ohio this year.

The welcome reception was held at the Cincinnati Museum Center, which proved to be an excellent venue, being spacious enough that it generally didn't feel too crowded even with about 1000 paleontologists crammed inside. The Cincinnati Museum Center is composed of several museums and other facilities, but for me going through just the Museum of Natural History and Science took up most of the evening. Here is the skull of Apatosaurus (with some other sauropod parts).

Two large theropods from the Morrison Formation are mounted side by side, the megalosauroid Torvosaurus and the allosauroid... Allosaurus. The Cincinnati Museum of Natural History and Science apparently has the world's only Torvosaurus mount on display, and it was certainly an impressive sight.

Nearby is the tyrannosaurid Daspletosaurus.

You know you study birds though when one of the theropod mounts you were most excited about was a chicken skeleton...

It wouldn't be a vertebrate paleontology trip to Ohio without saying hi to Dunkleosteus!

I'd had no idea that stem-amniote tracks had been found in Ohio. These ones are Carboniferous in age.

The museum has an excellent Pleistocene exhibit, including this cabinet of North American carnivoran skulls. These species lived alongside each other during the Pleistocene, but only half of them are still extant today.

To drive the point home, part of the gallery consists of a model Pleistocene forest where restorations of extinct and extant North American organisms are exhibited side by side. I noticed that the museum signage was remarkably up to date, for example reflecting the recent reclassification of the dire wolf in the genus Aenocyon, distinct from Canis.

Not every extinct species shown in this gallery died out thousands of years ago, as it also includes models of passenger pigeons, which went extinct in the early 20th Century. Imagery depicting passenger pigeons is not difficult to come by in Cincinnati—the Cincinnati Zoo was where the last one died. Speaking of which, SVP deserves props for organizing free entry to the zoo for conference attendees, an amenity that I made sure to capitalize on. That should probably be the subject of its own post though.

This year's SVP had no fewer than six talk sessions focused on dinosaurs. It would have been possible for one to attend almost nothing but dinosaur sessions and still be present at the conference throughout the entire time that talks were being held. As with last year, my labmates and I represented a large proportion of the speakers in the bird session (to the point where other speakers this year joked about it). 

We were also graced by the presence of a very special attendee. Session moderator Meig Dickson had brought along eir green-cheeked parakeet Ellie, making this the only time in SVP history I'm aware of that a dinosaur has helped moderate a talk session. Ellie was remarkably well behaved throughout the conference, for most part only vocalizing during audience applause. In any case, my own talk went well and received Ellie's blessing.

As others have written about, the Cincinnati/Northern Kentucky International Airport plays host to several skeletal mounts of Pleistocene megafauna, including a mastodon, the "stag-moose" Cervalces, and the ground sloth Megalonyx. I'd missed these on my way in to Cincinnati, but got to see them during my departure.

All in all, I had a lot of fun at SVP this year, but I know that not everyone came away from it so fortunate. As of the time of writing, the proportion of attendees who have reported contracting COVID-19 during or after the conference has been twice as high this year as it was in 2022. Notably, no masking mandate was in place for this SVP, and I hope the organization reconsiders this policy at future meetings. I managed to avoid the disease this time, but having caught it for the first time myself during SAPE earlier in the year, it's not an experience that I'm eager to repeat again, nor would I wish it on anyone else.

Tuesday, June 13, 2023

Prehistoric Planet Season 2

To the delight of many, Prehistoric Planet came back this year for a second season. Much of the praise that was lavished on the first season is equally applicable to this one, so I think it's fair to cut to the chase and dive into the specifics of its maniraptoran portrayals. Season 2 follows its predecessor in focusing on life during the Maastrichtian Age of the Late Cretaceous, and with this setting comes a whole cast of both new and returning maniraptoran taxa.

One of these returning maniraptorans is the oviraptorosaur Corythoraptor from the Nanxiong Formation of China, this time shown in a nesting colony. From an aesthetic standpoint, I'm very much a fan of the Corythoraptor model in this series, so it was fantastic to see it get some airtime besides becoming tyrannosaurid chow. This segment was also a great opportunity to showcase the relatively large amount of data we have on oviraptorid reproduction. Sharp-eyed viewers might notice that the Corythoraptor eggs have a rough surface texture composed of numerous tubercles and are arranged in multiple superimposed, partially buried rings with their narrower ends pointing outward and inclined downward, all of which are traits that have been observed in preserved oviraptorid nests. The narration informs us that male Corythoraptor are responsible for tending to the nests, a behavior that has been suggested for oviraptorids based on several lines of evidence. The posture of the nesting Corythoraptor and the presence of a central opening in the nest for them to sit in are also inspired by known oviraptorid specimens.

A Corythoraptor inspects its nest.

There are a few details in this depiction that may be controversial. One scene shows a Corythoraptor moving one of its eggs to a different spot in the nest, but it has been argued that most Mesozoic pennaraptorans likely did not manipulate their eggs after they were laid. In fact, a few studies have disputed the popular idea that oviraptorids incubated their own eggs at all, primarily on the basis of their nest architecture, though recent experimental evidence indicating that such eggs would have still benefited from the body heat of a parent and the absence of any obvious alternative means of incubation currently suggests to me that some level of contact incubation in these dinosaurs is more plausible. I was surprised that the Corythoraptor eggs were not colored blue-green following the inference of such coloration in eggs referred to Heyuannia. This is not a demonstrable inaccuracy (as egg coloration can be highly variable in closely related modern birds), but it seems like the type of scientific detail that Prehistoric Planet has otherwise fallen over itself to include.

Like nest-guarding animals of today, the Corythoraptor in the show have to contend with potential nest raiders. There is an irony to oviraptorids (once interpreted as specialist "egg thieves") being shown suffering from egg predation themselves. In this case, the main threat that appears is the dromaeosaurid Kuru, an unexpected choice given that this dinosaur was named in 2021 (almost certainly well into the production of the series) and is not known to have occurred at the same locality as Corythoraptor, instead having been discovered in the Barun Goyot Formation of Mongolia. I wasn't particularly bothered by this—if these taxa overlapped in time (which is a possibility given that the ages of many Upper Cretaceous fossil-bearing rock units in Asia are far from well constrained), it's entirely plausible that they had geographic ranges larger than their current fossil record would suggest. The Kuru is depicted using the cover of darkness to target the Corythoraptor nests, likely based on evidence from scleral ring anatomy that some dromaeosaurids, including the closely related Velociraptor, may have been primarily nocturnal. 

Speaking of Velociraptor, it is another returning face from the first season. In this season's outing, a family of Velociraptor are portrayed using steep terrain to their advantage while hunting the pachycephalosaur Prenocephale. It's a nice sequence that further showcases the potential diversity of hunting strategies in dromaeosaurids, and includes some cute baby Velociraptor to boot. New to the series (and to paleontology documentaries in general, as far as I'm aware) is the bizarre giant unenlagiine dromaeosaurid Austroraptor from the Allen Formation of Argentina, which is shown preying on fish, as has been hypothesized based on its long snout containing numerous conical teeth.

An Austroraptor dismembers a gar.

The final dromaeosaurid genus to appear in this season is Pyroraptor from southeastern France, with a few juveniles briefly showing up on a shoreline to eat some stranded ammonoid larvae. I'll admit, it did enter my head that Asteriornis* would have been an excellent fit for the geographic location and ecological role demanded by this scene, but I understand that reusing the existing juvenile dromaeosaurid model was undoubtedly much easier for the filmmakers than designing an entirely new animal for such a minor part.

*I'd originally assumed that Asteriornis, having been described in 2020, did not have a chance of being included in either season of Prehistoric Planet. However, the second season does extensively feature the Malagasy mammaliaform Adalatherium, which was described about a month after Asteriornis was.

Moving away from dromaeosaurids (probably), this season also depicts Imperobator from the Snow Hill Island Formation of Antarctica. Imperobator was a large paravian of unclear affinities, and unlike dromaeosaurids, its second toe on each foot was not specialized for being raised off the ground, which Prehistoric Planet portrays correctly. In one scene, several Imperobator are shown as though they were filmed using a thermal imaging camera, and later on, the group attempts to hunt the ornithopod Morrosaurus. Coincidentally, the events in this segment bear some resemblance those of to a fan-made storyboard sequence by Denver Humphries and Ilari Pätilä.

A pair of Imperobator catch their breath on the surface of a frozen lake.

Troodontids are represented in a couple of scenes. Several individuals investigate a fresh Alamosaurus carcass, but, consistent with studies on the structure and wear patterns of troodontid teeth, they are unable to penetrate its thick hide until a Tyrannosaurus opens it up. That troodontids coexisted with these larger dinosaurs is not in doubt, given that their teeth have been found in the Ojo Alamo Formation (or Kirtland Formation, depending on your terminology) of the southwestern United States. Pectinodon from the Lance and Hell Creek Formations further north appears in a separate segment, in which a group visits an alkaline lake to feed on swarms of brine flies, a behavior probably inspired by some extant gulls. The Pectinodon family here consists of a male and its offspring, reflecting the fact that male parental care has been inferred in troodontids for the same reasons that it has in oviraptorids.

Not content with only the flies as a food source, the adult Pectinodon is shown preying on some other paravians that have gathered at the lake, a flock of presbyornithids. An unusual decision was made here to refer to the presbyornithids by a name that has yet to be formally published, originally used for some undescribed pectoral girdle bones from the Hell Creek Formation. Regardless, the anatomy of the presbyornithids, like that of essentially all the other prehistoric animals in the show, is depicted very solidly. Details such as the webbed feet and the flattened, hook-tipped bill lined with keratinous plates for filter feeding were evidently referenced from more completely known Cenozoic presbyornithid fossils. In the interest of full disclosure, I should mention that conversations I've had with Darren Naish, the main scientific consultant on the series, were used to inform these presbyornithid restorations. So that's my informal and infinitesimally minuscule contribution to Prehistoric Planet, and I'm relieved that I no longer have to keep the existence of Season 2 a secret!

The flamingo-duck's smile.

Having said that, my most severe criticism of this season concerns this segment. After the presbyornithids are introduced, the narration follows up with, "Dinosaurs, are here, too" as an segue to the arrival of the Pectinodon, thus implying that the presbyornithids are not also dinosaurs! This is not only flat-out inaccurate, but not mentioning that birds are themselves a group of dinosaurs seems like a major missed educational opportunity, especially considering that Prehistoric Planet has not shied away from showing non-avialan dinosaurs with bird-like appearances and behaviors (including in this very same segment). Darren has hinted that wording more explicitly recognizing the dinosaurian nature of birds was met with pushback from other forces during show production, which I think is incredibly unfortunate given that Walking with Dinosaurs had no issue with referring to avialans as "flying dinosaurs" in 1999.

That's not to end on a dour note, however. There is one more maniraptoran featured in the series left to discuss: the flightless marine euornithean Hesperornis, probably one of the most aquatically-adapted dinosaurs ever to have lived. The best known fossils of Hesperornis are between Coniacian–Campanian in age, but undescribed specimens from the Hell Creek Formation have been mentioned in scientific literature. Having been named in 1872, Hesperornis is no stranger to science, nor to appearances in popular media. However, I feel reasonably secure in saying that the Hesperornis in this series is the most scientifically rigorous depiction of this taxon ever animated in a work of this genre. In Prehistoric Planet, Hesperornis has a beak occupying distinct parts of its jaws from its teeth (despite the narration incorrectly stating that it had a "beak full of [...] teeth"), sprawling hindlimbs with the shins held close to the body, and a rudder-like tail fan (suggested by its broad, flat tail vertebrae), all features that are commonly overlooked in artistic restorations.

A Hesperornis terrorizes a school of fish.

Last but not least, it should be said that Prehistoric Planet Season 2 goes a long way towards addressing both of my main concerns about the first season: the near-absence of tetrapods that weren't non-avialan dinosaurs, pterosaurs, or large marine reptiles, and the limited disclosure about the science behind creative decisions made for the show. This season not only includes two avialan species (the presbyornithids and Hesperornis), but also the crocodylomorphs Simosuchus and Shamosuchus, the snake Madtsoia, the mammaliaform Adalatherium, and (returning from the previous season) the large frog Beelzebufo in prominent roles. On a related note, though ammonoids were not tetrapods, Prehistoric Planet must also be commended again for highlighting the diversity and life history of these key players in Mesozoic marine ecosystems, in a way that no other paleontology documentary has done before.

As for scientific content, twice as many "Uncovered" videos explaining the scientific backing behind the series were produced for this season compared to the last. Half of these segments are also directly attached to the main series episodes (instead of only being available as separate videos), likely allowing them to reach a wider audience. Furthermore, a Prehistoric Planet podcast was released, explaining additional details about the show's creative process. These are welcome additions to the lineup of official Prehistoric Planet media, but they probably still just barely unveil the curtain on the substantial effort and decision-making that assuredly went into the series. I for one would be eager to see even more behind-the-scenes material.

Whether there's more in store from Prehistoric Planet, only time will tell. However, for now I wouldn't be too upset if this were all we ever got. The two seasons of this show have been a veritable triumph, representing by far the most scientifically grounded and up to date portrayal of life in the latest Cretaceous that's currently available as a documentary series.

Wednesday, May 24, 2023

Alvarezsaurid Paleobiology, an Update

It's been a few years since I wrote a post about alvarezsaurids on this blog, in which I discussed the interpretation of these dinosaurs as specialized insectivores that used their unusual forelimbs to dig for social insects, and why I happened to think that that was the most convincing model for their lifestyle and ecology. Back then, I had little expectation that there would be much alvarezsaurid news in the immediate future, so it was a pleasant surprise to see an alvarezsaurid featured prominently and portrayed as a termite-eating specialist in the high-profile documentary series Prehistoric Planet last year. Not only that, but many studies on alvarezsaurids have been published in the intervening time. Has this newfound information updated and modified my thoughts on how alvarezsaurids lived? Let's take a look.

The New Alvarezsaurs on the Block
To start off, several new alvarezsaurid taxa were named in recent years, including Trierarchuncus from the Hell Creek Formation of the western United States (Fowler et al., 2020; Freimuth and Wilson, 2021), Dzharaonyx from the Bissekty Formation of Uzbekistan (Averianov and Sues, 2021), and Khulsanurus and Ondogurvel from the Barun Goyot Formation of Mongolia (Averianov and Lopatin, 2022a; Averianov and Lopatin, 2022b). The braincase of an indeterminate alvarezsaurid from the Qiupa Formation of China was also described (Agnolín et al., 2022), and the anatomy of Parvicursor was reevaluated in detail (Averianov and Lopatin, 2021).

Nearly complete thumb claw of Trierarchuncus, from Fowler et al. (2020).

In general, most of these new findings don't substantially change what I previously talked about, but it's great to have more alvarezsaur anatomy to work with. Trierarchuncus deserves mention, however, in that one of the specimens referred to it represents the most completely preserved alvarezsaurid thumb claw to date. I noted in my previous post on alvarezsaurs that complete alvarezsaurid thumb claws are hard to come by, making it difficult to compare their shape to the claws of modern animals. This Trierarchuncus specimen reveals that at least some alvarezsaurid thumb claws were much more sharply curved than formerly expected, in line with their hypothesized function in hook-and-pull digging.

Reconstructed curvature of the thumb claw of Mononykus based on that of Trierarchuncus, from Fowler et al. (2020).

In addition, comparing the claws of differently sized Trierarchuncus individuals suggests that as alvarezsaurids aged, the bony core of their thumb claws became wider and gained a roughened texture (possibly induced by stress) near where the claw attached to the rest of the thumb, consistent with the use of the claws in strenuous activity (Fowler et al., 2020; Freimuth and Wilson, 2021).

Turning Tail
As I mentioned in my previous post, a long tail for balance can be a helpful adaptation to a hook-and-pull digger. Although alvarezsaurids were long tailed, however, a long tail is typical of diapsid reptiles, so this in itself was not necessarily a specific adaptation to digging in alvarezsaurids. That being said, alvarezsaurids did have a few unusual features of the tail that may shed light on their lifestyle, and Meso et al. (2021) provided a detailed description and functional interpretation of alvarezsaurid tail anatomy.

For one, alvarezsaurids were long-tailed even for theropods; relative to their body size, they probably had among the longest tails of any maniraptoran. For another, many features of their tail vertebrae suggest that their tails were more flexible from side to side than those of most other theropods. Probably to compensate for this increase in length and mobility, the muscles along the top of the tail that would have helped hold it aloft appear to have been well developed in alvarezsaurids. Also abnormal among theropods is the fact that alvarezsaurid tail vertebrae were procoelous: the back end of each vertebra fit into a socket in the front end of the vertebra behind it. (In typical theropods, the joint surfaces of the tail vertebrae tend to be flat or nearly so.) This may have been another way of reinforcing the tail, giving it further mobility without a correspondingly high risk of dislocating the vertebrae.

Reconstructed tail musculature of an early alvarezsaur (A), a patagonykine alvarezsaurid (B), and a parvicursorine alvarezsaurid (C), from Meso et al. (2021).

Meso et al. note that overall configuration of the alvarezsaurid tail exhibits similarities to that of the extant aardvark (Orycteropus afer), which also has large muscles running along the top of a long (by mammal standards), strong, and flexible tail. In aardvarks, the tail probably serves an important role as a brace while digging (Endo et al., 2013). This could have also been the case in alvarezsaurids, but Meso et al. suggest that their characteristic tail morphology might have provided another adaptive benefit. As I discussed previously, one of the many strange features seen in alvarezsaurids is their apparent specialization towards running, and it is likely that they relied on this as a means to escape predation. Meso et al. posit that their increased tail flexibility gave alvarezsaurids a smaller turning radius, granting them a further advantage in avoiding pursuers.

Diagram showing how the flexible tail of alvarezsaurids might have helped them avoid predators (with a successful escape making use of a reduced turning radius shown in B), from Meso et al. (2021).

Creatures of the Night
How an animal perceives the world naturally has a major influence on how it interacts with its environment and the other organisms in it, and Choiniere et al. (2021) offered some considerable insight into this aspect of alvarezsaurid biology. Their study was not focused solely on alvarezsaurs, but compared the proportions of the scleral ring (a bony ring embedded in the eyeball of many vertebrates, though not in mammals) and the anatomy of the inner ear in a wide variety of Mesozoic dinosaurs to those of extant birds and other reptiles. However, the results they found for the alvarezsaurid Shuvuuia (one of the few alvarezsaurs in which both of these elements are well preserved) were especially intriguing.

The anatomy of both the scleral ring and inner ear in Shuvuuia strongly suggests that it was primarily active at night. What's more, its inner ear exhibits an extreme morphology similar to that of the extant western barn owl (Tyto alba), a nocturnal predator that famously uses sensitive hearing to detect prey. It's important to emphasize here (as I've seen some popular retellings of this study misunderstand this) that the observed similarities between the inner ear of Shuvuuia and barn owls are due to the structure of the ear itself, not related to ear asymmetry.

Barn owls and their close relatives in the group Tytonidae are one lineage of owls that have evolved asymmetrical ears, which help them pinpoint the location of their prey. Ear asymmetry in owls can take several different forms, but in tytonids, the left ear opening is generally positioned higher than the right. However, this asymmetry is not reflected in their skull (Norberg, 2002). There are owls (such as those in the genus Aegolius) that do have asymmetrical skulls, but tytonids are not among them. Choiniere et al. also found no evidence of skull asymmetry in Shuvuuia. Might alvarezsaurids have instead had asymmetry in the soft tissue morphology of their ears, like barn owls? That seems like a possibility, but we may not be able to determine one way or another from their bony anatomy.

The back of the skull and inner ear of Shuvuuia (A–C) compared to that of a western barn owl (D–F), from Choiniere et al. (2021).

As noted by Choiniere et al., a combination of a nocturnal lifestyle with specialized hearing is widespread in tetrapods, especially in mammals, so the fact that Shuvuuia exhibits evidence of these traits does not by itself indicate that alvarezsaurids were myrmecophagous (feeding mostly on social insects). However, there is indeed an extant myrmecophage that often forages at night primarily by acoustic cues, the bat-eared fox (Otocyon megalotis) (Renda and le Roux, 2017). Therefore, I consider these new findings on alvarezsaurid sensory biology very much compatible with the hypothesis that they were myrmecophages. (In fact, I was aware of Choiniere et al.'s research before its formal publication, and deliberately inserted a reference to the bat-eared fox into my last alvarezsaur post as oblique foreshadowing.)

A bat-eared fox, photographed by Yathin S Krishnappa, under CC BY-SA 3.0. This species feeds mainly on termites, which it locates using its sensitive hearing. Perhaps alvarezsaurids did the same?

The Incredible Shrinking Alvarezsaurs
My previous post also mentioned small body size as as a curious evolutionary trend in later alvarezsaurids, and Qin et al. (2021) investigated this phenomenon in more detail. First of all, by inferring growth patterns based on the bone microstructure of individual alvarezsaur specimens, they confirmed that some alvarezsaurids, such as Albinykus and Xixianykus, had adult body masses of only about 1 kg or less. (It's worth noting though that one of the smallest known alvarezsaurids, the type specimen of Parvicursor, was subsequently reinterpreted as a juvenile instead of an adult by Averianov and Lopatin, 2021.)

Interestingly, Qin et al. indicated that Albinykus and Xixianykus had very different growth strategies from one another despite being of similar sizes. Whereas the type specimen of Albinykus had essentially stopped growing by the time it was three years old, the type specimen of Xixianykus grew more slowly but across a longer period of time, living for over a decade. This diversity in alvarezsaurid growth patterns was also observed more recently by D'Emic et al. (2023).

When Qin et al. plotted alvarezsaur body size across the evolutionary history of these dinosaurs, they found that not only were later alvarezsaurids smaller overall than their ancestors, but their small body size evolved very quickly in the early Late Cretaceous, about 90 million years ago. What may have caused this sudden miniaturization? Qin et al. point out that this event would have shortly followed the estimated diversification of both ants and termites during the Cretaceous, which may have created the opportunity for dedicated myrmecophagy to evolve in alvarezsaurs. Specialized insectivory as a driver of alvarezsaurid size reduction would also align with the median body size of insectivorous land vertebrates being consistently smaller than those with other dietary habits, a pattern that has been shown to hold true across different groups and biomes (Cooke et al., 2022).

Evolution of alvarezsaur body size over time, from Qin et al. (2021).

When All You Have is a Pick...
As I hope I reviewed thoroughly in my last alvarezsaur post, there are many features of alvarezsaurid forelimbs that suggest they functioned in digging. However, just how effective their claws would have been at this task had not been quantitatively tested until recently. Qin et al. (2023) applied finite element analysis (FEA) to alvarezsaur hand claws to compare how they performed under different scenarios. (This study also looked at the claws of therizinosaurs, but this post is not about them.) 

For each claw studied, three scenarios were tested: piercing (puncturing a substrate), pulling (using the underside of the claw to pull an object downward), and scratching (dragging the tip of the claw through a substrate), with the last being considered analogous to digging. When compared to a variety of mammals representing a range of claw morphologies and functions, the claws of alvarezsaurids were found to behave akin to those of pangolins: experiencing relatively low stress and similar stress distributions across all three scenarios. On the surface, it may sound surprising that such seemingly specialized claws would be well suited to multiple tasks in this way, but as Qin et al. point out, this makes a lot of sense if alvarezsaurids were diggers. Hook-and-pull digging in particular requires piercing, pulling, and scratching motions, so it would be potentially advantageous for animals adapted for this behavior to excel at all three.

Furthermore, Qin et al. found that the claws of alvarezsaurids performed better at scratching than those of earlier, non-alvarezsaurid alvarezsaurs, which had hands more broadly similar to those of typical theropods. This may suggest that increased digging specializations were specifically being selected for during alvarezsaur evolution.

Claw performance under different simulated scenarios in a selection of mammals (a), non-alvarezsaurian theropods (b), and alvarezsaurs (c), from Qin et al. (2023). What surprises me the most is that the fields for the pangolin and the tamandua plot so far apart, especially considering that, according to the supplementary material, the pangolin species studied was the tree pangolin (Phataginus ["Manis"] tricuspis), which, like tamanduas, spends a lot of time in trees. Maybe there is more functional variation among these superficially similar myrmecophages than commonly appreciated.

It makes me happy to see so much new alvarezsaur research being done lately, and new advances have been made in understanding the digging abilities of extinct mammals as well (Nakai and Fujiwara, 2023), which could perhaps be applied to alvarezsaurs in the future. I'm certainly pleased that essentially all of these new alvarezsaur studies either reinforce or are consistent with the hypothesis that alvarezsaurids were myrmecophages. However, what excites me most of all is that I might have had a very small hand in bringing some of this science to fruition. I have it on good authority from Qin Zichuan that he and his colleagues' study on alvarezsaurid claw biomechanics was directly inspired by my previous alvarezsaur post, in which I idly suggested some potential directions for future investigations into alvarezsaurid ecology. For my humble blog post, having stimulated actual scientific research on these remarkable dinosaurs is just about the highest honor that I can conceive of.

Excerpt from the acknowledgements of Qin et al. (2023).