One of the strangest theropod groups must have been the Alvarezsauridae, an unfortunately-named group of bizarre, stubby-armed cursorial dinosaurs who, upon initially described, were thought of as primitive flightless birds. Indeed, their exactly phylogenetic placement has always been tenuous, with some authors placing them near the Ornithomimidae while others prefer the stump-arms to sit at the base of the Maniraptora. That discussion is for another day, though. Today, we talk about their deviant lifestyles. Two animals in particular serve as baselines for the group, because they are the best-known members: Mononykus olecranus and Shuvuuia deserti. The former is known from more complete body remains, while the latter is known from good body remains and a wonderfully-preserved skull. Alvarezsaurs are small, with short tails, extremely bird-like pelves, ridiculously long hindlimbss, ridiculously short forelimbs, and lightweight, avian skulls.
In addition to their small size, alvarezsaur arms are unique in their construction: The humerus is a short but robust bone with a large deltopectoral crest. The radius is very short, while the ulna is fairly long, bearing a signficant olecranon process for the attachment of a powerful tricep muscle. The carpal block has been reduced to a single large, semirectangular bone. A single massive alular digit bears a similarly massive, blunt-tipped claw. Digits II and III were not completely absent--Shuvuuia and more basal forms have demonstrated that those digits were present but extremely reduced to the point of potentially not being visible on the living animal. On the other hand, Shuvuuia's second and third digits bear small, but sharp, claws, suggesting that perhaps they did have some function in life.
As early as 1994, Altangarel, et al. suggested that the forelimbs of Mononykus were similar to those of fossorial tetrapods, but noted that the rest of the animal's body contradicted a burrowing lifestyle, concluding with: "...and the forelimbs were presumably used for some other function requiring powerful movement." Various paleoartists and workers have informally suggested myrmecophagy (social insect-eating) over the years, but until just last week, a real discussion of such a hypothesis had gone unwritten.
In their new paper out in the new issue of Cretaceous Research, Nich Longrich and Phil Currie take a close look at the burrowing and myrmecophagy hypotheses. The authors note that the alvarezsaur humerus much resembles that of burrowing mammals such as the mole and echidna. In addition, alvarezsaurs feature other skeletal adaptations consistent with a digging habit, including an increased number of sacral vertebrae and a reduced pubic symphysis--both handy adapatations for hindlimb bracing. However, the rest of the alvarezsaur body, as lamented by Altangeral, et al., is woefully inconsistant with any sort of burrowing. The long legs are indicative of extremely cursorial habits. The long neck and pointy snout would have gotten in the way of any potential burrowing attempts. Worse yet, the forelimbs' basic form make for extremely ineffective digging instruments. Longrich & Currie write: "This morphology would be no more useful for burrowing through soft substrates than a geologist's pick is for moving soft earth or sand." Alvarezsaur arms probably couldn't fling material back, either, as they were essentially built for moving forward and inward, toward each other. Extension and flexion at the elbow would have been very limited, and I wonder how far that single alular digit could move (probably not much).
However, aside from a curiously fossorial humerus, alvarezsaurs have hands resembling those of nest-raiding insects like anteaters, pangolins, and the giant armadillo, an animal that is actually able to support its weight on its giant manual claw. To test the hypothesis that alvarezsaurs were myrmecophagous, the authors compared alvarezsaur anatomy to that of anatomical features shared by modern mammals that snack on social insects. For example, most myrmecophages have a toothless area at the front of the jaws so that the tongue can slurp up escaping bugs without having to tire the jaw muscles. Sloth bears and numbats ("marsupial anteaters") have reduced or moved their first pair of incisor teeth, while aardvarks, anteaters, pangolins, echidnas, and bug-eating armadillos have simply lost their front teeth entirely. While not entirely toothless at the front of the jaw, alvarezsaurs seem to have lost the teeth at the tip of the dentary. In addition, myrmecophages have extremely simplified dentition, when teeth are present at all. Alvarezsaurid teeth, exemplified by those of Mononykus and Shuvuuia, are simple needle-like structures lacking serrations and are very small. Interestingly, there are at least 50 teeth in each dentary, which gives alvarezsaurs one of the highest tooth counts among theropods.
Unsurprisingly, myrmecophages have long, narrow jaws, which allows the animal to stick the mouth into an insect nest and slurp up a gaggle of bugs with every lick. Shuvuuia, again representing the group, has an extremely long and slender snout. Additionally, Chiappe, et al. (1998) suggest that the snout was prokinetic as in many birds, perhaps a further adaptation for allowing the tongue to zip out of the mouth without the loss of the premaxillary teeth. However, despite the length of the jaws, myrmecophages have amazingly weak mandibles because they are not busy slicing meat or crushing bone. Shuvuuia has elongate, "delicately constructed" dentaries. And as a final point of comparison, myrmecophagous mammals have reduced jaw articulations, probably because they are not busily processing their food. And indeed, the joint surfaces of the quadrate and articular in Shuvuuia are highly reduced.
But wait--there's more! Not content with merely making valid comparisons between alvarezsaurs and modern myrmecophages (a la azharchid pterosaurs--another paper I love), Longrich & Currie tried to figure out what kind of social insects were available for harvest during the Maastrichtian in Alberta.* One might immediately shout out "ANTS!" but those well-intentioned people would be wrong. While ants were present in Alberta during the Late Cretaceous, they appear to be primitive forms that lacked the social organization so prevalent in Cenozoic. In fact, of the thousands of amberified insects known from Alberta, less than 0.1% are ants. What about termites--another popular myrmecophagous menu item? As it turns out, termites have evolved to inhabit virtually every kind of wood there is, from dampwood to drywood to subterranean wood to soil. Termites are freaking everywhere. But were they everywhere during the Late Cretaceous? Although termite body fossils are unknown in Alberta, their wood-borings are quite common, and curiously similar to the boring galleries of Zootermopsis, a dampwood termite that inhabits temperate regions of western North America. It seems probable, then, that North American alvarezsaurs dug into dead and/or rotting wood in search of dampwood termites.
Better still, fossil wood samples from the Nemegt Formation of Mongolia also appear to have been attacked by termites, which gives Mononykus something to eat as well. The explosion of insect evolution during and after the rise of flowering plants is well known, and it would seem that as insect groups rose in importance during the Mesozoic, dinosaurs also adapted to exploit these new food sources. I congratulate Dr. Longrich and Dr. Currie on a wonderful and informative paper!
The picture at the top is a preliminary drawing I did of Mononykus with a drawing of the critter's arm after Altangerel, et al., 1993. The middle picture was shamelessly stolen from the PaleoBlog (sorry!). Also note the similarities between alvarezsaur arms and heads with those same body parts of Drepanosaurus, a monkey-lizard from the Late Triassic with an exceptionally large 2nd manual claw, the same general "burrowing" adaptations in the forelimb, and a slender snout. I should really blog about this guy sometime. Until then, Matt Celesky has written up a beginner's guide to the group at the Hairy Museum of Natural History.
Altangerel, P., Norell, M. A., Chiappe, L. M. & Clark, J. M. (1993). Flightless bird from the Cretaceous of Mongolia. Nature 362: 623-626.
Altangarel, P., Chiappe, L. M., Rinchen, B., Clark, J. M. & Norell, M. A. (1994). Skeletal morphology of Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Museum Novitates 3105.
Chiappe, L. M., Norell, M. A. & Clark, J. M. (1998). The skull of a relative of the stem-group bird Mononykus. Nature 392: 275-278.
Longrich, N. R. & Currie, P. J. (2008). Albertonykus borealis, a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: implications for the systematics and ecology of the Alvarezsauridae. Cretaceous Research: Sent to me by the senior author.
Suzuki, S., Chiappe, L. M., Dyke, G. J., Watabe, M., Barsbold, R. & Tsogtbaatar, K. (2002). A new specimen of Shuvuuia deserti Chiappe et al., 1998 from the Mongolian Late Cretaceous with a discussion of the relationship of alvarezsaurids to other theropod dinosaurs. Contributions in Science 494.
*The paper introduces a new alvarezsaur taxon into the family: Albertonykus borealis. Annoyingly, the paper doesn't tell you where the animal was actually found. Wait, wait, there it is, in the "Geological Setting" section. Let's see here, Horseshoe Canyon Formation, Late Campanian to Early Maastrichtian...oh, oh here it is: south-central Alberta. So it's from Alberta. Have I mentioned before how much I despise naming an animal after it locality? Let's see how many dinosaurs we can name that lived in western Canada that are named after their respective regions: Albertosaurus, Albertaceratops, Albertonykus, Edmontonia, Edmontosaurus, Dromaeosaurus albertensis, Pachyrhinosaurus albertensis, Ornithomimus edmontonicus...am I missing any? Seriously, there are few things lazier (my opinion only) than naming an animal based on where it was found. "Well, there are all sorts of awesome things we could name this new ceratopsian with eight fucking horns on its head, but Albertaceratops should suffice." IT DOESN'T FUCKING SUFFICE! Use your goddamn imaginations!
I'm sorry! It's a thorn in my side!