Another Burrowing Ornithopod

Above: Type specimen of Koreanoaurus (minus referred pelvis and hindlimbs) from Huh et al. 2010.

Though Bob Bakker had been suggesting that small ornithpods dug burrows for years, on the basis of the kind of sediments the basal ornithopods of Montana were usually found in, we didn't get solid confirmation that these were really burrowing creatures until the discovery of Oryctodromeus. Here was an ornithopod with the same, vaguely burrower-like features as its relatives Orodromeus and Zephyrosaurus, but which was found inside an obvious burrow.

The burrow was the key, because aside from some fairly ambiguous skeletal details, these dinosaurs all had fairly standard ornithpod proportions: elongated necks, long (usually) stiffened tails (but see below), long hind limbs and short front limbs. The overall body plan was that of a bipedal runner, not a dinosaurian wombat.

While still far from mole or wombat like, the new dinosaur Koreanosaurus boseongensis (named by Min Huh, Lee Dae-Gil, Kim Jung-Kyun, Lim Jong-Deock and Pascal Godefroi) is even closer than its relatives. It has very robust forelimbs, and while the humerus is still long, the forearm is short and stout, with a massive scapula and coaracoid, and a big keeled breastbone, all of which indicate attachment sites for powerful muscles useful for digging. Interestingly, the hind limbs are also very specialized. They're relatively short compared to the forelimbs, with a low ratio between the femur and tibia lengths, and with short metatarsals. The length indicates that even if this wasn't a fossorial creature, it was probably a quadruped. The hip is especially interesting. The head of the femur, the bit which fits into the hip socket, is at a 135 degree angle to the rest of the bone. This would have given Koreanosaurus a very un-dinosaurian semi-splaying leg posture, similar to burrowing mammals. The authors speculate that it would have used its legs to brace itself inside an incipient burrow while it used its powerful forelimbs to shovel soil.

So, while the team of scientists was unable to locate any nearby fossil burrows big enough to have been made by this (roughly) meter long ornithopod, the skeletal details are more than enough to suggest a digging lifestyle. But it was no dinosaurian mole, as it still had many features in common with terrestrial dinosaurs, like a long neck and (presumably) long, partly stiffened tail. However, we shouldn't be so quick to assign stiffness to the tail just because this is found in other ornithopods. As I reported before, the Australian Leaellynasaura had an unusual, very long, very flexible tail.

Koreanosaurus was found in seaside cliffs of Boseong, on the south coast of Korea. It is the first Korean dinosaur known from good remains. It should be noted that another Korean dinosaur (a theropod) had previously been unofficially named "Koreanosaurus," but as this was a nomen nudum, it's no more a valid scientific name than "Sue," and is rightly ignored.

Quick News: SV-POW & SVP

Last time I reported on the odd case of a crazy new amateur paper on Morrison sauropod diversity, including the naming of a new species of Amphicloelias. I hoped that the SV-POWsketeers would comment on this situation, and they have. Be sure to check out this post and it's two follow-ups, as well as the comments (including comments by one of the paper's authors). The upshot is that "A. brontodiplodocus" has not been published, and the authors claim the current .pdf is an unfinished manuscript, but that they stand by their ridiculous conclusions nevertheless. As far as I know, because the name has only appeared in electronic form which is not recognized by the ICZN, "A. brontodiplodocus" can't even be considered a nomen nudum. It may be a nomen manuscriptum or something.

In more pleasant, mainly non-taxonomic quibbling news, SVP is happening right now! Those of us lucky enough to not be within a few hours drive of Pittsburgh for once in their lives (I kid!) but unlucky enough for that one time to coincide with the biggest paleo event of the year, can follow the interesting stuff in real time on Twitter, thanks largely to the efforts of Brian Switek of Lealaps, who is braving the conference's strict press policy and lack of free wifi to get the news out. Follow @Laelaps for hints about sampling biases, even more new, weird ceratopsians, how Euoplocephalus is over-lumped (early '80s favorite Scolosaurus coming back, I wonder?), and which bloggers are going to the bar tonight.

Brontodiplodocus

Above: "Ah love filter feedin' thorugh mah beak! Om nom nom." Apatosaurus louisae (or not?), photo by Tadek Kurpaski, licensed.

Dan Chure on the DML alerted us to this new, privately published monograph published without peer review (probably?) by an independent fossil digging/selling organization. It concerns a pretty damn remarkable looking bonebed from the lower Morrison Formation with several complete diplodocid specimens of various ages. This line from page 21 pretty much sums it up:

"The traditional approach would have provided us with two new species to add to the Morrison list of sauropods. Instead we employed a novel approach by attempting to fit previously reported Morrison fossils within the context of the A. brontodiplodocus sample. The results are astoundingly radical by comparison with previous studies."

I don't know what to say about this paper. It wouldn't surprise me THAT much if something like this were true but... really? Really guys? Fingers crossed that SV-POW or some some other sauropod experts take a look at this bad boy, though the stigma concerning privately held specimens may simply prompt everyone to ignore it. This is what Mike Taylor suggested on the DML, since he (correctly) pointed out that the hypothesis of the paper (all Morrison diplodocids are congeneric) is essentially unverifiable as long as the specimens are in a private collection and haven't been described in a peer-reviewed paper.

As it was independently published, questions about the newly named taxon's validity have been raised. But, really, is this any different from what Cope and Marsh were doing back in the 19th Century? If anything, cheap and easy publication has simply brought us back to those Wild West days of do-it-yourself science, spurious results and all.

Taxonomy aside, the baleobiological conclusions in this thing are... just... fascinating. and probably will NOT help the author's case.

You can read the pdf here:

Galiano, H. and Albersdorfer, R. (2010). "A new basal diplodocid species, Amphicoelias brontodiplodocus, from the Morrison Formation, Big Horn Basin, Wyoming, with taxonomic reevaluation of Diplodocus, Apatosaurus, and other genera." Dinosauria International, LLC. 44pp.

A Guide To Feather Colors

Above: Restoration of Changchengornis by Matt Martyniuk, all rights reserved. Even when the colors of a prehistoric feathered dinosaur haven't been revealed by studies of feather microstructure, there are ways to infer which colors were and were not likely.

The recent discovery of a fossil penguin from Peru, complete with preserved feathers and melanosomes that reveal their color, prompted me to dive a little deeper into this topic. Keep in mind this is an area I'm still learning about myself, so please feel free to add corrections or keep the discussion going in the comments.

For those of us interested in palaeontography,* the recent work by Jakob Vinther (see original post here) and others on reconstructing the life coloration of prehistoric animals has been some of the most exciting paleontology research of the decade. In the carefree halcyon days before fossil melanosomes were recognized, I and other artists were given free reign over the external appearance of our feathered dinosaurs. But since Vinther's paper, I've been inspired to look into exactly what biological factors go into bird coloration. Needless to say, this is something not a lot of others have probably looked at, as most paleoart follows the old philosophy that when it comes to color, anything goes. Apparently though, this was far from true even before Vinther and his colleagues came along.

* Since this is John Conway's term I reckon I'm stuck using the Queen's preferred spelling, sorta like "pycnofibres". Anyway, that's just a fancy way of saying "paleoart".

There are several processes that add color to the feathers of birds and, presumably, other feathered dinosaurs. At the most basic level, these can be divided into structural color and pigmentation (though sometimes it isn't hat simple, as I'll explain down the page).

Structural Colors
Structural colors come from the actual physical structure of the feather. At the microscopic level, feathers exhibiting structural color have a "foamy" texture of tiny spheres or channels which enclose minute air bubbles. Light scatters through these bubbles in various ways depending on the exact arrangement. The development of these extremely complex structures has recently been covered by Dufresne and colleagues (2009), so you can track down that paper for a technical treatment of structural feather colors.

(Right: Structurally colored blue feathers of a Blue-and-yellow Macaw Ara ararauna, by Jörg Groß, licensed.)

Basically, structural colors can do two things: produce colors not found among the various pigments, and enhance or change pigment colors. For example, among amniotes, there is no such thing as a blue biological pigment. The blue feathers of a bird are produced by scattering due to structural colors. Similarly, iridescence as seen in many birds comes from the feather structure. A bird with bright white or pitch black feathers likely uses structural colors to achieve this effect--without them, these colors would be flatter, duller, and less vivid. Structural coloration can act as a filter, combining with pigments to form new colors. In most birds that have them, green feathers are produced by layering yellow pigmentation nodules over a "blue" underlying structure.

Does it fossilize?: You bet! Iridescent feathers have been reported by Vinther, and it's often apparently to the naked eye alone. There are some stunning examples of iridescent insect fossils out there. Structurally colored feathers have been recognized by a distinct arrangement where a thin layer of densely aligned melanin overlies a looser conglomerate of melanosomes. This can be seen even if the overlying keratin scattering layer has degraded away (Vinther et al. 2008). This kind of structure-via-melanin is also found in the dazzlingly iridescent plumage of hummingbirds (Prum, 2006).

What it means for dinobirds: Blue, green, jet black and bright white can't be present in dinobirds that lack structural color in their feathers. I've said before that structural colors are impossible in the monofilament integument of primitive coelurosaurs. However, I'm not so sure that's true. The main difference between hair and feathers isn't the structure of the filaments, it's the structure of the underlying molecules. Hair is alpha-keratin, a helix-shaped molecule like DNA. beta-keratin, which makes up feathers, has a layered and pleated underlying molecular structure more conducive to structural scattering. So a blue-fuzzed Struthiomimus may be possible. However, in the iridescent fossil feathers studied by Vinther et al. (2008), the structural color was restricted to the barbules, which are not present in many primitive feathered dinosaurs.

Pigments
Most bird colors are due in whole or in part to pigmentation, or lack thereof. There are several different kinds of pigments, with the two most common being melanins and carotenoids.

Melanins are what all the hubbub is about. Not only are these easily identified in fossil feathers, but their shape and concentration can tell you what color they gave their feathers. Melanins are responsible for black (though not deep, solid black, which require an extra push from structural color), gray, and a wide variety of browns through rufous orange colors. Melanins are the main pigment in mammalian hair, so think of the spectrum of mammal colors when imagining what shades are possible with melanin. A lack of melanin will produce white.

(Right: Different types of melanin in the feather of a Zebra Finch Taeniopygia guttata, from Not Exactly Rocket Science/Zhang et al. 2010).

Does it fossilize?: Of course! I've discussed the relevant melanosome papers in the past, posts linked below.

What it means for dinobirds: For carnivorous dinobirds, these are where the action is. Pure carnivores will usually lack the dietary requirements for carotenoids, so structural colors plus melanin are all they've got (and maybe porphyrins, see below). It seems odd that of the three described prehistoric dinobirds with color, they all seem to have the same color palate. Sinosauropteryx (rufous and white), Anchiornis (gray, black, white, brown, and rufous), and now Inkayacu (gray, white, and rufous-brown). These are all carnivorous/fish eating species, so it makes sense that they don't exhibit any more exciting colors. However, it's also possible that we're missing something: In their 2009 Anchiornis paper, Li and colleagues specifically noted that they didn't test for carotenoids. However, I would imagine that given their prior 2008 paper, they did look for structural color or at least iridescence in fossil feathers.

Carotenoids are, by and large, what give birds their characteristically bright colors. The trick is that carotenoids can't be directly synthesized by the body in animals (some can, but there need to be other types of carotenoids present to convert). Carotenoids come almost exclusive from a diet of plants or, secondarily, of things that sequester a lot of carotenoids in their body tissues (like plant-eating invertebrates and some fish). Gulls living near salmon farms have begun shown hints of pink in their feathers: this is because farm-raised salmon are fed artificial carotenoid sources to make their flesh pink, and these are transferred to the birds. The most unusual source of carotenoids, this time among a carnivorous species, is the Egyptian Vulture Neophron percnopterus, which gets its bright yellow facial skin by eating the droppings of ungulates, dropping which yield no significant nutritional value and appear to be eaten only for the carotenoids (McGraw, 2006)! Indeed, while carnivores aren't usually brightly colored, McGraw noted that there may be selective pressures in some species to add weird things to a diet in order to become more colorful.

(Right: Egyptian Vulture, photo by Dezidor, licensed.)

Does it fossilize? Yes, but it looks the same as melanin, and unlike melanin, you can't tell a carotenoid by its shape. According to Li et al. (2009), special chemical tests would have to be run to determine if a melanosome is really a carotenoid, and what color it was.

What it means for dinobirds: Even though we haven't yet identified carotenoids in fossils, we know that they can only be present in animals that are herbivores or feed on herbivorous insects. Scansoriopterygids, for example, could have been brightly colored by carotenoids, since they presumably ate tree-dwelling arthropods. Alvarezsaurids would probably lack carotenoids if they are mainly termites and other social, non-colorful bugs, as has been suggested in the lit. Jeholornis and Jinfengopteryx, two dinobirds with direct evidence of seed eating, are prime candidates for reds, oranges, and yellows (or even greens, with added structural color). Also, keep in mind that red carotenoids from crustaceans, when eaten by birds with otherwise melanin-free feathers, are what give pink wading birds like flamingos their distinctive colors. This is why many artists restore some ctenochasmatid pterosaurs, especially Pterodaustro, as pink (though how all this applies to pycnofibres is still anyone's guess).

(Right: Speculative restoration of Jeholornis prima with red and yellow carotenoid crown by Matt Martyniuk, licensed.)

Carotenoids are often used by modern birds as a sign of fitness when choosing a mate. Because carotenoids have to be eaten, a bird with a poor diet will be drabber than a bird that is very successful at finding food. A flamingo kept in a zoo will turn white if its diet isn't artificially supplemented with red carotenoids.

Carotenoids can also impact the eye color of a bird, as well as beak color and the color of the scales on its feet... even the yellow yolk of a chicken egg is due to carotenoids (some birds use Flavin for yolk color, see below). Keep in mind that adding orange, yellow or green feathers, or red, orange or yellow beaks, implies your dinosaur is eating a diet containing carotenoids.

Porphyrins are perhaps most famous for lending blood its red color and leaves their green (both heme and chlorophyl are porphyrins), but it can also color feathers, adding browns and reds (and green, but only in the turacoverdin pigments found in Turacos). Interestingly, porphyrins may have a role in temperature regulation. In addition to insulating eggs (see below), they are mainly found in the downy feathers of nocturnal birds like owls, and those that are active in colder temperatures.

The blue of American Robin eggs is created by porphyrins, as is most other egg coloration. In fact, some researchers note a correlation between porphyrin in eggshells and nesting behavior. Pure white eggs are only found in birds which nest in shelter like under foliage, and which constantly attend their eggs. Species which leave the eggs partly exposed to the elements have colorful porphyrin-containing shells, possibly because of the supposed temperature regulating effect. Paleoartists might want to consider this when drawing various dinosaur nests.

Does it fossilize?: I'm guessing no, as we're dealing at the molecular level here. However, I wonder if porphyrins could be detected via chemical analysis, like the one used to detect beta keratin in the feathers of Shuvuuia deserti.

(Right: The juvenile Black-shouldered Kite Elanus axillarus uses porphyrins to achieve a red-brown color not found in adults. Photo by Mdk572, licensed.)

What it means for dinobirds: This one is the big question mark. I've never seen references that describe a method to detect porphyrins in fossils. Luckily, they're mainly only brown and dull red, colors that could conceivably be found with melanin alone. If anything, porphyrins give us license to add some extra reddish splashes to purely carnivorous dinobirds, especially those that may have been active at night or in cold climates, like troodontids.

Uncommon pigments: There are a variety of minor pigments that can color a bird's feathers. Pterins are responsible for the yellow, red, white, and orange colors of some bird eyes (in humans, eye color is controlled by melanin; low melanin results in blue eyes, and some babies eyes darken as the melanin levels increase). Flavin pigments cause many egg yolks to be yellow.

(Right: The feathers of a Yellow-headed Amazon Amazona oratrix. Parrots are so stingy with their carotenoids they had to evolve an entirely new pigment to color their feathers. Photo by Rei, licensed.)

Psittacofulvins are found only in (you guessed it) parrots, and create yellows oranges and reds in place of carotenoids, which parrots have evolved to sequester, possibly for nutritional reasons. There are some undescribed pigments known only in penguins that add florescence to their yellow display feathers.

The take-home message:
When you add color to a feathered dinosaur restoration, you're presenting an implicit hypothesis about its diet, lifestyle, and soft tissue anatomy. when doing serious paleoart, keep these constraints in mind, and use them to make your art more interesting and your science more rigorous. Nobody can tell you not to draw a Utahraptor with a bright red face, but if you're trying to make paleontography and not just a bit of fun, why not depict it munching on a big, red fish or a pile of Sauroposeidon poop?

References:
* Dufresne, Eric R., Heeso Noh, Vinodkumar Saranathan, Simon G. J. Mochrie, Hui Cao and Richard O. Prum (2009). "Self-assembly of amorphous biophotonic nanostructures by phase separation." Soft Matter, 5: 1792-179

* McGraw, K.J. and Nogare, M.C. (2004). "Carotenoid pigments and the selectivity of psittacofulvin-based coloration systems in parrots." Comparative Biochemistry and Physiology B, 138: 229–233.

* Li, Q., Gao, K.-Q., Vinther, J., Shawkey, M.D., Clarke, J.A., D'Alba, L., Meng, Q., Briggs, D.E.G. and Prum, R.O. (2009). "Plumage color patterns of an extinct dinosaur." Science, 327(5971): 1369 - 1372

* McGraw, K.J. (2006). "Mechanics of Carotenoid-Based Coloration." Pp. 177-242 in Hill, G.E. and McGraw, K.J. (eds.), Bird coloration, Volume I: Mechanisms and Measurements. Harvard University Press.

* McGraw, K.J. (2006). "Mechanics of Uncommon Colors: Pterins, Porphyrins, and Psittacofluvins." Pp. 354-398 in Hill, G.E. and McGraw, K.J. (eds.), Bird coloration, Volume I: Mechanisms and Measurements. Harvard University Press.

* Prum, R.O. (2006). "Anatomy, Physics, and Evolution of Structural Colors." In Hill, G.E. and McGraw, K.J. (eds.), Bird coloration, Volume I: Mechanisms and Measurements. Harvard University Press.
* Vinther, Jakob, Derek E.G. Briggs, Julia Clarke, Gerald Mayr and Richard O. Prum (2008). "Structural coloration in a fossil feather." Biology Letters, 6(1): 128-131.

* Zhang, F., Kearns, S.L., Orr, P.J., Benton, M.J., Zhou, Z., Johnson, D., Xu, X. and Wang, X. (2010). "Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds." Nature, 463: 1075-1078.

Nyctosaurus Update

Back in June, I reported on the sale of one of the famous crested Nyctosaurus specimens. A few weeks ago, Mark from a fossil selling site called DinoStar left this comment:

Not very accurate Matt. Frithiof is a very decent person who is doing a very legal business. Both KJ1 and KJ2 are still in the Brazos museum and are scheduled to be there for the next 2 years. KJ1 was sold to person who is constructing a museum. KJ2 is still for sale. Most private collections are eventually donated to a museum.

First, let me point out for the record I never said that what Frithiof and Mark do is illegal. As long as the fossils were obtained legally in the US, it's perfectly legal to sell them. That doesn't mean it isn't ethically questionable and actively detrimental to science, which I and pretty much 95% of the paleontological community would agree that it is.

Nevertheless, Mark shed some light on the nature of the infamous eBay sale. It's going to someone constructing a museum. What kind of museum or what kind of somebody remains a mystery. There are plenty of dodgy outfits out there calling themselves museums, and specimens in their collections may as well be in an 18th century cabinet of curiosity. A private "museum" is still a private collection unless the museum is somehow in the public trust, so it doesn't eliminate the hesitation most scientists would have of touching one of those specimens. But, as long as it's made freely available for study, this may be good news.

Or not, because even on Mark's own site, fossils currently on display in random small museums are being offered up for sale. For example, for only $19,000, I could become the proud owner of a Confuciusornis specimen with "scant restoration" and what looks to be an amazing anatomically incorrect ornithurine-like tail fan, straight "from China" which does not allow any exportation of fossils under any circumstances (though it does say it's from an "old collection", have the laws changed in 14 years since these birds first started turning up?). If I can buy my very own illegal Chinese pygostylian straight out of a museum's glass case, what's to keep the new owner of KJ1 from trying to cut his losses if the museum doesn't turn a profit?

KJ2, for what it's worth, is still for sale. So if you want to own a nearly one of a kind scientific treasure to hang up in your den, act soon. Maybe the Field Museum can take it off your hands in 50 years once you kick it?

What Is A Nomen Oblitum? Not What You Probably Think

Above: Illustration of Manospondylus gigas by Matt Martyniuk, copyright.

In a previous post, I brought up the taboo subject of Manospondylus gigas, the large Lancian theropod named by E.D. Cope in 1886 and a potential senior synonym of Tyrannosaurus rex. Google "manospondylus" and the first hit is an old Q&A post by Mike Taylor called "So why hasn't Tyrannosaurus been renamed Manospondylus?"

Good question!

Aside from cultural inertia, the answer Taylor gave is this: "As of 1st January 2000, a new ICZN ruling has come into effect, saying that a name that's been considered valid for fifty years can't now be replaced by one that's been considered invalid during that time." This is also the answer Mickey Mortimer gave in some (but not all) relevant entries on the current Theropod Database. Brian Switek, in an old Laelaps post, gave the same answer: Manospondylus hasn't been considered valid in 50 years, so it's a nomen oblitum ("forgotten name"). He also suggested that this means T. rex has "protected status" which would have to be overturned by the ICZN in order for M. gigas to become valid.

The problem is that none of that has any bearing on what "nomen oblitum" actually means according to the current ICZN code. The ICZN states that in order for a name to be declared a nomen oblitum, all of the following things need to be true:
1. the senior synonym or homonym has not been used as a valid name after 1899
2. the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years
3. a paper must be published citing evidence for #2, and citing both names together, declare that the junior synonym or homonym is being made a nomen protectum ("protected name") in accordance with ICZN article 23.9.

The confusion seems to come from #2. It's not a matter of the senior name not having been used in the past 50 years, but the junior name must have been used frequently enough in recent history in order to be eligible for conservation. Clearly, T. rex meets the second criteria. But what about the first and third?

For number one, the answer is I'm not sure, but I think so. It depends on what your definition if "is" is. Has Manospondylus been used since 1899? Definitely, yes, and in several papers, as Mickey Mortimer pointed out in the comments on my previous post on this topic. But was it used as valid? Most authors, even in the early 20th century, recognized that two vertebrae were pretty poor material to be basing a species on. Many of them considered it to be what we'd now call a nomen dubium ("dubious name"). For example, here's what Matthew & Brown (1922) had to say on the subject: "Osborn has already (1917) called attention to another fragmentary type, Manospondylus gigas, as possibly identical with Tyrannosaurus but based upon an inadequate type." They don't explicitly say it's valid or invalid (I'd like to know what Osborn 1917 actually aid on the matter but don't have that paper. Anyone?).

As for number three, it appears that the situation has never been adequately addressed in the literature, so the criterion is not satisfied. The issue came up in 2000, when Peter Larson claimed to have rediscovered the original Manospondylus locality, and more of the type specimen, confirming it is the same as T. rex. As far as I know, this has never seen print beyond an AP article (Anonymous, 2000. "Discovery could Endanger T.Rex Name." The Associated Press.)

In short, could M. gigas be a nomen oblitum? Maybe, depending on the meaning of "valid" and if somebody gets up the nerve to actually publish the case. Is Manospondylus a nomen oblitum as of right now? Definitely not, and it remains the valid senior synonym of Tyrannosaurus until and unless someone acts as revisor to the contrary.

Addendum: Just for fun, I looked up "valid" in the ICZN's glossary. Who better to determine what the ICZN means by valid than the ICZN? Here's the entry:
valid, a. (validity, n.) Of an available name or a nomenclatural act: one that is acceptable under the provisions of the Code and, in the case of a name, which is the correct name of a taxon in an author's taxonomic judgment.

Since the Code does not formally recognize nomina dubia as invalid names (or at all, really), this seems to indicate that M. gigas was considered valid by all those authors in the 20th century, at least under the ICZN's definition. Matthew and Brown, for example, did not consider M. gigas an invalid name (a junior homonym, improperly coined, etc.), just non-diagnostic. Which as far as the rules are concerned, is A-OK. Sorry kids, barring an act of ICZN, M. gigas looks like it's on solid footing as a currently valid name.

Balaur bondoc: Dodo-Raptor?

Here's a really interesting idea floated by Andrea Cau (with help from Mickey Mortimer, Ville Sinkkonen, Rutger Jansma and Zach Miller) over at his Theropoda blog.

Everyone is making a big deal about Balaur bondoc, the apparently double-sickle clawed dromaeosaur. The double sickle idea comes from the very strange nature of the foot (close-up image in the last post, linked above).

Above: Fossils of Balaur bondoc reconstructed as a standard dromaeosaur. Credit: PNAS/AP.

The first sickle claw is present on the first toe. Normally, in non-avian theropods, the first toe is small and placed high on the foot, like the dew claw of a dog. Basically, it's vestigial. However, in lineages where the first toe evolves some kind of use, the entire metatarsal supporting the toe tends to descend, placing the first toe at the same level as the others. This is seen in two lineages of theropod: birds, where the first toe is used for perching, and therizinosaurs, where it's been re-adapted for walking. Animals that use four toes in walking (rather than having four toes but only using three) are called functionally tetradactyl.

So, did Balaur not use its enlarged first and second toes to kill prey as in other dromaeosaurs, but simply as extra walking digits, as in therizinosaurs? Balaur is certainly built like a therizinosaur, with short, stocky hind limbs and very weird hips. The hip bones of Balaur, as you can see in the image above, are extremely swept back. Some of this may be due to crushing, but probably not all. Normally, such a hip arrangement is seen only in herbivorous dinosaurs, which need to clear out space in the torso for their expansive, plant-fermenting guts.

Tim Williams on the DML has also pointed out that the atrophied hands with reduced third finger and fused wrist elements are also indicators of decreased predatory ability. The only other dinosaurs I can think of off hand that have such reduced third fingers are avialans and the herbivorous Caudipteryx (and, of course, alvarezsaurids and tyrannosaurids, but for different reasons). Avialans and Caudipteryx are the only ones that retain fairly normal hand proportions while shrinking the third finger.

Much has been made of the fact Balaur lived on isolated Romanian islands, and that the "island rule" probably had a lot to do with its weird anatomy. Could similar effects have led to this line of dromaeosaurs becoming fully herbivorous? Weirder things have happened to island dinosaur lineages. Just look at the kiwi.

The parallels between Balaur and therizinosaurs are hard to ignore, but obviously it would be helpful to have some skull material before definitely drawing any conclusions about its diet. In the mean time, check out the cool reconstruction Andrea included in his post for what a therizinosaur-mimicking dodo-raptor may have looked like.