Balaur bondoc; a new Dromaeosaur from the late cretaceous of Romania!

Above; My own pencil reconstruction of Balaur bondoc.

Balaur bondoc; what a nice surprise when I connect up to the world wide web at 2 o'clock on a Wednesday afternoon. I was shocked almost to the point of disbelief. A Dromaeosaur with not one, but two killing claws on each foot! As well as the third finger atrophied leaving it with only two functional fingers! These sort of discoveries make me wonder what else we may be missing due to lack of animal remains that actually fossilized and what is yet to be discovered. Because of the original media blitz of the discovery it is hard to actually find any detail on Balaur. Sifting through the debris and brief mentioning has so far proved unsuccessful in gleaning anatomical info.Above; various skeletal elements of Balaur.

Island dwarfism

Above; outlines of various dinosaurs found in the Hateg basin drawn in scale with a human shape to give an impression of the size threshold in this prehistroic island community. Top is the Iguanodontian Rhabdodon. Upper middle is the sauropod Magyarosaurus. Lower middle is the Ankylosaur Struthiosaurus. Bottom is the Hadrosaur Telmatosaurus.

Balaur lived about 70 million years ago in the late Cretaceous period of Romania. It was found in the same assemblage of rocks as those dinosaurs described by Franz Baron Nopcsa in the early 20th century. Nopcsa suggested that these dinosaurs from the Hateg basin were an island fauna (as many showed characteristics of dwarfism). Dwarfs such as the 5 meter long Hadrosaur Telmatosaurus were present in the area. Although its size may not seem that small compared to many modern animals; relatives of Telmatosaurus elsewhere grew to over 10 meters in length. Other animals from the Hateg basin also include the 2.2 meter Struthiosaurus. This animal was an ankylosaur, yet again smaller than its relatives elsewhere. Balaur, however does not seem to have shrunk from its ancestors at all. 2 meters is an average size for a Dromaeosaur. One of the explanations for island dwarfism is that a given area of land can only provide a limited amount of nutrients for every cubed centimeter of biomass, and there is a given amount of nutrients required for every cubed centimeter of biomass occupying that area. When the area of land is small each individual organism must downsize to maintain the same wide gene pool within its population as a continental population would, whilst still maintaining the same quantity of biomass within the species; by downsizing, a smaller area of land can support more individual organisms, thus hedging the genetic bets of the species. Problems are created when there is limited genetic variability within a species. These include susceptibility to disease and an inability to evolve quickly.

Clearly the size threshold for land dwelling organisms is determined by the area of land in which they live. The explanation for the lack of downsizing in Balaur is that its continental ancestors did not exceed the size threshold of Hateg in the first place.

Balaur as a carnivore

Assuming that Balaur was a hypercarnivore as in other Dromaeosaurs it was clearly very specialised. The tibia is much broader than that of Velociraptor and huge muscle attachments are present on the hip bones for powerful leg muscles. These features suggest that Balaur was not as nimble as other Dromaeosaurs and relied more on strength than speed to kill its prey.

Above; the two fingered hands of Balaur seem to remind me of those belonging to Tyrannosaurs.

The hands of Balaur are extremely unusual and seem to have had little use in hunting. Many of the wrist movements were hampered by fusion of bones and the third digit was atrophied, leaving the hand with two functional digits. The arms, however, are not shrunk in any visible way and digits 1 and 2 are fairly large. This suggests that the arms still had some uses; perhaps in balance or gripping on when mating. Dromaeosaurs are known to have had feathery wings on their arms as in birds. These may have been important display structures and therefore their presence in evolutionary terms would have been secured. Big wings may have seemed "sexy" to a Balaur and therefore through sexual selection the relatively useless and inflexible arms would remain.

Above; the vertebrae (top) and lower leg (bottom) of Balaur. The erect posture of both the first and second digits as preserved suggess that they were held in a hyperextended position above the ground. If this were the case it would support the fact that the hallux had the same function as the killing claw on digit 2.

If the arms of Balaur were no longer that useful in prey aprehension this job was replaced almost entirely with the feet. If the animal was clinging onto the flanks of a larger animal the enlarged first toe would be an excellent gripping tool. By having 2 rather than 1 killing claw on the foot weight of the animal is distributed in smaller divisions to each claw, putting less stress on the individual toes. If the claws were instead slasing implements then a single kick may produce 2 deep slashes rather than one.

Balaur as an oppourtunistic omnivore

Above; could Balaur be a kind of Therizinosaur mimic? Top is a skeletal reconstruction of Nothronychus; a herbivorous Therizinosaur. Note the backward pointing pubis and enlarged first toe. Middle is a skeletal reconstruction of Balaur showing only the known elements. Like Nothronychus, Balaur has an enlarged hallux and a very backwardly pointing pubis, where as in Velociraptor (bottom) the pubis is oriented in a more vertical direction and the hallux is atrophied. By having a more posteriorly oriented pubis extra room for a large gut is created. This feature is also seen in birds and herbivorous Ornithischian dinosaurs, suggesting that Balaur may have been omnivorous.

The idea that Balaur could be omni/herbivorous was mentioned by Andrea Cau (writer of the blog Therapoda). He pointed out that the enlarged hallux was more similar to that of Therizinosaurs than a killing claw and could have been held on the ground as a support for the foot rather than above the ground as a killing claw. He suggests that the large, wide hips are an indication of a herbivorous lifestyle, allowing room for a long gut to ferment tough vegetation. The enlarged hallux would certainly have made Balaur slower than other Dromaeosaurs. Perhaps it was omnivorous; supplementing its diet with vegetation when meat was unavailable. This would explain the relatively disfunctional nature of the forelimbs, which seem inflexible and unsuited to a predatory lifestyle.

But why evolve a more herbivorous lifestyle? It is clear from the anatomy of Balaur that it had evolved features to deal with vegetation that impeded its ability to hunt prey. Perhaps when the ancestral Balaur became isolated on Hateg island prey was more scarce than in a continental setting. Therefore it may have been more advantageous to opportunistically feed on plant material when meat was in short supply. Eventually over millions of years Balaur sacrificed some of its adaptations as a predator to make processing plant material easier. If it had lived past the extinction 65 million years ago it may have either remained an oppourtunist like modern bears, or have become completely herbivorous as in the Therizinosaurs.

If Balaur was evolving towards being more herbivorous as in Therizinosaurs (which evolved from a group of carnivorous dinosaurs closely related to the Dromaeosaurs) then the coevolution of a re-enlarged first toe is interesting. Perhaps the enlargement of the hallux increased the area of the feet. This in turn would distribute the extra weight of an enlarged herbivorous gut over a wider area, decreasing pressure on the foot.

Conclusions

It seems likely that Balaur may well have been evolving towards herbivory. But until cranial material turns up the true nature of this animal will remain a relative mystery. Before I make too many assumptions I am going to find out more anatomical info on the animal, but for that I will have to wait for more to be published online.