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.

Were Deinonychosaurians and Avialians descended from an aboreal, sickle toed, four winged ancestor?

Above; Pedopenna is a typical example of a primitive Parave. It has four "wings", with long feathers on the hind and forelimbs. Uniquely this animal comes from the mind Jurassic of China and as a result lived at an earlier time than Archaeopteryx. Pedopenna was more primitive than later Paraves. It had a less developed "killing claw" and the feathers were symmetrical unlike the aerofoil asymmetrical feathers of other Paraves.

All early members of the Paraves share the features described in the title. Paraves as a group includes the raptors or Deinonychosaurs such as the Troodontids and Dromaeosaurs, and the Avialians which are birds and any other dinosaurs which have a closer common ancestry with birds than Deinonychosaurs.





Above; the skeleton of Anchiornis reconstructed. Troodontids had shorter arms and longer legs than other Paraves.

One of the earliest members of the Troodontidae is Anchiornis, a small animal from the late Jurassic of China's remarkable Liaoning province. Here, in a lake filled valley below a volcano, various animals, choked by ash, were washed into these lakes and buried in a sterile volcanic tomb. Because these creatures were covered over so quickly soft tissues were preserved. Anchiornis is covered in very advanced feathers on both its forelimbs and hind limbs. These "wings" would have probably allowed it to glide from tree to tree like a modern day gliding squirrel. The tail is also shortened and stiffened to enable more precise maneuverability amongst the branches and its arms are long for gripping on to branches. The pubis is pointed vertically as opposed to foreword in most meat eating dinosaurs, a step towards the bird like condition of a backward pointing pubis. The braincase and eyes are also enlarged, giving Anchiornis an almost bird like intelligence about it. In fact Troodontids are hailed as being the most intelligent dinosaurs that have ever lived on our planet, with brain relative to body ratios far surpassing other animals of their time.

Above; the advanced ground dwelling Troodontid Troodon from North America illustrates the general trend followed by Deinonychosaurian Paraves towards a predatory ground dewlling existence.

The retractable sickle shaped claw on the second toe was almost certainly present in Anchiornis as in other Troodontids, but was not as large as the classic raptorial claws of creatures like Velociraptor and was probably being used to help it to grip onto branches rather than to capture prey. The relatively long hind legs of Anchiornis made it unusual amongst basal Paraves and is a unique Troodontid condition that was probably the group's first step towards living a grounded existence as its descendants did. Other similar Troodontids have been found, but Anchiornis is the most unique in the respect that its melanosomes or pigment cells were preserved in its feathers, making its colouration deductible. Like modern birds Anchiornis clearly used its feathers for display; it possessed a huge brown crest of feathers on its head.

Above; Jinfengopteryx was once mistaken for a bird, but the long legs and short arms suggest it is a Troodontid.

Indications of a more bird like diet in Troodontids are seen in the seedy stomach contents of Jinfengopteryx. For a while this little Cretaceous Chinese dinosaur was mistaken for a bird, but recent analysis confirms its Troodontid position. The presence of seeds as part of this animals diet suggests that other Troodontids may not have been as carnivorous as was previously thought and probably lived much like bears, taking every opportunity to provide themselves with the energy to maintain a large bird like brain by becoming omnivorous.

Above; the foot of the advanced ground dwelling Dromaeosaurid Deinonychus. The inner toe has the large curved claw of its climbing ancestors, which in Dromaeosaurs has become greatly enlarged.

The Dromaeosaurs were the only group of Paraves to enlarge their second toe claw to the extent that they had, using it to its full potential as seen in the movie Jurassic Park being used to slash prey apart, but these gory portrayals are unfortunately fiction, not science. Rather than being used to climb trees as their ancestors did the Dromaeosaurs climbed their victims. The angulation of the curvature of their "killing claw" (160 degrees) matches that of a climbing animal rather than a ground dwelling slasher like a cassowary, and rather than being sharp sided as a slicing implement would be, the the claw has a blunt edge, but sharp tip. All this suggests that the Dromaeosaurian "killing claw" was a gripping claw rather than a slashing claw, a vestigial left over from their arboreal ancestors that was now used to grip on to and grab prey.

Above; a typical early arboreal Dromaeosaur from China; Microraptor.

Microraptor, an early Dromaeosaur from Liaoning was basically identical to Anchiornis in every way except perhaps its slightly shorter legs; Microraptor was probably more well adapted for an arboreal existence than Anchiornis because of this feature. Wind tunnel analysis of Microraptor models suggest that its winged hind legs played an important role in Microraptor's gliding cycle. The legs would swing foreword orienting the front wings upwards to give one final burst of lift before landing.

Above; the skeleton of the Madagascan flying Dromaeosaur, Rahonavis. Darker bones are speculative, but the lighter yellow bones represent the parts that were recovered from the fossil site.

An Uelangine Dromaeosaur from the late Cretaceous of Madagascar called Rahonavis is particularly interesting because it seems to have been capable of powered flight. The arm bones are much stronger than those of Archaeopteryx and are adorned with quill knobs for feather attachment. Rahonavis was clearly just as good as, if not better at flying than Archaeopteryx; the first "bird". It is ironic to think that a creature more closely related to Jurassic Park's Velociraptor was a better flier than some of the earliest birds. If Rahonavis is really a Dromaeosaur then it is possible that flight had evolved in Dinosaurs before Dromaeosaurs split off from the main bird family line and are therefore secondarily flightless animals. The gliding stage of Dromaeosaurs like Microraptor may not have been a step towards flight, but a step away from it. By 65 million years ago at the very end of the dinosaurs reign, all Dromaeosaurs were flightless ground dwelling carnivores rather than bird like climbers (except Rahonavis).

Above; the skeleton of Archaeopteryx is almost identical to that of the primitive Dromaeosaurs and Troodontids.

Archaeopteryx from the late Jurassic of Germany is commonly placed in the Avialian group with birds. Although this may not be true it possesses no features with either specifically Dromaeosaurs or Troodontids that aren't shared by all three, but does possess features shared with Deinonychosaurs (Troodonts and Dromaeosaurs) that are not present in birds: Archaeopteryx has a stiffened tail and a vertically oriented pubis. The boot of the pubis in all Paraves extends towards the ischium and in true birds fuses with the ischium completely. More recently discovered similarities between Dromaeosaurs, Troodontids and Archaeopteryx include lengthened feathers on the hind limbs and a retractable second toe. Although it lacks a large claw as in Deinonychosaurs one of the roller joints on this toe is extended far dorsally, allowing it to be flexed backwards. It is likely that Archaeopteryx did not really need this feature and that it was vestigial, birds certainly do not need it due to their array of foot tendons and this suggests a closer relationship with Avialians that Deinonychosaurs. On the other hand Archaeopteryx may have been evolving towards the sickle clawed condition seen in Deinonychosaurs. This feature was certainly useful as a climbing aid and by keeping the second toe off the ground and free of wear the first stage towards maintaining a "killing claw" was complete. If this is true then Archaeopteryx was more primitive than both Avialians and Deinonychosaurs.

I will try to explain the opposition to the theory of Deinonychosaurs and Avialians sharing a common volant ancestor in a later post, but for now my work is done. (The evolutionary relationships of Mahakala are hard to get your head around on a Sunday afternoon.)