Balaur (dinosaur)

Balaur bondoc is a uniquely specialized species of theropod dinosaur which lived in what is now Romania during the latter part of the Late Cretaceous. Balaur was described by scientists in August 2010, and was named after the balaur (Romanian pronunciation: [baˈla.ur][1]), a dragon of Romanian folklore. The species name "bondoc" means stocky, so Balaur bondoc means "Stocky dragon" in Romanian. This name refers to the greater musculature that Balaur had compared to its relatives. It is known from a single partial skeleton representing the type specimen.

Seventy million years ago, world sea levels were higher, and the location where its fossils are found was an off-shore part of the European archipelago called Hațeg Island which is also referred to as the "Island of the Dwarf Dinosaurs". Unlike other early members of the group Paraves, which includes Velociraptor, Troodon, and Archaeopteryx, this theropod had not just one but two large, retractable, sickle-shaped claws on each foot, and its limbs were proportionally shorter and heavier than those of its relatives. Given these and nearly twenty other specialized traits, the new genus Balaur was named for this one species. As with other dinosaurs from Hațeg, such as Magyarosaurus, a dwarf sauropod,[2] its aberrant features are argued to show the effects of its island habitat on its evolution.

Description
Balaur is a genus of theropod dinosaurs (either an early bird or a member of the more basal paravian lineage Dromaeosauridae) estimated to have lived about 70 million years ago in the late Cretaceous (Maastrichtian), and contains the single species B. bondoc.[3] The bones of this species were shorter and heavier than those of other basal paravians. While the feet of most early paravians bore a single, large "sickle claw" on the second toe which was held retracted off the ground, Balaur had large retractable sickle claws on both the first and second toes of each foot.[3] In addition to its strange feet, the type specimen of Balaur is unique for its status of being the most complete theropod fossil from the late Cretaceous of Europe. It also possesses a great number of additional autapomorphies, including a reduced and presumably nonfunctional third finger.[3][4]

The partial skeleton was collected from the red floodplain mudstone of the Sebeș Formation of Romania. It consists of a variety of vertebrae, as well as much of pectoral and pelvic girdles, and a large part of the limbs. It is the first reasonably complete and well-preserved theropod from the Late Cretaceous of Europe.[3] During the Maastrichtian much of Europe was fragmented into islands, and a number of the animal's bizarre features are thought to be a result of the relatively isolated conditions imposed on many of the populations in this area.[2] Species which are isolated on islands can be subject to the effects of genetic drift and the founder effect which can magnify the effect of mutations which might be diluted in a larger population. Other island effects such as Foster's rule, which describes how small mainland species become larger and large mainland species become smaller, can take effect. Cretaceous Romania is known, for example, for its dwarf sauropods.[2]

It is similar in size to Velociraptor, with Balaur's recovered skeletal elements suggesting an overall length of around 1.8–2.1 metres (5.9–6.9 ft).[3] Hence, in this case, Foster's effect is not evinced.[3] Yet, when compared to its related species, some 20 unique features were observed, including a re-evolved functional first toe with a large claw that can be hyperextended, short and stocky feet and legs, and large muscle attachment areas on the pelvis which indicate that it was adapted for strength rather than speed.[4][5] Csiki et al. describe this "novel body plan" as "a dramatic example of aberrant morphology developed in island-dwelling taxa."

Classification
The position of Balaur relative to other bird-like dinosaurs and early birds has been difficult to determine. The initial phylogenetic analysis placed Balaur closest to the Asiatic mainland dromaeosaurid species Velociraptor mongoliensis, but subsequent analyses found a variety of different possibilities for the classification of Balaur. A study by Brusatte et al. in 2013 found it in an unresolved close relationship with dromaeosaurids Deinonychus and Adasaurus, with some possible alternative trees suggesting it branched off before the common ancestor of Deinonychus and Velociraptor, while others maintained it as the closest relative of Velociraptor, with Adasaurus as their next closest relative.[6] A large analysis containing a wide variety of coelurosaurs and early birds published in 2013 found that Balaur was not a dromaeosaurid at all, but a basal avialan, more derived than Jeholornithiformes but more basal than Omnivoropterygiformes.[7] A study published in 2014 found Balaur to be sister to Pygostylia.