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Balaur bondoc is a species of Paraves theropod dinosaur from the late Cretaceous period, in what is now Romania. It is the type species of the monotypic genus Balaur, after the balaur (), a dragon of Romanian folklore. The specific 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. The genus, which was first described by scientists in August 2010, is known from two partial skeletons (including the type specimen). Some researchers suggest that the taxon might represent a junior synonym of Elopteryx.
Fossils of Balaur were found in the Densuș-Ciula and Sebeș Formations of Cretaceous Romania which correspond to Hațeg Island, a subtropical island in the European archipelago of the Tethys sea approximately 70 million years ago. Hațeg Island is commonly referred to as the "Island of the Dwarf Dinosaurs" on account of the extensive fossil evidence that its native dinosaurs exhibited island syndrome, a collection of morphological, ecology, physiology and ethology differences compared with their continental counterparts. Examples included island gigantism of Hatzegopteryx, island dwarfism of the titanosaur Magyarosaurus dacus, and a reduction in flight capacity in Balaur.
Balaur may have been a basal Avialae, a group that includes modern Aves based on phylogenetic analysis, though some researchers still include the genus within Dromaeosauridae dinosaurs, either Velociraptorinae as its original description or Unenlagiinae. This reduction in flight capacity is also seen in extant island birds including the ratites and insular Tytonidae as well as the extinct moa of New Zealand and the extinct dodo of Mauritius.
The generic name Balaur (three syllables, stressed on the second /a/) is from the Romanian word for a dragon of Romanian folklore, while the specific epithet bondoc (meaning "a squat, chubby individual") refers to the small, robust shape of the animal. As the mythological creature Balaur is a winged dragon, the name additionally hints at the close relation of the genus Balaur to the birds within Panaves. The species name bondoc was chosen by the discoverers also because it is derived from the Turkish language bunduk, "small ball", thus alluding to the probable Asian origin of the ancestors of Balaur.
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 girdle and Pelvis, and a large part of the limbs. It is the first reasonably complete and well-preserved theropod from the Late Cretaceous of Europe. It is similar in size to Velociraptor, with Balaur's recovered skeletal elements suggesting an overall length of around and a body mass of . Balaur had re-evolved a functional first toe used to support its weight, which bore a large claw that could be hyperextended. It had 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. Csiki et al. describe this "novel body plan" as "a dramatic example of aberrant morphology developed in island-dwelling taxa." The stocky feet are exemplified by the length of the metatarsus being only two times its width. It is 1.5 times wider than the lower leg. Both traits are unique in the Theropoda. The skeleton of Balaur also shows extensive fusion of limb bones. Wrist bones and the metacarpals are fused into a carpometacarpus. The pelvic bones are fused. The shinbone, calf bone and the upper ankle bones have been fused into a tibiotarsus and the lower ankle bones and the metatarsals into a tarsometatarsus. The degree of fusion is typical for the Avialae, the evolutionary branch of the birds and their direct relatives.
More recent analyses using different sets of anatomical data have since cast doubt on a dromaeosaurid classification for Balaur. In 2013, a larger analysis containing a wide variety of coelurosaurs found that Balaur was not a dromaeosaurid at all, but a basal Avialae, more closely related to modern birds than to Jeholornithiformes but more basal than Omnivoropterygiformes. A study published in 2014 found Balaur to be sister to Pygostylia. An independent analysis using an expanded version of the original data set (the one that found Balaur to be a dromaeosaurid) drew a similar conclusion in 2014. In 2015, researchers Andrea Cau, Tom Brougham, and Darren Naish published a study which specifically attempted to clarify which theropods were close relatives of Balaur. While their analysis could not completely rule out the possibility that B. bondoc was a dromaeosaurid, they concluded that this result was less likely than the classification of Balaur as a non- avialan based on several important bird-like features. Many of the presumed unique traits would in fact have been normal for a member of the Avialae. Typical bird features included the degree of fusion of the limb bones, the functional first toe, the first toe claw not being smaller than the second claw, a long penultimate phalanx of the third toe, a small fourth toe claw and a long fifth metatarsal. Some researchers continued to classify Balaur as a dromaeosaurid, with two separate studies published in 2021 placing Balaur within the Velociraptorinae, while the 2025 phylogenetic analysis recovered Balaur within Unenlagiinae.
Some researchers claim that Balaur may represent a junior synonym of Elopteryx. Brusatte and colleagues first mentioned the possibility in 2013, though they did not consider it the most likely case. In 2019, Mayr and colleagues claimed that the synonymy remains possible and more work is needed for confirmation. They also noted similarities with Gargantuavis and Elopteryx, indicating that the three taxa form a clade native to the Late Cretaceous European archipelago. In 2024, Stoicescu and colleagues suggested that Elopteryx is a member of the Avialae based on the new specimen from Romania, and that Balaur bondoc is probably a junior synonym of Elopteryx.
Italian paleontologist Andrea Cau has speculated that the aberrant features present in Balaur may have been a result of this theropod being omnivore or herbivore rather than carnivorous like most non-avian theropods. The lack of the third finger may be a sign of reduced predatory behavior, and the robust first toe could be interpreted as a weight-supporting adaptation rather than a weapon. These characteristics are consistent with the relatively short, stocky limbs and wide, swept-back pubis, which may indicate enlarged intestines for digesting vegetation as well as reduced speed. Cau referred to this as the "Dodoraptor" model.Cau, A (2010). Balaur: More than just a "Double-Sickle-Clawed Raptor" Theropoda, September 1, 2010. However, in light of the research done by Fowler et al., Cau has remarked that the anatomy of Balaur may be more congruent with the hypothesis that Balaur was predatory after all.Cau, A (2011). The Extinction of Dodoraptor (?) Theropoda, December 30, 2011.
In 2015, Cau et al. reconsidered the ecology of Balaur again in their reevaluation of its phylogenetic position, arguing that if Balaur was an avialan, it would be phylogenetically bracketed by taxa known to have been herbivorous, such as Sapeornis and Jeholornis. This suggests a non-hypercarnivorous lifestyle to be a more parsimonious conclusion and supports Cau's initial interpretations of its specializations. This is also indicated by the reduced third finger, the lack of a ginglymoid lower articulation of the second metatarsal and the rather small and moderately recurved second toe claw. Balaur had a broad pelvis, a broad foot, a large first toe, and broad lower ends of the metatarsals relative to the articulation surfaces; such a combination can in the remainder of the Theropoda only be found with the herbivorous Therizinosauridae.
In addition to island syndrome, species isolated on islands are also affected by genetic drift and the founder effect to a greater degree due to the small effective population size. This can magnify the effects of mutations which may otherwise be diluted in a larger population and may have given rise to some of the neomorphisms seen in Balaur like the retractable claw on its first toe.
In 2010, the increased robustness of Balaur was compared to parallel changes seen in isolated herbivorous mammals. In 2013, it was claimed that Balaur was the only predatory vertebrate known to have become more robust after invading an island niche and it was suggested that its broad feet had evolved to improve postural stability. The 2015 interpretation of Balaur as an omnivorous member of the Avialae, suggested it was the descendant of a flying species that had developed a larger size similar to the development in several other island herbivores. This would then be a rare instance of secondary flightlessness in a paravian to resemble a Dromaeosauridae, as predicted by Gregory S. Paul.
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