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The Phylogenetic Affinities of the Bizarre Late Cretaceous Romanian Theropod Balaur Bondoc (Dinosauria, Maniraptora): Dromaeosaurid or Flightless Bird?

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The Phylogenetic Affinities of the Bizarre Late Cretaceous Romanian Theropod Balaur Bondoc (Dinosauria, Maniraptora): Dromaeosaurid or Flightless Bird?

Andrea Cau et al. PeerJ.

Abstract

The exceptionally well-preserved Romanian dinosaur Balaur bondoc is the most complete theropod known to date from the Upper Cretaceous of Europe. Previous studies of this remarkable taxon have included its phylogenetic interpretation as an aberrant dromaeosaurid with velociraptorine affinities. However, Balaur displays a combination of both apparently plesiomorphic and derived bird-like characters. Here, we analyse those features in a phylogenetic revision and show how they challenge its referral to Dromaeosauridae. Our reanalysis of two distinct phylogenetic datasets focusing on basal paravian taxa supports the reinterpretation of Balaur as an avialan more crownward than Archaeopteryx but outside of Pygostylia, and as a flightless taxon within a paraphyletic assemblage of long-tailed birds. Our placement of Balaur within Avialae is not biased by character weighting. The placement among dromaeosaurids resulted in a suboptimal alternative that cannot be rejected based on the data to hand. Interpreted as a dromaeosaurid, Balaur has been assumed to be hypercarnivorous and predatory, exhibiting a peculiar morphology influenced by island endemism. However, a dromaeosaurid-like ecology is contradicted by several details of Balaur's morphology, including the loss of a third functional manual digit, the non-ginglymoid distal end of metatarsal II, and a non-falciform ungual on the second pedal digit that lacks a prominent flexor tubercle. Conversely, an omnivorous ecology is better supported by Balaur's morphology and is consistent with its phylogenetic placement within Avialae. Our reinterpretation of Balaur implies that a superficially dromaeosaurid-like taxon represents the enlarged, terrestrialised descendant of smaller and probably volant ancestors.

Keywords: Avialae; Deinonychosauria; Homoplasy; Mesozoic; Paraves.

Conflict of interest statement

Andrea Cau is a volunteer associate researcher at the Museo Geologico e Paleontologico ‘Giovanni Capellini, Bologna (Italy).

Figures

Figure 1
Figure 1. Comparison between the scapulocoracoid of Balaur and other paravians.
Comparison of the scapulocoracoid of (A) Balaur (lateral view) to that of (B) the pygostylian Enantiophoenix (medial view); and (C) the dromaeosaurid Velociraptor (lateral view); (A) after Csiki et al.. (, Fig. 1); (B) modified after Cau & Arduini (, Fig. 2); (C) after Norell & Makovicky (, Fig. 4). All scapulocoracoids are drawn with the proximal half of the scapular blade oriented horizontally to show relative placement of coracoid tubercle. Scale bar: 10 mm (A); 5 mm (B); 10 mm (C). Abbreviations: ac, acromion; co, coracoid; ct, coracoid tubercle; gl, glenoid; me, missing element; sc, scapula; snf, supracoracoid nerve foramen.
Figure 2
Figure 2. Comparison between the manus of Balaur and other paravians.
Comparison of the manus of (A) Balaur to those of (B) the enantiornithine Zhouornis; (C) the pygostylian Sapeornis; and (D) the dromaeosaurid Deinonychus, showing bird-like features of Balaur. (A) after Csiki et al.. (, Fig. 1, mirrored from original); (B) after Zhang et al. (, Fig. 7); (C) after Zhou & Zhang (, Fig. 7); (D) after Wagner & Gauthier (, Fig. 2). All drawn at the same metacarpal II length. Scale bar: 20 mm (A, D); 10 mm (B, C). Abbreviations: cis, closed intermetacarpal space; cmc, carpometacarpus; d3, reduced third digit; drc, distally restricted condyles; lsc, laterally shifted semilunate carpal; p1-III, first phalanx of manual digit 3; p2-III, second phalanx of manual digit 3; pec, proximally expanded extensor surface; pnm, proximally narrow metacarpal I; U, ungual; usc, unfused semilunate carpal.
Figure 3
Figure 3. Comparison between the pelvis of Balaur and other paravians.
Pelvis of Balaur in lateral view (A). Comparison of the pubes of Balaur in anteroventral view (B) to those of the pygostylian Sapeornis in anterior view (C), and the dromaeosaurid Velociraptor in posterior view (D). (C) after Zhou & Zhang (, Fig. 8); (D) after Norell & Makovicky (, Fig. 19). Scale bar: 10 mm (A, B, D), 2 mm (C). Abbreviations: aa, antitrochanter; ac, acetabulum; cf, cuppedicus fossa; dfi, dorsal flange of ischium; ipf, interpubic fenestra; is, ischium; pa, pubic apron; ps, pubic symphysis; pu, pubis, sv, sacral vertebrae.
Figure 4
Figure 4. Comparison between the metatarsus of Balaur and other paravians.
Comparison of the metatarsus and pes of (B) Balaur to that of (A) the dromaeosaurid Velociraptor; and (C) the pygostylian Zhouornis. (A) after Norell & Makovicky (, Figs. 6); (C) after Zhang et al.. (, Fig. 8, mirrored from original). Scale bar: 20 mm (A, B); 10 mm (C). Abbreviations: mt I, metatarsal I; mt V, metatarsal V; tt, tibiotarsus; U II: pedal ungual II; U IV, pedal ungual IV.
Figure 5
Figure 5. Updated dataset of Brusatte et al. (2014).
Reduced strict consensus of the shortest trees from the analysis of the modified Brusatte et al. (2014) matrix after pruning the ‘wildcard’ taxa Epidendrosaurus, Hesperonychus, Kinnareemimus, Pedopenna, Pyroraptor, and Shanag. Numbers adjacent to nodes indicate Decay Index values >1.
Figure 6
Figure 6. Updated dataset of Lee et al. (2014).
Strict consensus of the shortest trees from the analysis of the modified Lee et al. (2014) matrix. Numbers adjacent to nodes indicate Decay Index values > 1.
Figure 7
Figure 7. Skeletal reconstruction of Balaur.
Speculative skeletal reconstruction for Balaur bondoc, showing known elements in white and unknown elements in grey. Note that the integument would presumably have substantially altered the outline of the animal in life. Produced by Jaime Headden, used with permission.

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