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, 6 (3), e17932

New Information on the Cranial Anatomy of Acrocanthosaurus Atokensis and Its Implications for the Phylogeny of Allosauroidea (Dinosauria: Theropoda)

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New Information on the Cranial Anatomy of Acrocanthosaurus Atokensis and Its Implications for the Phylogeny of Allosauroidea (Dinosauria: Theropoda)

Drew R Eddy et al. PLoS One.

Abstract

Background: Allosauroidea has a contentious taxonomic and systematic history. Within this group of theropod dinosaurs, considerable debate has surrounded the phylogenetic position of the large-bodied allosauroid Acrocanthosaurus atokensis from the Lower Cretaceous Antlers Formation of North America. Several prior analyses recover Acrocanthosaurus atokensis as sister taxon to the smaller-bodied Allosaurus fragilis known from North America and Europe, and others nest Acrocanthosaurus atokensis within Carcharodontosauridae, a large-bodied group of allosauroids that attained a cosmopolitan distribution during the Early Cretaceous.

Methodology/principal findings: Re-evaluation of a well-preserved skull of Acrocanthosaurus atokensis (NCSM 14345) provides new information regarding the palatal complex and inner surfaces of the skull and mandible. Previously inaccessible internal views and articular surfaces of nearly every element of the skull are described. Twenty-four new morphological characters are identified as variable in Allosauroidea, combined with 153 previously published characters, and evaluated for eighteen terminal taxa. Systematic analysis of this dataset recovers a single most parsimonious topology placing Acrocanthosaurus atokensis as a member of Allosauroidea, in agreement with several recent analyses that nest the taxon well within Carcharodontosauridae.

Conclusions/significance: A revised diagnosis of Acrocanthosaurus atokensis finds that the species is distinguished by four primary characters, including: presence of a knob on the lateral surangular shelf; enlarged posterior surangular foramen; supraoccipital protruding as a double-boss posterior to the nuchal crest; and pneumatic recess within the medial surface of the quadrate. Furthermore, the recovered phylogeny more closely agrees with the stratigraphic record than hypotheses that place Acrocanthosaurus atokensis as more closely related to Allosaurus fragilis. Fitch optimization of body size is also more consistent with the placement of Acrocanthosaurus atokensis within a clade of larger carcharodontosaurid taxa than with smaller-bodied taxa near the base of Allosauroidea. This placement of Acrocanthosaurus atokensis supports previous hypotheses of a global carcharodontosaurid radiation during the Early Cretaceous.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Generalized theropod phylogenies.
Tree structures modified from Holtz et al. , O'Connor and Claessens , and the present analysis to illustrate the phylogenetic position of Allosauroidea (A) and relative placement of less-inclusive clades within Allosauroidea (B). 1, Theropoda; 2, Ceratosauria; 3, Tetanurae; 4, Allosauroidea.
Figure 2
Figure 2. Flesh reconstruction and line drawing of the skull of Acrocanthosaurus atokensis (NCSM 14345) in left lateral view.
Hatched lines represent missing bone. A, angular; aof, antorbital fenestra; AR, articular; D, dentary; emf, external mandibular fenestra; iop, intraorbital process of postorbital; J, jugal; L, lacrimal; lpr, lacrimal pneumatic recess; ltf, lateral temporal fenestra; M, maxilla; mf, maxillary fenestra; N, nasal; o, orbit; ob, orbital boss of postorbital; P, parietal; PM, premaxilla; pmf, promaxillary fenestra; PO, postorbital; PRE, prearticular; Q, quadrate; QJ, quadratojugal; SA, surangular; soc, supraoccipital; SQ, squamosal.
Figure 3
Figure 3. Left nasal of Acrocanthosaurus atokensis (NCSM 14345).
Nasal in (A) lateral and (B) medial views. Hatched lines represent broken surfaces; dashed lines represent material not in figure. en, external naris; fo, foramina; lrn, lateral ridge of nasal; m, maxillary contact; ms, medial symphysis; nf, narial fossa; nmp, naso-maxillary process.
Figure 4
Figure 4. Premaxillae of Acrocanthosaurus atokensis (NCSM 14345).
Premaxillae in (A) left lateral and (B) right lateral views. Dashed lines represent material not in figure. en, external naris; fo, foramina; m, maxillary contact; n, nasal contact; nf, narial fossa; v, vomeral contact.
Figure 5
Figure 5. Right maxilla of Acrocanthosaurus atokensis (NCSM 14345).
Maxilla in (A) lateral and (B) medial views. Dashed lines represent material not in figure. aof, antorbital fenestra; alf, accessory lateral fenestra of the maxilla; gdl, groove for dental lamina; ifs; interfenestral strut; j, jugal contact; lsm, lateral shelf; mf, maxillary fenestra; n, nasal contact; nvf, neurovascular foramina; pas, postantral strut; pdrm, posterodorsal ramus of the maxilla; pem, pneumatic excavation of the posterodorsal ramus; pfam, posterior fenestra of the maxilla; pm, premaxillary contact; pmf, promaxillary fenestra; prm; posterior ramus of the maxilla.
Figure 6
Figure 6. Left jugal of Acrocanthosaurus atokensis (NCSM 14345).
Jugal in (A) lateral and (B) medial views. Dashed lines represent material not in figure. aof, antorbital fenestra; dqjp, dorsal quadratojugal prong; l, lacrimal contact; ljf, lateral jugal foramen; ltf, lateral temporal fenestra; M, maxilla; mjf, medial jugal foramen; o, orbit; po, postorbital contact; pop, postorbital process of jugal; qj, quadratojugal contact; sap, small accessory prong; vqjp, ventral quadratojugal prong.
Figure 7
Figure 7. Left lacrimal of Acrocanthosaurus atokensis (NCSM 14345).
Lacrimal in (A) lateral and (B) medial views. Dashed lines represent material not in figure. aof, antorbital fenestra; fo, foramina; iopl, intraorbital process of lacrimal; j, jugal contact; llp, lacrimal lateral plate; lmp, lacrimal medial plate; lpr, lacrimal pneumatic recess; N, nasal; o, orbit; pf, prefrontal contact; po, postorbital contact.
Figure 8
Figure 8. Left postorbital of Acrocanthosaurus atokensis (NCSM 14345).
Postorbital in (A) lateral and (B) medial views. f, frontal contact; fo, foramina; iop, intraorbital process of postorbital; j, jugal contact; l, lacrimal contact; ls, laterosphenoid contact; ltf, lateral temporal fenestra; o, orbit; ob, orbital boss of postorbital; p, parietal contact; pf, prefrontal contact; sq, squamosal contact; vg, vascular groove.
Figure 9
Figure 9. Left prefrontal of Acrocanthosaurus atokensis (NCSM 14345).
Prefrontal in (A) medial and (B) lateral views. f, frontal contact; l, lacrimal contact; n, nasal contact; po, postorbital contact.
Figure 10
Figure 10. Left quadratojugal and quadrate of Acrocanthosaurus atokensis (NCSM 14345).
Quadratojugal and quadrate in (A) lateral, (B) posterior, and (C) medial views. Hatched lines represent missing material. j, jugal contact; lc, lateral condyle of quadrate; ltf, lateral temporal fenestra; mc, medial condyle of quadrate; mpr, medial pneumatic recess of quadrate; ppr, posterior pneumatic recess of quadrate; pt, pterygoid contact; Q, quadrate; qc, quadrate cotylus; QJ, quadratojugal; quf, quadrate foramen; sq, squamosal contact.
Figure 11
Figure 11. Left squamosal of Acrocanthosaurus atokensis (NCSM 14345).
Squamosal in (A) lateral, (B) ventral, and (C) medial views. ltf, lateral temporal fenestra; p, parietal contact; po, postorbital contact; pop, contact with paroccipital process; pqp; postcotyloid process of squamosal; q, quadrate contact; qj, quadratojugal contact; qjp, quadratojugal process of squamosal; sptf, supratemporal fossa; sqpr, squamosal pneumatic recess (foramen); tnc, transverse nuchal crest.
Figure 12
Figure 12. Frontals, parietals, and braincase of Acrocanthosaurus atokensis (NCSM 14345) in left lateral view.
Reconstructed from CT scan data. bpt, basipterygoid process; bt, basal tubera; BS, basisphenoid; F, frontal; I, olfactory nerve exit; ic, internal carotid artery entrance; II, optic nerve exit; III, oculomotor nerve exit; LS, laterosphenoid; n, nasal contact; OS, orbitosphenoid; P, parietal; PAS, parasphenoid; pf, prefrontal contact; pit, pituitary fossa; po, postorbital contact; popr; paroccipital process; pp, preotic pendant; PRO, prootic; SOC, supraoccipital; tnc, transverse nuchal crest; V, trigeminal nerve exit; VIIh, hyomandibular branch of facial nerve exit; VIIp, palatine branch of facial nerve exit.
Figure 13
Figure 13. Frontals, parietals, and braincase of Acrocanthosaurus atokensis (NCSM 14345) in anterior view.
Reconstructed from CT scan data. BS, basisphenoid; bsr, basisphenoid recess; F, frontal; I, olfactory nerve exit; LS, laterosphenoid; OS, orbitosphenoid; P, parietal; PAS, parasphenoid; pit, pituitary fossa; popr; paroccipital process; pp, preotic pendant; PRO, prootic; pt, pterygoid contact; tnc, transverse nuchal crest; V, trigeminal nerve exit.
Figure 14
Figure 14. Frontals, parietals, and braincase of Acrocanthosaurus atokensis (NCSM 14345) in dorsal view.
Reconstructed from CT scan data. bo, basioccipital; bpt, basipterygoid process; EO, exoccipital; F, frontal; fm, foramen magnum; LS, laterosphenoid; n, nasal contact; P, parietal; pf, prefrontal contact; po, postorbital contact; popr; paroccipital processes; SOC, supraoccipital; sptf, supratemporal fossa; sq, squamosal contact; tnc, transverse nuchal crest.
Figure 15
Figure 15. Frontals, parietals, and braincase of Acrocanthosaurus atokensis (NCSM 14345) in posterior view.
Reconstructed from CT scan data. bo, basioccipital; BS, basisphenoid; bsr, basisphenoid recess; EO, exoccipital; F, frontal; fm, foramen magnum; LS, laterosphenoid; P, parietal; popr; paroccipital process; pt, pterygoid contact; SOC, supraoccipital; sq, squamosal contact; vcd, vena capita dorsalis.
Figure 16
Figure 16. Frontals, parietals, and braincase of Acrocanthosaurus atokensis (NCSM 14345) in ventral view.
Reconstructed from CT scan data. bo, basioccipital; bpt, basipterygoid process; bsr, basisphenoid recess; bt, basal tubera; F, frontal; LS, laterosphenoid; OS, orbitosphenoid; P, parietal; popr; paroccipital processes; sptf, supratemporal fossa.
Figure 17
Figure 17. Digital endocranial endocast of the braincase of Acrocanthosaurus atokensis (NCSM 14345) in left lateral view.
ca, anterior semicircular canal; cbl, cerebellum; cer, cerebrum; ch, horizontal semicircular canal; cp, posterior semicircular canal; fm, foramen magnum; I, olfactory nerve; IV, trochlear nerve; obl, olfactory bulbs; pit, pituitary; V, trigeminal nerve; vcd, vena capita dorsalis; VI, abducens nerve.
Figure 18
Figure 18. Digital endocranial endocast of the braincase of Acrocanthosaurus atokensis (NCSM 14345) in right lateral view.
ca, anterior semicircular canal; cbl, cerebellum; cer, cerebrum; ch, horizontal semicircular canal; cp, posterior semicircular canal; faf, fossa acoustic-facialis; floc, flocculus; fm, foramen magnum; I, olfactory nerve; IV, trochlear nerve; obl, olfactory bulbs; pit, pituitary; V, trigeminal nerve; vcd, vena capita dorsalis; vf, vagus foramen; VI, abducens nerve.
Figure 19
Figure 19. Palatal complex of Acrocanthosaurus atokensis (NCSM 14345) in dorsal view.
bs, basisphenoid contact; ECT, ectopterygoid; EPI, epipterygoid; j, jugal contact; in, internal naris; iptv; interpterygoid vacuity; m, maxillary contact; PA, palatine; papr, palatine pneumatic recess; pm, premaxillary contact; PT, pterygoid; ptm, pterygoid medial process; ppp; pterygoid process of palatine; q, quadrate contact; stf, subtemporal fenestra; V, vomer; vpar, vomeropalatine ramus of the pterygoid.
Figure 20
Figure 20. Right pterygoid of Acrocanthosaurus atokensis (NCSM 14345) in lateral view.
Dashed lines represent material not in figure. ect, ectopterygoid contact; er, ectopterygoid ramus of pterygoid; epi, epipterygoid contact; pa, palatal contact; ptf; pterygopalatine fenestra; q, quadrate contact; qr; quadrate ramus of pterygoid; vpar, vomeropalatine ramus of pterygoid.
Figure 21
Figure 21. Left palatine of Acrocanthosaurus atokensis (NCSM 14345).
Palatine in (A) lateral and (B) medial views. Dashed lines represent anterior extension of vomer and posterior extension of vomropalatine ramus of pterygoid. in, internal naris; j, jugal contact; l, lacrimal contact; m, maxillary contact; pa, palatine contact; papr, palatine pneumatic recess; pt, pterygoid contact; ptf, pterygopalatine fenestra; ppp, pterygoid process of palatine; vpal, vomeropterygoid ramus of the palatine; vpar, vomeropalatine ramus of the pterygoid.
Figure 22
Figure 22. Reconstruction of right pterygoid, palatine, and vomer of Acrocanthosaurus atokensis (NCSM 14345) in lateral view.
Dashed lines represent hidden surfaces inferred by observing medial and ventral surfaces of shown elements. m, maxillary contact; j, jugal contact; PA, palatine; papr, palatine pneumatic recess; ptf, pterygopalatine fenestra; ppp, pterygoid process of palatine; V, vomer; vpar, vomeropalatine ramus of pterygoid.
Figure 23
Figure 23. Vomer of Acrocanthosaurus atokensis (NCSM 14345) in ventral view.
m, maxillary contact; PA, palatine; pm, premaxillary contact; V, vomer; vpar, vomeropalatine ramus of pterygoid.
Figure 24
Figure 24. Right pterygoid, palatine, and vomer of Acrocanthosaurus atokensis (NCSM 14345) in medial view.
Left vomer is figured until it reaches anterior extent of the right palatine. b, break; er, ectopterygoid ramus of the pterygoid; erf, fossa of the ectopterygoid ramus of the pterygoid; PA, palatine; pm, premaxillary contact; PT, pterygoid; ptm, pterygoid medial process; ptf, pterygopalatine fenestra; ppp; pterygoid process of palatine; qr, quadrate ramus of pterygoid; qrf; fossa of the quadrate ramus of the pterygoid; V, vomer; vpar, vomeropalatine ramus of the pterygoid.
Figure 25
Figure 25. Left ectopterygoid of Acrocanthosaurus atokensis (NCSM 14345).
Ectopterygoid in (A) dorsal, (B) medial, (C) ventral, and (D) lateral views. emr, ectopterygoid medial recess; fo, foramen; j, jugal contact; jr, jugal ramus of ectopterygoid; pt, pterygoid contact; stf, subtemporal fenestra.
Figure 26
Figure 26. Left epipterygoid of Acrocanthosaurus atokensis (NCSM 14345).
Epipterygoid in (A) lateral and (B) medial views. ls, laterosphenoid contact; pt, pterygoid contact.
Figure 27
Figure 27. Left mandible of Acrocanthosaurus atokensis (NCSM 14345) in medial view.
A, angular; AR, articular; abr, articular brace of the dentary; C, coronoid; D, dentary; emf, external mandibular fenestra; fo, foramen; fpct, foramen posterior chorda tympani; gl, glenoid region; imf, internal mandibular fenestra; mg, Meckelian groove; mhf, mylohyoid foramen; mssa, medial shelf of the surangular; PRE, prearticular; retp, retroarticular process of articular; SA, surangular; SPD, supradentary; SPL, splenial.
Figure 28
Figure 28. Left dentary of Acrocanthosaurus atokensis (NCSM 14345).
Dentary in (A) lateral and (B) medial views. Dashed lines represent material not in figure. a, angular contact; abr, articular brace of the dentary; emf, external mandibular fenestra; fan, foramina of the inferior alveolar nerve; lsd, lateral sulcus of the dentary; mg, Meckelian groove; ms, medial symphysis; nvf, neurovascular foramina; SA, surangular; sa, surangular contact; SPD, supradentary; spl, splenial contact.
Figure 29
Figure 29. Left splenial of Acrocanthosaurus atokensis (NCSM 14345).
Splenial in (A) medial and (B) internal views. a, angular contact; aps, angular process of the splenial; d, dentary contact; dps, dentary process of the splenial; iar, infra-angular ridge; mhf, mylohyoid foramen; pre, prearticular contact.
Figure 30
Figure 30. Left prearticular of Acrocanthosaurus atokensis (NCSM 14345).
Prearticular in (A) medial and (B) internal views. Dashed lines represent material not in figure. a, angular contact; c, coronoid contact; imf, internal mandibular fenestra; mhf, mylohyoid foramen; sa, surangular contact; spl, splenial contact.
Figure 31
Figure 31. Left angular of Acrocanthosaurus atokensis (NCSM 14345).
Articular in (A) lateral and (B) medial views. d, dentary contact; emf, external mandibular fenestra; fo, foramina; pre, prearticular contact; sa, surangular contact.
Figure 32
Figure 32. Left surangular, articular, and coronoid of Acrocanthosaurus atokensis (NCSM 14345).
Surangular, articular, and coronoid in (A) lateral, (B) medial, and (C) dorsal views. Hatched lines represent broken surfaces. a, angular contact; af, adductor fossa; apsa, angular process of surangular; AR, articular; asf, anterior surangular foramen; b, break; C, coronoid; D, dentary; d, dentary contact; emf, external mandibular fenestra; fpct, foramen posterior chorda tympani; gl, glenoid region of articular; lssa, lateral shelf of surangular; m, maxillary contact (mouth closed); mame, insertion for the M. adductor mandibulae externus; mssa, medial shelf of surangular; PRE, prearticular; psf, posterior surangular foramen; retp, retroarticular process; SA, surangular; SPD, supradentary.
Figure 33
Figure 33. Nine phylogenies recovered by systematic analyses of Allosauroidea and outgroup taxa.
Some trees have been pruned to show only those taxa incorporated into the present study. Analyses along the top two rows recover Acrocanthosaurus atokensis as a member of Carcharodontosauridae, whereas the bottom row represents those that place Acrocanthosaurus atokensis as close relative and/or sister taxon to Allosaurus.
Figure 34
Figure 34. Illustration of character 12 (Appendix S1).
Right maxilla of (A) Allosaurus fragilis (UUVP 5499) in posteromedial view and (B) Acrocanthosaurus atokensis (NCSM 14345) in posterior view. ascr, ascending ramus of the maxilla; pfam, posterior maxillary fenestra; ifs, interfenestral strut; pas, postantral strut.
Figure 35
Figure 35. Illustration of characters 13 and 14 (Appendix S1).
Left maxillae of (A) Allosaurus fragilis (figure modified from [69]), (B) Acrocanthosaurus atokensis (NCSM 14345) and (C) Carcharodontosaurus saharicus (SGM-Din 1) in medial view. Thick vertical dashed lines represent the anterior extent of palatal contact; thin dashed lines represent material not in the figure. aof, antorbital fenestra; gdl, groove for dental lamina of the maxilla; j, jugal contact; mf, maxillary fenestra; n, nasal contact; pa, palatine contact; pfam, posterior maxillary fenestra; pmf, promaxillary fenestra.
Figure 36
Figure 36. Illustration of characters 19, 25, and 26 (Appendix S1).
Left rostra of (A) Sinraptor dongi (figure modified from [16]) and (B) Acrocanthosaurus atokensis (NCSM 14345). aof, antorbital fenestra; en, external naris; mf, maxillary fenestra; nf, narial fossa; nmp, naso-maxillary process; nr, nasal recesses; pmf, promaxillary fenestra.
Figure 37
Figure 37. Illustration of character 31 (Appendix S1).
Right lacrimal of Acrocanthosaurus atokensis (NCSM 14345) in anterolateral view. j, jugal contact; llp, lateral lacrimal plate; lmp, lacrimal medial plate; lpr, lacrimal pneumatic recess; m, maxillary contact; n, nasal contact.
Figure 38
Figure 38. Illustration of characters 32 and 46 (Appendix S1).
Right lacrimals of (A) Sinraptor dongi (figure modified from [16]), (B) Allosaurus fragilis (UUVP 5198), (C) Acrocanthosaurus atokensis (NCSM 14345), and (D) Giganotosaurus carolinii (MUCPv-CH-1) in dorsal view. n, nasal contact; PF, prefrontal; pf, prefrontal contact; po, postorbital contact.
Figure 39
Figure 39. Illustration of characters 33 and 34 (Appendix S1).
Right jugals of (A) Sinraptor dongi (figure modified from [16]), (B) Allosaurus fragilis (figure modified from [69]), (C) Acrocanthosaurus atokensis (NCSM 14345), (D) Carcharodontosaurus saharicus (SGM-Din 1), and (E) Mapusaurus roseae (MCF-PVPH-108.167) in lateral view. dqjp, dorsal quadratojugal prong; pop, postorbital process of jugal; sap, small accessory prong; vqjp, ventral quadratojugal prong.
Figure 40
Figure 40. Illustration of characters 38, 39, and 43 (Appendix S1).
Left postorbitals of (A) Sinraptor dongi (modified from [16]), (B) Allosaurus fragilis (UUVP 5958); (C) Acrocanthosaurus atokensis (NCSM 14345), (D) Carcharodontosaurus saharicus (figure modified from [49]), and (E) Eocarcharia dinops (figure modified from [49]) in lateral view. iop, intra-orbital process; j, jugal contact; l, lacrimal contact; ob, orbital boss; sq, squamosal contact; vg, vascular groove.
Figure 41
Figure 41. Illustration of character 42 (Appendix S1).
Left postorbitals of (A) Allosaurus fragilis (UUVP 5160), (B) Acrocanthosaurus atokensis (NCSM 14345), (C) Carcharodontosaurus saharicus (figure modified from [49]), and (D) Eocarcharia dinops (figure modified from [49]) in dorsal view. f, frontal contact; l, lacrimal contact; ls, laterosphenoid contact; ob, orbital boss; p, parietal contact; pf, prefrontal contact; sptf, supratemporal fossa; sq, squamosal contact.
Figure 42
Figure 42. Illustration of character 45 (Appendix S1).
Left squamosals of (A) Allosaurus fragilis (AMNH 14554) and (B) Acrocanthosaurus atokensis (NCSM 14345) in lateral view. ltf, lateral temporal fenestra; po, postorbital contact; pqp, post-quadratic process ( = postcotyloid); q, quadrate contact; qjp, quadratojugal process.
Figure 43
Figure 43. Illustration of character 47 (Appendix S1).
Left prefrontals of (A) Allosaurus fragilis (image modified from [69]) and (B) Acrocanthosaurus atokensis (NCSM 14345) in medial view. f, frontal contact.
Figure 44
Figure 44. Illustration of character 59 (Appendix S1).
Left quadrates of (A) Allosaurus fragilis (UUVP 3082), (B) Acrocanthosaurus atokensis (NCSM 14345), and (C) Mapusaurus roseae (image modified from [36]) in posterior view. lc, lateral condyle; mc, medial condyle; ppr, posterior pneumatic recess; Q, quadrate; QJ, quadratojugal; qj, space occupied by quadratojugal; quf, quadrate foramen.
Figure 45
Figure 45. Illustration of character 60 (Appendix S1).
Braincase of (A) Acrocanthosaurus atokensis (NCSM 14345) and (B) Allosaurus fragilis (UUVP 3082) in anterior view. F, frontal; I, exit for olfactory nerve; n, nasal contact.
Figure 46
Figure 46. Illustration of characters 77 and 78 (Appendix S1).
Right pterygoids of (A) Sinraptor dongi (image modified from [16]), (B) Allosaurus fragilis (UUVP 5748), and (C) Acrocanthosaurus atokensis (NCSM 14345) in medial view. Dashed lines represent missing material. er, ectopterygoid ramus; erf, ectopterygoid ramus fossa; pmp, pterygoid medial process; qr, quadrate ramus; qrf, quadrate ramus fossa; vpar, vomeropalatine ramus.
Figure 47
Figure 47. Illustration of characters 79, 81, and 82 (Appendix S1).
Left ectopterygoids of (A) Sinraptor dongi (image modified from [16]), (B) Allosaurus fragilis (image modified from [69]), (C) Acrocanthosaurus atokensis (NCSM 14345), and (D) Giganotosaurus carolinii (MUCPv-CH-1) in dorsal (top row) and lateral (bottom row) views. Characters 81 and 82 appear to be dependent upon first examination, as both Acrocanthosaurus and Giganotosaurus share an ectopterygoid with a narrow jugal ramus (82∶1) that is rotated dorsally (81∶1), in contrast to the more robust jugal ramus (82∶0) that lies parallel to the main body of the ectopterygoid (81∶0) in Allosaurus and Sinraptor. However, the presence of a dorsally rotated jugal ramus (81∶1) in Carnotaurus (a basal theropod consistently recovered outside of Allosauroidea [81]) coinciding with a wide jugal ramus and narrow subtemporal fenestra (82∶0), suggests that the states are independent. j, jugal contact; jr, jugal ramus; pt, pterygoid contact; stf, subtemporal fenestra.
Figure 48
Figure 48. Illustration of character 83 (Appendix S1).
Left epipterygoids of (A) Allosaurus fragilis (UUVP 1414) and (B) Acrocanthosaurus atokensis (NCSM 14345) in medial view. Both elements shown are complete. ls, laterosphenoid contact.
Figure 49
Figure 49. Illustration of character 91 (Appendix S1).
Left dentaries of (A) Allosaurus fragilis (UUVP 871), (B) Sinraptor dongi (image modified from [16]), and (C) Acrocanthosaurus atokensis (NCSM 14345) in lateral view. a, angular contact; emf, external mandibular fenestra; lsd, lateral sulcus of the dentary; sa, surangular contact.
Figure 50
Figure 50. Illustration of character 99 (Appendix S1).
Left prearticulars of (A) Sinraptor dongi (image modified from [16]), (B) Allosaurus fragilis (image modified from [69]), and (C) Acrocanthosaurus atokensis (NCSM 14345) in internal views. Hatched lines indicate broken surfaces. a, angular contact; ar, articular contact; d, dentary contact.
Figure 51
Figure 51. Single most parsimonious tree resulting from the phylogenetic analysis.
Primary analysis included 177 characters evaluated for 18 taxa (Length  =  325 steps; CI  =  0.60; RI  =  0.66; RCI  =  0.39). Bootstrap values >50% are to the left of the nodes; Bremer support values >1 are to the right. The open circle represents Carnosauria; the closed circle represents Allosauroidea. The arrow shows the number of steps needed to remove Acrocanthosaurus from its current position within Carcharodontosauridae and place it as the sister taxon to Allosaurus, a relationship proposed by previous authors (Figure 33).
Figure 52
Figure 52. Phylogeny of Allosauroidea upon the inclusion of proposed taxa known from highly fragmentary specimens.
Included taxa are Siamotyrannus, Lourinhanosaurus, and Australovenator. A full phylogenetic analysis of 20 terminals recovered 11 MPT's (TL  =  338 steps), the strict consensus of which is shown. This phylogeny has been cropped to show only allosauroid relationships, as the outgroup taxa were unaffected.
Figure 53
Figure 53. Similarity in palatal morphology between basally-positioned and derived allosauroid taxa.
Palatal reconstructions of (A) Sinraptor dongi (image modified from [16]) and (B) Acrocanthosaurus atokensis (NCSM 14345).
Figure 54
Figure 54. Distribution across Allosauroidea of primary and ancillary characters diagnostic of the species Acrocanthosaurus atokensis.
Primary diagnostic characters are shown in boldface print. Refer to Appendix S2, Appendix S3, and Table S1 for character scorings.
Figure 55
Figure 55. Phylograms and comparisons of body size optimization across Allosauroidea.
Constructed from the trees recovered by (A) EAC, the present analysis, and (B) SET, Smith et al. . Silhouettes are to scale according to measurements listed in Table 6.

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References

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