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.

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.

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.

Copyright: © 2011 Eddy, Clarke. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

AMNH, American Museum of Natural History, New York, NY, USA; CMNH, Carnegie Museum of Natural History, Pittsburgh, PA, USA; CV, Municipal Museum of Chongqing, Chongqing, People's Republic of China; FWMSH, Forth Worth Museum of Science and History, Fort Worth, TX, USA; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, People's Republic of China; MCF-PVPH, Museo Carmen Funes, Paleontología de Vertebrados, Plaza Huincul, Neuquén, Argentina; MIWG, Museum of Isle of Wight Geology, Sandown, U.K.; MNN, Musée National du Niger, Niamey, Republic of Niger; MPEF-PV, Museo Paleontológico “Egidio Feruglio”, Trelew, Argentina; MUCPv-CH, Museo de la Universidad Nacional del Comahue, El Chocón Collection, Neuquén, Argentina; NCSM, North Carolina Museum of Natural Sciences, Raleigh, NC, USA; OMNH, Sam Noble Oklahoma Museum of Natural History, Norman, OK, USA; PVL, Instituto Miguel Lillo, Tucumán, Argentina; PVSJ, Instituto y Museo de Ciencias Naturales, San Juan, Argentina; SGM, Ministére de l'Energie et des Mines, Rabat, Morocco; SMU, Southern Methodist University, Dallas, TX, USA; USNM, United States National Museum, Smithsonian Institution, Washington D.C., USA; UUVP, Utah Museum of Natural History, Salt Lake City, UT, USA.

Acrocanthosaurus atokensis is the first-named and only species currently recognized as valid in the genus Acrocanthosaurus. The genus name stems from the Latin for “high-spined lizard”, as specimens referred to that taxon exhibit exceptionally tall neural spines along cervical and dorsal vertebrae [1] , [21] , [23] . The species name references Atoka County in southeastern Oklahoma, from which the holotype and paratype specimens were recovered. Reconstructions of the taxon upon its initial discovery were limited by a paucity of cranial material, although Acrocanthosaurus atokensis was suggested to be an intermediate form between allosauroids and tyrannosaurids [23] . Subsequent study suggested Acrocanthosaurus atokensis to be a tyrannosaurid due to similarities in size [55] . Conflicting phylogenetic placements of Acrocanthosaurus atokensis once prevented a consensus on relationships within Allosauroidea [22] . Previous analyses recovered this taxon alternatively as closely related to the smaller-bodied taxon Allosaurus fragilis from North America and Europe [1] , [13] , [36] , [39] , [56] , or placed within Carcharodontosauridae [10] , [12] , [17] , [19] – [22] , [25] , [42] , [49] . However, recent phylogenetic work has shown consistent support for Acrocanthosaurus atokensis as a carcharodontosaurid [10] , [22] , [25] , [37] , [42] .

Substantial taxonomic and phylogenetic modifications to Allosauroidea were proposed by Benson et al. [42] in their assessment of the relationships of several enigmatic Cretaceous theropod taxa with proposed allosauroid affinities. Although several of these taxa are known from largely incomplete specimens with little cranial material (e.g., Aerosteon riocoloradensis Sereno, Martinez, Wilson, Varricchio, Alcober, and Larsson 2008 [51] , Australovenator wintonensis [48] , Megaraptor namunhuaiquii Novas 1998 [52] , Fukuiraptor kitadaniensis Azuma and Currie 2000 [53] , Chilantaisaurus tashuikouensis Hu 1964 [50] ), a phylogenetic analysis combined with substantial postcranial data recovered within Allosauroidea the separate monophyletic group “Neovenatoridae” with Neovenator salerii Hutt, Martill, and Barker 1996 [54] as the most basal member [42] . Benson et al. [42] defined Neovenatoridae as the most inclusive clade containing Neovenator salerii, but not Carcharodontosaurus saharicus, Allosaurus fragilis, or Sinraptor dongi. Neovenatoridae is found to comprise the taxa Aerosteon, Australovenator, Chilantaisaurus, Fukuiraptor, and Megaraptor [10] , [25] . The recovery of “Neovenatoridae” as the sister taxon to Carcharodontosauridae further prompted the formation of the name “Carcharodontosauria” Benson, Carrano, and Brusatte 2009 [42] to describe the most inclusive clade comprising Carcharodontosaurus saharicus and Neovenator salerii, but not Allosaurus fragilis or Sinraptor dongi. Amendment of the name “Carcharodontosauridae” was also proposed in order to change its phylogenetic definition to the most inclusive clade comprising Carcharodontosaurus saharicus, but not Neovenator salerii, Allosaurus fragilis, or Sinraptor dongi [42] , and this distinction between Carcharodontosauridae and Carcharodontosauria is followed herein.

Stromer [43] coined the name “Carcharodontosauridae”, and Sereno [8] later gave it a phylogenetic definition as a stem-based name for a clade that includes all taxa more closely related to Carcharodontosaurus saharicus than to Sinraptor dongi, Allosaurus fragilis, or Passer domesticus. Discovery and subsequent phylogenetic placement of new allosauroid taxa (i.e., Australovenator wintonensis Hocknull, White, Tischler, Cook, Calleja, Sloan, and Elliott 2009 [48] ; Concavenator corcovatus Ortega, Escaso, and Sanz 2010 [25] ; Eocarcharia dinops Sereno and Brusatte 2008 [49] ; Mapusaurus roseae Coria and Currie 2006 [36] ; Shaochilong maortuensis Brusatte, Benson, Chure, Xu, Sullivan, and Hone 2009 [37] , [50] ; and Tyrannotitan chubutensis Novas, De Valais, Vickers-Rick, and Rich 2005 [39] ) has prompted the recognition of “Carcharodontosaurinae”, defined by Brusatte and Sereno [22] as a node-based name for the least-inclusive clade containing Carcharodontosaurus saharicus and Giganotosaurus carolinii Coria and Salgado 1995 [35] . Carcharodontosaurinae is consistently recovered as containing the derived carcharodontosaurid taxa Carcharodontosaurus, Giganotosaurus, and Mapusaurus [10] , [13] , [22] , [36] – [37] , [42] .

Significant new specimens have illuminated the diversity within Allosauroidea during the past fifteen years [1] , [20] , [25] , [35] – [37] . A consensus concerning the relationships of allosauroid taxa was problematic for some time [1] , [9] , [12] – [13] , [17] , [19] – [21] , [26] , [36] , [38] – [41] , but recent phylogenetic work has made substantial progress towards the resolution of the group [10] , [22] , [25] , [42] . Within Allosauroidea, four subclades have been recognized and are regularly differentiated by phylogenetic analyses: Allosauridae, Sinraptoridae, Carcharodontosauridae Stromer 1931 [43] , and Neovenatoridae Benson, Carrano, and Brusatte 2009 [42] ( Figure 1 ). The name “Allosauridae” has been applied to the clade including all taxa more closely related to Allosaurus fragilis than to Carcharodontosaurus saharicus Depéret and Savornin 1927 [44] and Sinraptor dongi [32] , [34] , but presently comprises only the taxon Allosaurus. “Sinraptoridae” defines the clade including all taxa more closely related to Sinraptor dongi than to Allosaurus fragilis and Carcharodontosaurus saharicus [34] , and frequently comprises the taxa Sinraptor and Yangchuanosaurus Dong, Chang, Li, and Zhou 1978 [45] , although recent analyses [10] , [25] found Sinraptoridae to also include Lourinhanosaurus Mateus 1998 [46] and Metriacanthosaurus Walker 1964 [47] .

“Allosauroidea” was coined by Currie and Zhao [16] to refer to a clade including Allosauridae Marsh 1878 [31] and Sinraptoridae Currie and Zhao 1993 [16] . Sereno [8] proposed a similar stem-based definition for the name “Allosauroidea” that Holtz and Padian [18] , [32] applied to the name “Carnosauria”: a clade including all taxa sharing a more recent common ancestor with Allosaurus fragilis than with Passer domesticus Linneaus 1758 [33] . In addition, Padian and Hutchinson [34] phylogenetically defined “Allosauroidea” prior to Sereno [8] as a node-based name for a clade including all descendants of the most recent common ancestor of Allosaurus fragilis and Sinraptor dongi Currie and Zhao 1993 [16] . The more restricted node-based name “Allosauroidea” and the stem-based name “Carnosauria” may both have utility in describing relationships among component taxa, although the presently known contents of these named clades may be identical. The present description and analysis follow the phylogenetic definitions for the names “Carnosauria” and “Allosauroidea” summarized in Padian et al. [32] , but prefer to employ “Allosauroidea” in place of “Carnosauria” to maintain congruence with previous work on allosauroids.

Carnosauria von Huene 1920 [15] ( = Allosauroidea Currie and Zhao [16] , see below) represents a particularly problematic theropod group that has historically fluctuated with respect to its included taxa and their interrelationships [1] , [14] , [17] – [25] . Gauthier's [14] early application of cladistic methodologies to estimate dinosaurian relationships led to his proposal that von Huene's name “Carnosauria” [15] be applied to a clade which excluded the basal theropods Megalosaurus and Streptospondylus, but included Allosaurus, Acrocanthosaurus Stovall and Langston 1950 [23] , and several other theropod taxa. Additionally, his cladistic analysis suggested that Carnosauria be placed within Theropoda as the sister taxon to Coelurosauria [14] , a hypothesis that has since been strongly supported ( Figure 1 ) [1] , [12] , [13] , [17] , [24] , [26] . However, Gauthier's proposed carnosaurian taxa [14] included several that are now recognized as coelurosaurs, such as Tyrannosaurus rex Osborn 1912 [27] , Daspletosaurus torosus Russell 1970 [28] , and Albertosaurus sarcophagus Osborn 1905 [29] , as well as the abelisaurids Indosuchus raptorius von Huene and Matley 1933 [30] and Indosaurus matleyi von Huene and Matley 1933 [30] . As a result, Gauthier's suggested contents for Carnosauria were determined to be paraphyletic [9] , [12] , [17] ; recognition of this paraphyly led to the practice of abandoning the name “Carnosauria” since it had become a “waste-basket” taxon for large-bodied theropods [8] .

Acrocanthosaurus atokensis is among the largest non-avian theropod dinosaurs, which were historically thought to be more closely related to one another than to smaller-bodied forms. This notion led von Huene [2] to apply the name “Carnosauria” to what has subsequently been discovered to comprise a paraphyletic assemblage, including the supraspecific theropod taxa Megalosaurus Buckland 1824 [3] , Spinosaurus Stromer 1915 [4] , Magnosaurus von Huene 1932 [2] , Dryptosaurus Marsh 1877 [5] , and Allosaurus Marsh 1877 [5] , and the rauisuchian Teratosaurus von Meyer 1861 [6] . This “carnosaurian” assemblage is now known to represent several independent origins of large size [7] – [11] . Although overall knowledge of non-avian theropod systematics has progressed substantially with discoveries of new species and specimens over the past 150 years, a detailed understanding of the evolutionary relationships of several theropod groups remains elusive [9] – [10] , [12] – [14] .

The most complete cranial specimen referred to the large-bodied theropod Acrocanthosaurus atokensis, NCSM 14345, comes from the Trinity Formation of North America (Aptian-Albian). The specimen was discovered along an incised creek bed southeast of Idabel, Oklahoma, with a nearly intact skull and associated, incomplete postcrania. Currie and Carpenter [1] originally described NCSM 14345, although the skull was incompletely prepared at that time. Sediment obscured the interior surfaces and, in some instances, entire views of cranial elements. Subsequent preparation of this specimen at the Black Hills Institute of Geological Research and the North Carolina Museum of Natural Sciences has allowed description and illustration of these previously undescribed cranial morphologies of Acrocanthosaurus. Here, we present a complete re-evaluation of the skull of Acrocanthosaurus, focusing on new data made available from NCSM 14345. From this morphological description, a suite of newly-recognized phylogenetic characters informative for allosauroid relationships is identified, and the phylogenetic position of Acrocanthosaurus is reassessed.

Despite a seemingly broad sample of comparative skull material, relatively few crania referred to taxa within Allosauroidea are extensively described or represented by multiple specimens. The most well-studied allosauroid skull is that of Allosaurus fragilis, known from several specimens with complete (or nearly complete) crania [27] , [68] – [70] . In addition to Allosaurus, four allosauroid taxa are known from specimens preserving relatively complete skulls (Sinraptor [16] , Yangchuanosaurus [45] , Carcharodontosaurus [20] , Acrocanthosaurus [1] ), as is one putative carnosaur (Monolophosaurus [71] ). Of these, only a skull referred to Sinraptor is monographed with multiple illustrations of every cranial element. Descriptions of partially prepared skulls of Monolophosaurus and Yangchuanosaurus are more limited, restricted to lateral and dorsal views of cranial, palatal, and mandibular elements, and medial views of the mandible [45] , [71] – [73] . Crania of specimens referred to several basally-positioned carcharodontosaurian taxa are largely incomplete (i.e., Neovenator [74] – [75] , Tyrannotitan [39] , Eocarcharia [49] , Australovenator [48] , and Shaochilong [37] , [76] ). Taxa recovered within Carcharodontosaurinae are known from more complete crania (i.e., Giganotosaurus [35] , [41] , Mapusaurus [36] , Carcharodontosaurus [20] , [75] , and Concavenator [25] ).

Comparisons with the skull of Acrocanthosaurus atokensis are drawn from cranial material referred to several taxa consistently recovered within Allosauroidea (e.g., Aerosteon riocoloradensis, Allosaurus fragilis, Australovenator wintonensis, Carcharodontosaurus saharicus, Eocarcharia dinops, Giganotosaurus carolinii, Mapusaurus roseae, Neovenator salerii, Shaochilong maortuensis, Sinraptor dongi, Tyrannotitan chubutensis, Yangchuanosaurus shangyouensis), as well as other taxa within Theropoda (e.g., Baryonyx walkeri Charig and Milner 1986 [65] , Coelophysis bauri Cope 1887 [66] , Herrerasaurus ischigualastensis Reig 1963 [67] , Tyrannosaurus rex). Table 3 provides a full list of evaluated cranial elements referable to Allosauroidea, and Table S1 describes the methods by which comparative material was assessed.

The past thirteen years have witnessed the description of new specimens crucial to understanding the morphology and phylogenetic affinities of Acrocanthosaurus atokensis (a list of specimens preserving material referable to the taxon is presented in Table 2 ). Harris [21] referred a specimen to Acrocanthosaurus atokensis from the Early Cretaceous of Texas that preserves a large amount of post-cranial material and several cranial elements (SMU 74646). Similar to the holotype specimen, the skull of SMU 74646 is largely incomplete and preserves only a fragmentary jugal, ectopterygoid, palatine, and posterior mandible. A postorbital is also preserved, but likely prepared after Harris' description.

The holotype specimen of Acrocanthosaurus atokensis (OMNH 10146) includes a braincase and fragmentary elements of the posterior skull and mandible recovered from the Trinity Formation (Aptian-Albian) of southeastern Oklahoma [23] ( Table 2 ). An additional specimen (OMNH 10147) preserving only post-cranial material was discovered in the same area and formation as the holotype, and designated as the paratype specimen of Acrocanthosaurus atokensis [23] . Material referred to Acrocanthosaurus atokensis between 1950 and the late 1990s was limited to various descriptions of tooth material tentatively assigned to the taxon [59] – [61] . One specimen was named during that interval as the holotype of a new European species Acrocanthosaurus altispinax Paul 1988 based on the presence of elongate neural spines on its dorsal vertebrae [62] . However, this specimen was later recognized as referable to a spinosauroid from England [12] , [14] , [63] – [64] , now called Becklespinax altispinax.

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.

Results

Cranial morphology of Acrocanthosaurus atokensis The following sections provide a detailed description of the cranial anatomy of Acrocanthosaurus atokensis specimen NCSM 14345. Unless otherwise indicated, descriptions of the morphology in Acrocanthosaurus focus on NCSM 14345. Cranial morphologies of Acrocanthosaurus described in previous works [1], [21], [23] are cited appropriately; all other observations are made by the authors. Traditional anatomical nomenclature is most often used over veterinary terminology (e.g., “anterior/posterior” instead of “rostral/caudal”).

Nasal The skull of NCSM 14345 (Figure 2) preserves the only nasal referable to Acrocanthosaurus atokensis. The left and right nasals are complete, but broken posteriorly near their contacts with the lacrimals. The left nasal is also broken anteriorly near its contact with the premaxilla (Figure 3), whereas the right nasal displays an additional break at mid-length. A portion of the ascending ramus of the right maxilla remains attached to the ventral surface of the right nasal, and the posterior portion of the left nasal is adhered to the medial surface of the left lacrimal horn. The nasal forms the posterior margin of the external naris with its contact to the subnarial processes of the premaxilla, excluding the maxilla from participating in the opening [1]. An elongated narial fossa extends posterodorsally from the rim of the external naris and depresses the lateral surfaces of the nasal (Figures 3A, 36B). Ridges border the narial fossa dorsally and ventrally, and converge at the posterior margin of the fossa. The thin ventral ridge articulates with the ascending ramus of the maxilla and contacts the premaxilla anteriorly [1], and the thicker dorsal ridge forms the upper rim of the external naris with the supranarial process of the premaxilla (Figure 2). The narial fossa is highly elongated in Acrocanthosaurus, Carcharodontosaurus, Concavenator, and Tyrannosaurus [20], [25], [27]. In Sinraptor, Allosaurus, Neovenator, and Monolophosaurus [16], [50], [69], [71]–[72], the reduced long axis of the narial fossa gives the depression a more rounded, ovular shape. Rounded narial fossae are also found in the coelurosaur Dilong paradoxus Xu, Norell, Kuang, Wang, Zhao, and Jia 2004 [77], and in basal theropods such as Herrerasaurus ischigualastensis and Coelophysis bauri. Giganotosaurus and Mapusaurus have highly rugose nasals that lack any expansion of the narial fossa. The lateral ridge of the nasal (Figure 3) participates in the dorsal margin of the antorbital fossa and contacts the lacrimal horn posteriorly [1]. In contrast to the rugose nasals of Mapusaurus, Neovenator, Carcharodontosaurus, Concavenator, and Giganotosaurus [20], [25], [35]–[36], [75], the nasal ridge of Acrocanthosaurus is relatively smooth as in Sinraptor, Monolophosaurus, and Allosaurus. Foramina above the antorbital fenestra perforate the nasal of Acrocanthosaurus [1]. These foramina are proportionally much smaller than the laterally-facing nasal pneumatic recesses of Sinraptor and Allosaurus (Figure 36A) which have been suggested to be homologous with ventrally-facing pneumatopores in Concavenator, Giganotosaurus, Mapusaurus, and Neovenator [36], [75]. However, these ventral pneumatopores are absent in Acrocanthosaurus. Along the ventral margin of the nasal, a narrow flange (referred to here as the “nasal-maxillary process”) projects anteroventrally to articulate with a notch along the dorsal margin of the ascending ramus of the maxilla (Figures 3, 36B). The nasal of Carcharodontosaurus (SGM-Din 1) also preserves this protrusion, but it is absent in specimens of Sinraptor, Neovenator, Allosaurus, and Monolophosaurus. Presence of the naso-maxillary process in Mapusaurus and Giganotosaurus is unclear, as rugosities cover the lateral surface of the nasals in these taxa. In medial view, a small ridge ventral and parallel to the roof of the nasal of Acrocanthosaurus flattens horizontally. The ridge is perforated posteriorly by three elongated foramina that open ventrally (Figure 3B) and likely represent foramina associated with the nasal vestibule [78]. Similarly positioned foramina also occur in Allosaurus.

Premaxilla The paired premaxillae preserved in NCSM 14345 (Figure 4) are the only premaxillary elements currently referred to Acrocanthosaurus (Table 2). In lateral view, the premaxillary body is taller than long (10.75×9.84 cm) [1], as in Giganotosaurus [35], Yangchuanosaurus [45], and several non-allosauroid theropods (e.g., Majungasaurus, Ceratosaurus Marsh 1884 [79], Tyrannosaurus [80]–[82]). In Allosaurus, Monolophosaurus, Neovenator, and Sinraptor, the premaxilla is longer than tall, and this condition is exaggerated in the spinosauroid Baryonyx walkeri [65]. The premaxilla of Acrocanthosaurus has four alveoli [1], as in Sinraptor and Giganotosaurus. Five premaxillary alveoli occur in Neovenator and Allosaurus. The supranarial and subnarial processes of the premaxilla of Acrocanthosaurus (Figure 2) extend posterodorsally to contact the nasal and form the anteroventral border of the external naris [1]. The subnarial process is dorsoventrally flattened, triangular in dorsal view, and excludes the maxilla from participating in the ventral margin of the external naris. The anterior region of the narial fossa depresses the rostrum between the supranarial and subnarial processes of the premaxilla (Figure 4). The medial view of the premaxilla is partially obscured in NCSM 14345, as the element is in contact with its counterpart to strengthen the mounted specimen. In posterior view, the small maxillary process articulates posteromedially with the maxilla, but does not surpass the posterior margin of the premaxillae as in Sinraptor and the tetanuran Duriavenator [83]. Foramina perforate the lateral surface of the premaxillary body and likely accommodated branching of the medial ethmoidal nerve and subnarial artery [1]. These premaxillary foramina in Acrocanthosaurus are shallower and less abundant than those in Allosaurus and Neovenator. An isolated, larger depression is present at the base of the right supranarial process (Figure 4B). Sinraptor, Neovenator and some specimens of Allosaurus (CM 1254; UUVP 1863) also possess a large foramen near the base of the supranarial process [16], [74].

Jugal Both jugals of NCSM 14345 are complete and appear morphologically similar to the left jugal of the holotype specimen of Acrocanthosaurus [23] and the right jugal of SMU 74646 [21]. The jugal from the holotype specimen is missing the posterior region, including the quadratojugal prongs, whereas the jugal of SMU 74646 lacks most of its postorbital and anterior processes. The jugal of Acrocanthosaurus (Figure 6) is laterally compressed and tripartite. The anterior jugal process forms the posteroventral corner of the antorbital fenestra as the process broadly contacts the descending process of the lacrimal and is supported ventrally by the posterior ramus of the maxilla [1]. The antorbital fossa is demarcated by a curved ridge on the jugal that expands dorsally onto the lacrimal and anteriorly onto the maxilla (Figure 2). A foramen penetrates the jugal medial to this ridge (Figure 6B), as in Sinraptor [16], Mapusaurus [36], and Monolophosaurus [71] (although see [75]); the jugal of Allosaurus is apneumatic [72]. Posterior to the anterior jugal process in Acrocanthosaurus, a triangular postorbital process contacts the anterodorsal margin of the postorbital ventral ramus [1]. The posterior process of the jugal is split into two quadratojugal prongs that fit tongue-in-groove with the anterior ramus of the quadratojugal (Figure 6). The dorsal quadratojugal prong is more than twice as tall as the ventral prong in Acrocanthosaurus (4.4 cm and 2.17 cm, respectively). This ratio is observed in most allosauroid taxa except for Allosaurus, in which the ventral quadratojugal prong is consistently shorter (Figure 39). The ventral quadratojugal prong of the jugal in Acrocanthosaurus is thin, elongated, and overlaps most of the ventral margin of the anterior process of the quadratojugal. Between the two quadratojugal prongs, a small, rounded accessory prong is present laterally, but partially obscured in lateral view by overlap of the quadratojugal (Figure 6). This prong has not been described for Acrocanthosaurus, because the holotype specimen and SMU 74646 fail to preserve the posterior region of the jugal [21], [23]. The accessory prong on the lateral surface of the jugal of Acrocanthosaurus is distinct from the jugal of Sinraptor, in which the ventral quadratojugal prong is split into two processes and includes an exaggerated medial process overlapping the medial surface of the quadratojugal [16]. A small accessory prong is also preserved in Mapusaurus [36], Tyrannotitan, and possibly in Carcharodontosaurus (SGM-Din 1), but is absent in Allosaurus. In medial view, the medial jugal foramen penetrates the jugal posterior to the junction of the quadratojugal prongs (Figure 6B). This foramen is expressed in SMU 74646 [21], and its presence in the holotype specimen of Acrocanthosaurus is likely because the jugal is highly pneumatic [23]. Sinraptor and Carcharodontosaurus also preserve a medial jugal foramen [16]. The left jugal of NCSM 14345 preserves an additional recess similar in size to the medial jugal foramen, but situated along the contact with the posterior ramus of the maxilla. Sinraptor also possesses a pneumatic opening in this region [16].

Lacrimal In addition to the left and right lacrimals of NCSM 14345, only the holotype specimen preserves lacrimal material referable to Acrocanthosaurus. The left lacrimal of the holotype is morphologically similar to those of NCSM 14345, although it is not as well-preserved and has a narrower descending process. Currie and Carpenter [1] described the lateral surface of the left lacrimal; medial surfaces were not visible at that time. Aside from the dorsal boss of the postorbital, the lacrimal horn is one of the more laterally prominent features of the facial region in Acrocanthosaurus. Projection of the horn above the dorsal margin of the skull is reduced (Figure 2), consistent with Carcharodontosaurus, Giganotosaurus, Concavenator, and Sinraptor, but unlike the raised lacrimal horn of Allosaurus [1]. The anterior ramus of the lacrimal is relatively straight and long in dorsal view (∼32.5 cm), but the ramus curves laterally dorsal to the lacrimal pneumatic recess (Figure 38C). Acrocanthosaurus shares this curvature with Carcharodontosaurus and Giganotosaurus, whereas the lacrimal horns of Sinraptor and Allosaurus are straight in dorsal view. The internal structure of the lacrimal pneumatic recess is well-preserved (Figure 7A). The lacrimal recess is assessable in the holotype specimen of Acrocanthosaurus, but the delicate septa dividing the openings have been crushed. Stovall and Langston [23] describe the pneumatic recess of the holotype as preserving two main openings, which differs from the single opening in NCSM 14345 [1]. However, both left and right lacrimal pneumatic recesses in NCSM 14345 are tri-radiate and divided by septa into three distinct cavities that extend anterodorsally, posteriorly, and posteroventrally. A single opening was also likely present in the holotype specimen, although breakage of the cavity caused it to appear to preserve multiple openings. Tri-radiate lacrimal pneumatic recesses are also present in Allosaurus, Sinraptor, and the coelurosaur Tyrannosaurus [84]. The lacrimal pneumatic recess in Giganotosaurus is also divided by at least one septum, but this condition is unknown for other carcharodontosaurids due to breakage of the lacrimal horns of Carcharodontosaurus and Mapusaurus. Anterior to the primary lacrimal recess, additional openings are visible in both lacrimals of NCSM 14345, a feature not present in the holotype specimen of Acrocanthosaurus. These openings also occur in Giganotosaurus, Concavenator, Sinraptor, and some specimens of Allosaurus. In posterior view, the naso-lacrimal canal (‘lacrimal duct’ [16]) perforates the lacrimal of Acrocanthosaurus with a single foramen extending anterodorsally, as in Allosaurus. However, Allosaurus preserves this naso-lacrimal canal and several ‘orbital recesses’ that excavate the posterior margin of the lacrimal [84]. Multiple posterior lacrimal foramina are similarly present in Sinraptor [16] and Mapusaurus [36], but these features are absent in Acrocanthosaurus. The lateromedially-flattened descending process of the lacrimal articulates broadly with the jugal [1]. This process is comprised by distinct medial and lateral layers that are separated by a deep sulcus along the anterior margin of the lacrimal (Figure 37). Acrocanthosaurus shares this characteristic with Carcharodontosaurus, Concavenator, and Giganotosaurus. In Sinraptor, Allosaurus, and Monolophosaurus, the descending process is not separated by a sulcus and instead has a rounded anterior margin. The lateral layer of the descending process in Acrocanthosaurus protrudes anteriorly into the antorbital fenestra [1] to demarcate the posterior margin of the antorbital fossa, while the medial layer occupies the edge of the antorbital fenestra (Figures 2, 7). The lateral layer also protrudes posteriorly into the orbital fenestra, as in Giganotosaurus and Mapusaurus. In contrast, the posterior margin of the lacrimal of Allosaurus, Monolophosaurus, Concavenator, and Sinraptor is nearly straight. Medially, the lacrimal of Acrocanthosaurus preserves several anteroposteriorly-oriented ridges along the medial surface of the lacrimal horn that articulate with the nasal and maxilla anteriorly (Figure 7B). The ridges contact the prefrontal posterior to their contact with the nasal, at which point the ridges display a ventral curvature. The posterior margin of the lacrimal horn contacts the postorbital in Acrocanthosaurus, as in Giganotosaurus, Carcharodontosaurus, and Mapusaurus [1], [20], [35]–[36]. The lacrimal and postorbital are separated by a gap in Sinraptor, Allosaurus, and Monolophosaurus [16], [69], [71] that permits the prefrontal to be seen when the skull is in lateral view.

Postorbital The left and right postorbitals of NCSM 14345 are complete. The holotype specimen of Acrocanthosaurus preserves a left postorbital [23], although the orbital brow and anterior margin of the postorbital are weathered and broken. Additionally, SMU 74646 has an undescribed, fragmentary right postorbital with a reconstructed ventral ramus and a tall dorsal boss. The postorbital of Acrocanthosaurus is a robust, tripartite element that protrudes laterally from the dorsal margin of the skull (Figures 8, 40, 41). A rugose, sinusoidal orbital boss is present posterior to contact with the lacrimal and forms the roof of the orbit. The boss is split in lateral view by a sinuous vascular groove that extends along its entire length anteroposteriorly. The morphology and vascularization of this boss in Acrocanthosaurus is similar to that observed in Concavenator, Mapusaurus and Carcharodontosaurus [22], [25], and its presence is attributed to the possible fusion of a palpebral bone to the postorbital [36]. The postorbital in Eocarcharia displays a vascular groove along the anterior half of the orbital boss. Giganotosaurus lacks this vascular groove completely, although weathering of the bone surface may have removed this feature. The likely presence of a vascular groove on the postorbital of Giganotosaurus is supported by the presence of a palpebral bone covering the dorsal surface of its postorbital [41]. Palpebral-postorbital fusion is probable in Acrocanthosaurus as well, and although no sutures between the elements are visible, small fossae along the posterior termination of the dorsal boss of the postorbital may indicate postorbital-palpebral contact as in Mapusaurus and Eocarcharia [36], [49]. Postorbital rugosity has been noted in specimens of Allosaurus [69], although this taxon and Monolophosaurus lack a laterally expanded, vascularized postorbital boss. Posterior to the orbital boss of Acrocanthosaurus, a triangular, tapering process fits into a grooved articulation with the squamosal (Figure 8). The descending ramus of the postorbital tapers along its posterior margin near the contact with the jugal. Together these elements form the anterior edge of the lateral temporal fenestra. The left postorbital preserves a triangular flange (‘intraorbital process’ [49]) anteriorly along the descending ramus, a feature not previously described for Acrocanthosaurus. This flange protrudes into the orbital fenestra and denotes the lower limit of the ocular cavity with the posterior projection of the descending process of the lacrimal (Figures 2, 8). The right postorbital of NCSM 14345 and the postorbital of the holotype specimen have broken anterior margins, inferred by Brusatte and Sereno [22] to represent missing intraorbital processes. The robustness of the intraorbital process in Acrocanthosaurus resembles that of the abelisaurid Carnotaurus sastrei. The carcharodontosaurian taxa Carcharodontosaurus, Concavenator, Eocarcharia, and Giganotosaurus also possess postorbitals with an intraorbital process [13]–[14], [35], [49], although the protrusion is laterally compressed, triangular, and proportionally smaller in these taxa (Figure 38). A lateromedially-flattened intraorbital process is also present in Tyrannosaurus and Majungasaurus [27], [80], although the process is dorsoventrally taller in these taxa than in members of Allosauroidea. In Allosaurus, Monolophosaurus, Sinraptor, and Yangchuanosaurus, the anterior margin of the postorbital is smooth and lacks an intraorbital process (although a small convexity is present in Monolophosaurus and Sinraptor [72]), a condition shared with Herrerasaurus, Coelophysis, and most other non-allosauroid theropods [88]–[89]. The medial surface of the postorbital of Acrocanthosaurus has a medially-expanded shelf that contacts the prefrontal and frontal anteriorly. Several small fossae are tucked beneath the margin of the shelf near its contact with the parietal and laterosphenoid (Figure 8B). Here, the shelf is divided into a posterior shelf and a ventral ridge. The posterior extension of the shelf parallels the dorsal surface of the postorbital, and is overlapped laterally by the squamosal. The ventral ridge is curved and terminates along the ventral ramus of the postorbital near contact with the jugal. Similar medial shelf morphologies are present on the postorbitals of Giganotosaurus, Mapusaurus, and Sinraptor. The anterior portion of this shelf that contacts the prefrontal of Acrocanthosaurus appears similar in morphology and location to a shelf figured for the postorbital of Eocarcharia, although in Eocarcharia this shelf contacts the frontal [49]. In dorsal view, the postorbital is lateromedially expanded, pitted with small fossa, and flattened except for the raised orbital boss along its lateral margin. The posterior region of the dorsal postorbital surface is depressed by the supratemporal fossa. The margin of this depression is curved medially near its expansion onto the frontal and parietal (Figure 41). Acrocanthosaurus shares a posteriorly-positioned depression of the dorsal surface of the postorbital with Carcharodontosaurus, Mapusaurus, and Eocarcharia. In Allosaurus and Sinraptor, the expression of the supratemporal fossa on the dorsal surface of the postorbital expands further anteriorly to approach the anterior margin of the postorbital [10], [42].

Prefrontal The first prefrontal material referred to Acrocanthosaurus is from NCSM 14345 (Figure 9), of which the dorsal surfaces have been described. Prefrontal exposure in dorsal view is minimal, and the relatively small element appears as a triangular wedge between the lacrimal horn, frontal, postorbital, and posterior process of the nasal [1]. The prefrontal contacts the lacrimal with a flattened articular surface pitted by numerous small depressions (Figure 9B). The prefrontal-lacrimal contact is not fused in Acrocanthosaurus, Sinraptor, Eocarcharia and Allosaurus; the prefrontal is fused to the lacrimal in Giganotosaurus, Mapusaurus, and Carcharodontosaurus [1], [22], [49]. The medial articular surface of the prefrontal is auriform in Acrocanthosaurus (Figure 9A), similar to Sinraptor [16] but unlike the triangular prefrontal of Allosaurus [69]. Blade-like processes extend anteriorly and ventrally from the main body of the prefrontal to contact the frontal and nasal, respectively. These processes converge upon the body of the prefrontal. Posteriorly, the anterior blade forms a ridge upon the body of the prefrontal that curves posteromedially to surround a deep sulcus. The ventral blade of the prefrontal contacts the frontal and curves posterodorsally to meet the rounded ridge formed by the anterior blade. Posterior to this ridge, a small flange contacts the postorbital.

Quadratojugal The skull of NCSM 14345 preserves the only quadratojugal material referred to Acrocanthosaurus. The dorsal rami of both quadratojugals are broken (Figure 10), and the medial surfaces of the quadratojugals are obscured by close contact with the quadrate. The quadrate and quadratojugal were cast in articulation. The quadratojugal of Acrocanthosaurus is an L-shaped bone at the posteroventral corner of the cranium that forms the majority of the posterior margin of the lateral temporal fenestra. The lateral surface of the quadratojugal is relatively smooth and unornamented. The right quadratojugal, at the base of its dorsal ramus, preserves a small lateral fossa. This ramus would have likely contacted the precotyloid process of the squamosal dorsally, and the articulation of these processes is inferred to have curved anteriorly into the lateral temporal fenestra [1] to create a convex posterior margin of the fenestra in lateral view. The shape of the quadratojugal immediately below its dorsal breakage suggests that Acrocanthosaurus likely has a narrow dorsal quadratojugal ramus, as in Sinraptor, Monolophosaurus, and Yangchuanosaurus, but unlike the anteroposteriorly broader dorsal rami of Allosaurus and Tyrannosaurus [90]. The anterior ramus of the quadratojugal is trifurcated to fit tongue-in-groove between the dorsal and ventral quadratojugal prongs of the jugal (Figures 10A, 10C). The medial projection of the anterior ramus overlaps the medial surface of the jugal. The forked lateral projection sutures tightly between the quadratojugal prongs of the jugal and covers the small accessory prong of the jugal in lateral view. The convex posterior surface of the quadratojugal is curved along its contact with the quadrate (Figures 10A, 10B). The quadratojugal terminates ventrally at the dorsolateral surface of the lateral condyle of the quadrate. This quadrate-quadratojugal suture extends dorsally and terminates as the quadratojugal is deflected anterolaterally to contact the squamosal.

Quadrate Both left and right quadrates of NCSM 14345 are relatively intact (Figure 10), although the pterygoid wing of the right quadrate is broken and reconstructed. The posterior surface of the quadrate of Acrocanthosaurus was previously described [1], and NCSM 14345 preserves the only quadrate material referred to the taxon. The medial condyle of the quadrate is positioned further posteriorly than the lateral condyle (Figure 10C), although the lateral condyle is wider in posterior view (Figure 10B), similar to the condition in most theropods [9]. The quadratojugal overlaps the quadrate and obscures most of the lateral surface of the quadrate from view. Along the quadrate-quadratojugal suture, the quadrate is pierced posteriorly by the quadrate foramen. Similar to Allosaurus, Giganotosaurus, Mapusaurus, and Sinraptor [16], [35]–[36], [69], most of the quadrate foramen of Acrocanthosaurus is enveloped by the quadrate, with the quadratojugal forming a reduced portion of the lateral rim of the opening (Figure 10B). In Monolophosaurus and Tyrannosaurus, the quadratojugal participates more extensively in the lateral rim of the quadrate fenestra [9], [72], [82]. Dorsal to the quadrate fenestra, a large depression (5.80 cm tall×3.44 cm wide) referred to here as the ‘posterior pneumatic recess’ penetrates the body of the quadrate and is split into two blind cavities by a thin septum. Acrocanthosaurus shares the presence of posterior quadrate pneumaticity with Aerosteon, Giganotosaurus, and Mapusaurus [10]. In Giganotosaurus and Mapusaurus, the pneumatic recess lacks a visible septum (Figure 44). The quadrate fenestra of Aerosteon is of a similar size and position to the posterior pneumatic recess of Acrocanthosaurus, although in Aerosteon the quadrate fenestra opens completely through the quadrate and is accompanied ventrally by a large blind fossa (‘pneumatocoel’ [51]). The lateromedially-flattened pterygoid wing projects anteriorly from the quadrate to articulate with the quadrate ramus of the pterygoid. In medial view, the left quadrate preserves a large, rounded pneumatic recess (3.27 cm tall×3.73 cm wide) within the posteroventral corner of the pterygoid wing (Figure 10C). The ventral surface of the pterygoid wing of the quadrate forms the floor of this depression, referred to here as the ‘medial pneumatic recess’. A septum splits the medial pneumatic recess of the quadrate of Acrocanthosaurus, similar to the posterior pneumatic recess. Giganotosaurus and Mapusaurus also preserve a medial pneumatic recess in a comparable position. However, in Giganotosaurus the recess is small and round, and in Mapusaurus the recess is anterodorsally elongated and undivided. Quadrate material referred to Allosaurus, Shaochilong, and Sinraptor preserves a shallow depression in this region [16], [37], [69], but lacks a sharply defined medial pneumatic recess.

Squamosal The intact left and right squamosals of NCSM 14345 are the most complete squamosal elements referred to Acrocanthosaurus. Squamosal material from the holotype specimen includes a fragmentary left squamosal missing most of the quadratojugal and postcotyloid processes [23]. Both squamosals of NCSM 14345 are tri-radiate elements at the posterodorsal margin of the skull that form the posterodorsal corners of the lateral temporal fenestrae (Figures 2, 11). The dorsal process of the squamosal is lateromedially broad, and its lateral surface bears a rectangular suture for the posteriorly projecting squamosal process of the postorbital (Figure 11A). The medial surface of the dorsal process contacts the parietal with an anteriorly tapering, flat surface. The lateral margin of the nuchal crest is also preserved on the left squamosal. The quadratojugal process of the squamosal extends anteroventrally into the lateral temporal fenestra [1]. The postcotyloid process of the squamosal is expanded and triangular in lateral view (3.70 cm wide neck, 6.27 cm wide distal expansion; Figure 11A), and wraps around the posterodorsal edge of the quadrate cotyle. The expanded distal end of the postcotyloid process in Acrocanthosaurus contrasts with distally-tapering processes in Allosaurus and Monolophosaurus (Figure 42). An expanded postcotyloid process is interpreted as being present in Sinraptor ([16]: p. 2048), but missing squamosal material prevents the confirmation of this morphology. Because the postcotyloid process is not distally expanded in Yangchuanosaurus, it is therefore possible that in Sinraptoridae (i.e., Sinraptor and Yangchuanosaurus in this analysis) the postcotyloid process of the squamosal is tapered, as in other basally-positioned allosauroids. In ventral view, the squamosal appears triangular and quadri-radiate (Figure 11B). A small, blind fossa penetrates the posterodorsal corner formed by the junction of the dorsal and quadratojugal processes of the squamosal. This opening occurs in Tyrannosaurus and Majungasaurus [80], [82], but not in the allosauroids Allosaurus and Sinraptor [16], [69].

Frontal The frontal, parietal, and braincase elements of NCSM 14345 are fused, as in the holotype specimen of Acrocanthosaurus [23]. The paired frontals of Acrocanthosaurus are dorsoventrally flattened and form the majority of the cranial surface dorsal to the orbital and olfactory regions of the braincase (Figures 12–16). The frontals were cast and are presently mounted in articulation with the parietal and orbitosphenoid, obscuring the connective surfaces among those elements. The suture between the frontals is completely fused [1], and the paired frontals form a triangular shape in dorsal view (Figure 14). Frontal fusion also occurs in the holotype specimen of Acrocanthosaurus [23], and in Carcharodontosaurus, Giganotosaurus, Eocarcharia, and Shaochilong [41], [49], [75], [76]. The frontals of the allosauroids Allosaurus and Sinraptor are unfused [76]. The frontal of Acrocanthosaurus contacts the posterior margin of the nasal with a flange-like triangular process. This process is exposed dorsally and slightly underlies the nasals, as in Allosaurus, Eocarcharia, Giganotosaurus, and Shaochilong [41], [49], [69], [76]. The nasal process of the frontal of Sinraptor extends proportionally further anteriorly beneath the nasal [16]. The frontal of Acrocanthosaurus contacts the prefrontal and postorbital anterolaterally and exhibits a shallow depression at the junction of these elements [1]. Posterior to this contact, the frontal displays a steep rim that flattens near its lateral contact with the postorbital. This rim forms the anteromedial margin of the supratemporal fossa (Figure 14). Posterior to this rim, the frontoparietal suture forms a sharply-raised ridge that expands laterally across the supratemporal fossa of the frontal and shallows near the postorbital contact. This ridge is pronounced and appears as a protuberance adjacent to the laterally-facing shelf of the supratemporal fossa, a condition also present in Eocarcharia [49] and Sinraptor. In ventral view, the frontal is separated from the orbitosphenoid by an anteroposteriorly-oriented sulcus that curves laterally near its contact with the laterosphenoid (Figure 16).

Parietal Similar to the frontals of NCSM 14345, the parietals are also fused. This occurs in the holotype specimen of Acrocanthosaurus and the carcharodontosaurids Carcharodontosaurus, Giganotosaurus, and Shaochilong [41,75, –76]. The anterior portion of the parietal contacts the frontal and extends laterally to contact the postorbital (Figures 12, 14, 16). The parietals contact each other along the midline of the skull, forming a flat, anteroposteriorly-oriented crest between the transverse nuchal crest and the frontals (Figure 14). In Acrocanthosaurus, the lateral margin of this crest is oriented vertically to form the medial wall of the supratemporal fossa, as in Allosaurus, Monolophosaurus, and Sinraptor. This crest is proportionally wider in Carcharodontosaurus and Giganotosaurus, and in these taxa the transverse nuchal crest is shifted forward to overlap the posteromedial corner of the supratemporal fossa [41], [75]. The relative size and length of the supratemporal fossa in Acrocanthosaurus are similar to that of Eocarcharia, and both taxa have proportionally longer and larger fossae than in Carcharodontosaurus, Giganotosaurus, and Shaochilong, but smaller than in Sinraptor and Allosaurus [76]. The parietal of Acrocanthosaurus forms the posterior wall of the supratemporal fossa. The transverse nuchal crest extends posterolaterally from the midline to contact the dorsal surface of the exoccipital process. Anteroventral to the nuchal crest, the parietal-laterosphenoid contact is slightly distorted by posterior crushing of the skull. Posterodorsally, the parietals contact the supraoccipital process near the midline of the braincase, although the lateral extent of this contact is also damaged (Figures 14, 15). The nuchal crest surrounds the supraoccipital of Acrocanthosaurus and exhibits a squared morphology in posterodorsal view as in Sinraptor [16], but unlike the rounded nuchal crest in Allosaurus, Giganotosaurus, and Monolophosaurus [19], [41], [71]. Additionally, the dorsal margins of the parietals are even with or slightly lower than the supraoccipital in Acrocanthosaurus. In Allosaurus and Sinraptor, the nuchal crest of the parietals extends above the supraoccipital.

Vomer The only vomeral material referred to Acrocanthosaurus comes from NCSM 14345. Anteriorly, the elongated vomer (46.1 cm in length) contacts the medial symphysis of the premaxilla with a short, tapering process that is ventrally deflected (Figures 19, 23, 24). Posterior to its contact with the premaxilla, the vomer flattens dorsoventrally and widens laterally (‘rhomboid flange’ [97]) near a possible contact point with the anteromedial processes of the maxillae, as in Sinraptor and Tyrannosaurus. Further posteriorly, the lateromedially-compressed ‘posterior stem’ of the vomer [97] is expanded dorsoventrally near its contact with the palatine (Figure 24). Anterior to this contact, the posterior stem is split along its midline into left and right processes by a deep sulcus that extends further anteriorly in ventral view than in dorsal view (Figures 19, 23). PPT PowerPoint slide

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larger image TIFF original image Download: 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. https://doi.org/10.1371/journal.pone.0017932.g024 Anterior to the palatine-vomer contact, the vomeropalatine rami of the pterygoid overlap the vomer laterally (Figures 19, 22). Unlike in Sinraptor, ventral troughs for contact with the pterygoid are not visible on the vomer of Acrocanthosaurus. Acrocanthosaurus shares the plesiomorphic condition of the vomer not contacting the pterygoids with Sinraptor and the coelurosaur Tyrannosaurus [97], but unlike Allosaurus in which the two elements contact each other [16], [98]. In ventral view, the vomeropalatine rami of the pterygoids are visible within the vomeral sulcus and are overlapped by the medial surfaces of the split posterior stem of the vomer. The vomer terminates posteriorly and is overlapped by the medial surfaces of the vomeropterygoid processes of the palatines (Figure 22), a condition also described in the palate of Tyrannosaurus [97]. A similar arrangement of palatal elements occurs in Sinraptor [16], although this taxon preserves dorsal troughs on the vomer that are not visible in Acrocanthosaurus due to palatine-vomer fusion.

Ectopterygoid Ectopterygoid material was previously referred to Acrocanthosaurus, including a partial right ectopterygoid from the holotype specimen [23] and a right ectopterygoid from SMU 74646 [21]. The ectopterygoid from SMU 74646 was mislabeled as a left element, but a suture on the medial surface of the right jugal articulates with the jugal ramus of the ectopterygoid, confirming its identification as a right element. The well-preserved ectopterygoids of NCSM 14345 are morphologically similar to those referred to both the holotype specimen and SMU 74646, but were not visible during the description by Currie and Carpenter [1]. The ectopterygoid of Acrocanthosaurus is hook-shaped in dorsal view [21], and the jugal ramus extends posterolaterally from the medial body of the bone to contact the medial surface of the jugal (Figure 25). In Allosaurus, Sinraptor, and the megalosauroid taxon Dubreuillosaurus valesdunensis Allain 2002 [56], the ectopterygoid contacts the jugal with an expanded, triangular ramus (Figure 47). In these taxa, the angle of the jugal ramus also parallels the ventral margin of the main body of the ectopterygoid. In Acrocanthosaurus, Giganotosaurus, and a probable partial ectopterygoid from Carcharodontosaurus (SGM-Din 1), the jugal ramus is rectangular in lateral view, with parallel dorsal and ventral margins. In Acrocanthosaurus and Giganotosaurus the ramus is inclined dorsally by approximately twenty degrees to the ventral margin of the ectopterygoid (Figure 47). PPT PowerPoint slide

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larger image TIFF original image Download: 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. https://doi.org/10.1371/journal.pone.0017932.g025 The medial surface of the ectopterygoid extensively contacts the pterygoid between the vomeropalatine and ectopterygoid rami (Figure 25B). The ectopterygoid is perforated medially by an elongate, ovular vacuity within the main body of the element that expands into the base of the jugal ramus, unlike the sub-circular ectopterygoid recess of Tyrannosaurus [99]. The ectopterygoid of Acrocanthosaurus is also characterized by small fossae along the medial edge of the main body, positioned posterior to the medial vacuity. These fossae are not preserved in either the holotype specimen or SMU 74646, but occur in both left and right ectopterygoids of NCSM 14345. The fossae open medially and are in close association with the fossae that perforate the ectopterygoid ramus of the pterygoid. Giganotosaurus preserves two accessory fossae on the medial surface of the ectopterygoid, whereas Allosaurus and Sinraptor preserve none.

Epipterygoid The left epipterygoid of NCSM 14345 is the only such element currently referred to Acrocanthosaurus. The triangular epipterygoid is laterally compressed and overlaps the lateral surface of the quadrate ramus of the pterygoid (Figures 19, 26). In medial view, a thin ridge is expands posteriorly along the anteromedial margin of the epipterygoid. Ventral to this ridge, a short, rounded process overlaps the pterygoid. Dorsally, the epipterygoid tapers to a point as in Ceratosaurus magnicornis Madsen and Welles 2000 [81], Cryolophosaurus ellioti Hammer and Hickerson 1994 [100], and some tyrannosauroids [101]. In contrast, some specimens of Allosaurus (UUVP 1414; BYU 671/8901) [102] and Tyrannosaurus (FMNH PR2081) [82] have a wide, bulbous dorsal tip of the epipterygoid (Figure 48). PPT PowerPoint slide

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larger image TIFF original image Download: Figure 26. Left epipterygoid of Acrocanthosaurus atokensis (NCSM 14345). Epipterygoid in (A) lateral and (B) medial views. ls, laterosphenoid contact; pt, pterygoid contact. https://doi.org/10.1371/journal.pone.0017932.g026 Dorsally, the epipterygoid of Acrocanthosaurus approaches, but does not articulate with the laterosphenoid; this contact is present in other theropods (e.g., Tyrannosaurus [82], Cryolophosaurus [13]). In an articulated skull of Allosaurus (BYU 571/8901) [102], an undescribed epipterygoid articulates with the pterygoid and is in close proximity to the laterosphenoid, but does not contact the braincase. This is inconsistent with the interpretation [69] of a highly kinetic epipterygoid-laterosphenoid contact in Allosaurus. However, absence of epipterygoid-laterosphenoid contact in Allosaurus and Acrocanthosaurus supports the model proposed by Holliday and Witmer of a reduction in the size and kinetic nature of the epipterygoid in non-avian theropods [103].

Supradentary and Coronoid The supradentary (‘intercoronoid’ [69]) and coronoid of NCSM 14345 are the only such elements referred to Acrocanthosaurus and are described here for the first time in this taxon. In medial view, the thin supradentary process overlaps the dentary immediately ventral to the alveolar margin (Figures 27, 28B). The anterior margin of the supradentary is positioned ventral to the fourth dentary alveolus in Acrocanthosaurus, as in Monolophosaurus. The lateromedially-flattened supradentary extends past the posterior margin of the dentary alveoli, at which point it displays a posteroventral curvature and narrowly contacts the medial surface of the surangular. The supradentary is fused with the dorsal process of the coronoid (Figures 27, 32B). Supradentary-coronoid continuity has been noted in Monolophosaurus [71] and some specimens of Tyrannosaurus [82]. This continuity may be more broadly distributed within Theropoda, but because coronoid and supradentary elements are prone to disarticulation due to their ligamentous attachment to the mandible, such fusion is not commonly preserved [82]. Propensity for this disarticulation in allosauroids is supported by the presence of isolated supradentaries and coronoids in specimens of Allosaurus [69], and the lack of these elements in an otherwise nearly complete skull of Sinraptor (IVPP 10600). The coronoid of Acrocanthosaurus is lateromedially-flattened like the supradentary, sub-rectangular, and split anteriorly by a narrow sulcus that separates the element into dorsal and ventral processes. As previously mentioned, the dorsal process is continuous with the supradentary; the ventral process of the coronoid tapers to a point and dorsomedially overlaps an anteriorly-projecting process of the surangular (Figure 32B). Although the supradentaries of Monolophosaurus, Tyrannosaurus, and Allosaurus are similar in morphology to that of Acrocanthosaurus, coronoids from these taxa appear triangular in medial view and display a tapering posterodorsal flange that is absent in Acrocanthosaurus. Coronoid material has not been described for Sinraptor, Yangchuanosaurus, or taxa within Carcharodontosauria, and therefore the extent of morphological variation of the coronoid within Allosauroidea is uncertain.

Splenial Splenial material previously referred to Acrocanthosaurus (SMU 74646) [21] is highly fragmentary, and its identification as a splenial is equivocal. Both splenials of NCSM 14345 are well preserved but were not visible during the original description of the specimen [1]. The right splenial is mounted in articulation with the dentary, obscuring its internal surface, although the left splenial is isolated from the mandible. The splenial is a long (∼60.5 cm), medially convex sheet of bone that articulates with the medial surface of the dentary anteriorly and the articular posteriorly (Figures 27, 29). The ventral margin of the splenial is curved posteroventrally in Acrocanthosaurus, Mapusaurus, and Allosaurus; in Sinraptor, Yangchuanosaurus, and Monolophosaurus, this ventral margin is straight. The dentary process of the splenial is forked anteriorly into separate prongs in Acrocanthosaurus. The dorsal prong of the splenial terminates below the eighth dentary alveolus. The smaller ventral prong does not project as far anteriorly and ends below the ninth alveolus (Figure 27). Along the ventral margin, the anteroposteriorly-elongated anterior mylohyoid foramen is completely enclosed by the splenial (Figure 29). The splenial also entirely encloses the mylohyoid foramen in Mapusaurus, Sinraptor, and Tyrannosaurus. In Allosaurus, Monolophosaurus, and some non-allosauroid theropods (e.g., Dubreuillosaurus, Majungasaurus) this anterior mylohyoid foramen is present, but its anterior margin is open and not surrounded by the splenial. PPT PowerPoint slide

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larger image TIFF original image Download: 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. https://doi.org/10.1371/journal.pone.0017932.g029 Posterodorsally, a squared splenial process contacts the prearticular of Acrocanthosaurus. The splenial surface that contacts the rounded tip of the prearticular is slightly concave (Figures 27, 29), and proportionally shorter than the posterodorsal projections of the splenial in Allosaurus, Monolophosaurus, Sinraptor, and Tyrannosaurus. Acrocanthosaurus shares a short posterodorsal projection of the splenial with Mapusaurus. In Acrocanthosaurus, the posteroventrally-downturned angular process of the splenial contacts and parallels the posteroventral curvature of the dentary. Internally, an infra-angular ridge is developed along the posteroventral margin of the splenial and contacts the dentary and angular. This ridge is relatively thin in Acrocanthosaurus, Sinraptor, and Allosaurus, compared to the thicker, raised infra-angular ridges of Ceratosaurus, Tyrannosaurus, and Mapusaurus. The shape of the infra-angular ridge is not known in Giganotosaurus and Carcharodontosaurus, because no splenial material has yet been referred to these taxa. The posterior margin of the splenial forms the anterior margin of the internal mandibular fenestra (Figure 27; ‘Meckelian fossa’ [82]). This opening is also present between the splenial and prearticular of Mapusaurus, Monolophosaurus, and Sinraptor, but is greatly reduced in Yangchuanosaurus. This fenestra in the aforementioned allosauroid taxa is not homologous to the internal mandibular fenestra described for Majungasaurus by Sampson and Witmer [80], which is instead attributed to the open region dorsal to the prearticular and ventral to the medial surangular shelf.

Angular The lateral surface of the angular has been described for Acrocanthosaurus [1]. The intact left and right angulars of NCSM 14345 represent the most complete material referred to the taxon, although a fragmentary angular is recognized from the holotype specimen [23]. The angular is flattened lateromedially and parallels the curvature of the prearticular to form the ventral margin of the mandible (Figures 2, 31). Anteriorly, the angular narrows and curves anterodorsally to contact the medial surface of the dentary and internal surface of the splenial. The angular overlaps the lateral surface of the surangular posteriorly (Figure 2), and contacts the prearticular medially [1]. The medial surface of the angular preserves a thin ridge along its ventral margin that articulates with the medial ridge of the prearticular. The dorsal surface of the angular forms the ventral margin of the external mandibular fenestra. Anteroposteriorly-elongated fossae are visible below the dorsal margin of both angulars. It is unclear whether these depressions are accessory pneumatic structures of the external mandibular fenestra or simply neurovascular foramina. Fossae are absent in the angulars of Sinraptor, Monolophosaurus, and Allosaurus, although angulars referred to Tyrannosaurus preserve large openings in this region [82]. PPT PowerPoint slide

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larger image TIFF original image Download: 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. https://doi.org/10.1371/journal.pone.0017932.g031

Surangular The surangular, articular, and posterior portion of the prearticular of NCSM 14345 are preserved in articulation (Figure 32), similar to material referred to Acrocanthosaurus from the posterior mandible of the holotype specimen and SMU 74646. Incomplete surangular material has been previously described, including a partial left surangular from the holotype and a partial right surangular from SMU 74646. Both surangulars are intact in NCSM 14345, with only the lateral surfaces previously described [1]. PPT PowerPoint slide

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larger image TIFF original image Download: 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. https://doi.org/10.1371/journal.pone.0017932.g032 The surangular is an elongated element (∼59.1 cm) composed of a lateromedially- flattened anterior sheet of bone bordered dorsally by expanded lateral and medial surangular shelves. The medial shelf of the surangular is positioned further dorsally than the lateral shelf, and the M. adductor mandibulae externus presumably attaches dorsally and laterally along a depression between these shelves [80]. Here, the surangular also nears the mass occupied by the quadratojugal, jugal, and the posterior ramus of the maxilla when the mouth of Acrocanthosaurus is closed. A knob located near the posterior margin of the lateral surangular shelf of NCSM 14345 [1] is also present in SMU 74646 [21]. Ventral and slightly anterior to this knob, a rounded posterior surangular foramen is visible (Figure 32A). The angular laterally overlaps a flat, anteroventrally-projecting flange of the surangular to form the posterior and ventral margins of the external mandibular fenestra (Figure 2). Anteriorly, two large, irregularly-shaped openings perforate the surangular and the thin posterior process of the dentary (Figures 28, 32A). Similarly-positioned openings in greater abundance are described from the surangular of Tyrannosaurus as lesions with surrounding rings of inflated bone [82]. In Acrocanthosaurus, the openings exhibit flat margins and are suggested to represent post-depositional damage [1]. The anterior process of the surangular is tall (>16 cm) and contacts the dentary along a posteroventrally-sloped margin (Figure 32C). The anterior tip of the surangular participates in the external mandibular joint (Figures 27, 32A) with a thin, blade-like flange that overlaps the lateral surface of the dentary [1]. The dorsal margin of the flange terminates posteriorly at the entrance for the anterior surangular foramen. In anteromedial view, another large foramen opens anteriorly between the right surangular and the prearticular; this depression is absent in the left prearticular. Dorsal to this foramen, the medial shelf of the surangular splits anteriorly into two processes near lateral contact with the coronoid. The anterior extent of the dorsal process is obscured laterally by the supradentary (Figures 27, 32B); the ventral process contacts the internal surface of the coronoid and extends anteriorly past the mandibular joint to overlap the medial surface of the dentary.