Chilantaisaurus maortuensis Hu 1964
Shaochilong maortuensis (Hu, 1964) Figs 1–15 Lectotype. IVPP V.2885.1, well preserved and nearly complete braincase, including parts of the parietals, supraoccipital, exoccipital-opisthotics, basioccipital, basisphenoids, parasphenoid, prootics, and orbitosphenoids; IVPP V.2885.2, paired frontals, p...
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| Summary: | Shaochilong maortuensis (Hu, 1964) Figs 1–15 Lectotype. IVPP V.2885.1, well preserved and nearly complete braincase, including parts of the parietals, supraoccipital, exoccipital-opisthotics, basioccipital, basisphenoids, parasphenoid, prootics, and orbitosphenoids; IVPP V.2885.2, paired frontals, paired parietals, and posterior end of right nasal. Taxonomic note and paralectotype series. Hu (1964) erected Chilantaisaurus maortuensis on the basis of cranial bones, an axis and six caudal vertebrae. Although the material was collected from a single locality, Hu (1964) did not explain the degree of association of the bones or provide quarry maps. It is considered likely that the bones represent a single individual, but to provide for the possibility that they will be shown to belong to multiple taxa in future studies we designate the braincase (IVPP V.2885.1) and skull roof fragment (IVPP V.2885.2) as the lectotype (name-bearing type specimen) and consider the remaining material to belong to the paralectotype series: left and right quadrates (IVPP V.2885.3), a right maxilla (IVPP V.2885.4), an axis vertebra (IVPP V.2885.5) and six caudal vertebrae (IVPP V.2885.6– 7). The braincase and skull roof piece are both assigned to the lectotype because they clearly fit together as a single specimen (broken along the parietals, which are shared between both pieces). The remaining skull bones and axis probably belong to the same individual as the braincase and skull roof, due to similar size, proximity in the skeleton, non-duplication of elements, and similar phylogenetic affinities indicated by all elements. The caudal vertebrae are referred with less certainty, as they do not show unambiguous evidence for carcharodontosaurid affinities and are from a more distant part of the skeleton. In an unpublished thesis, Chure (2000) briefly described the lectotype series of Shaochilong and provided a new generic name, “Alshansaurus”. Chure’s (2000) thesis was circulated to many dinosaur researchers and is often cited. Although the name “Alshansaurus” was never formally published, it has been used by many dinosaur workers as an informal name for the specimen. Type locality and horizon. Ulansuhai Formation, Maortu, Inner Mongolia Autonomous Region, People’s Republic of China (60 km north of Chilantai). The Ulansuhai Formation is often regarded as Aptian-Albian (late Early Cretaceous) based on perceived faunal similarities to other deposits of this age (e.g., Weishampel et al. 2004 b). However, radiometric dating of underlying strata indicates a maximum age of approximately 92 Ma (Turonian, early Late Cretaceous [“mid Cretaceous”]; Kobayashi & Lu 2003, Benson & Xu 2008). We prefer the Turonian date, as it is tied to explicit radiometric data. Original diagnosis. “Skull small, occipital condyle comparatively large, maxilla with 12 teeth, quadrate relatively small” (Hu 1964: 59). Emended diagnosis. Allosauroid theropod possessing the following autapomorphies: maxillary antorbital fossa reduced in extent and nearly absent; paradental groove absent on the medial surface of the maxilla; deep, dorsoventrally oriented grooves located dorsally on maxillary interdental plates; pneumatic recess penetrates to posterior end of nasal; dorsoventrally deep sagittal crest on the frontal; large pneumatic foramen (pneumatopore) in the anterodorsal corner of the dorsal tympanic recess of the prootic (Brusatte et al. 2009 a). Nomenclatural note. Clade names and phylogenetic definitions follow a recent revision of basal tetanuran taxonomy proposed by Benson et al . (2009). Allosauroidea is comprised of four “family”-level clades: Sinraptoridae, Allosauridae, Carcharodontosauridae, and Neovenatoridae. The latter two clades are sister taxa and comprise the rank-free Carcharodontosauria, which is equivalent in definition to Carcharodontosauridae as commonly used by previous authors (e.g., Holtz et al . 2004; Sereno et al . 2005). These taxonomic changes do not severely affect the terminology used in this monograph. However, Neovenator is no longer a “carcharodontosaurid,” as commonly considered under previous taxonomies, but rather a “neovenatorid carcharodontosaurian”. Description and comparisons. Skull. A skull reconstruction of Shaochilong , drawn by Brett Booth, is presented in Figure 1. The snout of Shaochilong is shortened relative to other carcharodontosaurids, which generally possess long snouts despite their large body size (e.g., Sereno et al. 1996; Currie & Carpenter 2000; Eddy 2008). In concert with small body size (see below), the short-snouted skull suggests that Shaochilong possessed a unique morphotype, and perhaps ecotype, among carcharodontosaurids. Maxilla. Hu (1964: 59) listed both a right maxilla and a “fragmental left maxilla” among the “material” (taken here as the syntype series) of Shaochilong maortuensis . However, we could only locate the right maxilla (IVPP V2885.4; Figs 2–3), which is nearly complete and well preserved. This element was illustrated by Hu (1964: fig. 10), but it is difficult to distinguish original bone and broken margins in this figure. A revised version of this figure was published by Zhao et al. (2008: fig. 325) and a photograph was provided by Dong (1992). However, other than a paragraph in the original description (Hu 1964: 60), this bone has not been thoroughly described in the literature. The right maxilla is nearly complete but is missing the dorsal part of the ascending process as well as the posterior portion of the jugal process bearing the articular facet for the jugal. As preserved the maxilla is 290 mm long anteroposteriorly and 77 mm deep dorsoventrally at the anterior margin of the antorbital fenestra. The tooth row in Shaochilong is complete and the jugal process extends 35 mm posterior to it as preserved. However, a substantial portion is missing in this region, as in other allosauroids there is an extensive margin of non-dentigerous bone posterior to the posteriormost alveolus (e.g., Madsen 1976; Currie & Zhao 1993 a; Sereno et al. 1996; Eddy 2008; Sereno & Brusatte 2008). The main body tapers in depth as it continues posteriorly and becomes confluent with the jugal process, thinning to a depth of 38 mm at the posterior broken margin. A tapering main body and jugal process is common among theropods but contrasts with the condition in abelisaurids (e.g., Lamanna et al. 2002; Sampson & Witmer 2007), some coelurosaurs (e.g., Dromaeosaurus : Currie 1995), Monolophosaurus (Brusatte et al. in press), and Zupaysaurus (Ezcurra 2007), which possess maxillae that maintain a relatively constant depth across their length. Posteriorly the jugal process is deflected posteroventrally, beginning at the anterior end of the jugal articulation. Only the base of this deflection is preserved but this region is oriented at an angle of approximately 20 degrees from the anteroposterior trend of the main body. A similar deflection is present in the carcharodontosaurids Acrocanthosaurus (Eddy 2008: fig. 14) and Eocarcharia (Brusatte & Sereno 2008: figs. 11-13), as well as those megalosaurids in which this region is preserved ( Afrovenator , UC OBA 1; Megalosaurus , OUNHM J. 13506), and was employed as a phylogenetic character by Sereno & Brusatte (2008: ch. 8). In contrast, other allosauroids and basal tetanurans only exhibit ventral deflection at the far posterior tip of the jugal process (e.g., Madsen 1976; review in Sereno & Brusatte 2008). As in many other basal tetanurans there is a distinct anterior ramus of the maxilla that projects from the main body anterior to the ascending ramus (e.g., Madsen 1976; Sereno et al. 1994; Holtz et al. 2004). The separation between the ascending ramus and anterior ramus is slight in Shaochilong and the anterior ramus is tall relative to its length (78 mm deep by 35 mm long). It is proportionally taller than in most other theropods that possess an anterior ramus that is taller than long, such as Ceratosaurus (Madsen & Welles 2000), and its shape and size are similar to those of some individuals of Mapusaurus (MCF-PVPH- 108.115; Coria & Currie 2006: fig. 2 B). In other Mapusaurus specimens the anterior ramus is essentially absent, as it is confluent with the anterior rim of the maxillary body and ascending process (MCF-PVPH- 108.169; Coria & Currie 2006: fig 2 A). Similarly, the anterior ramus is deep and either confluent with the ascending ramus or weakly demarcated in most other carcharodontosaurids, including Acrocanthosaurus (Currie & Carpenter 2000), Carcharodontosaurus (Sereno et al. 1996; Brusatte & Sereno 2007), and Eocarcharia (Sereno & Brusatte 2008). However, Neovenator exhibits a prominent anterior ramus (Brusatte et al. 2008). The shape of the ramus is variable in non-carcharodontosaurian allosauroids, as it is prominent in Allosaurus (Madsen 1976) but confluent with the ascending ramus in Sinraptor (Currie & Zhao 1993 a). The ascending ramus extends posterodorsally at approximately 45 degrees from the anteroposterior trend of the main body. This is the case in most basal tetanurans, but differs from the nearly vertical orientation of the ramus in most abelisaurids (e.g., Chatterjee 1978; Bonaparte 1985; Bonaparte et al. 1990; Lamanna et al. 2002; Canale et al. 2009). In Shaochilong the ascending ramus is broken dorsally, but by this point it has already strongly tapered (minimum axis “width” measurement in lateral view of 12 mm, compared with 50 mm at its base at the anteroventral corner of the antorbital fenestra). The entire ramus is very thin anteroposteriorly across its length. The overall proportions of the ramus are narrower than those of Carcharodontosaurus (Brusatte & Sereno 2007), Giganotosaurus (MUCPv-CH- 1), and Mapusaurus (Coria & Currie 2006), which have relatively narrow ascending rami and narrow antorbital fossae (see below). In contrast, Acrocanthosaurus (Currie & Carpenter 2000; Eddy 2008), Allosaurus (Madsen 1976), Eocarcharia (Sereno & Brusatte 2008), Neovenator (Brusatte et al. 2008), and Sinraptor (Currie & Zhao 1993 a) have proportionally wider ascending rami that accommodate a more extensive antorbital fossa. Only some regions of the surfaces for contact with the premaxilla, nasal, and jugal are observable. The premaxilla is contacted via the anterior surface of the anterior ramus of the maxilla, which is broadly convex in lateral view. Furthermore, when seen in lateral view, the premaxilla-maxilla suture trends strongly posterodorsally. This is also the case in most other allosauroids (e.g., Currie & Zhao 1993 a; Brusatte & Sereno 2007), but differs from the more vertical contact in Allosaurus (Madsen 1976) and Neovenator (Brusatte et al. 2008). The nasal articulates with the anterior surface of the ascending ramus and may have continued onto the dorsal surface of the ramus more posteriorly, although this region is broken in the paralectotype maxilla (IVPP V.2885.4). Few details of the nasal suture are evident and it is unclear whether the maxilla contributed to the floor of the external naris. However, it is evident that the nasal articulation is located solely on the anterior surface of the anterior ramus and does not face laterally, unlike in abelisaurids (Wilson et al. 2003; Sereno et al. 2004; Sereno & Brusatte 2008). Furthermore, the nasal articulation does not terminate ventrally in the blunt pit that is characteristic of abelisaurids (Wilson et al. 2003; Sereno et al. 2004). Although most of the jugal articulation is broken, the jugal clearly sat within a deep trough on the posterior part of the jugal process of the maxilla. Whether this trough was partially exposed laterally as in some allosauroids (e.g., Acrocanthosaurus : Eddy 2008; Eocarcharia : Sereno & Brusatte 2008), due to a lower lateral wall, is unclear. However, it is evident that the most anterior region of the trough is a deep embayment hidden in lateral view, and thus the complete articular surface on the maxilla is not entirely laterally facing as is often considered a synapomorphy of abelisaurids (e.g., Wilson et al. 2003; Sereno et al. 2004). The lateral surface of the maxilla is generally smooth, although it is slightly rugose anteriorly and above the tooth row. This form of sculpturing is similar to that of most theropods, and is not as extensive as in the derived carcharodontosaurids Carcharodontosaurus , Giganotosaurus , and Mapusaurus (Brusatte & Sereno 2008) and abelisaurids (Lamanna et al. 2002; Sampson & Witmer 2007; Sereno & Brusatte 2008). In Carcharodontosaurus , elongate grooves and ridges ornament most of the lateral surface, a texturing that has been described as autapomorphic for the genus (Brusatte & Sereno 2007). The surface texture is mottled, with random rugosities that do not form distinct ridges or grooves, in Giganotosaurus (Coria & Salgado 1995), Mapusaurus (Coria & Currie 2006), and the neovenatorid carcharodontosaurian Neovenator (Brusatte et al. 2008). In contrast, the lateral surface of the maxilla in Acrocanthosaurus (Currie & Carpenter 2000), Allosaurus (Madsen 1976), Eocarcharia (Sereno & Brusatte 2008), and Sinraptor (Currie & Zhao 1993 a) is smooth and little different in texture from that in Shaochilong . The lateral surface of the maxilla of Shaochilong is pierced by numerous foramina, which are especially abundant immediately dorsal to the tooth row. These foramina form two distinct series: a primary series that is approximately 10 mm dorsal to the tooth row and a secondary series that is positioned 35 mm above the tooth row. Foramina in both rows are large, measuring up to 5 mm in diameter, and form a linear series that approximately parallels the tooth row. The two rows merge posterior to the eighth alveolus, and the final foramen in the conjoined rows (located above the ninth alveolus) opens posteriorly into a deep and elongate groove. A discrete secondary row is also present in Acrocanthosaurus (Eddy 2008), Carcharodontosaurus saharicus (Brusatte & Sereno 2007), Eocarcharia (Sereno & Brusatte 2008), and possibly Mapusaurus (Coria & Currie 2006: fig. 2). Foramina are located in this region in other taxa (e.g., Allosaurus : Madsen 1976), but are not always set into a discrete row. However, whether this represents random variation or a phylogenetically informative signal is difficult to determine in the small samples for most theropod taxa. Furthermore, the final foramen of the conjoined row also opens into a deep groove in Eocarcharia (Sereno & Brusatte 2008). Unfortunately, this region of the maxilla is missing in many closely related taxa, precluding comparison. Finally, the primary row of Shaochilong , like those of other allosauroids, is positioned several millimetres above the tooth row, not immediately above the alveolar margin as in abelisaurids (Sereno & Brusatte 2008). The antorbital fossa is not extensive on the lateral surface of the maxilla, although this appearance is partially exaggerated by breakage. As preserved, the fossa only extends for approximately 7 mm ventral to the antorbital fenestra across most of the main body. However, the dorsal edge of the fossa is a broken surface, which is quite thick in mediolateral width. It is possible to link this broken surface with original bone on the dorsal margin of a small flange that projects dorsally at the anteroventral corner of the antorbital fenestra. This was not a flange in life, but rather is a preserved flake of bone, completely covered by the smooth fossa, that remains in isolation after the rest of the bone in this area has been broken away. Furthermore, the original dorsal surface of this flange can be linked to original bone surface on the posterior margin of the ascending ramus, giving a complete and fairly accurate reconstruction of the true dimensions of the antorbital fossa (Fig 2). In life, the fossa extended only 10-15 mm ventrally from the antorbital fenestra along the main body of the maxilla. Similar ventral reduction is present in other carcharodontosaurids such as Carcharodontosaurus (Brusatte & Sereno 2007), Giganotosaurus (MUCPv-CH- 1), and Mapusaurus (Coria & Currie 2006), as well as abelisaurids (e.g., Bonaparte et al . 1990; Sereno & Brusatte 2008) and the megalosaurid Torvosaurus (Britt 1991). In contrast, Allosaurus (Madsen 1976), Sinraptor (Currie & Zhao 1993 a), and the basal carcharodontosaurians Acrocanthosaurus (Currie & Carpenter 2000; Eddy 2008), Eocarcharia (Sereno & Brusatte 2008), and Neovenator (Brusatte et al. 2008) have a ventrally extensive antorbital fossa. The antorbital fossa extends anteriorly onto the ascending ramus of the maxilla, but only excavates approximately 15 % of the width of the base of the ramus (Table 1). In most allosauroids, including basal carcharodontosaurians such as Acrocanthosaurus , Eocarcharia , and Neovenator , this proportion is 50-65 %. A more extreme condition, an extensive fossa along the entire ascending ramus, is a synapomorphy of Coelurosauria (Sereno et al. 1996; Rauhut 2003 a; Holtz et al. 2004). In Carcharodontosaurus and other carcharodontosaurines the fossa is reduced on the ascending ramus (Table 1), but not to the extent seen in Shaochilong . Thus, the extremely limited antorbital fossa on the ascending ramus is an autapomorphy of Shaochilong among allosauroids. The antorbital fossa and the subcutaneous surface of the main body of the maxilla are not separated by a sharp rim or a swollen ridge (as in Carcharodontosaurus saharicus : Sereno et al. 1996; Brusatte & Sereno 2007), but rather by an abrupt change in bone texture. Anteriorly, the rim surrounding the antorbital fossa is rounded, not squared-off as in some carcharodontosaurians ( Eocarcharia , Neovenator : Sereno & Brusatte 2008), as well as megalosaurids ( Afrovenator : UC OBA 1; Dubreuillosaurus : Allain 2002), coelophysids (Rauhut 2003 a) and Eoraptor (Sereno et al. 1993). There is a distinct foramen within the fossa, which faces laterally and posteriorly, level with the region between the eighth and ninth alveoli. No accessory antorbital openings are readily visible within the antorbital fossa. However, as the anteroventral region of the fossa—the location of these openings in other theropods—is broken, this absence is potentially artifactual. Indeed, the broken medial surface of the maxilla shows that the base of the ascending ramus and the promaxillary process were inflated. These two regions are usually inflated by the maxillary and promaxillary fenestrae, respectively (Witmer 1997). 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