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Implications for Theropod Phylogeny and the Validity of the Genus Nanotyrannus Bakker Et Al., 1988

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Implications for Theropod Phylogeny and the Validity of the Genus Nanotyrannus Bakker Et Al., 1988 See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/290522732 Distribution of the dentary groove of theropod dinosaurs: Implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al., 1988 Article in Cretaceous Research · June 2016 Impact Factor: 1.9 · DOI: 10.1016/j.cretres.2015.12.016 CITATIONS READS 2 66 2 authors, including: Bruce Rothschild Carnegie Museum Of Natural History 97 PUBLICATIONS 2,771 CITATIONS SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Bruce Rothschild letting you access and read them immediately. Retrieved on: 06 July 2016 Cretaceous Research 61 (2016) 26e33 Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes Distribution of the dentary groove of theropod dinosaurs: Implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al., 1988 * Joshua D. Schmerge a, , Bruce M. Rothschild b a Biodiversity Institute, University of Kansas, 1345 Jayhawk Blvd., Lawrence, KS, 66045, USA b Department of Vertebrate Paleontology, Carnegie Museum, 4400 Forbes Ave., Pittsburgh, PA, 44272, USA article info abstract Article history: This study examines the phylogenetic distribution of a morphologic character, described as a groove Received 3 September 2015 containing pores, on the lateral surface of the dentary bone in theropod dinosaurs. The nature of this Received in revised form groove is a feature unique to theropods. Of the 92 theropod taxa examined for the presence and absence 27 December 2015 of this feature, 48 possessed and 44 lacked this feature. Distribution of this character was compared Accepted in revised form 27 December 2015 to published phylogenetic analyses of theropods, in order to evaluate the utility of the dentary groove Available online xxx as a diagnostic feature. 80% of pre-Tyrannoraptoran theropods possessed the dentary groove, with only 6 reversals in basal theropod clades. Theropods with beaks or edentulous jaws all lacked a dentary Keywords: Tyrannosaurus groove. Tyrannosauroidea is marked by mosaic distribution of this character. Among tyrannosauroids, þ Tyrannosaurinae the dentary groove occurs only in Dryptosaurus and the Albertosaurinae (Albertosaurus Gorgosaurus). Albertosaurinae Nanotyrannus lancensis, sometimes described as representing juvenile Tyrannosaurus rex, also possesses Neurovascular foramina this groove, unlike the remainder of the Tyrannosaurinae. Nanotyrannus lancensis was included in a Mandible phylogenetic analysis of Tyrannosauroidea and was recovered within Albertosaurinae. We recommend Ontogeny that Nanotyrannus stand as a valid taxon nested within the Albertosaurinae, based on the presence of this groove, as well as other features of the skull. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction occurrence of the dentary groove in theropod dinosaurs, and to discuss its phylogenetic and behavioral inferences. The past decade has witnessed increased attention to the Recognition and description of the dentary groove has varied in anatomy, ontogeny, and phylogeny of theropod dinosaurs, espe- previous studies. As such, there is no standard nomenclature to cially tyrannosaurids, with speculation as to their physiology and describe this feature found in some theropods. Various descriptions behavior (Brochu, 2003; Erickson et al., 2004; Weishampel et al., used to describe grooves on the lateral surface of the dentary 2004; Snively et al., 2006; Larson and Carpenter, 2008; Brusatte include “longitudinal groove that supports the nutritious foramina” et al., 2010; DePalma et al., 2013; Parrish et al., 2013). Theropod (e.g. Calvo and Coria, 1998), “dentary sulcus” (e.g. Novas et al., 2005; phylogeny based on cladistic analysis has shifted attention to large Sampson and Witmer, 2007; Eddy and Clarke, 2011), “lateral ridge” character datasets (e.g., Rauhut, 2003; Brusatte et al., 2010; Carrano with a “row of foramina above the ridge” (e.g. Coria and Currie, et al., 2012; Turner et al., 2012), with less consideration of func- 2006), “groove” (Novas et al., 2009), “groove for nutrient tional morphology. Such seems to be the case for a groove on the foramina” (e.g., Benson, 2010), “neurovascular groove” (Brusatte lateral aspect of the dentary, herein called the dentary groove, et al., 2012a), and “mental groove” (Carrano et al., 2012). Previous which appears to have functional and phylogenetic utility for the detailed consideration has apparently been limited to the Troo- Tyrannosauroidea. The purpose of this paper is to provide an dontidae, in which the groove bears neurovascular foramina that anatomical definition of the theropod dentary groove, trace the are connected with the inferior alveolar canal (e.g. Barsbold et al., 1987; Currie, 1987; Makovicky and Norell, 2004). The dentary groove was not included in reported cladistic analyses of theropods, until Brusatte et al. (2010) study of Tyrannosauroidea and Eddy and * Corresponding author. E-mail address: [email protected] (J.D. Schmerge). Clarke (2011) study of Allosauroidea. Neither study posited a http://dx.doi.org/10.1016/j.cretres.2015.12.016 0195-6671/© 2015 Elsevier Ltd. All rights reserved. J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33 27 function for the feature. Eddy and Clarke (2011) determined three Institutional AbbreviationsdLACM, Los Angeles County character states: absence of a groove and either a foramina- Museum, Los Angeles, CA; BHI, Black Hills Institute of Geological containing groove on the posterolateral portion of the dentary or Research, Inc., Hill City, SD; BMR, Burpee Museum, Rockford, IL; one running the length of the dentary. Brusatte et al. (2010) and BSP, Bayerische Staatsammlung für Palaontologie€ und historische Carrano et al. (2012) utilized a binary (presence vs. absence) coding Geologie, Munich, Germany; CMNH, Cleveland Museum of Natural of this character in their respective phylogenies, which is the History, Cleveland, OH; FMNH, Field Museum of Natural History, approach adopted here. This was one of more than 350 other Chicago, IL; KUVP, University of Kansas Museum of Natural History characters utilized in the aforementioned studies. Whether the and Biodiversity Institute, Lawrence, KS; NMMNH, New Mexico groove was a phylogenetically-informative character was not Museum of Natural History, Albuquerque, NM; TCM, The Children's discussed. Museum, Indianapolis, IN. The phylogenetic distribution of the lateral dentary groove was investigated to determine the polarity of the character, to interpret 3. Results the implication of its losses, and to assess its potential as a diag- nostic feature for re-classifying the position of “Jane” (BMR The dentary groove is a unique structure with multiple pores P2002.4.1) and other specimens labeled Nanotyrannus within aligned within the depression and sub-equally spaced. The groove Tyrannosauroidea. “Jane” is clearly a tyrannosaurid (Brusatte et al., originates near the anterior portion of the bone from a position 2010), but its taxonomic affiliation has been highly disputed approximately underneath the 2nd to 4th dentary alveolus. It (Erickson et al., 2006; Snively and Russell, 2007; Larson, 2008; extends caudally along the lateral surface, terminating near the Henderson and Harrison, 2008; Peterson et al., 2009; Larson, end of the tooth row (Fig. 1; see also figure 1a of Sampson and 2013a). The debate over “Jane” is a smaller part of a larger debate Witmer, 2007). This groove does not extend beyond the dentary about the validity of Nanotyrannus lancensis as a separate genus in examined theropods. This contrasts with the sauropodomorphs (Bakker et al., 1988). Three specimens have taken center stage in Eoraptor and Panphagia, wherein a groove extends onto the sur- this debate: “Jane”, the holotype specimen of N. lancensis (CMNH face of the surangular and terminates in a large foramen. Multiple 7531), and the theropod described as one of the “Dueling Di- pores were noted along the length of the groove in theropod nosaurs” (BHI 6437; Larson, 2013b). The debate presently rests on dinosaurs. three competing hypotheses, either that 1) Nanotyrannus stands as The dentary groove was present on 48 of 92 sampled theropod a valid taxon (Currie, 2003a; Larson, 2013a); 2) these individuals taxa (see supplemental data). No groove was observed in the represent the taxon Tyrannosaurus lancensis (Currie et al., 2005); or crocodylomorph Shuvosaurus or the early dinosaur Herrerasaurus, 3) these individuals are juvenile Tyrannosaurus rex (Carr, 1999; strengthening the hypothesis that this feature is unique to Brochu, 2003; Carr and Williamson, 2004; Holtz, 2004). A goal of theropod taxa. The groove was also not observed in Daemonosaurus this study is to attempt to clarify the relationship of N. lancensis or Tawa, the most primitive true theropods that we sampled. with other tyrannosaurids using the dentary groove and other Allosaurus, Baryonyx, Monolophosaurus, Ornitholestes, Spinosaurus, cranial characters (Larson, 2013a). and Torvosaurus were the only observed pre-Tyrannoraptoran taxa that reverted to the primitive state (Fig. 2). 2. Materials and methods The single most parsimonious tree recovered by our phyloge- netic analysis of tyrannosauridae is reported in Fig. 3A. The mini- We investigated 92 theropod taxa for presence or absence of a mum tree length was found after 5 replicates, and the most groove on the
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