New Reconstruction of Cranial Musculature in Ornithischian Dinosaurs: Implications for Feeding Mechanisms and Buccal Anatomy

Abstract

The charismatic and diverse ornithischian dinosaurs exhibited some of the most extreme examples of cranial anatomy, inspiring decades of investigation into their muscular anatomy. Current ornithischian jaw muscle reconstructions, although parsimonious, pose concerns of small adductor muscles and caudally displaced insertions relative to mandibular proportions. Here, craniomandibular material of ornithischian genera spanning all subclades is reexamined for osteological correlates indicative of intracranial and oral soft tissues. M. adductor mandibulae externus (mAME) has traditionally been reconstructed as solely inserting along the caudal margin of the coronoid process for jaw closure. Here, a new mAME reconstruction is proposed in derived ornithischians, with the superficial‐most mAME layer reconstructed as a rostrolabial expansion of muscle, exiting the cranium rostroventrally beneath a unique, laterally flaring jugal and inserting along the lateral surface of the coronoid process and its rostrally extending, shelf‐like labial dentary ridge (LDR). Through previous dental microwear and morphological studies, ceratopsians, hadrosaurids, and ankylosaurs are known to have implemented a major palinal feeding component in their jaw motions, unlike other primarily basal ornithischians. This rostral fan‐like extension of muscle in these derived clades would create a greater mandibular support system and mechanical advantage along the labial margin of the jaw, cradling the entire mandible while lifting it up into occlusion and retracting it. In hadrosaurids and ankylosaurs, this rostrolabially expanding muscle also acts in medial rotation of the dentaries about their long axes. With these new reconstructions, the notion of a novel, unparsimonious “cheek” muscle is rejected, with further discussion of plausible buccal soft tissues. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 303:347–362, 2020. © 2018 American Association for Anatomy

Tooth replacement in Manidens condorensis: baseline study to address the replacement pattern in dentitions of early ornithischians

“…2011; Hummel & Clauss 2011; Barrett 2014; Wings 2014; Erickson et al . 2016; Nabavizadeh 2020). When dental evolution is considered in herbivorous lineages of amniotes, high rates of oral processing and food ingestion are linked to the appearance of high‐crowned dentitions, in which precise dental occlusion is enabled by extensive wear, and hardened dental tissues increase resistance (Janis & Fortelius 1988; Sander 1999; Kaiser et al .…”

Modularity and heterochrony in the evolution of the ceratopsian dinosaur frill

“…Muscle scarring is much less developed in the expanded frills of coronosaurs. Current reconstructions of jaw muscles in these animals place the jaw adductor origins closer to the rostral edge of the supratemporal fenestra, along the midline keel of the frill, and along the inner surface of the squamosal, with the majority of the frill, including its caudal margin, free of musculature (Dodson, 1996; Nabavizadeh, 2018). These differences in frill anatomy between early and late diverging ceratopsians prompted Makovicky (2012) and Makovicky and Norell (2006) to propose that the ceratopsian frill underwent a shift in its primary function within Neoceratopsia from providing an increased area for attachment of jaw adductor muscles as (Haas, 1955; Lull, 1908; Ostrom, 1964; Xu et al., 2002), to serving as a species‐specific display structure with possible social and/or sexual functions (Dodson, 1976, 1993, 1996; Farlow & Dodson, 1975; Hone, Naish, & Cuthill, 2012; Hone et al., 2016; Horner & Goodwin, 2006, 2008; O'Brien et al., 2018; Padian & Horner, 2011; Sampson, Ryan, & Tanke, 1997).…”

Cranial Musculature in Herbivorous Dinosaurs: A Survey of Reconstructed Anatomical Diversity and Feeding Mechanisms

“…Museumhoused skulls of herbivorous dinosaurs were examined either firsthand (in person or using photographs) or referenced from the literature (see Supporting Information Table S1; nearly half of taxa used were only studied by referencing previously published descriptions while the rest additionally were either studied in person or with photographs provided). Using previous knowledge of muscle attachment sites (e.g., Holliday, 2009;Nabavizadeh, 2018; see below for more detailed account of muscle reconstruction references), anatomical traits were chosen according to their correspondence to muscle architecture. Cranial traits are examined qualitatively, including those of the temporal region and supratemporal fenestra (e.g., rostrocaudal breadth of supratemporal bar; transverse breadth of the temporal region), the adductor chamber and infratemporal fenestra (e.g., correlates of volume and muscle body orientation), the quadrate (e.g., muscle attachment and how its orientation informs us of muscle architecture), maxilla-jugal junction (e.g., how it bounds the rostral extent of temporal muscle mandibular insertion), overall palatal breadth and plane (e.g., to show volume and breadth of palatal muscle origin), the presence or absence of an epipterygoid, and the paroccipital processes.…”

“…Mediolateral attachment sites of mAME along the coronoid elevation have been described for heterodontosaurids (Norman et al, ; Sereno, ), thyreophorans (Haas, ; Lautenschlager et al, ), pachycephalosaurs (Sues and Galton, ), and ornithischians as a whole (Nabavizadeh, , ). Crompton and Attridge () noted differences in muscle body sizes among ornithischians.…”

“…He described the insertion sites of mPTD and mPTV on the medial and lateral aspect of the retroarticular process and caudal mandibular body, respectively. Aside from the extra slip of mPTD on the maxillary shelf (only described for hadrosaurs by Ostrom [1961]), these reconstructions have been consistent in all other ornithischians in more recent studies (Ostrom, 1964 [in Triceratops, with only one mPT muscle body described]; Thulborn, 1971;Galton, 1974 [in Hypsilophodon]; Weishampel, 1984 [in ornithopods]; Sues and Galton, 1987 [in pachycephalosaurs]; Witmer, 1997 [describing mPTD in a range of mostly basal ornithischians]; Rybczynski et al, 2008 [in hadrosaurs];Holliday, 2009;Bell et al, 2009 [in ceratopsids and hadrosaurs]; Tanoue et al, 2009 [in ceratopsians, with one mPT muscle body]; Mallon and Anderson, 2015;Nabavizadeh, 2016Nabavizadeh, , 2018 [in ornithischians as a whole]). Holliday (2009) additionally hypothesized the possibility that caudoventrally oriented rugose spurs on the jugal flange of some hadrosaurs (as well as tyrannosaurid theropods) may indicate a dorsal expansion of mPTV insertion; although, this requires further study.…”