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Bite me: Biomechanical models of theropod mandibles and implications for feeding behavior

Authors:
  • Royal Tyrrell Museum of Palaeontology

Abstract

A biomechanical approach is used to study feeding behavior in non-avian theropods. Mandibles can be modeled as beams undergoing bending loads during food ingestion. The bite force applied at any given point along the mandible should be proportional to the external dimensions of the mandibular ramus at that location. Thus, patterns of variation in these dimensions reflect the adaptation of the jaw to specific loads, which are related to the method of killing prey. These beam models were compared to those of the extant varanids Varanus komodoensis (Komodo dragon, an ambush predator with a slashing bite) and Varanus niloticus (Nile monitor, molluscivorous) to gain insight into the feeding behavior of theropods. On the basis of our results, we identified five distinct theropod feeding categories: (1) the allosauroid "Antrodemus valens" and the abelisaurids Majungatholus atopus and Carnotaurus sastrei share the mandibular properties of the Komodo dragon, and these shared properties, combined with the similarity between the craniodental morphology of "Antrodemus" and Majungatholus and that of the varanid, suggest that they were probably large-prey hunters delivering slashing bites; (2) dromaeosaurids have mandibular properties reminiscent of those of V. komodoensis for slashing bites, but differences can be identified between Dromaeosaurus and velociraptorines, the former having a stronger bite than the latter, suggesting that it possibly relied more on its jaws to capture and kill prey; (3) Suchomimus tenerensis and Dilophosaurus wetherilli exhibit mandibular adaptations related to the capture of prey smaller than themselves, the former probably practicing a bite-and-hold strategy and the latter finishing its prey with slashing bites; (4) Ceratosaurus nasicornis, Allosaurus fragilis, Acrocanthosaurus atokensis, and Giganotosaurus carolinii demonstrate adaptations of the anterior extremity of the mandible for capturing prey and delivering powerful bites to bring down prey or deliver the final blow; and (5) tyrannosaurids, unlike other theropods, exhibit mandibular adaptations to resist high torsional stresses at the anterior of the mandible induced during prey capture, bone crushing, or both. The mandibular models were also used to infer relative bite force in theropods. Velociraptorines appear to have had a maximum bite force similar to that of V. komodoensis, whereas that of Dromaeosaurus was three times as great. Suchomimus, Allosaurus, "Antrodemus," and Ceratosaurus were capable of exerting maximum bite forces as great as A. mississippiensis, and those of abelisaurids and Albertosaurus were twice as powerful. Among the largest theropods, Acrocanthosaurus and Giganotosaurus were surpassed by Daspletosaurus and Tyrannosaurus. The high estimates obtained for tyrannosaurids are consistent with previously published values that suggest bone-cracking abilities. Growth series for Allosaurus fragilis, Albertosaurus sarcophagus, Gorgosaurus libratus, and Tyrannosaurus rex were also studied in order to determine whether mandibular properties changed during ontogeny. Significant changes were observed in Allosaurus, especially in bending rigidity, indicating that juveniles did not feed the same way as adults; juveniles probably delivered simple slashing bites. Unfortunately, the question of parental care in Allosaurus cannot be resolved in light of our results. The mandibular properties of tyrannosaurids were not found to vary significantly during ontogeny, other than in terms of bite force. This finding strongly suggests that juveniles were apt predators, capable of subduing their own prey rather than relying on carrion or parental care to survive.
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... In contrast, Irritator shows the third setup, in which the dorsal contact of the quadrate is positioned more posteriorly than the ventral one (in relation to the long axis of the mandible, the downward orientation of the snout mentioned above notwithstanding), as can be seen in some maniraptoriformes, e.g., Archaeopteryx (Rauhut, 2014) and Ornithomimosauria (Makovicky et al., 2004). Assuming a comparable relative position of the coronoid eminence and thus the insertions of the jaw muscles, this third arrangement of bones results in different jaw mechanisms, which likely produced a weak bite (lower mechanical advantage) in comparison to other large-bodied theropods (see Henderson, 2002;Therrien et al., 2005). This is partially because, in comparison to theropods with posteroventrally inclined or straight quadrates, the leverage for the jaw closing muscles (the input moment arm) is shortened in taxa with an anteroventrally inclined quadrate, such as spinosaurids, as the jaw joint moves closer to the insertion areas of the main jaw closing muscles. ...
... Reaching the point of maximal tension, the relaxation of the abductor muscles would reinforce the contraction speed of the adductor muscles, further supporting the hypothesis of a very rapid jaw closure. In summary, the skull morphology of Irritator indicates fast, rather than strong biting, supporting previous studies on skull strength and bite force in nonavian theropods (Henderson 2002;Therrien et al., 2005;Rayfield 2011). ...
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Although originally described almost three decades ago, the holotype of Irritator challengeri from the Lower Cretaceous Romualdo Formation of Brazil still represents the most complete spinosaurid skull known to science. Here, we present a detailed description of the skull of Irritator based on digital reconstructions from medical and micro computed tomography (μCT) data. Segmentation reveals the near-complete palatal complex and braincase, an unusual morphology of the retroarticular process, a large, ventrally inclined surangular shelf and the tooth replacement pattern. The digitally reconstructed skull anatomy indicates a robust dentition, a field of binocular vision in front of the skull with an inclined snout orientation, a relatively weak but fast bite, as well as laterally spreading and rotating lower jaw rami during jaw opening. We modified an existing phylogenetic matrix of Tetanurae to account for new observations on the morphology of Irritator and analysed this using parsimony and Bayesian methods. Results support Spinosauridae as members of Megalosauroidea and recover a monophyletic Carnosauria (Megalosauroidea + Allosauroidea). Parsimony analysis recovers Monolophosaurus nested within Megalosauroidea as sister taxon to spinosaurids, but this is not supported by the Bayesian analysis. Bayesian time-calibration and evolutionary rate analysis indicate that spinosaurid evolution happened fast, despite a long ghost lineage of at least 35 million years. High evolutionary rates over a prolonged time can explain the highly derived skull morphology of spinosaurids. This study provides an in-depth look into the evolution of spinosaurid skull anatomy and refines our understanding of these specialized Mesozoic predators.
... Materials resist or dissipate stress in diverse ways, influenced by their ultrastructure or their geometry (Wainwright, 1992). The jaws of vertebrates experience both compressive and shear stresses during feeding (Therrien et al., 2005). Bills are subject to an additional constraint: being lightweight to support flight. ...
... In contrast, the relatively broad, weak bill of the moa genus Euryapteryx may have handled softer food (Attard et al., 2016). Beam theory suggests similar functions in theropod jaws that are weak mediolaterally, with high dorsoventral strength putatively indicating cutting or slashing feeding mechanisms (Therrien et al., 2005). The success of these mechanisms in birds, along with the stress resistance provided by the rhamphotheca, may have been key morphological innovations leading to modern bird diversity. ...
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... Some studies have proposed specific behaviors for abelisaurids based on the peculiar features of the caudal portion of their skull, cervical vertebrae, and ribs (e.g., hypertrophied epipophyses, low neural spines, ribs with aliform processes; O' Connor, 2007;Sampson & Witmer, 2007;Delcourt, 2018;González, Baiano & Vidal, 2021). Hence, behavioral inferences, especially as related to feeding habits and intraspecific behaviors, were tested by biomechanical analyses of the skull and/or the cervical portion of the axial skeleton (Mazzetta, Fariña & Vizcaíno, 1998;Mazzetta et al., 2009;Therrien, Henderson & Ruff, 2005;Snively et al., 2011). ...
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... Having a deeply robust cranium and an expanded mandible allowed robust-snouted tyrannosauroids such as an adult Tyrannosaurus, Tarbosaurus, Daspletosaurus, and Gorgosaurus to resist high forces and deliver powerful bite forces to similarly sized prey (Gignac & Erickson, 2017;Hurum & Sabath, 2003;Rowe & Snively, 2021;Snively et al., 2006;Therrien et al., 2005Therrien et al., , 2021. ...
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Tyrannosaurus has been an exemplar organism in feeding biomechanical analyses. An adult Tyrannosaurus could exert a bone‐splintering bite force, through expanded jaw muscles and a robust skull and teeth. While feeding function of adult Tyrannosaurus has been thoroughly studied, such analyses have yet to expand to other tyrannosauroids, especially early‐diverging tyrannosauroids ( Dilong, Proceratosaurus , and Yutyrannus ). In our analysis, we broadly assessed the cranial and feeding performance of tyrannosauroids at varying body sizes. Our sample size included small ( Proceratosaurus and Dilong ), medium‐sized ( Teratophoneus ), and large ( Tarbosaurus, Daspletosaurus , Gorgosaurus , and Yutyrannus ) tyrannosauroids, and incorporation of tyrannosaurines at different ontogenetic stages (small juvenile Tarbosaurus, Raptorex , and mid‐sized juvenile Tyrannosaurus ). We used jaw muscle force calculations and finite element analysis to comprehend the cranial performance of our tyrannosauroids. Scaled subtemporal fenestrae areas and calculated jaw muscle forces show that broad‐skulled tyrannosaurines ( Tyrannosaurus , Daspletosaurus , juvenile Tyrannosaurus , and Raptorex ) exhibited higher jaw muscle forces than other similarly sized tyrannosauroids ( Gorgosaurus, Yutyrannus , and Proceratosaurus ). The large proceratosaurid Yutyrannus exhibited lower cranial stress than most adult tyrannosaurids. This suggests that cranial structural adaptations of large tyrannosaurids maintained adequate safety factors at greater bite force, but their robust crania did not notably decrease bone stress. Similarly, juvenile tyrannosaurines experienced greater cranial stress than similarly‐sized earlier tyrannosauroids, consistent with greater adductor muscle forces in the juveniles, and with crania no more robust than in their small adult predecessors. As adult tyrannosauroid body size increased, so too did relative jaw muscle forces manifested even in juveniles of giant adults.
... For example, they were unlikely to have been obligate scavengers as their large size and the diversity of smaller, contemporaneous terrestrial carnivores (ornithosuchids, crocodylomorphs, theropods, and therapsid cynodonts) would have made carrion an unreliable exclusive food source (Carbone et al., 2011;Ezcurra et al., 2017;Martínez et al., 2013;Nesbitt et al., 2013). Regular osteophagy is unlikely for two main reasons: (i) Saurosuchus does not possess the incredibly high bite forces needed to crack bones as deployed by tyrannosaurids and modern crocodilians (Erickson et al., 2012(Erickson et al., , 2014Gignac & Erickson, 2017;Meers, 2002;Therrien et al., 2005), although small bones could have potentially been swallowed whole; (ii) Saurosuchus does not possess morphological features that typically facilitate bone cracking and shearing. For example, spotted hyenas have lower bite forces than African lions but their robust carnassial teeth and their enlarged zygomatic arches and sagittal crests for larger adductor muscle attachment areas all aid in producing the necessary tooth pressures to crack bone (Christiansen & Adolfssen, 2005;Christiansen & Wroe, 2007;Schubert et al., 2010). ...
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... It would be extremely speculative to link this with inferred parental behaviour (or the lack of it), and dietary characteristics related to cochlear duct dimensions failed to produce significant signals (Hanson et al., 2021). Nevertheless, low-frequency hearing in baryonychines as an adaptation for detecting large prey would not corroborate with evidence of spinosaurids as predators of generally small vertebrates (Hone & Holtz Jr, 2017;Therrien et al., 2005), as shown by their relatively weak bite force for instance (Sakamoto, 2022). Indeed, small prey items were likely commonly selected and depredated by carnivorous theropods in general (Hone & Rauhut, 2010). ...
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The digital reconstruction of neurocranial endocasts has elucidated the gross brain structure and potential ecological attributes of many fossil taxa, including Irritator, a spinosaurine spinosaurid from the "mid" Cretaceous (Aptian) of Brazil. With unexceptional hearing capabilities, this taxon was inferred to integrate rapid and controlled pitch-down movements of the head that perhaps aided in the predation of small and agile prey such as fish. However, the neuroanatomy of baryonychine spinosaurids remains to be described, and potentially informs on the condition of early spinosaurids. Using micro-computed tomographic scanning (μCT), we reconstruct the braincase endocasts of Baryonyx walkeri and Ceratosuchops inferodios from the Wealden Supergroup (Lower Cretaceous) of England. We show that the gross endocranial morphology is similar to other non-maniraptoriform theropods, and corroborates previous observations of overall endocranial conservatism amongst more basal theropods. Several differences of unknown taxonomic utility are noted between the pair. Baryonychine neurosensory capabilities include low-frequency hearing and unexceptional olfaction, whilst the differing morphology of the floccular lobe tentatively suggests less developed gaze stabilisation mechanisms relative to spinosaurines. Given the morphological similarities observed with other basal tetanurans, baryonychines likely possessed comparable behavioural sophistication, suggesting that the transition from terrestrial hypercarnivorous ancestors to semi-aquatic "generalists" during the evolution of Spinosauridae did not require substantial modification of the brain and sensory systems.
... In predatory mammals, the symphyseal shape indicates the feeding behaviour and cranial resistance during predation. Equidimensional symphyses resist to torsional stresses on prey capture or bone-cracking, while a longer symphysis indicates that stresses are predominantly induced by pressure made by the canine or the anteriormost teeth (Hylander, 1984;Therrien et al., 2005). If this symphyseal shape proportion is also maintained, Itasuchids can be considered generalist predators, which could retain their prey in the anterior portion of their snouts, possibly drowning terrestrial prey as an alternative to inertial feeding, also observed to extant crocodilians. ...
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... This relationship has been postulated for non-avialan theropods (Powers et al., 2020) and exists in extant taxa as divergent as crocodilians (Walmsley et al., 2013) and canids (Slater et al., 2009), although seemingly not in felids (Sakamoto et al., 2010). A simple but powerful mechanical explanation for the connection between jaw length and preferred prey type arises from lever mechanics and beam theory, as applied to the tetrapod jaw apparatus (Ostrom, 1964;Bock, 1966;Thomason, 1991;Preuschoft & Witzel, 2002;Therrien, 2005;Therrien et al., 2005Therrien et al., , 2021. The bite force that can be applied to prey by a given tooth in a predator's mouth is inversely proportional to the distance between the jaw joint and the position of the tooth in question. ...
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