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The ability to accurately and reliably estimate body mass of extinct taxa is a vital tool for interpreting the physiology and even behavior of long-dead animals. For this reason, paleontologists have developed many possible methods of estimating the body mass of extinct animals, with varying degrees of success. These methods can be divided into two main categories: volumetric mass estimation and extant scaling methods. Each has advantages and disadvantages, which is why, when possible, it is best to perform both, and compare the results to determine what is most plausible within reason. Here we employ volumetric mass estimation (VME) to calculate an approximate body mass for previously described specimens of Anzu wyliei from the Carnegie Museum of Natural History. We also use extant scaling methods to try to obtain a reliable mass estimate for this taxon. In addition, we present the first digital life restoration and convex hull of the dinosaur Anzu wyliei used for mass estimation purposes. We found that the volumetric mass estimation using our digital model was 216-280kg, which falls within the range predicted by extant scaling techniques, while the mass estimate using minimum convex hulls was below the predicted range, between 159-199 kg . The VME method for Anzu wyliei strongly affirms the predictive utility of extant-based scaling. However, volumetric mass estimates are likely more precise because the models are based on comprehensive specimen anatomy rather than regressions of a phylogenetically comprehensive but disparate sample.

The tyrannosaurids are among the most well-studied dinosaurs described by science, and analysis of their feeding biomechanics allows for comparison between established tyrannosaurid genera and across ontogeny. 3D finite element analysis (FEA) was used to model and quantify the mechanical properties of the mandibles (lower jaws) of three tyrannosaurine tyrannosaurids of different sizes. To increase evolutionary scope and context for 3D tyrannosaurine results, a broader sample of validated 2D mandible FEA enabled comparisons between ontogenetic stages of Tyrannosaurus rex and other large theropods. It was found that mandibles of small juvenile and large subadult tyrannosaurs experienced lower stress overall because muscle forces were relatively lower, but experienced greater simulated stresses at decreasing sizes when specimen muscle force is normalized. The strain on post-dentary ligaments decreases stress and strain in the posterior region of the dentary and where teeth impacted food. Tension from the lateral insertion of the looping m. ventral pterygoid muscle increases compressive stress on the angular but may decrease anterior bending stress on the mandible. Low mid-mandible bending stresses are congruent with ultra-robust teeth and high anterior bite force in adult T. rex. Mandible strength increases with size through ontogeny in T. rex and phylogenetically among other tyrannosaurids, in addition to that tyrannosaurid mandibles exceed the mandible strength of other theropods at equivalent ramus length. These results may indicate separate predatory strategies used by juvenile and mature tyrannosaurids; juvenile tyrannosaurids lacked the bone-crunching bite of adult specimens and hunted smaller prey, while adult tyrannosaurids fed on larger prey.

SynopsisTyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods.MethodsTo compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilitiesa(p) to stringently test the null hypothesis of equal agility.ResultsTyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration).ImplicationsThe capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.

Tyrannosaur bite marks on several bones, including ceratopsian (horned dinosaur) occipital condyles scattered in bone beds, have recently been attributed to play behavior. In the cited cases, play ...

Gobekko cretacicus, a Cretaceous lizard from the Gobi Desert of Mongolia, is a key fossil for understanding gecko phylogeny. We revisit this fossil using high-resolution X-ray computed tomography. The application of this imaging method reveals new information about sutures, bone shape, and structural details of the palate and basicranium. These data were used to assess the phylogenetic affinities of Gobekko in the context of an existing squamate data set. The effects of character ordering, search strategy, and the addition of another putative gekkonomorph (Hoburogekko suchanovi) on inferred gekkonomorph relationships were explored. Available specimens of G. cretacicus are skeletally mature but have unfused nasals, frontals, and parietals, and (possibly) a persistent basicranial fenestra. Some putative gekkonomorphs are not consistently supported as closer to crown clade gekkotans than to autarchoglossans. In a strict consensus both Gobekko and Hoburogekko form a polytomy with extant geckos. Some of the adult character states of Gobekko are observable in embryos of extant species. The evolution of tubular frontals and dentaries in gekkotans may be structurally related to the loss of the postorbital and supratemporal bars in this lineage. The complete lack of a parietal foramen, and presumably a light-sensitive parietal eye, in this clade is of interest and could indicate an early origin of nocturnality in geckos.

Hadrosaurid and ceratopsid dentaries display traits that suggest divergent functions toward broadly similar diets of fibrous plants. Computed tomographic scans of dentaries of a lambeosaurine and a centrosaurine (Centrosaurus aptertus) were used to compare feeding function of these animals using finite element analysis (FEA). In the hadrosaur, mediolateral expansion of the dentary and elongation of the coronoid process of the surangular were optimally developed to withstand torsion associated with transverse-isognathous jaw occlusion. FEA results strongly suggest longitudinal rotation of the hadrosaurid mandible. Mediolaterally compressed mandibles and FEA stress for the ceratopsid are in compliance with purely isognathous jaw adduction, although palinal retraction during the powerstroke cannot be ruled out. The ceratopsid dentary is further reinforced by a longitudinal ridge on the lateral surface of that element. Surface texture indicating Sharpey's fibers within the ceratopsid coronoid process suggests greater bite force than in lambeosaurines. These findings corroborate previous interpretations and suggest complementary or alternative kinematics to maxillary pleurokinesis in hadrosaurs.

Ankylosaurid dinosaurs have modified distal caudal vertebrae (the handle) and large terminal caudal osteoderms (the knob) that together form a tail club. Three-dimensional digital models of four tail clubs referred to Euoplocephalus tutus were created from computed tomography scans of fossil specimens. We propose to use finite element modeling to examine the distribution of stress in simulated tail club impacts in order to determine the biological feasibility of hypothesized tail clubbing behavior. Results show that peak stresses were artificially high at the rigid constraint. The data suggest that tail clubs with small and average-sized knobs were unlikely to fail during forceful impacts, but large clubs may have been at risk of fracture cranial to the knob. The modified handle vertebrae were capable of supporting the weight of even very large knobs. Long prezygapophyses and neural spines in the handle vertebrae helped distribute stress evenly along the handle. We conclude that tail swinging-behavior may have been possible in Euoplocephalus, but more sophisticated models incorporating flexible constraints are needed to support this hypothesis.

Cretaceous polar dinosaur faunas were taxonomically diverse, which suggests varied strategies for coping with the climatic stress of high latitudes. Some polar dinosaurs, particularly larger taxa such as the duckbill Edmontosaurus Lambe, 1917, were biomechanically and energetically capable of migrating over long distances, up to 2600 km. However, current evidence strongly suggests many polar dinosaurs (including sauropods, large and small theropods, and ankylosaurs of New Zealand) overwintered in preference to migration. Certain groups also appear more predisposed to overwintering based on their physical inability (related to biomechanics, natural history, or absolute size) to migrate, such as ankylosaurs and many small taxa, including hypsilophodontids and troodontids. Low-nutrient subsistence is found to be the best overwintering method overall, although the likelihood that other taxa employed alternative means remains plausible. Despite wide distribution of some genera, species-level identification is ...

With multiple well-preserved specimens, tyrannosaurid dinosaurs are ideal subjects for exploring the role of the neck for feeding in an extinct amniote clade. Detailed exploration of the morphology of avian and crocodilian neck muscles grounds phylogenetic inference of musculature of tyrannosaurids. Introduced methods of kinematic and physiological inference explicate action and function of tyrannosaurid neck muscles, and the explicit method of extant behavioural interpolation (EBI) allows inference of generalized behaviours. Anteriorly originating craniocervical muscles of tyrannosaurids, and insertions of the neck dorsiflexor m. transversospinalis cervicis, were similar to those of birds, while the major respective head dorsiflexor and lateroflexor, m. transversospinalis capitis and m. longissimus capitis superficialis, resembled their homologues in crocodilians. Because the proposed inferential methods are cumulative and emphasize mutual falsification, behavioural effects of muscle function become immediately testable rather than assumed. Kinematic inference of muscle moment topology, and bracketed and unipolar inference of muscle activity, facilitate EBI of rapid gaze shifts, lateral and sagittal strikes, inertial feeding, and tearing of flesh by specific permutational actions of tyrannosaurid neck muscles. Several examples of EBI are tested further through biomechanical considerations. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 151, 759‐808. ADDITIONAL KEYWORDS: Aves ‐ behaviour ‐ Crocodylia ‐ electromyography ‐ feeding ‐ muscle.

Tyrannosaurus rex and other tyrannosaurid theropods exerted high bite forces, and large muscle attachments suggest that the tyrannosaurid neck was a concomitantly powerful component of the feeding apparatus. We examine accelerative and work-generating capacity (WGC) of neck muscles in adult Tyrannosaurus rex, using a 3-D vector-based method that incorporates aspects of muscle force generation, reconstruction of muscle morphology and moment arms, and rotational inertias of the head and neck. Under conservative assumptions, radial accelerations of the head by large superficial muscles (M. transversospinalis capitis, M. complexus, and M. longissimus capitis superficialis) enabled rapid gaze shifts and imparted high tangential velocities to food sufficient for inertial feeding. High WGC by these and deeper muscles under eccentric contraction indicate high efficacy for tearing flesh, especially with the head and neck in an extended posture. Sensitivity analyses suggest that assigned density of the antorbital region has substantial effects on calculated rotational inertia, and hence on the accuracy of results. However, even with high latitude for estimation errors, the results indicate that adult T. rex could strike rapidly at prey and engage in complexly modulated inertial feeding, as seen in extant archosaurs.

We present a hypothesis of tyrannosaurid foot function termed the "tensile keystone model," in which the triangular central metatarsal and elastic ligaments dynamically strengthened the foot. The tyrannosaurid arctometatarsus, in which the central metatarsal is proximally constricted, displays osteological correlates of distal intermetatarsal ligaments. The distal wedge-like imbrication of tyrannosaurid metatarsals indicates that rebounding ligaments drew the outer elements towards the middle digit early in the stance phase, unifying the arctometatarsus under high loadings. This suggests increased stability and resistance to dissociation and implies, but does not demonstrate, greater agility than in large theropods without an arctometatarsus.

Tyrannosaurid dinosaurs possess a metatarsus with an arctometatarsalian proximal constriction of metatarsal III, and strongly interlocking proximal articulations. Bone and inferred ligament morphologies are suggestive of modes of locomotor energy transmission. CT scanning and Finite Element Analysis (FEA) ofGorgosaurus libratus metatarsals test two hypotheses of tyrannosaurid arctometatarsus function: ligaments mediated transfer of energy from the central metatarsal to the outer elements, and ligaments arrested anterodorsal rotation of the distal portion of the central metatarsal. The results have implications for the use of FEA in functional morphology: 1) strain artifacts are identifiable under low-resolution modeling, but higher resolution is better; and 2) bone strain aids in testing hypotheses of ligament function. Concentrations of bone strain energy under postulated loading regimes forGorgosaurus support the hypothesis of axial energy transmission for the tyrannosaurid metatarsus, and indirectly support the rotation damping hypothesis. Palaeopathology provides a vital complement to engineering tests of these hypotheses.

We examine the influence of neck muscle topology and kinematics on feeding function of Tyrannosauridae. Cervical musculature intrinsic to the neck and craniocervical muscles from the neck to the head, are here collectively termed ‘neck muscles’. Tyrannosaurid neck muscles are reconstructed by morphological comparisons with extant archosaurs, and their function and role in feeding behaviour are inferred by newly proposed modes of kinematic, electromyographic and observational inference. To ground the subsequent explorations of functional morphology, we first summarize fundamental divisions of neck axial musculature, and introduce inferential methods in detail. FUNCTIONAL DIVISIONS OF AMNIOTE NECK MUSCLES Amniote axial muscle groups are functionally and anatomically classifiable by serial homology and innervation (Tsuihiji, 2005). Transversospinalis and longissimus muscles (Fig. 1) are primarily involved with dorsiflexion and lateral flexion, respectively, while the iliocostalis group (Fig. 1) brings about ventrolateral and, to a lesser extent, ventral flexion. Unsurprisingly, muscles running along the underside of the neck are responsible for most of the ventral flexion. These include the longus colli system and m. rectus capitis anterior/ventralis, serial homologues of prevertebral muscles such as m. psoas and m. iliopsoas (Rosse & Gaddum-Rosse, 1997). INFERRING FUNCTION OF NECK MUSCLES OF EXTINCT VERTEBRATES The conservatism of neck muscle development, innervation and topology in extant amniotes engenders confidence that these general characteristics are also applicable to the muscles of extinct taxa. More specific reconstruction of cervical muscle morphology in an extinct taxon requires focused comparisons with its closest extant relatives. Dissection and osteological examination of extant relatives reveal constancy and variation in muscle origins and insertions. In a formalized extension of the comparative method (Bryant & Russell, 1992; Witmer, 1995), osteological correlates of muscle attachment common to the fossil taxon and one or more extant groups indicate the muscle’s presence in the extinct form. An extant member of the clade of interest, and the extant sister taxon of this couplet, form the extant phylogenetic bracket (EPB) of their extinct counterpart (Witmer, 1995). An osteological correlate of a given muscle in all three taxa provides the most robust inference of its presence in the fossil animal (a Level I inference: Witmer, 1995). Less definitive evidence of correlates in any of the taxa, such as when there is no discrete attachment scar for a muscle, leads to inference of increasing ambiguity (Level II and III inferences: Witmer, 1995). If the extinct taxon shares a correlate with one, but not both, poles of the extant bracket, a Level II inference is possible by what is herein termed ‘unipolar’ comparison or inference. Extrapolatory inference of a muscle’s morphology is still possible given a unique correlate in the extinct taxon, through explicit mechanical considerations and morphological comparison with more distantly related clades (Bryant & Russell, 1992). KINEMATIC AND PHYSIOLOGICAL INFERENCE OF MUSCLE FUNCTION Comparative morphology is the foundation for a hierarchy of inference (Witmer, 1995) about how animals use their muscles, from the kinematic action of individual muscles moving origin and insertions together, to the integration of the work and power of multiple muscles towards a desired behavioural end. A further development of Bryant & Russell’s (1992) and Witmer’s (1995) methods enables inference of a muscle’s action and function. Function incorporates aspects of a structure’s form and physiology to define what the structure does and how it works (Bock & von Wahlert, 1965). As the basic description of what muscles do, their action in extant vertebrates often is inferred based on the relative positions of a muscle’s origin and insertion and the effect of the resulting lever mechanics on skeletal movement (e.g. Burton, 1974). This method is reliable for inferring potential muscle action, although it often ignores the stabilizing action of shunt muscles that perform little work (Rosse & Gaddum-Rosse, 1997) or the damping action of contralaterally contracting muscles (Cleuren & De Vree, 2000). Applying musculoskeletal lever mechanics to the inference of action in extinct taxa is here termed ‘kinematic inference’. This is an explicit subset of extrapolatory inference proposed by Bryant & Russell (1992). Similar muscle attachment topology and leverage in extant relatives allow kinematic inference of these moment relationships in the extinct form, by extant phylogenetic bracketing. However, if the muscle’s origin and insertion are reasonably deduced, the robustness of kinematic inference is independent of differences in skeletal morphology between the extant and extinct clades. Non-topological characteristics also contribute to a given muscle’s action. Applying these factors to deduce muscle action in extinct animals is here termed ‘physiological inference’. Physiological tests, such as electromyography (EMG), magnetic resonance imaging (MRI) and assays for muscle fibre type, can reveal patterns of muscle activation, activity and force-generating capacity in extant animals. EMG records the occurrence, timing and general intensity of muscle activity. MRI is useful for deducing fibre direction and pennation, and hence muscle architecture, which influences force-generating capacity. Fibre types dictate speed, and hence power, of contraction and fatigability of the muscle. If homologous muscles in the extant bracket display similar in vivo characteristics, physiological inference will facilitate deduction of these features in their extinct relative. Physiological inference is more sensitive to extant comparison than kinematic inference; differing muscle physiology in the extant groups will reduce confidence in the inference. Also, differential modifications in closely related extant taxa will render the inference more tenuous. Lability of physiological influences on muscle function is likely to be higher than with gross morphology, because muscles fatigue, recover and experience different metabolic regimes. However, high potential for disparity increases the power of bracketed physiological inference if similarities between extant taxa are established confidently. Witmer (1995) established inference Levels I′, II′ and III′ for attributes that are variably present in extant comparative clades, but that have no osteological correlates. A strong, Level I′ physiological inference is possible when both poles of the extant bracket show homologous attributes of muscle activity. If tests have been carried out on members of only one clade of the extant bracket, ‘unipolar (Level II′) physiological inference’ will tentatively indicate presence of the same physiological characteristic in the extinct taxon. While most aspects of physiological muscle function will be difficult to bracket in fossil forms, EMG offers a clear route to physiological inference. EMG activity is an immediate, measurable correlate of muscle activity as extant animals carry out behaviour in real time. If EMG of homologous muscles in crocodilians and birds reveals electrical activity during similar functions (such as dorsiflexion and lateroflexion), bracketed inference of these functions is strengthened for homologous muscles of tyrannosaurids. Strict morphological and physiological bracketing is ideal. However, if a Level II morphological inference (Witmer, 1997) indicates the presence of a muscle in tyrannosaurids and only one member of the phylogenetic bracket, EMG can facilitate a Level II′ physiological inference in which tyrannosaurid muscle activity is less stringently inferred. Physiological inference is testable and falsifiable by kinematic inference. Functional bracketing by EMG might signal inference of a function in the extinct form, but its moment arm topology would render this function impossible or unrealistically limited. An example would be EMG indicating a muscle’s activity strictly during head lateroflexion in crocodilians and birds (such as m. longissimus capitis superficialis and a hypothetical avian equivalent). Morphological and physiological bracketing would indicate this function in the carnosaur Allosaurus fragilis. The insertion of the muscle is lateral to the occipital condyle, which kinematically supports the physiological bracket. However, Bakker’s (2000) analysis of osteology indicates that this muscle has a long ventroflexive lever arm as well. By kinematic inference we would deduce that the muscle was capable of strong head ventroflexion in Allosaurus as well as lateroflexion, and that Allosaurus was more versatile than extant archosaurus when employing this muscle. Conversely, physiological inference can strongly inform, and even falsify, kinematic inferences. This will occur when moment arms indicate that a muscle would facilitate a certain action, but EMG indicates that the muscle stabilizes a joint posturally or against the actions of other muscles. One example of this is the human m. brachioradialis. Lever arms might indicate that this muscle is capable of high-geared, rapid flexion of the elbow, but EMG indicates that it acts as a shunt muscle that stabilizes the joint during both rapid flexion and extension by the biceps and triceps, respectively (Rosse & Gaddum-Rosse, 1997). Corroborating EMG of m. brachioradialis in gibbons would allow bracketing of its shunt function in fossil apes. BEHAVIOURAL INTERPOLATION: INFERRING THE CONSEQUENCES OF MUSCLE FUNCTION A collective subset of extrapolatory and phylogenetic inference (Bryant & Russell, 1992), termed ‘extant behavioural interpolation’ or EBI, expands upon deducing the basic function of muscles to facilitate inference of faculty, biological role (Bock & von Wahlert, 1965) and behaviour. EBI involves observing a behaviour in extant groups, correlating that behaviour with a reductive muscle function, and inferring the potential for that function and behaviour in the extinct taxon. The actual behaviour can be inferred in the extinct taxon only if the extant groups carry it out, using structural and physiological capabilities observed and inferable as common to all three clades. If a muscle’s morphology and action are known for extant groups, observations of their behaviour can elucidate how the muscle contributes to a given biological role. An example would be a muscle able to dorsiflex the head that facilitates inertial feeding in birds and crocodilians. If both birds and crocodilians use a homologous muscle for inertial feeding, and the muscle is inferred as being present in tyrannosaurids, the behaviour and biological role are also inferred as potentially present in tyrannosaurids. The inference is not absolute, as the bracket cannot predict loss of a function or behaviour in the extinct taxon. The robustness of behavioural interpolation will be greatest when the EPB indicates similarity in the extant taxa of all correlates for activity, including kinematic morphology, muscle physiology and the behaviour itself. Potential behaviours are falsifiable with refined information on structure, and inference of overly specific behaviours will not be tenable. Inertial feeding may be acceptable as an inference involving herbivorous hadrosaurian dinosaurs, employing the extant bracket of crocodiles and eagles. However, morphological considerations rule out the inference of hadrosaurs tearing flesh (Weishampel, 1983). A muscle’s function for a given behaviour in extant animals is deducible from analysis of morphology and kinematics, physiological data (as from EMG), and observed behaviour. The degree of corrobaration for these phenomena, ranging from initial speculation to corroboration by multiple independent authors, can be plotted along respective axes to create a box that subtends a qualitative ‘behavioural inference space’ (Fig. 2A). The size and shape of the box, respectively, represent the degree of certainty about a muscle’s behavioural role, and the relative contributions of observation, morphology and physiology to our understanding of that utility. A similar behavioural inference space for extinct taxa is shown in Figure 2B. Two axes represent inferability of morphology (to a maximum Level I inference) and physiology (maximum of Level I′ without preserved correlates). The third axis represents certainty of kinematic interpretation, which is independent of specific morphologies in the extant relatives if EPB unambiguously suggests the muscle’s presence. EBI facilitates inference of behaviour from bracketed muscle function, but the behavioural hypothesis becomes more useful and specific when tested biomechanically. Capabilities and limits of an animal’s behavioural performance are quantifiable through biomechanical methods, such as quasi-static (Hutchinson & Garcia, 2001) and dynamic (Snively, 2006) modelling and finite element analysis (Rayfield, 2004). Figure 3 presents a scheme for arriving at the initial hypothesis of EBI based on preceding inferences. Behaviour is emergent from numerous factors in addition to muscle kinematics and physiology and will remain less testable than other inferences, unless other drivers of behaviour are shown to be homologous in the extant taxa. Nevertheless, these methods facilitate, as far as possible, reconstruction of neck muscles for tyrannosaurids, inference of their action and instigation of biomechanical hypotheses, and discussion of feeding variation within Tyrannosauridae and between tyrannosaurids and other large theropods. MATERIAL AND METHODS MUSCLE RECONSTRUCTION Cervical and craniocervical muscles of tyrannosaurids were reconstructed with reference to dissections and literature descriptions of extant archosaurians, lepidosaurians and mammals, and identification of muscle scars on recent and fossil bone. The guiding principles for reconstruction were EPB and extrapolatory inference (Bryant & Russell, 1992; Witmer, 1995). Through the EPB approach, muscles present in both crocodilians and modern dinosaurs are robustly inferred as being present in tyrannosaurids. Examination of successive extant outgroups to Archosauria, such as varanid lizards and mammals, reinforces the immediate bracket if a homologous muscle is present in more distant groups, and polarizes the sequence of a muscle’s evolution and modification. When precise homologies are ambiguous in any of the taxa, less certain reconstructions are possible through higher levels of inference (by greater osteological similarity between the extinct taxon and one of the extant groups; osteological and arthrological analogues; and mechanical considerations: Bryant & Russell, 1992; Witmer, 1995). Autapomorphic loss of a muscle is considered to be a less parsimonious interpretation in the absence of osteological evidence. Specimens for dissection and osteological examination were obtained by the University of Calgary Museum of Zoology (UCMZ), primarily through the Calgary Zoo, Alberta Department of Fish and Wildlife, and Reptile World, Drumheller, Alberta. Two specimens of Varanus dumerilii were dissected and described as part of an assessment of intervertebral flexion (our unpubl. data). Dissection of commercially obtained, preserved domestic cats and three human cadavers (generously bequeathed to the Department of Cell Biology and Anatomy, University of Calgary Faculty of Medicine: UCDA), and examination of muscle attachments on cat and human skeletal preparations (UCDA, UCMZ), introduced the context of a more distal outgroup. Dissected crocodilians were Alligator mississippiensis and Caiman crocodylus, and muscle scars were assessed on skulls and cervical vertebrae of Caiman crocodylus (two), Alligator mississippiensis (two) and Crocodylus acutus. Birds dissected were Struthio camelus (two adults and two late-stage embryos), Leptoptilos crumeniferus, Pelicanus occidentalis, Cygnus columbianus, Cygnus buccinator, Anas platyrhynchos (two), Gallus gallus (three), Falco columbarius, Aquila chrysaetos, Haliaeetus leucocephalus, Asio flammeus, Halcyon cinnamomina and Pica pica. Skeletal material consisted of cervical vertebrae and skulls of skeletonized dissection specimens, and additional skeletal specimens of numerous avian clades curated at UCMZ. The Appendix lists the examined skeletal material of extant reptiles. Specimens of tyrannosaurids and other Mesozoic theropods were examined at numerous institutions. These included specimens of Tyrannosaurus rex (six; seven if this taxon subsumes Nanotyrannus lancencis), Tarbosaurus bataar (two), Daspletosaurus torosus (four, possibly including other Daspletosaurus species), Gorgosaurus libratus (four) and Albertosaurus sarcophagus (two). Non-tyrannosaurid specimens included many smaller coelurosaurs, and large noncoelurosaurian theropods including Acrocanthosaurus atokensis, Allosaurus fragilis, three Ceratosaurus species, Dilophosaurus wethereli and D. sinensis, Giganotosaurus carolinii, Monolophosaurus jiangi, Sinraptor dongi and Torvosaurus tanneri. The Appendix summarizes specimen information for examined fossil theropods. MUSCLE SYNONYMIES AND NAMING CONVENTIONS Neck muscles are complex and have a rich history of description in multiple languages. Consequently, consistency of muscle names between taxa, or even within well-defined clades, has been difficult for researchers to maintain. Avian muscle names, in particular, conflict with those for homologues in other amniotes, but precision within the avian literature is stabilizing with the adoption of the Nomina Anatomica Avium (Baumel et al., 1993). To avoid ambiguity between conflicting names, for a given homologous muscle or bone of insertion in birds, crocodilians and tyrannosaurids we use both the term from Nomina Anatomica Avium (Vanden Berge & Zweers, 1993) and the synonym most commonly applied for the muscle in other reptiles (usually from Nomina Anatomica Veteri- naria: ICVGAN, 1994). However, we do list syn- onyms and include their citation when authors diverge from the ICVGAN nomenclature. For muscles present in either birds or other reptiles but not universally present in sauropsids, we apply the name given to the muscle in the respec- tive group if correlates indicate its presence in tyrannosaurids. Neck muscles have often been described as inserting on the exoccipitals. However, the exoccipitals and opisthotics are ambiguously differentiable in many dinosaurs, and the opisthotics may be more the prominent lateral components of the occiput (A. Carabajal, pers. comm.). We therefore follow Seidel (1978) and refer to these muscles as inserting on the paroccipital processes. There is general agreement between the names used here for muscles and those in work recently published and in preparation by Tsuihiji (2002, 2003, 2005). Tsuihiji’s research encompasses the evolution and homologies of neck muscles throughout Sauria, while the present work emphasizes analogous functions of these muscles in tyrannosaurids and their extant relatives. With its broader systematic scope Tsuihiji’s nomenclature will be more comprehensively applicable, and should be adopted in the case of conflicting names. Muscle descriptions of extant archosaurs and tyr- annosaurids are organized by topographic and func- tional divisions. Descriptio

Eric Snively, Anthony P. Russell (2007): Functional morphology of neck musculature in the Tyrannosauridae (Dinosauria, Theropoda) as determined via a hierarchical inferential approach. Zoological Journal of the Linnean Society 151, No. 4: 759-808, DOI: 10.1111/j.1096-3642.2007.00334.x

Reconstructed neck muscles of large theropod dinosaurs suggest influences on feeding style that paralleled variation in skull mechanics. In all examined theropods, the head dorsiflexor m. transversospinalis capitis probably filled in the posterior dorsal concavity of the neck, for a more crocodilian- than avian-like profile in this region. The tyrannosaurine tyrannosaurids Daspletosaurus and Tyrannosaurus had relatively larger moment arms for lateroflexion by m. longissimus capitis superficialis and m. complexus than albertosaurine tyrannosaurids, and longer dorsiflexive moment arms for m. complexus. Areas of dorsiflexor origination are significantly larger relative to neck length in adult Tyrannosaurus rex than in other tyrannosaurids, suggesting relatively large muscle cross-sections and forces. Tyrannosaurids were not particularly specialized for neck ventroflexion. In contrast, the hypothesis that Allosaurus co-opted m. longissimus capitis superficialis for ventroflexion is strongly corroborated. Ceratosaurus had robust insertions for the ventroflexors m. longissimus capitis profundus and m. rectus capitis ventralis. Neck muscle morphology is consistent with puncture-and-pull and powerful shake feeding in tyrannosaurids, relatively rapid strikes in Allosaurus and Ceratosaurus, and ventroflexive augmentation of weaker jaw muscle forces in the nontyrannosaurids. Anat Rec, 290:934–957, 2007. © 2007 Wiley-Liss, Inc.

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Descriptive, principal component (PCA), and thin-plate spline (TPS) analyses of theropod third metatarsals (MT III) definitively segregate the arctometatarsus from other theropod pedal morphologies and reveal variation within phylogenetic and functional subgroups of metatarsi. PCA indicates that the arctometatarsalian MT III differs in shape from the nonarctometatarsalian condition independently of size, indicating that allometric differences among taxa produced this divergence in MT III shape. TPS indicates substantial transfer of footfall force from MT II to MT III in ornithomimids and tyrannosaurids and from MT IV to MT III in troodontids. The study suggests different modes of ligament-damped sagittal rotation of MT III in tyrannosaurids, ornithomimids, and troodontids. Deinonychus had a large MT II−MT III articulation consistent with resisting forces of predatory strikes, while MT III of some large carnosaurs are less robust than expected. Phylogenetic bracketing suggests that proximal intermetatarsal ligaments in theropods were a key innovation preceding arctometatarsus evolution. A Bayesian phylogenetic analysis indicates that an arctometatarsus evolved in the common ancestor of the Tyrannosauridae + (Ornithomimosauria + Troodontidae) clade, but other optimizations are plausible. The most likely selective benefit of the structure was increased agility; if so, homoplasy indicates multiple exaptive and adaptive pathways towards predation and escape roles. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 142, 525−553.

Donald M. Henderson, Eric Snively (2004): Tyrannosaurus en pointe: allometry minimized rotational inertia of large carnivorous dinosaurs. Proceedings of the Royal Society, Series B 271: 57-60, DOI: 10.1098/rsbl.2003.0097, URL: http://dx.doi.org/10.1098/rsbl.2003.0097

Gekkota is often interpreted as sister to all remaining squamates, exclusive of dibamids, or as sister to Autarchoglossa. It is the only diverse lineage of primarily nocturnal lizards and includes some of the smallest amniotes. The skeleton of geckos has often been interpreted as paedomorphic and/or “primitive” but these lizards also display a wide range of structural specializations of the postcranium, including modifications associated with both scansorial locomotion and limb reduction. Although the concept of “Gekkota” has been variously applied by different authors, we here apply a rigorous apomorphy based definition, recent advances in gekkotan morphology and phylogenetics, and diverse comparative material to provide a comprehensive assessment of 28 known pre-Quaternary geckos, updating the last such review, published three decades ago. Fossils evaluated include both sedimentary fossils and amber-embedded specimens. Known Cretaceous geckos are exclusively Asian and exhibit character combinations not seen in any living forms. Cenozoic gekkotans derive from sites around the world, although Europe is especially well represented. Paleogene geckos are largely known from disarticulated remains and show similarities to Sphaerodactylidae and Diplodactylidae, although resemblances may be plesiomorphic in some cases. Many Neogene gekkotans are referable to living families or even genera, but their geographic occurrences are often extralimital to those of modern groups, as is consistent with paleoclimatic conditions. The phylogenetic placement of fossil gekkotans has important repercusions for timetree calibration, but at present only a small number of fossils can be confidently assigned to even family level groupings, limiting their utility in this regard. Anat Rec, 00:000–000, 2014. V C 2014 Wiley Periodicals, Inc.