Comparative Biomechanical Modeling of Metatherian and Placental Saber-Tooths: A Different Kind of Bite for an Extreme Pouched Predator

Questions surrounding the dramatic morphology of saber-tooths, and the presumably deadly purpose to which it was put, have long excited scholarly and popular attention. Among saber-toothed species, the iconic North American placental, Smilodon fatalis, and the bizarre South American sparassodont, Thylacosmilus atrox, represent extreme forms commonly forwarded as examples of convergent evolution. For S. fatalis, some consensus has been reached on the question of killing behaviour, with most researchers accepting the canine-shear bite hypothesis, wherein both head-depressing and jaw closing musculatures played a role in delivery of the fatal bite. However, whether, or to what degree, T. atrox may have applied a similar approach remains an open question. Here we apply a three-dimensional computational approach to examine convergence in mechanical performance between the two species. We find that, in many respects, the placental S. fatalis (a true felid) was more similar to the metatherian T. atrox than to a conical-toothed cat. In modeling of both saber-tooths we found that jaw-adductor-driven bite forces were low, but that simulations invoking neck musculature revealed less cranio-mandibular stress than in a conical-toothed cat. However, our study also revealed differences between the two saber-tooths likely reflected in the modus operandi of the kill. Jaw-adductor-driven bite forces were extremely weak in T. atrox, and its skull was even better-adapted to resist stress induced by head-depressors. Considered together with the fact that the center of the arc described by the canines was closer to the jaw-joint in Smilodon, our results are consistent with both jaw-closing and neck musculature playing a role in prey dispatch for the placental, as has been previously suggested. However, for T. atrox, we conclude that the jaw-adductors probably played no major part in the killing bite. We propose that the metatherian presents a more complete commitment to the already extreme saber-tooth ‘lifestyle’.     

Radiographs Reveal Exceptional Forelimb Strength in the Sabertooth Cat,Smilodon fatalis

Background

The sabertooth cat, Smilodon fatalis, was an enigmatic predator without a true living analog. Their elongate canine teeth were more vulnerable to fracture than those of modern felids, making it imperative for them to immobilize prey with their forelimbs when making a kill. As a result, their need for heavily muscled forelimbs likely exceeded that of modern felids and thus should be reflected in their skeletons. Previous studies on forelimb bones of S. fatalis found them to be relatively robust but did not quantify their ability to withstand loading.

Methodology/Principal Findings

Using radiographs of the sabertooth cat, Smilodon fatalis, 28 extant felid species, and the larger, extinct American lion Panthera atrox, we measured cross-sectional properties of the humerus and femur to provide the first estimates of limb bone strength in bending and torsion. We found that the humeri of Smilodon were reinforced by cortical thickening to a greater degree than those observed in any living felid, or the much larger P. atrox. The femur of Smilodon also was thickened but not beyond the normal variation found in any other felid measured.

Conclusions/Significance

Based on the cross-sectional properties of its humerus, we interpret that Smilodon was a powerful predator that differed from extant felids in its greater ability to subdue prey using the forelimbs. This enhanced forelimb strength was part of an adaptive complex driven by the need to minimize the struggles of prey in order to protect the elongate canines from fracture and position the bite for a quick kill.     

Directional selection in the evolution of elongated upper canines in clouded leopards and sabre‐toothed cats

Extremely developed or specialized traits such as the elongated upper canines of extinct sabre‐toothed cats are often not analogous to those of any extant species, which limits our understanding of their evolutionary cause. However, an extant species may have undergone directional selection for a similar extreme phenotype. Among living felids, the clouded leopard, Neofelis nebulosa, has exceptionally long upper canines for its body size. We hypothesized that directional selection generated the elongated upper canines of clouded leopards in a manner similar to the process in extinct sabre‐toothed cats. To test this, we developed an approach that compared the effect of directional selection among lineages in a phylogeny using a simulation of trait evolution and approximate Bayesian computation. This approach was applied to analyse the evolution of upper canine length in the Felidae phylogeny. Our analyses consistently showed directional selection favouring longer upper canines in the clouded leopard lineage and a lineage leading to the sabre‐toothed cat with the longest upper canines, Smilodon. Most of our analyses detected an effect of directional selection for longer upper canines in the lineage leading to another sabre‐toothed cat, Homotherium, although this selection may have occurred exclusively in the primitive species. In all the analyses, the clouded leopard and Smilodon lineages showed comparable directional selection. This implies that clouded leopards share a selection advantage with sabre‐toothed cats in having elongated upper canines.     

Variation in Craniomandibular Morphology and Sexual Dimorphism in Pantherines and the Sabercat Smilodon fatalis

Sexual dimorphism is widespread among carnivorans, and has been an important evolutionary factor in social ecology. However, its presence in sabertoothed felids remains contentious. Here we present a comprehensive analysis of extant Panthera and the sabertoothed felid Smilodon fatalis. S. fatalis has been reported to show little or no sexual dimorphism but to have been intraspecifically variable in skull morphology. We found that large and small specimens of S. fatalis could be assigned to male and female sexes with similar degrees of confidence as Panthera based on craniomandibular shape. P. uncia is much less craniomandibularly variable and has low levels of sexual size-dimorphism. Shape variation in S. fatalis probably reflects sexual differences. Craniomandibular size-dimorphism is lower in S. fatalis than in Panthera except P. uncia. Sexual dimorphism in felids is related to more than overall size, and S. fatalis and the four large Panthera species show marked and similar craniomandibular and dental morphometric sexual dimorphism, whereas morphometric dimorphism in P. uncia is less. Many morphometric-sexually dimorphic characters in Panthera and Smilodon are related to bite strength and presumably to killing ecology. This suggests that morphometric sexual dimorphism is an evolutionary adaptation to intraspecific resource partitioning, since large males with thicker upper canines and stronger bite forces would be able to hunt larger prey than females, which is corroborated by feeding ecology in P. leo. Sexual dimorphism indicates that S. fatalis could have been social, but it is unlikely that it lived in fusion-fission units dominated by one or a few males, as in sub-Saharan populations of P. leo. Instead, S. fatalis could have been solitary and polygynous, as most extant felids, or it may have lived in unisexual groups, as is common in P. leo persica.     

Comparative bite forces and canine bending strength in feline and sabretooth felids: implications for predatory ecology

The sabretooth felids were widespread across much of the world in the Late Tertiary, and appear to have been an important group of large predators. Owing to the substantially different skull morphology of derived sabretooths compared with extant felids, there has been considerable debate over the killing mode, bite forces, and bending strength of the large upper canines, and over the implications of these characteristics on feeding ecology. Debates have, however, usually been based on indirect comparisons of force vectors. In this paper, I provide assessments of the estimated force output from the jaw adductor muscles, based on estimates of muscle cross-sectional areas and force vectors, along with canine bending strengths, in a variety of sabretooth felids, in comparison with extant felids. In general, sabretoothed felids had moderately powerful bites, albeit with less jaw adductor power for their body sizes compared with extant felids, sometimes markedly so. Less derived sabrecats appear to have had proportionally higher bite forces than derived forms. The length of the upper canines seemingly compromised their bending strength at any given body size, and again this was most marked in derived forms. However, compared with estimated jaw adductor forces, the canines of sabrecats appear, if anything, to have been stronger than those of extant conical-toothed felids. It has previously been suggested that large sabretoothed felids hunted large prey with a canine shearing bite, powered in part by the jaw adductors and in part by the muscles of the upper neck–occipital region. The present results of canine bending strengths versus the predicted bite force from the jaw adductors supports this suggestion.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 151 , 423–437.     

A forceful upper jaw facilitates picking-based prey capture: biomechanics of feeding in a butterflyfish,Chaetodon trichrous

Biomechanical models of feeding mechanisms elucidate how animals capture food in the wild, which, in turn, expands our understanding of their fundamental trophic niche. However, little attention has been given to modeling the protrusible upper jaw apparatus that characterizes many teleost species. We expanded existing biomechanical models to include upper jaw forces using a generalist butterflyfish, Chaetodon trichrous (Chaetodontidae) that produces substantial upper jaw protrusion when feeding on midwater and benthic prey. Laboratory feeding trials for C. trichrous were recorded using high-speed digital imaging; from these sequences we quantified feeding performance parameters to use as inputs for the biomechanical model. According to the model outputs, the upper jaw makes a substantial contribution to the overall forces produced during mouth closing in C. trichrous. Thus, biomechanical models that only consider lower jaw closing forces will underestimate total bite force for this and likely other teleost species. We also quantified and subsequently modeled feeding events for C. trichrous consuming prey from the water column versus picking attached prey from the substrate to investigate whether there is a functional trade-off between prey capture modes. We found that individuals of C. trichrous alter their feeding behavior when consuming different prey types by changing the timing and magnitude of upper and lower jaw movements and that this behavioral modification will affect the forces produced by the jaws during prey capture by dynamically altering the lever mechanics of the jaws. In fact, the slower, lower magnitude movements produced during picking-based prey capture should produce a more forceful bite, which will facilitate feeding on benthic attached prey items, such as corals. Similarities between butterflyfishes and other teleost lineages that also employ picking-based prey capture suggest that a suite of key behavioral and morphological innovations enhances feeding success for benthic attached prey items.     

Craniodental indicators of prey size preference in the Felidae

In the present study, we used linear morphometrics of the crania, mandible and dentition to explore the association between craniodental shape and prey size among 35 species of living felids. To accomplish this, felids were divided into three prey-size groups: (1) large prey specialists; (2) small prey specialists; and (3) mixed prey feeders. From these linear measurements, large prey specialist felids can be distinguished from small and mixed prey feeders by their relatively robust canines and incisors and relatively wide muzzles. These cranial characters are advantageous when dispatching large prey, due to the stranglehold that cats employ during this activity. Robust canines resist the bending and torsional forces applied by struggling prey and a wider muzzle helps to stabilize grip and distribute bite forces more evenly during the killing bite. Small prey specialists had smaller canines, narrower muzzles and slightly longer jaws for a speed advantage when catching small, quick prey. Mixed prey feeders were intermediate between large and small prey specialists, indicating they are adapted to killing both sizes of prey. Given the success of this ecomorphological analysis of living felids that specialize on different prey sizes, we look forward to applying this same approach to extinct species.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 784–799.     

Canine Evolution in Sabretoothed Carnivores: Natural Selection or Sexual Selection?

The remarkable elongated upper canines of extinct sabretoothed carnivorous mammals have been the subject of considerable speculation on their adaptive function, but the absence of living analogues prevents any direct inference about their evolution. We analysed scaling relationships of the upper canines of 20 sabretoothed feliform carnivores (Nimravidae, Barbourofelidae, Machairodontinae), representing both dirk-toothed and scimitar-toothed sabretooth ecomorphs, and 33 non-sabretoothed felids in relation to body size in order to characterize and identify the evolutionary processes driving their development, using the scaling relationships of carnassial teeth in both groups as a control. Carnassials display isometric allometry in both sabretooths and non-sabretooths, supporting their close relationship with meat-slicing, whereas the upper canines of both groups display positive allometry with body size. Whereas there is no statistical difference in allometry of upper canine height between dirk-toothed and scimitar-toothed sabretooth ecomorphs, the significantly stronger positive allometry of upper canine height shown by sabretooths as a whole compared to non-sabretooths reveals that different processes drove canine evolution in these groups. Although sabretoothed canines must still have been effective for prey capture and processing by hypercarnivorous predators, canine morphology in these extinct carnivores was likely to have been driven to a greater extent by sexual selection than in non-sabretooths. Scaling relationships therefore indicate the probable importance of sexual selection in the evolution of the hypertrophied sabretooth anterior dentition.     

A dynamic model for the evolution of sabrecat predatory bite mechanics

The ability of sabretoothed felids to achieve sufficiently high bite forces for predation at extreme gape angles has been the subject of decades of debate. Previous studies have indicated that bite forces in derived sabretoothed felids would have been low, but that they were probably augmented by head depressing muscles. However, bite mechanics is a dynamic process, and mechanical properties change with changes in gape angles. In this study, I present the first comprehensive model of bite mechanics, vector angles, and forces about the temporomandibular joint at gape angles from occlusion to maximal inferred gape in sabretoothed felids. Primitive sabrecats (Machairodus, Paramachairodus) appear broadly comparable to extant large felids (Panthera, Puma), but derived sabrecats in the groups Homotherini (Amphimachairodus, Homotherium, Xenosmilus) and Smilodontini (Megantereon, Smilodon) are often substantially different from either of the former. The ability of the mandibular adductors to generate torque changes with gape angle, indicating that previous models fail to capture potentially important differences in bite function. Inferred muscle sizes and the angles of effective torque from individual adductor fibres in derived sabrecats are different from those of primitive sabrecats and extant large felids, but they had evolved a number of compensatory adaptations for maximizing force output at the canine and carnassial, primarily changes in muscle fibre angles and more compact crania. Inferred outforces at the canines and carnassials were comparable amongst all groups at low gape angles, but at extreme gape angles outforces would have been low, supporting previous hypotheses of head flexor contribution during initial stages of the killing bite in sabrecats. Mandibular adduction in extant carnivores is a complicated pattern of differences in twitch tension and electromyographical activity at different gape angles, and inference of maximal isotonic bite forces from reconstructed mandibular adductor sizes in fossils will give estimates primarily suitable for comparative purposes. Potentially, derived sabrecats could have evolved differences from extant felids in adductor histochemistry or pinnation angle of individual fibres. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 162 , 220–242.     

Jaw Function in Smilodon fatalis: A Reevaluation of the Canine Shear-Bite and a Proposal for a New Forelimb-Powered Class 1 Lever Model

The jaw function of Smilodon fatalis has long been a source of debate. Although modern-day lions subdue large prey through the use of a suffocating throat bite, the dramatically elongated maxillary canines of S. fatalis suggest an alternative bite mechanism. The current literature favors a “canine shear-bite,” in which the depression of the cranium by the ventral neck flexors assists the mandibular adductors in closing the jaws. Although the model makes intuitive sense and appears to be supported by scientific data, the mechanical feasibility of “neck-powered” biting has not been experimentally demonstrated. In the present study, the computer-assisted manipulation of digitized images of a high-quality replica of an S. fatalis neck and skull shows that a rotation of the cranium by the ventral neck flexors will not result in jaw closure. Instead, the cranium and mandible rotate ventrally together (at the atlantooccipital joint), and the jaws remain in an open configuration. The only manner by which rotation of the cranium can simultaneously result in jaw closure is by an anterior rotation at the temporomandibular joint. Based on this finding, the author proposes a new Class 1 lever mechanism for S. fatalis jaw function. In this model, the mandible is immobilized against the neck of the prey and a dorsally directed force from the extension of the forelimbs rotates the cranium anteriorly at the temporomandibular joint. The maxillary canines pierce the prey’s neck and assist in clamping the ventral neck structures. The model is based on a maximum gape angle of approximately 90° and incorporates a secondary virtual point of rotation located slightly anteroventral to the temporomandibular joint. The Class 1 Lever Model is mechanically feasible, consistent with current data on S. fatalis anatomy and ecology, and may provide a basis for similar studies on other fossil taxa.     

A unique feeding strategy of the extinct marine mammal Kolponomos: convergence on sabretooths and sea otters

Mammalian molluscivores feed mainly by shell-crushing or suction-feeding. The extinct marine arctoid, Kolponomos, has been interpreted as an otter-like shell-crusher based on similar dentitions. However, neither the masticatory biomechanics of the shell-crushing adaptation nor the way Kolponomos may have captured hard-shelled prey have been tested. Based on mandibular symphyseal morphology shared by Kolponomos and sabre-toothed carnivores, we hypothesize a sabretooth-like mechanism for Kolponomos prey-capture, whereby the mandible functioned as an anchor. Torque generated from jaw closure and head flexion was used to dislodge prey by prying, with prey then crushed using cheek teeth. We test this hypothesized feeding sequence using phylogenetically informed biomechanical simulations and shape analyses, and find a strongly supported, shared high mandibular stiffness in simulated prey-capture bites and mandibular shape in Kolponomos and the sabre-toothed cat Smilodon. These two distantly related taxa converged on using mandibles to anchor cranial torqueing forces when prying substrate-bound prey in the former and sabre-driving forces during prey-killing in the latter. Simulated prey-crushing bites indicate that Kolponomos and sea otters exhibit alternative structural stiffness-bite efficiency combinations in mandibular biomechanical adaptation for shell-crushing. This unique feeding system of Kolponomos exemplifies a mosaic of form-function convergence relative to other Carnivora.     

Pleistocene Extinctions and the Perceived Morphofunctional Structure of the Neotropical Felid Ensemble

The felid Neotropical ensemble has experienced important changes from the Pleistocene to the present, the extinction of sabertooth cats being the most significant. Assuming that the morphofunctional structure of the Neotropical felid ensemble was maximally expressed when machairodontines were present, we added specimens of Smilodon fatalis and S. populator to a morphological dataset inclusive of all extant species of Neotropical felids to explore the hypothetical effect of extinction of machairodontines on the perceived morphofunctional structure of the ensemble. We studied 321 specimens of 12 extant species of Neotropical felids plus S. populator and S. fatalis by measuring 123 skull and postcranial morphofunctional variables. We used Principal Component Analysis to find morphofunctional patterns of the skull, postcranium, and both combined (with and without correction for size) of both ensembles, past (with Smilodon) and present (with Smilodon lost to extinction). Canonical Phylogenetic Ordination was performed to assess the degree of phylogenetic influence on this morphospace. We also explored the effect of including homotherines in the Pleistocene guild by including specimens of North American Homotherium serum. Size was the principal factor structuring ensembles. Important morphological characters were associated to ecological performance of species. A pattern of bodily proportions was apparent: more stout species with larger skull, longer rostrum, stronger bite force, and longer pectoral crests, versus gracile, jumping species with opposing characters. The felid morphospace shrank after the extinction of Smilodon and Homotherium, but the configuration of the portion of space containing extant felids remained stable. This pattern is associated with deep phylogenetic roots.     

A New Machairodont from the Palmetto Fauna (Early Pliocene) of Florida,with Comments on the Origin of the Smilodontini (Mammalia,Carnivora, Felidae)

South-central Florida’s latest Hemphillian Palmetto Fauna includes two machairodontine felids, the lion-sized Machairodus coloradensis and a smaller, jaguar-sized species, initially referred to Megantereon hesperus based on a single, relatively incomplete mandible. This made the latter the oldest record of Megantereon, suggesting a New World origin of the genus. Subsequent workers variously accepted or rejected this identification and biogeographic scenario. Fortunately, new material, which preserves previously unknown characters, is now known for the smaller taxon. The most parsimonious results of a phylogenetic analysis using 37 cranio-mandibular characters from 13 taxa place it in the Smilodontini, like the original study; however, as the sister-taxon to Megantereon and Smilodon. Accordingly, we formally describe Rhizosmilodon fiteae gen. et sp. nov. Rhizosmilodon, Megantereon, and Smilodon ( =  Smilodontini) share synapomorphies relative to their sister-taxon Machairodontini: serrations smaller and restricted to canines; offset of P3 with P4 and p4 with m1; complete verticalization of mandibular symphysis; m1 shortened and robust with widest point anterior to notch; and extreme posterior “lean” to p3/p4. Rhizosmilodon has small anterior and posterior accessory cusps on p4, a relatively large lower canine, and small, non-procumbent lower incisors; all more primitive states than in Megantereon and Smilodon. The former also differs from Megantereon and Smilodon gracilis by having a very small mandibular flange. Rhizosmilodon is the oldest known member of the Smilodontini, suggesting that the tribe originated in North America. Two more derived, similar-sized species evolved in parallel during the Blancan, Megantereon hesperus and Smilodon gracilis. The former is rarer, known only from the north-central and northwestern US, and presumably dispersed into the Old World. The latter is known from the eastern and southern US, and dispersed into South America.     

Phylogeny of the sabertoothed felids (Carnivora: Felidae: Machairodontinae)

In recent years, advances in our understanding of feline relationships have cast light on their evolutionary history. In contrast, there have been no phylogenetic analyses on machairodont felids, making it difficult to develop an evolutionary hypothesis based on the recent surge of studies on their craniomandibular morphology and functional anatomy. In this paper, I provide the first phylogenetic hypothesis of machairodont relationships based on 50 craniomandibular and dental characters from a wide range of sabercats spanning more 11 Myr. Exact searches produced 19 most‐parsimonious trees, and a strict consensus was well resolved. The Machairodontinae comprise a number of basal taxa (Promegantereon, Machairodus, Nimravides, Dinofelis, Metailurus) and a well‐supported clade of primarily Plio‐Pleistocene taxa (Megantereon, Smilodon, Amphimachairodus, Homotherium, Xenosmilus) for which the name Eumachairodontia taxon novum is proposed. Previous phenetic grouping of machairodont taxa into three distinct groups, the Smilodontini, Homotherini and Metailurini, was not supported by cladistic parsimony analysis, and forcing monophyly of these groups was significantly incompatible with character distribution. Machairodonts as a clade are not characterized by saberteeth, i.e. hypertrophied, blade‐like upper canines, but by small lower canines, as well as small M¹; and large P³ parastyle. True saberteeth arose later and are a synapomorphy of the Eumachairodontia.     

Molecular evidence that the deadliest sea snake Enhydrina schistosa (Elapidae: Hydrophiinae) consists of two convergent species

We present a striking case of phenotypic convergence within the speciose and taxonomically unstable Hydrophis group of viviparous sea snakes. Enhydrina schistosa, the ‘beaked sea snake’, is abundant in coastal and inshore habitats throughout the Asian and Australian regions, where it is responsible for the large majority of recorded deaths and injuries from sea snake bites. Analyses of five independent mitochondrial and nuclear loci for populations spanning Australia, Indonesia and Sri Lanka indicate that this ‘species’ actually consists of two distinct lineages in Asia and Australia that are not closest relatives. As a result, Australian “E. schistosa” are elevated to species status and provisionally referred to Enhydrina zweifeli. Convergence in the characteristic ‘beaked’ morphology of these species is probably associated with the wide gape required to accommodate their spiny prey. Our findings have important implications for snake bite management in light of the medical importance of beaked sea snakes and the fact that the only sea snake anti-venom available is raised against Malaysian E. schistosa.     

On the ecological scenario of the first hominin dispersal out of Africa

The archaeopaleontological site of Dmanisi in Georgia, dated to ～1.8 Ma, provides evidence on the first hominin dispersal out of Africa, while the sites of Barranco León and Fuente Nueva-3 in Spain, dated to ～1.4 Ma, record the earliest hominin settlements in Europe. However, a number of issues related to the dispersal route, the climatic conditions and the ecological scenario of this dispersal event are subject to debate. In a recent paper in L’anthropologie, Agustí and Lordkipanidze (2019) proposed an alternative scenario for the arrival of hominins in the Caucasus, which they conceived as a forest refugium area during the Early Pleistocene, and discarded that their dispersal coincided with that of other members of the Ethiopian and Asian faunas, like the sabertooth Megantereon whitei or the giant hyena Pachycrocuta brevirostris. Our review of these issues suggests that: (i) the elongated sabers and reduced postcanine teeth of African M. whitei limited the ability of this predator to process the prey carcass, which resulted in scavengeable resources for the Dmanisi hominins; (ii) the mass estimate in excess of 100 kg obtained for the trochlear perimeter of the distal humerus of the hyena from Dmanisi shows that it can be confidently ascribed to the genus Pachycrocuta; (iii) the postcranial anatomy of the Dmanisi hominins was not advantageous for scavenging tree-stored prey; (iv) the laterally flattened upper canines of M. whitei could not withstand the loads that would result from climbing a prey carcass into a tree; (v) paleobotanical analyses suggest a temperate grassland ecosystem in Dmanisi, not dominant forest conditions, with enhanced aridity in the level of hominin occupation; (vi) similarly, the low frequency of arboreal pollen in the Levantine Corridor at ～1.8 Ma points to more arid conditions than today in this area; (vii) many archaeopaleontological sites of the Rift Valley and its extension towards the Red Sea, the Levant and the Caucasus show evidence of tectonic, volcanic and/or hydrothermal events; and (viii) the delay of 400 ka in the arrival of hominins in Western Europe did not result from a lower availability of scavengeable resources.     

Maximum Bite Force and Prey Size of Tyrannosaurus rex and Their Relationships to the Inference of Feeding Behavior

The feeding behavior of the theropod dinosaur Tyrannosaurus rex is investigated through analysis of two variables that are critical to successful predation, bite force and prey body mass, as they scale with the size of the predator. These size-related variables have important deterministic effects on the predator’s feeding strategy, through their effects on lethal capacity and choice of prey. Bite force data compiled for extant predators (crocodylians, carnivorans, chelonians and squamates) are used to establish a relationship between bite force and body mass among extant predators. These data are used to estimate the maximum potential bite force of T. rex, which is between about 183,000 and 235,000 N for a bilateral bite. The relationship between maximum prey body mass and predator body mass among the same living vertebrates is used to infer the likely maximum size of prey taken by T. rex in the Late Cretaceous. This makes it possible to arrive at a more rigorous assessment of the role of T. rex as an active predator and/or scavenger than has hitherto been possible. The results of this analysis show that adult Triceratops horridus fall well within the size range of potential prey that are predicted to be available to a solitary, predaceous T. rex. This analysis establishes boundary conditions for possible predator/prey relationships among other dinosaurs, as well as between these two taxa.     

Bone-Breaking Bite Force of Basilosaurus isis (Mammalia,Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis

Bite marks suggest that the late Eocence archaeocete whale Basilosaurus isis (Birket Qarun Formation, Egypt) fed upon juveniles of the contemporary basilosaurid Dorudon atrox. Finite element analysis (FEA) of a nearly complete adult cranium of B. isis enables estimates of its bite force and tests the animal’s capabilities for crushing bone. Two loadcases reflect different biting scenarios: 1) an intitial closing phase, with all adductors active and a full condylar reaction force; and 2) a shearing phase, with the posterior temporalis active and minimized condylar force. The latter is considered probable when the jaws were nearly closed because the preserved jaws do not articulate as the molariform teeth come into occulusion. Reaction forces with all muscles active indicate that B. isis maintained relatively greater bite force anteriorly than seen in large crocodilians, and exerted a maximum bite force of at least 16,400 N at its upper P³. Under the shearing scenario with minimized condylar forces, tooth reaction forces could exceed 20,000 N despite lower magnitudes of muscle force. These bite forces at the teeth are consistent with bone indentations on Dorudon crania, reatract-and-shear hypotheses of Basilosaurus bite function, and seizure of prey by anterior teeth as proposed for other archaeocetes. The whale’s bite forces match those estimated for pliosaurus when skull lengths are equalized, suggesting similar tradeoffs of bite function and hydrodynamics. Reaction forces in B. isis were lower than maxima estimated for large crocodylians and carnivorous dinosaurs. However, comparison of force estimates from FEA and regression data indicate that B. isis exerted the largest bite forces yet estimated for any mammal, and greater force than expected from its skull width. Cephalic feeding biomechanics of Basilosaurus isis are thus consistent with habitual predation.     

Sabretoothed carnivores and the killing of large prey

Sabre-like canines clearly have the potential to inflict grievous wounds leading to massive blood loss and rapid death. Hypotheses concerning sabretooth killing modes include attack to soft parts such as the belly or throat, where biting deep is essential to generate strikes reaching major blood vessels. Sabretoothed carnivorans are widely interpreted as hunters of larger and more powerful prey than that of their present-day nonsabretoothed relatives. However, the precise functional advantage of the sabretooth bite, particularly in relation to prey size, is unknown. Here, we present a new point-to-point bite model and show that, for sabretooths, depth of the killing bite decreases dramatically with increasing prey size. The extended gape of sabretooths only results in considerable increase in bite depth when biting into prey with a radius of less than ～10 cm. For sabretooths, this size-reversed functional advantage suggests predation on species within a similar size range to those attacked by present-day carnivorans, rather than "megaherbivores" as previously believed. The development of the sabretooth condition appears to represent a shift in function and killing behaviour, rather than one in predator-prey relations. Furthermore, our results demonstrate how sabretoothed carnivorans are likely to have evolved along a functionally continuous trajectory: beginning as an extension of a jaw-powered killing bite, as adopted by present-day pantherine cats, followed by neck-powered biting and thereafter shifting to neck-powered shear-biting. We anticipate this new insight to be a starting point for detailed study of the evolution of pathways that encompass extreme specialisation, for example, understanding how neck-powered biting shifts into shear-biting and its significance for predator-prey interactions. We also expect that our model for point-to-point biting and bite depth estimations will yield new insights into the behaviours of a broad range of extinct predators including therocephalians (gorgonopsian + cynodont, sabretoothed mammal-like reptiles), sauropterygians (marine reptiles) and theropod dinosaurs.     

Interpopulation variation in prey use and feeding biomechanics in Caribbean triggerfishes

The relationships between prey utilization and jaw biomechanics were explored in two Caribbean populations (La Parguera and Mona Island) of four trigger-fishes. The volumetric contribution of major prey types and six biomechanical features of the jaws that characterize biting strength were contrasted between populations. At Mona, Xanthichthys ringens ate 45% benthic organisms, whereas conspecifics at La Parguera fed exclusively on plankton. Balistes vetula at Mona consumed 63% soft and nonelusive invertebrates, in contrast to their La Parguera conspecifics, which consumed 62% hard prey. Differences in diet between populations were associated with differences in jaw biomechanics. Xanthichthys at Mona had jaw muscles, bones, and closing-lever ratios larger than those of fish at La Parguera, indicating a stronger bite. Balistes at Mona had 50% smaller jaw bones, muscles, and closing-lever ratios than their La Parguera conspecifics, indicating a weaker but swifter bite. Melichthys niger and Cantherhines macrocerus ate similar prey at the two locations and showed little difference in trophic anatomy. We hypothesize that the interpopulation differences in morphology are induced by the activities of feeding on different prey and enhance the feeding ability of fishes for locally dominant prey. Plasticity of the feeding mechanism may be a widespread attribute of fish feeding systems that promotes the ability of species to occupy multiple habitat types successfully.