Thylacosmilus Research Articles

Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta)

The evolution of mammalian vision is difficult to study because the actual receptor organs—the eyes—are not preserved in the fossil record. Orbital orientation and size are the traditional proxies for inferring aspects of ocular function, such as stereoscopy. Adaptations for good stereopsis have evolved in living predaceous mammals, and it is reasonable to infer that fossil representatives would follow the same pattern. This applies to the sparassodonts, an extinct group of South American hypercarnivores related to marsupials, with one exception. In the sabertooth Thylacosmilus atrox, the bony orbits were notably divergent, like those of a cow or a horse, and thus radically differing from conditions in any other known mammalian predator. Orbital convergence alone, however, does not determine presence of stereopsis; frontation and verticality of the orbits also play a role. We show that the orbits of Thylacosmilus were frontated and verticalized in a way that favored some degree of stereopsis and compensated for limited convergence in orbital orientation. The forcing function behind these morphological tradeoffs was the extraordinary growth of its rootless canines, which affected skull shape in Thylacosmilus in numerous ways, including relative orbital displacement.

An eye for a tooth: Thylacosmilus was not a marsupial "saber-tooth predator".

BackgroundSaber-toothed mammals, now all extinct, were cats or “cat-like” forms with enlarged, blade-like upper canines, proposed as specialists in taking large prey. During the last 66 Ma, the saber-tooth ecomorph has evolved convergently at least in five different mammalian lineages across both marsupials and placentals. Indeed, Thylacosmilus atrox, the so-called “marsupial saber-tooth,” is often considered as a classic example of convergence with placental saber-tooth cats such as Smilodon fatalis. However, despite its superficial similarity to saber-toothed placentals, T. atrox lacks many of the critical anatomical features related to their inferred predatory behavior—that of employing their enlarged canines in a killing head strike.MethodsHere we follow a multi-proxy approach using canonical correspondence analysis of discrete traits, biomechanical models of skull function using Finite Element Analysis, and 3D dental microwear texture analysis of upper and lower postcanine teeth, to investigate the degree of evolutionary convergence between T. atrox and placental saber-tooths, including S. fatalis.ResultsCorrespondence analysis shows that the craniodental features of T. atrox are divergent from those of placental saber-tooths. Biomechanical analyses indicate a superior ability of T. atrox to placental saber-tooths in pulling back with the canines, with the unique lateral ridge of the canines adding strength to this function. The dental microwear of T. atrox indicates a soft diet, resembling that of the meat-specializing cheetah, but its blunted gross dental wear is not indicative of shearing meat.ConclusionsOur results indicate that despite its impressive canines, the “marsupial saber-tooth” was not the ecological analogue of placental saber-tooths, and likely did not use its canines to dispatch its prey. This oft-cited example of convergence requires reconsideration, and T. atrox may have had a unique type of ecology among mammals.

Caudal Cranium of Thylacosmilus atrox (Mammalia, Metatheria, Sparassodonta), a South American Predaceous Sabertooth

The caudal cranium of the South American sabertooth Thylacosmilus atrox (Thylacosmilidae, Sparassodonta, Metatheria) is described in detail, with emphasis on the constitution of the walls of the middle ear, cranial vasculature, and major nerve pathways. With the aid of micro-CT scanning of the holotype and paratype, we have established that five cranial elements (squamosal, alisphenoid, exoccipital, petrosal, and ectotympanic) and their various outgrowths participate in the tympanic floor and roof of this species. Thylacosmilus possessed a U-shaped ectotympanic that was evidently situated on the medial margin of the external acoustic meatus. The bulla itself is exclusively composed of the tympanic process of the exoccipital and rostral and caudal tympanic processes of the squamosal. Contrary to previous reports, neither the alisphenoid nor the petrosal participate in the actual tympanic floor, although they do contribute to the roof. In these regards Thylacosmilus is distinctly different from other borhyaenoids, in which the tympanic floor was largely membranous (e.g., Borhyaena) and lacked an enlarged ectotympanic (e.g., Paraborhyaena). In some respects Thylacosmilus is more similar to hathliacynids than to borhyaenoids, in that the former also possessed large caudal outgrowths of the squamosal and exoccipital that were clearly tympanic processes rather than simply attachment sites for muscles. However, hathliacynids also exhibited a large alisphenoid tympanic process, a floor component that is absent in Thylacosmilus. Habitual head posture was inferred on the basis of inner ear features. Large paratympanic spaces invade all of the elements participating in bounding the middle ear, another distinctive difference of Thylacosmilus compared to other sparassodonts. Arterial and venous vascular organization is relatively conservative in this species, although some vascular trackways could not have been securely identified without the availability of CT scanning. The anatomical correlates of the internal carotid in relation to other basicranial structures, the absence of a functional arteria diploetica magna, and the network for venous return from the endocranium agree with conditions in other sparassodonts.

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’.

Form and function within a phylogenetic framework: locomotory habits of extant predators and some Miocene Sparassodonta (Metatheria)

In this study, we analysed locomotory habits in extant predators and Sparassodonta species through geometric morphometric techniques and discriminant analyses of the distal humerus in anterior view, proximal ulna in lateral view, and tibia in proximal view. We included a wide sample of extant predators, and considered the phylogenetic and allometric structure in the data sets. We also included some Sparassodonta, a group of carnivorous metatherians that inhabited South America during the Cenozoic, and inferred their locomotory habits. Results suggest the presence of a close relationship between shape and locomotory habits, even after removing the shape component explained by phylogeny in the three postcranial elements. Terrestrial habits were inferred for Arctodictis sinclairi, Borhyaena tuberata, ‘Lycopsis’ longirostrus, and Thylacosmilus atrox. Some degree of cursoriality was highlighted in B. tuberata and T. atrox, and climbing abilities in ‘L.’ longirostrus, and to a lesser degree in B. tuberata. Scansorial habits were inferred for Cladosictis patagonica, Sipalocyon gracilis, Prothylacynus patagonicus, and Pseudonotictis pusillus, and in the case of C. patagonica, some digging ability was also tentatively inferred.

Estimación de Masa de las Especies de Sparassodonta (Mammalia, Metatheria) de Edad Santacrucense (Mioceno Temprano) a Partir del Tamaño del Centroide de los Elementos Apendiculares: Inferencias Paleoecológicas

BODY-MASS ESTIMATION OF SPARASSODONTA SPECIES (MAMMALIA, METATHERIA) OF SANTACRUCIAN AGE (EARLY MIOCENE) BASED ON CENTROID SIZES OF POSTCRANEAL ELEMENTS: PALAEOECOLOGICAL INFERENCES. Sparassodonts have been described as a group with a large body-mass diversity, but few works had focused on body mass estimations. In this contribution, we calculated the centroid size of the anterior view of the distal humerus, lateral view of the proximal ulna, and proximal articular view of the tibia. This data together with the known body-mass for many current predators (marsupials and placentals) allowed us to derive new allometric equations to estimate body size in the Sparassodonta. We used comparative analyses to quantify the effect of the phylogenetic structure. The relationships between body mass, centroid size, and locomotory habits were explored. The phylogenetic effect was significant for all regressions. In the humerus analysis, we found an important relationship between body mass and locomotory habit, suggesting adaptive causes. We used the ulna and tibia regressions to predict body mass, since they were less influenced by locomotory habits. Our equations produced the following masses: Arctodictis sinclari Marshall, 39.9 kg; Borhyaena tuberata Ameghino, 36.4 kg; Cladosictis patagonica Ameghino, 6.6 kg; Lycopsis longirostrus Marshall, 29.77 kg; Prothylacynus patagonicus Ameghino, 31.79 kg; Sipalocyon gracilis Ameghino, 2.11 kg; and Thylacosmilus atrox Riggs, 117.36 kg. For Borhyaena and Thylacosmilus, we obtained values higher than previously published masses, while for Pseudonotictis pusillus Ameghino, we have made the first body mass estimation of 1.17 kg. The results suggest that the guild of the santacrucian predators was structured by the body mass and other variables, such as diet and locomotion.

A new thylacosmilid (Mammalia, Metatheria, Sparassodonta) from the Miocene of Patagonia, Argentina

A new genus and species, Patagosmilus goini, of the family Thylacosmilidae (Mammalia, Metatheria, Sparassodonta) is described here. The new taxon is based on a single specimen collected from the west margin of the Río Chico, in Río Negro Province, Argentina, from the middle Miocene Colloncuran SALMA. Until now, two formally recognized species were encompassed in the family Thylacosmilidae: Thylacosmilus atrox, from the late Miocene-late Pliocene Huayquerian to Chapadmalalan SALMAof Argentina and probably Uruguay; and Anachlysictis gracilis, from the middle Miocene Laventan SALMA of Colombia. Recognition of the Patagonian taxon, Patagosmilus, provides new anatomical data, likely to be included in future phylogenetic analyses. The overall morphology of Patagosmilus suggests that it has a more generalized anatomy than Thylacosmilus. The dental morphology suggests the new Patagonian taxon was probably closer to Thylacosmilus than Anachlysictis. Saber-tooth thylacosmilids have several autapomorphic features in the skull that differentiate them from other sparassodonts, including the delayed replacement or non-replacement of the deciduous last premolar.

Feeding behaviour and bite force of sabretoothed predators

The feeding behaviour of extinct sabretoothed predators (machaeroidines, nimravids, barbourofelids, machairodonts and thylacosmilines) is investigated using beam theory. Because bite force applied along the mandible should be proportional to the external dimension of the mandibular corpus, patterns of variation in these dimensions at interdental gaps will reflect the adaptation of the jaw to specific loads, related to killing methods. Comparison of the mandibular force profiles of sabretooths to those of extant conical-toothed carnivorans of known feeding behaviour reveals that sabretooths had a powerful bite, as strong or stronger than extant felids of similar mandibular length. Loads exerted at the lower canine were better constrained in the sagittal plane than in extant conical-toothed carnivorans, indicating that prey was efficiently restrained when the sabre bite was delivered. The mandibular symphysis is generally better buttressed dorsoventrally in dirk-toothed sabretooths than in scimitar-toothed sabretooths, implying different killing strategies for the two ecomorphs: dirktooths delivered powerful sabre bites on prey they restrained with their forelimbs, while scimitartooths delivered slashing sabre bites and may have used their incisor battery to subdue their prey. The mandibular symphysis of Smilodon fatalis is less buttressed dorsoventrally than that of other dirk-toothed sabretooths, possibly as a consequence of the greater torsional stresses induced while feeding rapidly on carcasses in response to intense competition. The mandibular symphysis of Thylacosmilus atrox is better buttressed dorsoventrally in juveniles than in adults, suggesting that young marsupial sabretooths underwent an extended period of parental care as typically observed in modern felids and inferred for eutherian sabretooths. Finally, machaeroidines and the nimravid Nimravus brachyops are exceptional in exhibiting a degree of dorsoventral buttressing of the mandibular symphysis that is intermediate between advanced sabretooths and conical-toothed felids but similar to the extant Neofelis nebulosa, suggesting that the latter taxon may be close to the ancestral condition of a new sabretooth radiation. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 145, 393–426.

Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa

We provide the first predictions of bite force (BS) in a wide sample of living and fossil mammalian predators. To compare between taxa, we calculated an estimated bite force quotient (BFQ) as the residual of BS regressed on body mass. Estimated BS adjusted for body mass was higher for marsupials than placentals and the Tasmanian devil (Sarcophilus harrisii) had the highest relative BS among extant taxa. The highest overall BS was in two extinct marsupial lions. BFQ in hyaenas were similar to those of related, non-osteophagous taxa challenging the common assumption that osteophagy necessitates extreme jaw muscle forces. High BFQ in living carnivores was associated with greater maximal prey size and hypercarnivory. For fossil taxa anatomically similar to living relatives, BFQ can be directly compared, and high values in the dire wolf (Canis dirus) and thylacine (Thylacinus cynocephalus) suggest that they took relatively large prey. Direct inference may not be appropriate where morphologies depart widely from biomechanical models evident in living predators and must be considered together with evidence from other morphological indicators. Relatively low BFQ values in two extinct carnivores with morphologies not represented among extant species, the sabrecat, Smilodon fatalis, and marsupial sabretooth, Thylacosmilus atrox, support arguments that their killing techniques also differed from extant species and are consistent with 'canine-shear bite' and 'stabbing' models, respectively. Extremely high BFQ in the marsupial lion, Thylacoleo carnifex, indicates that it filled a large-prey hunting niche.

Evolution of South American mammalian predators (Borhyaenoidea): anatomical and palaeobiological implications

The evolution of South American carnivorous marsupials, the borhyaenoids, has been investigated through the functional analysis of postcranial adaptive traits and palaeobiological data. There is evidence that the evolutionary history of Borhyaenoidea proceeded from a noncursorial ancestor. The locomotion and habits of the early Palaeocene Mayulestes ferox probably approached a generalized plesiomorphic pattern for marsupial locomotion, i.e. primarily terrestrial with secondary arboreal adaptations. An exceptionally rich early Miocene Patagonian fauna has yielded various morphological predator types, from scansorial ambusher to terrestrial, incipiently cursorial, taxa. The most specialized borhyaenoid was the powerful sabretooth Thylacosmilus atrox that survived until the late Pliocene. The evolution from a scansorial pattern towards a cursorial trend, illustrated by Borhyaena tuberata, and towards the probable development of postlactational parental care in Thylacosmilus, required by its dental specialization and killing strategy, suggests a modification of the selective pressures and predatory activities of the group over evolutionary time. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 140, 487–521.

Functional-adaptive features and palaeobiologic implications of the postcranial skeleton of the late Miocene sabretooth borhyaenoid Thylacosmilus atrox (Metatheria)

A functional-adaptive study of the postcranium of two late Miocene sabretooth borhyaenoid specimens (Mammalia, Metatheria) is presented. Thylacosmilus atrox developed a longer neck than in non-sabretooth borhyaenoids and was capable of strong flexion of the head. The lower back is well-stabilized and more rigid than in the other borhyaenoids. The forelimb appears well-suited for manipulating and capturing prey, with a probably well-developed deltoid and pectoral musculature. Compared with other Miocene borhyaenoids, the hip joint of Thylacosmilus is modified to allow greater postural flexibility (e.g. possibility of erect stances). The low greater femoral trochanter, the short and sigmoid tibia, and the semiplantigrade hind foot of Thylacosmilus precluded fast running. Thylacosmilus killed by stabbing, a peculiar mechanism that evolved in parallel in many other sabretooth taxa. This technique has significant functional-adaptive consequences on the postcranial skeleton, superimposed on the generalized morphological pattern reflected in its non-sabertooth relatives. The superficial similarities observed between Thylacosmilus and Smilodon overshadow real differences (at the level of joint patterns and muscular groups involved in particular movements), a condition that suggests the development in the marsupial form of a morphological type unique to the thylacosmilid lineage within Borhyaenoidea.

Dental functional morphology in the marsupial sabre-tooth Thylacosmilus atrox (Thylacosmilidae) compared to that of the felid sabre-tooths.

The South American Middle to Late Pliocene fauna of Argentina included the borhyaenid marsupials Thylacosmilus atrox and Notosmilus pattersoni, the latter known only from a single Late Pliocene occurrence. Thylacosmilus atrox has been interpreted as a marsupial with a carnivorous specialisation convergent on the fissipede and machairodont sabre-tooths of Eurasia, Africa and North America. Thylacosmilus atrox superficially resembles placental sabre-tooths in general conformation but also shows remarkable divergences, some of which are attributable to its marsupial ancestry.