Thylacosmilus atrox

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Thylacosmilus atrox


Reconstruction of T. atrox. Image source->

Temporal range: Late Miocene to mid-Pliocene; Huayquerian to Chapadmalalan (~9-3 Ma)[1]

Scientific classification:

Life
Clade: Neomura
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Phylum: Chordata
Subkingdom: Eumetazoa
(unranked): Bilateria
Superphylum: Deuterostomia
Phylum: Chordata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Suborder: Cynodontia
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Infraclass: Metatheria
Order: †Sparassodonta
Superfamily: †Borhyaenoidea[2]
Family: †Thylacosmilidae
Genus: †Thylacosmilus
Species: †T. atrox


Thylacosmilus atrox (“cruel pouch knife”) is an extinct metatherian carnivorous mammal that belonged to the extinct order Sparassodonta, a sister taxon to crown marsupials.[3] It lived in Argentina and Uruguay[4] from the earliest Huayquerian to the latest Chapadmalalan South American Land Mammal Ages (corresponding to the Late Miocene to mid-Pliocene epochs) from ~9-3 million years ago[1], existing for ~6 million years.

Description and paleobiology:
Thylacosmilus is arguably the most famous sparassodont for its close resemblance to the famous saber-toothed cat Smilodon. Its most distinctive feature are its enlarged upper canines similar to the saber teeth of machairodontines, and this has commonly led to assertions that Smilodon and Thylacosmilus represent an example of convergent evolution.[5]


Direct comparison of the skulls of the sparassodont Thylacosmilus (left), the nimravid Hoplophoneus (middle), and the felid Smilodon (right). Image source->

Unlike the upper canines of machairodontines, these saber teeth were perpetually growing and deeply anchored in the maxilla.[4] They were more sub-triangular in cross section for most of their length[6], as opposed to elliptical like the canines of machairodontines. Also unlike Smilodon, but like some earlier carnivoran sabertooths (i.e. nimravids and barbourofelids), Thylacosmilus had prominent bony symphyseal flanges that protected its upper canines when the mouth was closed. The enamel was extremely thin and generally restricted to the lateral surface, being no more than a fifth or a quarter millimeter thick.[6][7] The denticulations (i.e. denticles or serrations) on the upper canines are described as being very minute and barely visible to the naked eye.[6] These canines were sharpened by wear from the far smaller lower canines (this is known as thegosis).[7]


Comparison of the canine teeth of Smilodon fatalis and Thylacosmilus atrox. The latter’s teeth are far more deeply rooted. Fig. 1 of [5].

Thylacosmilus’ dental formula has been rather contentious, particularly in the number of incisors. One specimen clearly had at least one pair of lower incisors, possibly more. Wear facets on the lower incisors indicate that upper incisors were present as well[4] (upper incisors from Thylacosmilus have not been found so far). Two premolars existed in each jaw; the last upper premolar has been interpreted as a retained deciduous element.[4]

Physical observations and biomechanical modeling have revealed the function of Thylacosmilus’ jaws and neck. Thylacosmilus had a relatively long and flexible neck (longer and more flexible than those of other borhyaenoids) with hyper-developed neck musculature. This neck musculature not only allowed for extremely powerful ventroflexion, but also controlled the head and canines with great precision, to protect the canines from fracture when attacking.[8] The canine bite force of Thylacosmilus at maximum gape was an astonishingly low 38 Newtons. This was far lower than even the 519 N estimated for Smilodon fatalis, and proportionately far less than the 484 N estimated for Panthera pardus (“At maximum gape, Thylacosmilus atrox would need to generate 14.5 times the jaw adductor muscle force of P. pardus in order to achieve a bite force consistent with its body mass.”). However, when forces from the neck muscles were applied, “mean” brick element stresses were comparable in S. fatalis and P. pardus, and relatively low in T. atrox. The skull of Thylacosmilus was even better able to resist stress from head depressors than that of Smilodon. This has been taken to mean that, while Smilodon would have still had some use for its mandible for a “canine shear bite” in conjunction with its neck, almost all of the saber-driving force in Thylacosmilus was from its neck, and therefore that its mandible played no major role in prey dispatch. As such, Thylacosmilus represents an even more extreme commitment to the sabertooth lifestyle than does Smilodon.[9] Interestingly, the skull morphology of Thylacosmilus and other metatherians seem to overlap more with those of caniforms (particularly canids, ursids, and amphicyonids) in having a narrower, pointier skull, in contrast with those of felids. Despite convergent evolution between Thylacosmilus and machairodontines, their skull morphologies do not overlap (interestingly, in another principle component analysis, Thylacosmilus finds itself isolated away from all other carnivores, seemingly closer to a long, narrower, and flatter skull).[10]

Body mass estimates for T. atrox vary. Ercoli and Prevosti (2011) obtained an estimate of 117.4 kg; this was similar to a femoral variable-based estimate of 116 kg by Wroe et al. (1999), but not similar to a femoral circumference-based estimate of 54 kg (Wroe et al., 2004). Argot (2004) used other postcrania-based equations to obtain estimates of 47.5-49.5 kg, 82-86.7 kg, and 108 kg for the holotype. The largest estimate obtained was 150 kg based on condylobasal skull length (Sorkin, 2008).[4] It seems that estimates considered plausible gravitate within the 80-120 kg range.[5]

The manus was either digitigrade or semidigitigrade, while the pes was plantigrade.[4] A single proximal end of an ungual phalanx was similar in structure to that of Borhyaena and other borhyaenoids.[6][8] The flexor tubercle was well developed, and its articular facet was wider than tall, a condition, more similar to that of Borhyaena than that of Prothylacinus.[8] Overall, the unguals were probably similar in structure to those of Borhyaena.[6] The claws were clearly non-retractile, a stark contrast to the claws of eutherian sabertooths (i.e. machairodontines, nimravids, and barbourofelids).[8] Historically, this has been seen as a handicap on Thylacosmilus’ part, hindering its ability to hunt like a true saber-toothed cat and forcing it to adopt a different method of hunting. This has led to bizarre proposals for Thylacosmilus modus operandi when hunting, such as the idea that it rammed its prey.[11] However, despite the lack of grasping claws, Thylacosmilus seems to have had a pseudo-opposable pollex that would have improved the grasping ability of the manus.[8] Furthermore, the forelimbs possessed adaptations allowing them to not only have the power and flexibility to wrestle with prey (e.g. robust humerus, powerful deltoid and pectoral muscles), but also to absorb shock from impacting prey. The highly flexible hindlimb and hip joints, the robust and sigmoid tibia, very low position of the greater trochanter (both of which preclude fast running ability), and rigid lumbar region are consistent with the hindlimbs being used as braces when wrestling with prey.[8]

The tympanic floor and roof were morphologically complex, having at least five elements whose outgrowth participate in their formation (it should be noted, though, that the actual shape of the floor and roof is largely a function of extensive pneumatization of the middle ear’s bony walls). The tympanic floor was distinctly different from those of other borhyaenoids; in Thylacosmilus, it was largely bony (as opposed to largely membranous in other borhyaenoids). Estimation of the habitual head posture suggests that Thylacosmilus had the ability to significantly incline the head, maximizing its visual field.[1]

Thylacosmilids evolved alongside large ground sloths, large meridiungulates (e.g. toxodontids and litopterns), and giant rodents. A throat bite with the saber teeth would seem well suited against meridiungulates and giant rodents, but not armored glyptodonts and pampatheres, nor large-clawed ground sloths.[8]

Extinction:
It is traditionally thought that the native South American apex predators went extinct through competition with eutherian carnivores from North America. Thylacosmilus in particular used to be thought to have succumbed to competition from its carnivoran ecological vicar Smilodon.

These claims are unsupported. Thylacosmilus remains are known from the Chapadmalalan[1][12] (which ended 3 million years ago), but do not appear in sediments dating to any later time.[1][12] By contrast, felids (including Smilodon, Puma, Panthera, and Homotherium) are unknown from South America until the Ensenadan (which began 1.2 million years ago) onwards.[12][13][14][15] Possible reports of Chapadmalalan age felids have been debunked.[12] Therefore, there is a 1.8 million year gap between the last thylacosmilids and the first South American felids (this figure is consistent with a range of a gap of 1.2-2 million years claimed in [13], and >1.5 million years claimed in [16]). This does not appear to be a product of fossil record sampling bias. The Chapadmalalan is well known and has been extensively explored for more than a century.[12][13] The following is worth noting: small hypercarnivores (mustelids and Galictis spp.) are known in South America since the Vorohuean (=Uquian[17]), even if they were rare. Their fossil record during the Ensenadan-Holocene is also sparse and sporadic. Foxes are known in the continent since the Uquian and during the Ensenadan-Lujanian ages, and foxes and Smilodon are the most commonly found carnivorans from the Ensenadan. Therefore, if Smilodon was present in South America before the Ensenadan, its remains should be expected to be found with those of smaller carnivorans in older levels.[13]

It is possible that Thylacosmilus competed with the large procyonid Chapalmalania, but the fact that these mammals were of very different ecotypes makes this unlikely (Thylacosmilus was an apex predatory hypercarnivore, while Chapalmalania was a large omnivore).[13]

Instead, it appears that the extinction of the sparassodonts, including Thylacosmilus, was a result of a combination of physical and biological factors (i.e. changes in prey community and environmental changes).[13]


Hypothesized interactions between Thylacosmilus and Smilodon like that depicted here are anachronistic; the former went extinct 1.8 million years before the latter appeared on the continent. Screen capture from Life After Dinosaurs (link->).

References:

[1] Forasiepi, A. M., Macphee, R. D., & del Pino, S. H. (2019). Caudal Cranium of Thylacosmilus atrox (Mammalia, Metatheria, Sparassodonta), a South American Predaceous Sabertooth. Bulletin of the American Museum of Natural History, 2019(433), 1-66.
[2] Forasiepi, A. M. (2009). Osteology of Arctodictis sinclairi (Mammalia, Metatheria, Sparassodonta) and phylogeny of Cenozoic metatherian carnivores from South America. Monografías del Museo Argentino de Ciencias Naturales, 6, 1-174.
[3] Vizcaíno, SF, Kay, RF, & Bargo, MS (Eds.). (2012). Early Miocene paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge University Press. p. 174
[4] Prevosti, F. J., & Forasiepi, A. M. (2018). Evolution of South American mammalian predators during the Cenozoic: paleobiogeographic and paleoenvironmental contingencies. Springer. p. 70
[5] Wroe, S., Chamoli, U., Parr, W. C., Clausen, P., Ridgely, R., & Witmer, L. (2013). Comparative biomechanical modeling of metatherian and placental saber-tooths: a different kind of bite for an extreme pouched predator. PLoS One, 8(6).

[6] Riggs, E. S. (1934). A new marsupial saber-tooth from the Pliocene of Argentina and its relationships to other South American predacious marsupials. Transactions of the American Philosophical Society, 24(1), 1-32.
[7] Turnbull, W. D., Butler, P. M., & Joysey, K. A. (1978). Another look at dental specialization in the extinct saber-toothed marsupial, Thylacosmilus, compared with its placental counterparts. Development, function and evolution of teeth. Academic Press, London, 399-414.
[8] Argot, C. (2004). Functional-adaptive features and palaeobiologic implications of the postcranial skeleton of the late Miocene sabretooth borhyaenoid Thylacosmilus atrox (Metatheria). Alcheringa, 28(1), 229-266.
[9] Wroe, S., Chamoli, U., Parr, W. C., Clausen, P., Ridgely, R., & Witmer, L. (2013). Comparative biomechanical modeling of metatherian and placental saber-tooths: a different kind of bite for an extreme pouched predator. PLoS One, 8(6).
[10] Goswami, A., Milne, N., & Wroe, S. (2011). Biting through constraints: cranial morphology, disparity and convergence across living and fossil carnivorous mammals. Proceedings of the Royal Society B: Biological Sciences, 278(1713), 1831-1839.
[11] Goin, F. J., & Pascual, R. (1987). News on the biology and taxonomy of the marsupials Thylacosmilidae [Late Terciary of Argentina]. In Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales de Buenos Aires (No. 39).
[12] Prevosti, F. J., & Forasiepi, A. M. (2018). The fossil record of mammalian carnivores in south America: Bias and limitations. In Evolution of South American Mammalian Predators During the Cenozoic: Paleobiogeographic and Paleoenvironmental Contingencies (pp. 137-154). Springer, Cham.
[13] Prevosti, F. J., Forasiepi, A., & Zimicz, N. (2013). The evolution of the Cenozoic terrestrial mammalian predator guild in South America: competition or replacement?. Journal of Mammalian Evolution, 20(1), 3-21.
[14] Manzuetti, A., Perea, D., Ubilla, M., & Rinderknecht, A. (2018). First record of Smilodon fatalis Leidy, 1868 (Felidae, Machairodontinae) in the extra-Andean region of South America (late Pleistocene, Sopas Formation), Uruguay: taxonomic and paleobiogeographic implications. Quaternary Science Reviews, 180, 57-62.
[15] Rincón, A. D., Prevosti, F. J., & Parra, G. E. (2011). New saber-toothed cat records (Felidae: Machairodontinae) for the Pleistocene of Venezuela, and the Great American Biotic Interchange. Journal of Vertebrate Paleontology, 31(2), 468-478.
[16] Forasiepi, A. M. (2009). Osteology of Arctodictis sinclairi (Mammalia, Metatheria, Sparassodonta) and phylogeny of Cenozoic metatherian carnivores from South America. Monografías del Museo Argentino de Ciencias Naturales, 6, 1-174.
[17] MacFadden, B. J. (2013). Dispersal of Pleistocene Equus (Family Equidae) into South America and calibration of GABI 3 based on evidence from Tarija, Bolivia. PloS one, 8(3).

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Well, this new paper flips a lot of things in the OP above on its head.

Background
Saber-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.

Methods
Here 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.

Results
Correspondence 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.

Conclusions
Our 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.

peerj.com/articles/9346/?utm_source=Twitter&utm_medium=organic&utm_campaign=Justpublished&utm_term=PeerJ&utm_content=9346

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The following paper has data from a stable isotope analysis with Thylacosmilus.

www.nature.com/articles/s41598-020-58575-6#Sec16

See SI 7. As the spreadsheet says, the greatest prey contributions are marked in bold, while the lowest are marked in italics. Thylacosmilus from two different formations were analyzed.

1.) In Salinas Grandes de Hidalgo, Thylacosmilus was mainly feeding upon notoungulates. Rodents, cingulates, and litopterns were also eaten, but not as much as notoungulates. Pilosans were eaten the least.

2.) In Quehué, Thylacosmilus was again mainly feeding upon notoungulates. Rodents were eaten, but not as much as notoungulates. Cingulates were eaten the least.

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The neurocranium, nasals, and maxillary regions show no bearing on convergence among sabre-tooths and set Thylacosmilus apart (Table 4, Fig. 2). Intriguingly, the barbourofelid Barbourofelis sits closer to the felid Homotherium than it does to Smilodon, probably because of similarity in their incisor arcades (Biknevicius et al. 1996). We detected greater similarity between Thylacosmilus and Barbourofelis, suggesting that the metatherian sabre-tooth converged more completely on the highly specialized barbourofelid. Previous work has suggested that these taxa converge on the presence of very long canines, a postorbital bar, mandibular flanges and a number of postcranial characters (Prevosti et al. 2021).

The most obvious example here is that the most phylogenetically distant of sabre-tooth taxa, Thylacosmilus, is strongly convergent on placental sabre-tooths across regions of the cranium widely considered to be functionally significant. However, a recent study addressing the mechanical performance of Thylacosmilus (Janis et al. 2020) proposes a very different ecology. Their FEA-based analysis suggested a greater capacity to exert pulling forces in Thylacosmilus than in Smilodon. It was inferred on this basis that Thylacosmilus did not deploy killing behaviour common to other sabre-tooths that distinguished them from conical-toothed predators and that it may have been a scavenger. We do not contest the possibility that Thylacosmilus was better adapted to exert pulling behaviour, because the only other FEA-based study to include these two taxa did not apply a neck-pulling load case (Wroe et al. 2013). On the other hand, we do note that the study of Janis et al. (2020) did not include any conical-toothed taxa by way of comparison, whereas the earlier study (Wroe et al. 2013) included the conical-toothed Panthera pardus. Wroe et al. (2013) found that Thylacosmilus was better adapted to deliver a stabbing bite that was reliant on head depressing neck musculature than was Smilodon, but that both were superior in this respect to Panthera pardus. We suggest that Thylacosmilus was better adapted to deliver both neck-driven head pulling and depressing functions, but without applying a head-pull load case to a conical-toothed cat as well as to other sabre-tooths there is no mechanics-based evidence to indicate whether a capacity to deliver a head pull is a further defining feature of sabre-tooths or not. As well as FEA-based analysis, an intriguing feature of Thylacosmilus identified by Janis et al. (2020), determined on the basis of dental microwear patterns, was that Thylacosmilus did not habitually consume meat or bone. This raises the possibility that the metatherian sabre-tooth's diet was largely restricted to soft internal organs such as the heart, lungs and liver. These characteristics would make Thylacosmilus truly unique among mammalian scavengers, which typically do consume both bone and meat, as well as internal organs. However, large extant mammalian predators commonly consume these internal organs preferentially over meat and bone (Schaller 1972). We consider it more likely that Thylacosmilus may have concentrated on the internal organs of prey it had killed, rather than the carcasses of animals killed by other large South American carnivores (e.g. phorusrhacid birds), which were unlikely to have eaten only meat and/or bone and left the internal organs untouched. Sabre-tooths are characterized by relatively weak jaw closing muscles but large head depressors (Wroe et al. 2005; Christiansen 2011). Thylacosmilus is extreme with regard to both features (Wroe et al. 2013), and we contend that together with the evidence for a diet comprised largely of soft internal organs, these characteristics point to an even more extreme example of sabre-tooth killing and feeding behaviour, as opposed to an entirely divergent ecomorph and diet.

onlinelibrary.wiley.com/doi/full/10.1111/pala.12542

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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.

www.nature.com/articles/s42003-023-04624-5?utm_medium=affiliate&utm_source=commission_junction&utm_campaign=CONR_PF018_ECOM_GL_PHSS_ALWYS_DEEPLINK&utm_content=textlink&utm_term=PID100052172&CJEVENT=5b7feff3c83f11ed813c3b850a82b820