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The brain of Thylacosmilus atrox. Extinct South American saber-tooth carnivore marsupial
Abstract
Two endocasts of the extinct marsupial Thylacosmilus atrox are studied. The specimens were found in outcrops at present considered of Pliocene Age. The followings are the conclusions arrived in the study: 1) The encephalon of Thylacosmilus atrox shows a well developed neocortex, with a deep rhinal fissure and the major marsupial neocortical sulci well observable. The homologies of the sulci was certainly easy in the major part of them, and consequently, functional inferencies were possible. 2) The somatic neocortical area shows certain distinctive characteristics. It appears that the size proportions between the mandibular and maxillary areas is reversed in T. atrox, which appears congruent with the infered function of its peculiar cranial anatomy. 3) The relative brain size of T. atrox is very heigh for a Polyprotodont marsupial being its encephalization quotients and progression indexes at the level of the living Diprotodonta. 4) Thylacosmilus atrox was a very peculiar extinct carnivore marsupial, which at the Late Tertiary in South America reached higher levels of encephalization than the posterior or living representatives of its same Order.
... However, natural and artificial endocasts from some South American species have been described. For example, two endocasts of Thylacosmilus atrox (Sparassodonta) from skulls from the Pliocene of Argentina were described by Quiroga and Dozo (1988). Dozo (1994) expanded the description of the endocasts of Thylacosmilus atrox and provided a description of the cranial endocast of Borhyaena tuberata (Sparassodonta) from the Miocene of Argentina. ...
... 11.4 and 11.5). Similarly, the South American Dromiciops gliroides, the monito del monte, shows substantial gyrification and encephalization of the brain and corresponding endocast (Macrini 2006;Macrini et al. 2007a;Rowe et al. 2011) as do the endocasts of the extinct sparassodonts Thylacosmilus atrox and Borhyaena tuberata (Quiroga and Dozo 1988;Dozo 1994). Gyrification of the brain has occurred independently in multiple lineages of marsupials and stem marsupials, likely as a function of brain size and packaging within the cranial cavity (Macrini et al. 2007a;Mota and Herculano-Houzel 2015;Weisbecker et al. 2021). ...
... Similarly, subsequent descriptions of natural cranial endocasts of metatherians from South America (e.g. Quiroga and Dozo 1988;Dozo 1989Dozo , 1994 have added to the knowledge of the paleoneurology of marsupials and their closest extinct relatives. ...
The metatherians (crown-clade marsupial mammals and their fossil relatives) originated in the Late Jurassic or Early Cretaceous of Laurasia, and have since spread worldwide with diversification in South America and Australasia during the Cenozoic. Despite this long evolutionary history, paleoneurology is known for a few taxa from the Americas and Australia, with most work being published in the last 40+ years. Here, we contextualise research on metatherian paleoneurology with traditional tenets that marsupials are developmentally constrained in their brain size and advanced cognitive and sensorimotor capabilities. We summarize recent research on marsupial neuromorphology with a perspective on how these insights apply to extinct species. We describe a digital cranial endocast of the didelphid Caluromys philander to compare with endocasts of crown and stem marsupials. Although endocasts of basal metatherians morphologically resemble those of didelphids, there is significant variation in brain shape and cerebrum gyrification among marsupials, possibly due to differences in how neural tissue is distributed within limited braincase space. Lastly, we examined existing endocranial volume and body mass estimates for crown and stem marsupials. The earliest metatherians have substantially smaller relative brain sizes than recent species, although this may relate to errors in estimating metatherian mass and endocast volumes.
... Formulae of Encephalization Quotients must be changed, as suggested by Kielan-Jaworowska (1983), Martin (1981) and Quiroga and Dozo (1988), in order to make this method useful for students of relative brain size and its evolution. ...
... The proper slope was used by Kielan-Jaworoski (1983) and Quiroga and Dozo (1988 category does not separate Birds from Mammals, creates the false impression that a brain mass can be in the "bird-mammal" range, whereas in fact a relative brain size may be in the bird (table 10; figs 6-7) but not the mammal range. ...
This study proposes formulae for three new Encephalization Quotients (EQ's), one each for Reptiles (REQ's), Birds (BEQ's) and Mammals (MEQ's), to replace those of Jerison (1973), which were calculated for Higher Vertebrates (HVEQ's) and Lower Vertebrates (LVEQ's). EQ's are measures of relative brain size, and are calculated by dividing actual brain mass by the brain mass predicted for the average reference animal of its body mass. Predicted brain mass is calculated by entering body mass into the brain-body equation for a group of reference animals. Reptiles are the reference group for REQ's, birds for BEQ's, and mammals for MEQ's. The brain-body equations in this study are from much larger and more taxonomically comprehensive samples than those of Jerison (1973). These revised EQ's permit comparisons with birds distinct from mammals, which was impossible with the "Higher Vertebrates" category. REQ's are calculated for all reptiles in the study, as are BEQ's of the bird sample and MEQ's of the mammal sample. This gives ranges of relative brain size in each group, and facilitates comparison of relative brain size of other animals to reptiles, birds and mammals. All reptiles have smaller absolute brain masses than either birds or mammals of the same body mass. With relative brain size measured as REQ's, pterosaurs have the brain size expected for reptiles of their body mass, whereas Archaeopteryx has the brain size expected for both birds and mammals of its body mass. All dinosaurs have the brain mass expected for, or greater than, that expected for reptiles of their body mass, by use of the REQ method. Jerison's method, in contrast, showed some dinosaurs have LVEQ's less than those of the least encephalized Recent reptiles. Different estimates of body weight within an order of magnitude have only minor effects on determination of EQ's.
... Regarding the Cenozoic mammals of South America, a considerable number of paleoneurological studies have been carried out. They include marsupials, xenarthrans (Gervais, 1869;Dechaseaux, 1958Dechaseaux, , 1962Quiroga, 1980Quiroga, , 1988Dozo, 1987Dozo, , 1989aDozo, , 1989bDozo, , 1994aDozo, , 1994bDozo, , 1998Quiroga and Dozo, 1988;Macrini et al., 2007b), notoungulates, litopterns, rodents, and carnivorans (Simpson, 1933a(Simpson, , 1933bDechaseaux, 1962;Jerison, 1973;Radinsky, 1981;Dozo, 1997aDozo, , 1997bDozo et al., 2004). Within xenarthrans, glyptodonts are the group in which the fewest paleoneurological studies have been carried out. ...
Glyptodonts are a group of extinct xenarthrans with several anatomical features that make them one of the
most bizarre groups of mammals. By the late 19th century, some authors began to analyze the brain of Pleistocene glyptodonts using natural endocranial casts. These studies revealed the small size of the brain of the large Pleistocene forms. However, the evolution of the brain in glyptodonts and how it fits in a phylogenetic context has not been analyzed. In order to evaluate the evolution of the brain in this group, we described the first digital endocranial cast of the late Miocene glyptodont Pseudoplohophorus absolutus and compared it with digital endocranial casts of the Pleistocene glyptodonts Glyptodon, Doedicurus, and Panochthus and the extant armadillos Dasypus, Euphractus, Chaetophractus, and Zaedyus. The endocast morphology of P. absolutus is similar to that of Pleistocene glyptodonts: large olfactory bulbs, a small cerebrum with a single neocortical sulcus, and a large cerebellum. However, the relative brain size is larger than in the Pleistocene forms, with values of the encephalization quotient (EQ) close to that of extant armadillos. A comparison between xenarthrans orders shows that Cingulata (glyptodonts and armadillos) have lower EQ values than Pilosa (sloths and vermilinguas). This could possibly be related to certain restrictions and benefits imposed by the presence of the carapace in cingulates. Furthermore, because the carapace restricts the development of the cervical musculature that supports the skull, the small size of the brain in glyptodonts could be a trade-off (along with others) to reduce the weight of the skull.
... El trabajo de Ortiz Jaureguizar (1986) representa el primer antecedente de análisis integral de las comunidades de mamíferos cenozoicos sudamericanos con una aproximación paleoecológica. Asimismo estudios paleoneurológicos en marsupiales (Quiroga y Dozo, 1988; Dozo, 1989) y notoungulados (Dozo, 1997 ) neógenos permitieron evaluar las respuestas neurológicas de diferentes grupos de mamíferos extinguidos como consecuencia de sus diferentes roles ecológicos. En la década de 1990 el paleontólogo uruguayo Richard Fariña realizó dos novedosas propuestas que produjeron dispares reacciones entre la comunidad de paleomastozoólogos. ...
Ameghiniana was created 50 years ago. During this lapse, the late Cenozoic paleomastozoology developed exponentially in Argentina. Many of the papers dealed with systematics. However, fossil mammals were
also used for establish the chronological scale based on a biostratigraphic sequence of faunas mainly from the Pampean area. This scale proved valid for other South American areas. Besides, mammals were used
as proxies for understanding the continental climatic evolution and, at the same time, biogeographic stud-ies flourished. In recent years, many paleoecological and morphofunctional studies were carried out. Certainly, the last 50 years were the most productive time in Argentina since the pioneering work of Florentino Ameghino.
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In this contribution, we investigate two sparassodonts from the Sarmiento Formation (Colhuehuapian Age; Early Miocene) recovered at the Patagonian locality of Sacanana, Chubut Province, Argentina. The first specimen (MACN-Pv CH1911), identified as Sipalocyon externus, is an almost complete cranium with upper dentition. The second specimen (MACN-Pv CH40), referred to Borhyaena macrodonta, consists of a fragmentary rostral portion of the face with associated m3–m4. The cranium of S. externus was studied through µCT-scanning to investigate its internal anatomy and infer paleoecological aspects of olfaction and hearing. In general, the endocranial anatomy and encephalization quotient are like those of other metatherians. We report the presence of an accessory transverse diploic sinus that has not previously been described in marsupials. Elements of the nasal cavity (e.g., turbinals, ossified nasal septum, cribriform plate) of S. externus exhibit features that are widespread among marsupials. Sense of olfaction, as evaluated from the cribriform plate and the 3D models of the olfactory bulbs, appears to have grossly resembled that of the domestic cat, a well-researched therian proxy. The dimensions of the tympanic membrane, as estimated from the preserved ectotympanic, suggest that the optimal hearing frequency of S. externus would have been similar to that of extant carnivorous marsupials. In sum, our study suggests that S. externus was a typical small-bodied (2–3 kg) hypercarnivorous metatherian that primarily relied on vision to hunt its prey, complemented by other sensory capabilities (olfaction and hearing), in a similar manner to small felids.
Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.
Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mam-malian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.
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.
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