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The brain of Thylacosmilus atrox. Extinct South American saber-tooth carnivore marsupial

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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. ...
Chapter
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. ...
Thesis
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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. ...
Article
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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. ...
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