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Evolutionary constraints and ecological pressures influence species' morphological diversity. The aim of this study was to explore patterns of morphological variation, to investigate the influence of environmental variables on morphological differentiation, and assess the strength of phylogenetic signal for the cranium and mandible in spiny rats of the genus Trinomys. We examined 377 crania and mandibles of all species of the genus Trinomys. We used geometric morphometric methods based on two-dimensional anatomical landmarks to describe cranium and mandible shape and size. Phylogenetic signal was tested using the K statistics and associations between morphology and environmental variables were made using a phylogenetic partial least squares analysis. We found marked differences in size and shape in the synchranium among species, especially in the segregation of T. albispinus and T. yonenagae from the other species, which in turn greatly overlapped both in size and shape spaces. We found a weak phylogenetic signal for size of the synchranium and intermediate phylogenetic signals for shape, a pattern similar to other studies. Differentiation of the synchranium shape in Trinomys was associated largely with precipitation-related variables, while the association between size and environmental variables was weak. Notably, Trinomys species inhabiting xeric environments had relatively larger tympanic bullae.

Até meados da década de 1990, predominava na literatura especializada que o Novo Mundo teria sido colonizado por tres levas distintas, todas com origem no nordeste asiático. Na segunda metade da década, dois modelos alternativos começaram a desfrutar de grande popularidade entre a comunidade acadêmica internacional. O primeiro deles, denominado \"Modelo dos Dois Componentes Biológicos Principais\", baseado na variabilidade craniométrica de populações nativas americanas extintas, sugere que a América teria sido colonizada por pelo menos duas populações morfologicamente distintas vindas do nordeste asiático. O segundo, gerado por pesquisas sobre a variabilidade do DNA mitocondrial e do cromossomo Y de populações indígenas atuais, defende que o continente americano teria sido colonizado por apenas uma migração, também de origem asiática. Alguns especialistas acreditam que a compatibilização desses dois modelos é simples: as duas morfologías que se sucederam no tempo no Novo Mundo são resultado de um processo microevolutivo local, independente daquele que ocorreu, em paralelo, na Ásia. Uma outra maneira de compatibilizar os dois cenários é assumir que morfologia craniana e linhagens de DNA são entidades evolutivamente independentes, com histórias, modos, tempos e tendências próprias. Este trabalho apresenta novas evidências de que dois padrões morfológicos cranianos de fato se sucederam no Novo Mundo. Um relacionado às populações mais antigas (paleoíndias) e um relacionado a populações arcaicas e agrocerâmicas. Esses resultados são analisados à luz da discussão acima caracterizada. Until mid 1990s the prevailing model to explain the early colonization of the Americas rested on the assumption that three different migrations were involved in the process. The first migration would have given rise to most of the modern Native Americans, and is known as \"Amerind\"; the second migration would have given rise only to the Na-Dene Indians of the northern pacific; while the third would have given rise to the Eskimos and Aleuts. Known as the Three Migrations Model, the model was said to rest on convergent evidences coming from dental morphology, linguistics and the gene pool of living Native Americans. By the time the model was formulated, genetic diversity of living humans was studied by means of gene products, like serum proteins, and not by means of DNA itself. From mid 1990s on, two other models to explain the origin of Native Americans started competing with the Three Migration Model. They are known today as The Two Main Biological Components Model, and The Single Migration Model. The first one rests on the analysis of the cranial morphology of extinct and extant Native Americans along time, while the second has emerged from the study of DNA polymorphisms of living populations, mainly from mitochondrial and Y chromosome DNA sequencing. In other words, evidence coming from cranial morphology and its variation along time sustains that two Northern Asian populations entered the continent: the first one exhibiting a more generalized cranial pattern, and a second one exhibiting a more specialized architecture. On the other hand, the distribution of DNA haplogroups in modern Native American populations is easily explained by the entrance of only one mother population from Northern Asia. In this study we present new evidence that two very distinct cranial morphologies are indeed found among extinct Native Americans along time: a more generalized cranial pattern typifying the first newcomers, known in the literature as Paleoindians (12 to 8 thousand years ago), and a more specialized pattern typifying latter groups, since the Archaic period (<8 thousand years). Although exceptions to this temporal logic have already been found, the chronology proposed seems to work in the vast majority of the American continent. The results obtained are discussed under the light of those generated by Molecular Biology of extant Native Americans.

Nós investigamos as afinidades morfológicas das populações esqueletais mais antigas da Arnérica do Norte e do Sul, com base na variabilidade craniana mundial, através de uma Análise de Componentes Principais. Duas análises foram efetuadas, uma usando-se tamanho e forma e outra usando somente informação quanto a forma. Os resultados obtidos para as mulheres, em ambas as análises, não permitiram qualquer interpretação factível sobre as afinidades extra-continentais dos primeiros americanos. Quanto aos homens, a análise baseada em tamanho e forma mostrou que as séries arqueológicas americanas ocupam uma posição intermediária entre os grupos do Pacífico Sul e do Sul da Ásia/Europa. Quando a informação quanto a tamanho foi retirada da análise, as quatro séries esqueletais exibiram uma associação mais pronunciada com o Pacífico Sul. Esses resultados sugerem que tanto a América do Norte, quanto a do Sul foram, ambas, ocupadas por populações pré-mongoloides similares, cuja morfologia demonstra uma semelhança acentuada com aquela das populações do Pacífico Sul