Abstract
Understanding the mechanisms responsible for phenotypic diversification, and the associated underlying constraints and ecological factors represents a central issue in evolutionary biology. Mammals present a wide variety of sizes and shapes, and are characterized by a high number of morphological convergences that are hypothesized to reflect similar environmental pressures. Extinct South American notoungulates evolved in isolation from northern mammalian faunas in highly disparate environments. They present a wide array of skeletal phenotypes and convergences, such as ever-growing dentition. Here, we focused on the origins of the rostral diversity of notoungulates by quantifying the shape of 26 genera using three-dimensional geometric morphometric analysis. We tested the influence of allometry and phylogeny on rostral shape and evaluated rates of evolutionary change in the different clades. We found strong allometric and phylogenetic signals concerning the rostral shape of notoungulates. Despite convergent forms, we observed a diffuse diversification of rostral shape, with no significant evidence of influence by large-scaled environmental variation. This contrasts with the increase in dental crown height that occurred in four late-diverging families in response to similar environmental pressures. These results illustrate the importance of considering both biological components and evolutionary rates to better understand some aspects of phenotypic diversity.
Highlights
During their evolutionary history, mammals underwent numerous events of diversification that produced a large variety of shapes, including spectacular examples of morphological convergence (e.g. [1,2])
The native South American ungulates have long puzzled palaeontologists, including Simpson [5], for their impressive morphological dualism: ‘on one hand, they are remarkably exotic in comparison with the fossil or recent mammals of any other continent, and on the other they parallel these mammals in many features, considered largely adaptive or secondary, in a way often amazing’
None of these studies focused on the origins and evolution of the wide range of shapes of the masticatory apparatus in the entire group, which would contribute to a better understanding of how such a morphological diversity could arise in South America
Summary
Mammals underwent numerous events of diversification that produced a large variety of shapes, including spectacular examples of morphological convergence (e.g. [1,2]). Late-diverging families of notoungulates (Toxodontidae, Interatheriidae, Hegetotheriidae, Mesotheriidae) present some morphological and ontogenetic dental convergences including highcrowned teeth [9,10], and fast dental eruption [11] These dental innovations, which probably reflect repeated ecological and biological specializations (e.g. specialized herbivory, fast growth; [11]), largely coincide with changing environments and climates starting by the end of the Palaeogene in South America ([12,13], figure 1). It is interesting to know if this diversity, especially cranial convergences, arose in relation to similar external pressures (e.g. large-scale environmental and climatic variations), as suggested for convergent dental crown height increases [11] None of these studies focused on the origins and evolution of the wide range of shapes (e.g. convergences) of the masticatory apparatus in the entire group, which would contribute to a better understanding of how such a morphological diversity could arise in South America
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