Although sexual size dimorphism is an important component of the biology of many living New World metatherians, little is known about sexual dimorphism in extinct members of this group. Here, we describe morphometric variation in two traditionally recognized species of the paucituberculatan Acdestis from the early Miocene Santa Cruz Formation of Patagonia, A. owenii and A. spegazzinii (=A. lemairei). The results of this morphometric study show that specimens of Acdestis from the Santa Cruz Formation form a single morphological continuum, with no features that clearly distinguish the two recognized morphotypes besides overall body size and the depth of the mandibular ramus, both features which are sexually variable in extant marsupials. This suggests that the two species of Acdestis from the Santa Cruz Formation are better regarded as gender morphs of a single dimorphic species rather than two highly similar monomorphic species. The degree of sexual dimorphism inferred here for Acdestis is comparable to that seen in living sexually dimorphic marsupials. The identification of sexual dimorphism in Acdestis suggests that extinct paucituberculatans may have exhibited much greater variation in their ecology and life history than currently predicted based on extant members of this group.
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