A new subadult partial skeleton of a pantolestid is reported from the late early Eocene Fossil Butte Member of the Green River Formation. Although the specimen displays a full permanent dentition, the presence of unfused elbow epiphyses (typically the first epiphyses to close), together with a short skull, shallow dentaries, and incompletely erupted third molars, indicates that it was a young juvenile. This adds to evidence that pantolestids had relatively early eruption of permanent premolars and late epiphyseal closure. Based on molar size and morphology, including presence of a well-developed paraconid, the specimen is attributed to Palaeosinopa didelphoides. Despite differences in skeletal size and robustness among the three known Fossil Butte pantolestid skeletons, all of which are skeletally subadult, similarity in molar size (and morphology, to the extent that it is visible) suggests that the three skeletons represent three growth stages of P. didelphoides. If this interpretation is correct, it has implications for recognition of fossil species based on such traits as jaw depth, presence/absence of diastemata, and relative robustness of skeletal features.
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