Despite being the dominant group of South American mammalian carnivores for much of the Neogene, most post-early Miocene sparassodonts (Metatheria) are poorly known. Here, we describe new specimens of the hathliacynid sparassodont Acyon myctoderos Forasiepi et al. from the middle Miocene locality of Quebrada Honda, Bolivia, including a juvenile specimen that preserves the first known hathliacynid DP3. This specimen is unique among sparassodonts in preserving both upper and lower deciduous premolars and suggests that hathliacynids may have differed from borhyaenoid sparassodonts in having simultaneous eruption of M3 and m4 rather than M4/m4. Another specimen from Quebrada Honda tentatively assigned to A. myctoderos preserves the first known long bones (femur, tibia) of this genus, allowing us to estimate its body mass (13–17.5 kg) based on postcranial data. These new specimens document further morphological and ontogenetic variation within A. myctoderos as well as the Sparassodonta as a whole.
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