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Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy.