We present new data on the tooth attachment histology of the Late Cretaceous marine lizard Platecarpus (Mosasauridae). Examination of thin sections of a right dentary reveals the presence of a woven-fiber bone matrix that forms the margins and floor of the tooth alveolus; this bony matrix is traditionally identified as bone of attachment. We identify it as alveolar bone based on its histologic and topologic similarities to archosaurian and mammalian alveolar bone. We also identify a cribiform plate, a structure usually associated with the periodontal ligament. Parallel fibers present in multiple, non-resorbed generations of alveolar bone are tentatively identified as remnants of mineralized portions of collagen fiber bundles, or Sharpey’s fibers. Along the sides of the dentine root we identify a thin layer of acellular cementum. The acellular cementum is surrounded by an enormous mass of cellular cementum tissue that fills the alveolus. This cementum mass is composed of two histologically distinct forms: (1) a loosely organized cellular cementum ground matrix; (2) a laminar form surrounding the vascularization (cementeons) that we term osteocementum. Mosasaurs possess the attachment tissues that are used to diagnose thecodont ankylosis. Mosasaur thecodonty is derived within the Mosasauroidea (aigialosaurs mosasaurs).