All scale bars, 5 mm. Angled arrows indicate the lingual organ, and vertical arrows indicate teeth (not all of which are marked). a, FMNH PE45419, dark stain in the base of the apparatus that may represent a remnant of the lingual organ. The base and bifurcate structure are both asymmetric; the thicker (presumably dorsal) element of the bifurcate structure occurs on the same side as the dorsal bulge in the base. Three teeth (indicated by arrows) lie within the base rather than along the bifurcate structure. Teeth are present as far as the distal-most end of the bifurcate structure. The teeth are preserved as three-dimensional moulds and casts, occasionally as pyrite infills, and are situated on raised soft tissue areas (blue arrow in bifurcate structure). b, FMNH PE28739, lingual organ and two rows of teeth associated with the ventral element of the bifurcate structure. Two teeth in each row are indicated by arrows; the rows are offset and indicated by black and yellow arrows. Teeth are preserved as moulds. c, FMNH PE31057, teeth with white kaolinite infill of negative-relief moulds. Other teeth are preserved as casts or moulds, beyond the distal-most preserved outline of the bifurcate structure. At the distal-most end, the teeth of each element of the bifurcate structure are offset (alternating), whereas for most of the length of the buccal apparatus they are not offset. The apparent asymmetry of the elements of the buccal apparatus is exaggerated as the kaolinite does not reveal the complete outline. d, A Tullimonstrum tooth in three dimensions as a slightly hooked hollow cone with a bulbous base. e–i, The three-dimensional tooth in d is digitally ‘rotated’ and ‘sliced’, to show how a two-dimensional representation of a three-dimensional structure may result in a variety of morphologies. These morphologies match those seen in preserved Tullimonstrum teeth, suggesting that the three-dimensional reconstruction in d might represent the original shape.