A mechanism for tooth resorption in arthrodires

Arthrodira are the only placoderms in which teeth comparable to those of crown-group gnathostomes, with a bony base, mineralised crown, dentine, and a central pulp canal, have been confirmed (Rücklin et al., 2012). The arthrodires from the Gogo Formation exhibit a wide variety of tooth morphologies (Long & Trinajstic, 2010) in different species, such as the unusual dentition of Holonema comprising a series of vertical tubes, the cusped toothplates of Compagopiscis and Incisoscutum, multidentate crushing toothplates of Bullerichthyes and shearing blades of Eastmanosteus. However an understanding of the underlying developmental mechanism that can explain the diversity in the arthrodire dentition is lacking (Johnason & Smith, 2005). Using a combination of synchrotron, micro-CT and traditional thin sectioning we demonstrate that although there are different tooth morphologies amongst taxa, arthrodires share a common tooth developmental model. We confirm the presence of resorption surfaces and remodelling within the toothplates and show how tooth morphologies differ in adults and juveniles, suggesting different feeding strategies depending on the life stage. The level of resorption present is dependent on the ontogenetic stage of the individual, being most evident in subadults and decreasing dramatically in older individuals in which the biting division of the infragnathal comprises mainly dermal bone, the teeth having being fully resorbed.