Dental microstructure in Palaeanodon and Tubulodon (Palaeanodonta) and bioerosional tunneling as a widespread phenomenon in fossil mammal teeth

The early Eocene genus Tubulodon (Epoicotheriidae) from Wyoming is one of a few primitive genera of the Palaeanodonta that retained tooth enamel. Contemporary Palaeanodon lacked enamel, as presumably did allmetacheiromyid palaeanodonts and some derived epoicotheriids. The enamel of Tubulodon is relatively thin and therefore often transparent. Thus, unusual tubes in the orthodentine are often visible. These tubes were initially interpreted as primary dental structures, being pre-stages to the hexagonal dentinal prisms in aardvarks, and inspired the name Tubulodon. Our detailed scanning electron microscopic study showed that individual tubes are surrounded by a hypermineralized rim and enclose a cluster of much smaller elongated structures, or filaments, each residing in a small tunnel. The tubes and filaments do not belong to the natural orthodentine structure and are here interpreted as early post-mortem bioerosional phenomena. Size and morphology of the filaments affiliates them with actinomycetal bacteria. Bioerosional destruction is not unique to Tubulodon. Additional evidence that this is a widespread taphonomical phenomenon comes from numerous specimens of other early Eocene teeth from theWillwood Formation of the Bighorn Basin,Wyoming, as well as diverse mammalian taxa from different stratigraphic levels and geographic locations where we observed varying degrees of tunneling, and even substantial tube development in the dentary itself. In contrast to the orthodentine, the enamel of Tubulodon is not altered. It can be characterized as a primitive stage of radial enamel with substantial interprismatic matrix.