The short-beaked echidna, Tachyglossus aculeatus, is one of five species in the mammalian order Monotremata. The other living members of the order are the three long-beaked echidnas: Zaglossus spp., as well as the platypus, Ornithorhynchus anatinus. Monotremes diverged from therian mammals approximately 187 million years ago and echidnas diverged from the platypus lineage around 55 million years ago. Until recently, it has not been possible to breed echidnas in captivity. However, there is now a successful breeding colony at Currumbin Wildlife Sanctuary (CWS) where echidna reproductive behaviour is monitored under continuous video recording. This has enabled us, for the first time, to obtain precisely staged young throughout egg incubation and pouch life. This would be extremely challenging in the wild because echidnas lay their eggs and then incubate them in their pouch while hidden in a burrow. There has been a reduction in the tooth roots and molar numbers of the monotremes in the fossil record with the development of hardened keratinised palatal spines, which aid in mastication in modern monotremes. A monotreme fossil Steropodon (85 MYA) had few teeth, showing the evolution away from tooth development and towards hardened pads. In contrast, all echidna fossils identified to date are edentulous so teeth were lost very early in this lineage. Since modern platypus nestlings still possess 3 cusped molars which are lost by adulthood (Green, 1937), the echidna is assumed to have lost cheek teeth after diverging from the platypus. Monotremes have also lost the teeth-related genes ODAM (odontogenic ameloblast-associated protein) and its derivative SCPPPQ1. However, the platypus, which still has a thin enamel layer on their cheek teeth, has retained two other related enamel genes, namely enamelin (ENAM) and ameloblastin (AMBN), but these have been lost in the echidna.

The only tooth which is known to be retained in modern echidnas is the egg tooth, which is lost some time shortly after hatching. An ancestral amniote characteristic conserved in monotremes and reptiles that distinguishes them from therian mammals is oviparity. At the time the echidna egg is laid in the pouch it is 15–17 mm in diameter and the embryo is at an early somite stage. Once the egg is laid, the female remains inactive in a burrow and forgoes food and does not defecate for the duration of the short incubation period until the egg hatches. During incubation the embryo develops into a fetus and the young hatches from the egg after only 10–10.5 days. The monotreme eggshell is leathery, porous and consists of loosely wound keratinous fibres. Notably, monotremes develop both an egg tooth and a caruncle to escape from their egg. Tooth development in therian mammals is initiated by ligand-receptor interactions between the oral epithelium and mesenchyme during embryonic development. The tooth itself forms in different stages called the bud, cap and bell stages. During the bell stage cyto-differentiation occurs and it is here that odontoblasts and ameloblasts form, which produce dentine and enamel respectively.

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Regards,
Catherine
Journal Co-Ordinator
Journal of Orthodontics and Endodontics