External resorption normally signals the onset of tooth replacement, and visible resorption pits do occur in virtually all other toothed amniotes 14, 17. Snakes can have multiple generations of replacement teeth forming behind each functional tooth, indicating very frequent replacement, but they show no external signs of resorption until an old tooth is about to be shed. One of the most conspicuous features of a snake’s dentition is a lack of external resorption pits along the lingual surfaces of the teeth, the absence of which also occurs in Heloderma, Lanthanotus, and Varanus 3, 5, 7, 14, 15, 16, 17, 18, 19. For this reason, we investigated the development and evolution of a well-known, but poorly understood feature of extant snake dental anatomy, their tooth replacement mode, and compared this with key snake fossils. We would therefore expect to observe changes in dental anatomy early in their evolutionary history that are present in stem- and crown-group snakes 13. However, as limb-reduced, and eventually limbless predators, novel craniodental morphologies were particularly important to the evolutionary success of early and modern snakes 11, 12. Furthermore, the fossil record of early snakes is fragmentary, represented by isolated or associated bones, which hinders our ability to confidently identify fossil snakes or establish the relative timing of the appearance of key snake features 7, 9, 10. Despite their unique bauplan, the evolutionary relationships of snakes to other lizards have been extensively debated 2, 3, 4, 5, 6, 7, 8, 9. Snakes are among the most speciose groups of reptiles, numbering over 3700 extant species 1. We then detected internal tooth resorption in the fossil snake Yurlunggur, and one of the oldest snake fossils, Portugalophis, suggesting that it is one of the earliest innovations in Pan-Serpentes, likely preceding limb loss. Internal tooth resorption is widespread in extant snakes, differs from replacement in other reptiles, and is even detectable via non-destructive μCT scanning, providing a method for identifying fossil snakes. We reveal through histological analysis that the lack of resorption pits in snakes is due to the unusual action of odontoclasts, which resorb dentine from within the pulp of the tooth. One promising candidate is their unusual mode of tooth replacement, whereby teeth are replaced without signs of external tooth resorption. Identifying features shared between extant and fossil snakes is therefore key to unraveling the early evolution of this iconic reptile group. Whether snakes evolved their elongated, limbless bodies or their specialized skulls and teeth first is a central question in squamate evolution.
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