Investigating the Morphogenesis and Replacement of Lamprey Toothlets Using Synchrotron Imaging.

Teeth are a key innovation that underpinned the adaptive radiation of jawed vertebrates; however, their evolutionary origin must lie with the diverse tooth-like structures of living and fossil jawless vertebrates. Most previous studies have focussed on the extinct stem-gnathostomes that phylogenetically intercalate the living jawed and jawless vertebrates. The only two extant jawless cyclostome lineages, the lampreys and hagfish bearing keratinous toothlets, have long been overlooked, though they possess complex (but unmineralised) toothlets that some have interpreted as precursors to the teeth of jawed vertebrates. Regardless of whether the toothlets of cyclostomes are homologous or convergent on the teeth of jawed vertebrates, they have the potential to offer unparalleled molecular developmental insights into the evolutionary origin of teeth. To that end, we provide a synthesis of classical literature on cyclostome toothlet structure and development, as a basis for informing future molecular studies, to which we add new insights from X-ray microtomography of three parasitic lamprey species spanning the breadth of the lamprey crown group. Based on detailed morphological analysis we describe their toothlet replacement mechanism at tissue level and uncover a relationship between toothlet size and the number of replacement cones. All examined species reveal the presence of replacement toothlets, suggesting this replacement mode is a conserved feature of the lamprey crown group. We discuss these results in comparison to hagfish, and conclude that toothlet replacement is a symplesiomorphy of cyclostomes. By describing lamprey toothlet development and replacement and comparing it with gnathostome teeth, this study lays the foundation for research into the development and evolution of teeth and tooth-like structures across vertebrate lineages.