Most jawed vertebrates replace their teeth throughout life (polyphyodonty) and there
is a great drive to understand the developmental basis of this mechanism. The
extreme diversity of fish dentitions offers rich opportunities for investigation. Here,
surface feature observations and X-ray micro-CT virtual sections are used to identify
tooth replacement mechanisms in fossil and modern fish, which are evaluated in
light of existing research. A consensus exists that tooth replacement requires a
‘dental lamina’; an epithelial connection between predecessor and replacement tooth,
which provides the putative stems cells required for long-term tooth renewal. This
single epithelial connection also enables only one tooth to be replaced by one
successor, at any one time. The findings herein show this is not the case in the
crushing dentitions of an extinct group of fishes, the pycnodonts. Instead, tooth
positioning suggests an opportunistic, gap-filling addition, where teeth fill space
arising from tooth damage, loss, and the geometry of neighbouring teeth.
Contrastingly, in the modern fish specimens, the mechanisms by which teeth are
regenerated are recognisable. However, the crushing dentitions of seabream show
occasional unusual change in tooth size, shape, and positioning, over one tooth
generation. These crushing dentitions, and those of two other modern specimens,
exhibit a close-packed, near-tessellating ‘anamestic’ patterning. A range of research
is drawn on to propose hypotheses for these observations. In pycnodonts, I propose
that gap-filling was enabled by the oral epithelium retaining an odontogenic potential
throughout life, possibly facilitated by stem cells that generate taste buds. I propose
that tooth positioning and morphology in pycnodont, seabream and other crushing
dentitions is an adaptive phenotypic response to mechanical strain at the crushing
surface, a known phenomenon in cichlids. I suggest that alternative sources of stem cells to predecessor teeth, and mechanoreception-mediated tooth morphology and
patterning, are promising areas for future study.