Unique damage‐related, gap‐filling tooth replacement in pycnodont fishes

Abstract

Most jawed vertebrates (gnathostomes) replace their teeth throughout life (polyphyodonty) and there is currently great interest in its molecular and cellular basis, particularly in fish. While much has still to be elucidated, it appears that whichever tooth replacement mechanism is used, only one tooth replaces one predecessor, at any one time. Here we present fossil crushing dentitions of two extinct pycnodont fishes, Pycnodus zeaformis and Pycnodus maliensis. Their surface features and x‐ray micro‐CT virtual sections show no evidence of one‐for‐one replacement. Instead, individual large teeth were replaced by multiple small teeth, for which, as far as we could ascertain, there is no known mechanism. This occurred where underlying dentigerous bone was damaged. Small teeth also developed where parts of large teeth had broken off, and in gaps between large teeth created by the geometry of their close alignment in rows. We compared the virtual sections to those of functionally analogous crushing dentitions of three modern fishes. Contrasting greatly to the pycnodonts, each showed an orderly, one‐for‐one replacement, typical of osteichthyans. We propose that the pycnodont specimens exhibit a gap‐filling tooth addition hitherto unseen in gnathostomes, and that the oral epithelium retained an initiatory competence throughout life, with a programming of ‘if a gap exists, fill it’. This would also have facilitated the addition of large teeth in rows, in space provided by ontogenetic growth. We hypothesize that gaps were registered as an absence of pressure at the crushing surface, initiating tooth development, as in the modern cichlid Astatoreochromis alluaudi.

The dental system of †Kazanichthys viatkensis (Actinopterygii, Acrolepididae) from the middle Permian of European Russia: palaeobiological and palaeoecological inferences

“…Moreover, the shallow dental plates do not provide enough space for intraosseous tooth development, further supporting our interpretation of extraosseous tooth formation and complete basal resorption of the teeth in † K. viatkensis . Also, there is no evidence of gap‐filling tooth replacement in † K. viatkensis (large, damaged teeth being replaced by multiple small teeth), as was recently described for the pycnodonts, unique for this particular group of fossil neopterygians (Collins & Underwood 2021). Instead, † K. viatkensis had one‐for‐one tooth replacement.…”

“…Leuzinger et al . 2020; Collins & Underwood 2021), replacement teeth develop from a crypt inside the dentigerous bone (intraosseous). But tooth germs of † K. viatkensis primarily formed in the soft tissue outside the bone (extraosseously) and became attached to the bone when the base was developed.…”

“…The molariform teeth of † K. viatkensis are peculiar and quite different from most flat molariform teeth of pycnodonts (Kriwet 2005; Vullo et al . 2017; Collins & Underwood 2021), dapediids (Smithwick 2015), some semionotiforms (López‐Arbarello & Sferco 2011), lepisosteiforms (Leuzinger et al . 2020), sparids (El Bakary 2012; Elgendy et al .…”

Continuous tooth replacement: what can teleost fish teach us?

“…It is important to distinguish tooth replacement from tooth addition, i.e. the addition of new teeth as space is provided by the growth of the underlying jaw (Chen et al ., 2016; Collins & Underwood, 2021). The question is, how is an increase in tooth number achieved during growth?…”

The moment of tooth: rate, fate and pattern of Pacific lingcod dentition revealed by pulse-chase

“…A fundamental question in polyphyodont dentitions is whether replacement is driven by damage [9,10]-does a new tooth grow in response to a broken tooth, or does it appear because certain regions of the jaw experience greater mechanical stress [8,9,11]? This complexity is best showcased in the en bloc replacement of onequarter of the entire dental battery by piranhas [11].…”