Cretaceous Mammal Fossil Shows Lifelong Tooth Replacement, Extending Reptilian Trait into Mammalian Lineage

The primary study, published in Nature, used high-resolution micro-CT imaging on multiple jaw elements to count replacement teeth at successive stages, revealing at least five cycles in adult individuals. This polyphyodont pattern contrasts with the diphyodont condition fixed in crown mammals by the Late Jurassic, indicating that dental renewal persisted in stem lineages after the mammal-reptile split. Sample size was limited to six partial skulls, all from one locality, leaving open whether the trait was widespread or clade-specific.

Contextual analysis links this to broader patterns in early mammal diversification, where fossils like Morganucodon already show reduced replacement, yet Origolestes retained reptile-grade renewal while evolving mammalian jaw joints. The finding challenges strict dichotomies in textbooks that tie diphyodonty directly to endothermy and lactation. It also aligns with recent genomic work on tooth-development genes showing retained ancestral regulatory pathways in monotremes.

Next steps require targeted fieldwork in additional Early Cretaceous basins and comparative developmental studies in living mammals to test whether regulatory gene reactivation could restore limited replacement. A threshold of ten independent polyphyodont fossils would substantially strengthen the inference that continuous renewal was not lost until after the divergence of crown Mammalia.