Enamel formation and growth in non-mammalian cynodonts

MARTINELLI, AGUSTÍN G.; DIRKS, WENDY; O'MEARA, RACHEL N.

Royal Society Open Science

Royal Society of London

Año: 2018 vol. 5

The early evolution of mammals is associated with the linkedevolutionary origin of diphyodont tooth replacement, rapidjuvenile growth and determinate adult growth. However,specific relationships among these characters during nonmammalian cynodont evolution require further exploration.Here, polarized light microscopy revealed incrementallines, resembling daily laminations of extant mammals, inhistological sections of enamel in eight non-mammaliancynodont species. In the more basal non-probainognathiangroup, enamel extends extremely rapidly from cusp to cervix.By contrast, the enamel of mammaliamorphs is graduallyaccreted, with slow rates of crown extension, more typical of themajority of non-hypsodont crown mammals. These results areconsistent with the reduction in dental replacement rate acrossthe non-mammalian cynodont lineage, with greater rates ofcrown extension required in most non-probainognathians, andslower crown extension rates permitted in mammaliamorphs,which have reduced patterns of dental replacement incomparison with many non-probainognathians. The evolutionof mammal-like growth patterns, with faster juvenile growthand more abruptly terminating adult growth, is linked withthis reduction in dental replacement rates and may providean additional explanation for the observed pattern in enamelgrowth rates. It is possible that the reduction in enamelextension rates in mammaliamorphs reflects an underlyingreduction in skeletal growth rates at the time of postcanineformation, due to a more abruptly terminating patternof adult growth in these more mammal-like, crownwardspecies.