Enamel formation and growth in non-mammalian cynodonts

Abstract

The early evolution of mammals is associated with the linked evolutionary origin of diphyodont tooth replacement, rapid juvenile growth and determinate adult growth. However, specific relationships among these characters during non-mammalian cynodont evolution require further exploration. Here, polarized light microscopy revealed incremental lines, resembling daily laminations of extant mammals, in histological sections of enamel in eight non-mammalian cynodont species. In the more basal non-probainognathian group, enamel extends extremely rapidly from cusp to cervix. By contrast, the enamel of mammaliamorphs is gradually accreted, with slow rates of crown extension, more typical of the majority of non-hypsodont crown mammals. These results are consistent with the reduction in dental replacement rate across the non-mammalian cynodont lineage, with greater rates of crown extension required in most non-probainognathians, and slower crown extension rates permitted in mammaliamorphs, which have reduced patterns of dental replacement in comparison with many non-probainognathians. The evolution of mammal-like growth patterns, with faster juvenile growth and more abruptly terminating adult growth, is linked with this reduction in dental replacement rates and may provide an additional explanation for the observed pattern in enamel growth rates. It is possible that the reduction in enamel extension rates in mammaliamorphs reflects an underlying reduction in skeletal growth rates at the time of postcanine formation, due to a more abruptly terminating pattern of adult growth in these more mammal-like, crownward species.

Dental histology of late Miocene hipparionins compared with extant Equus, and its implications for Equidae life history

“…The fact that the deposition of bone lamella is coupled with the Retzius line formation (Bromage et al, 2009), and that there is an endogenous rhythm controlling both functions (Bromage et al, 2016), endorses the relationship between enamel growth rates with bone growth and, then, the general somatic growth. The same idea has recently been suggested due to the discovery of lower EER in mammaliamorphs that experienced an earlier reduction of the overall skeletal growth rates (O'Meara et al, 2018). The influence of growth hormones (GH and IGF-1) and their…”

“…A N U S C R I P T extension rates of the dwarf late Turolian Spanish forms further suggest a slower pace of growth compared to H. concudense. Some authors have considered the existence of an allometric coupling of the EER with total crown height (Dirks et al, 2012) or body mass (O'Meara et al, 2018). Taking this into account, the lower EERs of the hipparionins compared to extant equids, and the lower EER of dwarf hipparionins compared to larger representatives, could represent the consequence of the allometric scaling of this parameter.…”

Histological and developmental insights into the herbivorous dentition of tapinocephalid therapsids

“…In thin section, regular incremental lines are visible in the enamel with a distance of about 6.5 μm between each line and as many as 60 lines at the thickest point (Fig 3C). We interpret these incremental lines as striae of Retzius, given their similarity to structures seen in other non-mammalian synapsids [43].…”

Influences of Domestication and Island Evolution on Dental Growth in Sheep

“…The average rate of ameloblast differentiation of the molars, i.e., the crown/enamel extension rate, averaged across cuspal and cervical enamel regions, is 150 μm d −1 in a mainland Catalonian O. aries and 80 μm d −1 in Myotragus (Jordana and Köhler 2011). The average crown extension rate of m1 in Soay sheep has been found to be 111 μm d −1 (Kierdorf et al 2013), thus lying in between the comparatively slow rate of Myotragus and the relatively fast rate of the Catalonian sheep (O'Meara et al 2018). The differences of enamel extension rates among these caprines are comparable in magnitude to interordinal differences, as exemplified by Equus with an average enamel extension rate of 178.3 μm d −1 (Nacarino-Meneses et al 2017).…”