Abstract
Studying dental ontogeny in mammals can provide valuable insight on the evolution of their masticatory apparatus and their related adaptations. The multiple acquisitions of a prolonged to continuous growth of teeth in herbivorous mammals in response to high abrasion represent an intensively investigated issue. However, the ontogenetic and architectural patterns associated with these repeated dental innovations remain poorly known. Here, we focused on two case studies corresponding to distant mammalian clades, the extinct Mesotheriidae (Notoungulata), which shared some striking dental features with the extant Ctenodactylidae (Rodentia). We studied the impact of prolonged to continuous growth of molars on their occlusal complexity, their relative size and their dynamics in the jaw. We found that variations of occlusal complexity patterns are the result of paedomorphic or peramorphic heterochronic processes impacting dental crown. We showed that variations in both upper and lower molar proportions generally follow the inhibitory developmental cascade model. In that context, prolonged dental growth implies transitory adjustments due to wear, and also involves dental migration and loss when combined with molar lengthening. Interestingly, these features may be present in many mammals having prolonged dental growth, and emphasize the crucial need of considering these aspects in future evolutionary and developmental studies.
Highlights
Studies of dental development in mammals have long been interested in the growth, modes of replacement and structural morphology of teeth, in order to better understand their evolution [1,2,3,4,5,6]
The extinct clade of South American notoungulates is a suitable model to study the ontogenetic variations and structural adjustments associated with increasing crown height, inasmuch as they acquired prolonged to ever-growing dentitions several times during their evolutionary history [18,19]
It is generally thought that many rodents have teeth with enamel infoldings, whereas ungulates generally show isolated fossae [9,33]
Summary
Studies of dental development in mammals have long been interested in the growth, modes of replacement and structural morphology of teeth, in order to better understand their evolution [1,2,3,4,5,6]. The underlying developmental mechanisms and associated gene signalling pathways involved in the prolonged growth of teeth gained more attention in order to better understand how these innovations convergently appeared and evolved in different clades of mammals [10,11,12]. The study of some of these aspects in relation to the known developmental mechanisms governing prolonged to continuous dental growth could be of high interest to understand how they contribute to both homeostasis and evolution of an efficient dentition in extinct and extant mammals. The extinct clade of South American notoungulates is a suitable model to study the ontogenetic variations and structural adjustments associated with increasing crown height, inasmuch as they acquired prolonged to ever-growing dentitions several times during their evolutionary history [18,19]. The convergent increase of crown height in association with faster molar eruption recently evidenced in four main clades of notoungulates may have been influenced by these environmental pressures, and facilitated by inherent ontogenetic and structural adjustments within their masticatory apparatus, which remained to be evidenced [19]
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