Dental peculiarities in the silvery mole-rat: an original model for studying the evolutionary and biological origins of continuous dental generation in mammals.

Helder Gomes Rodrigues,Radim Šumbera

doi:10.7717/peerj.1233

Abstract

Unravelling the evolutionary and developmental mechanisms that have impacted the mammalian dentition, since more than 200 Ma, is an intricate issue. Interestingly, a few mammal species, including the silvery mole-rat Heliophobius argenteocinereus, are able to replace their dentition by the addition of supernumerary molars at the back of jaw migrating then toward the front. The aim here was to demonstrate the potential interest of further studying this rodent in order to better understand the origins of continuous dental replacement in mammals, which could also provide interesting data concerning the evolution of limited dental generation occurring in first mammals. In the present study, we described the main stages of the dental eruptive sequence in the silvery mole-rat and the associated characteristics of horizontal replacement using X-ray microtomography. This was coupled to the investigation of other African mole-rats which have no dental replacement. This method permitted to establish evidence that the initial development of the dentition in Heliophobius is comparable to what it is observed in most of African mole-rats. This rodent first has premolars, but then identical additional molars, a mechanism convergent to manatees and the pygmy rock-wallaby. Evidence of continuous replacement and strong dental dynamics were also illustrated in Heliophobius, and stressed the need to deeply investigate these aspects for evolutionary, functional and developmental purposes. We also noticed that two groups of extinct non-mammalian synapsids convergently acquired this dental mechanism, but in a way differing from extant mammals. The discussion on the diverse evolutionary origins of horizontal dental replacement put emphasis on the necessity of focusing on biological parameters potentially involved in both continuous and limited developments of teeth in mammals. In that context, the silvery mole-rat could appear as the most appropriate candidate to do so.

Highlights

Mammals differs from their extinct non-mammalian synapsid relatives by numerous anatomical, physiological and biological innovations (Kielan-Jaworowska, Cifelli & Luo, 2004; Kemp, 2005)
It would allow to compare the mechanisms involved in the continuous development of teeth in a few extant mammals, with those involved in the regression of the dental lamina, which prevents dental replacement and additional development of teeth distally in most mammals (Buchtovaet al., 2012)
The first specimen shows remnants of teeth in resorption (Fig. 2D, ID13), while the mesial most teeth are highly reduced in the second specimen (Fig. 2E, ID178)

Summary

Introduction

Mammals differs from their extinct non-mammalian synapsid relatives by numerous anatomical, physiological and biological innovations (Kielan-Jaworowska, Cifelli & Luo, 2004; Kemp, 2005). Continuous dental replacement (CDR) is very rare in mammals It was first described in manatees (Trichechus sp.; Stannius, 1845) and in the pygmy rock-wallaby (Petrogale concinna; Thomas, 1904). This mechanism is described as a continuous development of teeth at the back of the jaw, horizontally migrating toward the front, and replacing anterior cheek teeth shedding (Domning & Hayek, 1984). The development of additional teeth distally reminds of a mechanism lost more than 200 Ma ago in first mammals (e.g., Morganucodon†; Luo, Kielan-Jaworowska & Cifelli, 2004). It would permit to better understand what it happened in first mammalian dentition more than 200 Ma ago

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Conclusion