Abstract
In mammals that grow up more slowly and live longer, replacement teeth tend to appear earlier in sequence than in fast growing mammals. This trend, known as ‘Schultz's Rule’, is a useful tool for inferring life histories of fossil taxa. Deviations from this rule, however, suggest that in addition to the pace of life history, ecological factors may also drive dental ontogeny. Myotragus balearicus is an extinct insular caprine that has been proved to be an excellent test case to correlate morphological traits with life history. Here we show that Myotragus balearicus exhibits a slow signature of dental eruption sequence that is in agreement with the exceptionally slow life history of this species, thus conforming to ‘Schultz's Rule’. However, our results also show an acceleration of the absolute pace of development of the permanent incisors in relation to that of the posterior teeth. The rodent-like incisors of Myotragus balearicus erupted early not only in relative but also in absolute terms (chronological age), suggesting that feeding characteristics also plays an important role in dental ontogeny. This is in agreement with ecological hypotheses based on primates. Our study documents a decoupling of the pace of development of teeth in mammals that is triggered by different selection pressures on dental ontogeny. Moreover, we show that Myotragus kopperi from the early Pleistocene (a direct ancestor of the late Pleistocene-Holocene M. balearicus) follows the pattern of first incisor replacement known in living bovids. Hence, the advance in the eruption sequence of the first incisors occurs along the Myotragus evolutionary lineage over a period of about 2.5 Myr. To our knowledge, this is the first fossil evidence of an advance of the emergence of the permanent first incisor along an anagenetic mammalian lineage.
Highlights
Most placental mammals are diphyodonts as they produce two generations of teeth: the deciduous or milk dentition, which erupts around the time of birth, and a permanent dentition, including molars and replacement teeth, that appears after weaning
We scanned a juvenile mandible of M. kopperi from the early Pleistocene, as well as a mandible of an extant domestic dwarf goat (Capra hircus)
The material of M. balearicus comes from late Pleistocene-Holocene sites of Mallorca (Cova de Moleta, Cova Estreta and Cova des Moro) and is housed in the collections at the Institut Mediterrani d’Estudis Avancats (IMEDEA), Esporles, Mallorca, and at the Museu Balear de Ciencies Naturals (MBCN), Soller, Mallorca
Summary
Most placental mammals (eutherians) are diphyodonts as they produce two generations of teeth: the deciduous or milk dentition, which erupts around the time of birth, and a permanent dentition, including molars and replacement teeth, that appears after weaning. Humans represent the extreme case of slow dental eruption signature with the second molar erupting only after all the deciduous teeth have been replaced [2]. Schultz [3] proposed that a prolonged growth period places an extra load on deciduous teeth and that species adapt by replacing them relatively early. This rule, states that teeth eruption sequence in eutherians is a functional system adapted to the rate of post-natal growth. Useful for making inferences about life histories of fossil taxa [1,2]
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