Myotragus balearicus, the endemic dwarf goat from the Balearic Islands, is proved again to be a suitable model to validate theories that relate life history of the species with some morphological features. In an article published today in PLOS ONE, ICP researchers demonstrate that this extinct bovid fits the Schultz's rule, which relates the growth rate of a species and the sequence of substitution of milk teeth and eruption of molars.
Most mammals have two dentitions: the deciduous or milk dentition, which erupts shortly after birth, and permanent dentition, which replaces the first one after weaning in a process that ends when all teeth have been replaced and molars have erupted. In mammals with slow life histories (late sexual maturity, long lifespan, slow growth, etc.) permanent teeth tend to erupt relatively early and simultaneously with the molars. Humans are an extreme example of this slowness, since the second molar does not appear until all the milk teeth have been replaced by permanent teeth. In contrast, in mammals with a faster pace of life, molars and replacing teeth erupt sequentially.
The relationship between the pace of life history and the sequence in the eruption of teeth is known as the Schultz’s rule and could be explained as a functional adaptation to minimize wear and work of milk teeth in species in which the growth period is long. Shultz's rule has been confirmed in most primates although in some groups, such as lemurs and tarsiers, the anterior dentition appears regardless of the pace of their life history. So, in most cases, it is possible to infer species’ life histories from the analysis of the emergence of permanent teeth and molars.
The article published today in PLOS ONE by Xavier Jordana, Marin Nekane Moratalla, Blanca Moncunill-Solé and Meike Köhler from the Institut Català de Paleontologia Miquel Crusafont (ICP) and researchers from the Institut Mediterrani d’Estudis Avançats (CSIC-UIB), confirms that Schultz's rule is also fulfilled in the case of Myotragus balearicus the endemic extinct dwarf goat from the Balearic Islands. Unlike other bovines, this goat had a slow life cycle, a common feature in many species that evolve in a context of island with food scarcity. This species survived completely isolated on Mallorca and Menorca for over five million years, from the Pliocene to the Holocene and became extinct about 3,000 years.
Several jaws of Myotragus balearicus have been analyzed and researchers have found that the sequence in which teeth emerge in this species is different from the observed in extant bovines, as both permanent incisors and premolars erupt relatively early. This would suggest that M. balearicus had a slow life history, which is consistent with other previous studies. The researchers were able to compare the pattern of emergence of teeth Myotragus kopperi, a direct ancestor of M. balearicus late Pleistocene, which has the same substitution pattern as current cattle. This would mean that the advancement of the sequence of eruption of incisors occurred during the evolution of lineage Myotragus over a period of 2.5 million years.
Limited food resources and lack of predators as a consequence of geographic isolation has led to consider islands a kind of natural laboratory and the fossil genus Myotragus has been confirmed as a suitable model for conducting studies of evolution in these environments. This species has a number of special adaptations: dwarfism (weighed only 25 kilos and had a height to its withers of 70 cm), hypsodonty (increased dental crown height) and an extremely modified lower dentition that has not been observed in any other known ruminant. Adults display a single rodent-like evergrowing incisor, a single premolar and three molars on each side of the jaw.
Some authors proposed that, like in rodents, the ever-growing incisor of adult specimens of M. balearicus could be a milk tooth. Although this would explain the early emergence of these teeth, this study shows that this fast developing incisive is an adaptation to the chronic scarcity of resources of this insular environment.
+ info: Jordana X, Marín-Moratalla N, Moncunill-Solé B, Bover P, Alcover JA, et al. (2013) First Fossil Evidence for the Advance of Replacement Teeth Coupled with Life History Evolution along an Anagenetic Mammalian Lineage. PLoS ONE 8(7): e70743. doi:10.1371/journal.pone.0070743