The evolution and ontogeny of the dentition of Myotragus balearicus Bate, 1909 (Artiodactyla, Caprinae): evidence from new fossil data

Abstract

M y o t r a g u s balearicw Bate 1909 is an artiodactyl Caprinae endemic to the Balearic Islands (Spain), which became extinct more than 4000 years ago. It is characterized by a series of very unusual apomorphies acquired throughout its insular evolution, one of which is the presence of a single evergrowing incisor (with an open root) in each dentary. This incisor has been classically considered as I,. The study of recently discovered fossils of this species, which have been collected from the excavation of Holocene cave sediments in Cova Estreta (Pollenqa, Mallorca) and in Cova des Moro (Manacor, Mallorca), together with the reexamination of materials belonging both to this species and to its ancestors, allowed us to study the ontogeny and evolution of the Myot7agu.s dentition. The replacement of premolars differs only slightly from the pattern recorded in other bovids. Nevertheless, there are significant differences with other bovids regarding the incisiform series. M y o t r a g u s balearicw lacks secondary incisors. Through a neotenic process, which started during the Upper Pliocene, M. babaricw acquired a monophyodontic incisiform dentition, reducing the number of incisiforms to only one, identified as d12. The richness of the finds allows us to describe the different steps in this evolution. The only incisiiorm that appears to have been lost is dI,. The identification of the evergrowing incisor of M. balearicus as dI, reinforces its convergence with rodents postulated by Bate and there is discussion regarding homologies of incisors of rodents and lagomorphs.

First Fossil Evidence for the Advance of Replacement Teeth Coupled with Life History Evolution along an Anagenetic Mammalian Lineage

“…Contrary to the suggestion in Bover and Alcover [17], the present study provides further evidence for the successional or secondary nature of the adult evergrowing incisors of M. balearicus . Based on the CT-scan study on dental development in M. balearicus , the more lingual position within the jaw of the adult incisor germ (i1) relative to the erupted incisor (di1) lends support for its successional nature.…”

“…This tooth is present in the similarly aged specimen MNCM 39318 from Bover and Alcover [17].bVestigial tooth present in juvenile specimens.…”

“…An important argument for the primary or deciduous nature of the incisors of adult specimens of M. balearicus [17] came from embryological studies in rodents and lagomorphs that show that their large evergrowing incisors are deciduous [23]. However, incisor enlargement in mammals can occur at different dental positions and generations [22].…”

“…The second and third incisor, the canine and the third premolar are lost during anagenetic evolution of the Myotragus lineage. Their eruptive sequence is also unusual among living bovids, as the permanent incisors and the premolars erupt relatively early [13], [17]. In all living bovids the permanent first incisor appears after the eruption of the second molar and the fourth premolar is the last tooth to emerge after or subsequently to the third molar [18].…”

“…However, Bover and Alcover [17] and subsequently other authors [19] proposed that, like in rodents and lagomorphs, the evergrowing incisor of adult specimens of M. balearicus is a primary tooth, specifically the second deciduous incisor, which would explain its early emergence. In a reply, Moyà-Solà et al [20] rebutted this interpretation on the basis of the dental evolution in the Myotragus anagenetic lineage and on the standard pattern of dental eruption in extant bovids.…”