Abstract
Notoungulates are an extinct clade of South American mammals, comprising a large diversity of body sizes and skeletal morphologies, and including taxa with highly specialized dentitions. The evolutionary history of notoungulates is characterized by numerous dental convergences, such as continuous growth of both molars and incisors, which repeatedly occurred in late-diverging families to counter the effects of abrasion. The main goal of this study is to determine if the acquisition of high-crowned incisors in different notoungulate families was accompanied by significant and repeated changes in their enamel microstructure. More generally, it aims at identifying evolutionary patterns of incisor enamel microstructure in notoungulates. Fifty-eight samples of incisors encompassing 21 genera of notoungulates were sectioned to study the enamel microstructure using a scanning electron microscope. We showed that most Eocene taxa were characterized by an incisor schmelzmuster involving only radial enamel. Interestingly, derived schmelzmusters involving the presence of Hunter-Schreger bands (HSB) and of modified radial enamel occurred in all four late-diverging families, mostly in parallel with morphological specializations, such as crown height increase. Despite a high degree of homoplasy, some characters detected at different levels of enamel complexity (e.g., labial versus lingual sides, upper versus lower incisors) might also be useful for phylogenetic reconstructions. Comparisons with perissodactyls showed that notoungulates paralleled equids in some aspects related to abrasion resistance, in having evolved transverse to oblique HSB combined with modified radial enamel and high-crowned incisors.
Highlights
Notoungulates constitute an extinct clade of South American ungulates showing a wide range of skeletal and cranial shapes that are in some aspects reminiscent of rhinos, horses, and even rabbits or rodents, but for which ecological affinities need to be more accurately understood (Scott 1932, 1937; Simpson 1980; Bond et al 1995; Reguero and Prevosti 2010; Cassini and Vizcaíno 2012; Giannini and GarcíaLópez 2013; Croft 2016)
A significant crown height increase of incisors is known in some other mammalian clades, such as elephants, hippos, hyraxes, rodents, lagomorphs, or some marsupials (e.g., Koenigswald 2011; Renvoisé and Michon 2014), in which it is often viewed as a major functional innovation
Our study shows that there is a high number of convergences in the enamel microstructure and crown height of incisors in Notoungulata (Figs. 6 and 8)
Summary
Notoungulates constitute an extinct clade of South American ungulates showing a wide range of skeletal and cranial shapes that are in some aspects reminiscent of rhinos, horses, and even rabbits or rodents, but for which ecological affinities need to be more accurately understood (Scott 1932, 1937; Simpson 1980; Bond et al 1995; Reguero and Prevosti 2010; Cassini and Vizcaíno 2012; Giannini and GarcíaLópez 2013; Croft 2016). A significant crown height increase of incisors is known in some other mammalian clades, such as elephants, hippos, hyraxes, rodents, lagomorphs, or some marsupials (e.g., Koenigswald 2011; Renvoisé and Michon 2014), in which it is often viewed as a major functional innovation. As dental enamel microstructure displays highly complex patterns in mammals, it can provide information on phylogenetic affinities (Korvenkontio 1934; Koenigswald and Clemens 1992; Koenigswald et al 1993; Clemens 1997). Incisor and molar enamel microstructure have been investigated in numerous groups of hoofed mammals (Euungulata and Paenungulata; e.g., Pfretzschner 1993; Koenigswald et al 2011; Alloing-Seguier et al 2014; Tabuce et al 2017). Microstructural characteristics of the dental enamel in notoungulate incisors remain understudied, because the rare studies have focused almost exclusively on molars (Pfretzschner 1992; Maas 1997; Lindenau 2005)
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