Enamel microstructure of permanent and deciduous teeth of notoungulate Toxodon: development, functional and evolutionary implications

Abstract

Enamel microstructure is studied here in Toxodon sp., a notoungulate from the Pleistocene of South America. The material includes 13 specimens from outcrops in São Paulo and Rio Grande do Sul states, Brazil. Analyses of ground sections of upper and lower incisors, premolars, molars, and deciduous premolars by scanning electron microscopy reveal Schmelzmuster with three enamel types: modified radial enamel (MRE), associated with the enamel-dentine junction (EDJ); Hunter-Schreger bands (HSB), an intermediary layer with decussating prisms; and radial enamel (RE), a layer placed next to the outer enamel surface. Microstructural features vary in each tooth category, on the buccal and lingual sides, as well as in the different regions of each tooth. The proportion of RE increases in the occlusal area of I2, which commonly exhibits intense wear, and may be related to abrasion resistance. HSB thickness ranges from 6 to 20 prisms, with the thickest portions placed in areas with intense masticatory loads. The most concentrated packing densities of HSB in the upper incisors and lower premolars suggest these teeth bore the greatest biomechanical demands. Incisors and cheek teeth show differentiation of Schmelzmuster, suggesting dental antagonistic contact areas, as well as leading and trailing edges. Deciduous premolars exhibit enamel in the early stages of development, and a neonatal line is observed almost parallel to the EDJ, possibly related to biological stress during birth. The EDJ is scalloped in all dental categories, with varying sizes and shapes. Larger and more pronounced scallops are observed in I2, i3, p4, and in the enamel folds of the upper and lower cheek teeth, associated with microstructural features indicative of greater biomechanical demands. Microstructural enamel findings presented here corroborate morphological trends in the dentition of Notoungulata related to hypsodonty, providing greater resistance to the consumption of abrasive diets in these euhypsodont-toothed herbivores.

Developmental models shed light on the earliest dental tissues, using Astraspis as an example

“…primates (Whittaker 1978), notoungulates (Braunn et al . 2021), litopternes (Oliveira et al . 2001)).…”

Stable isotope analysis evidences dietary changes in toxodontids (Mammalia, Notoungulata) across the Neogene-Quaternary from Argentina

“…This taxon was very relevant in favor of Darwin´s evolutionary thinking and their ideas on the transmutation of species (Fernicola et al, 2009;Brinkman, 2010). Ever since the 19 th century, the representatives of Toxodontidae have been widely studied from systematic and phylogenetic perspectives (e.g., Van Frank & Simpson, 1957;Bond et al, 2006;Bonini et al, 2017;Ferrero et al, 2022); however, paleoecological analysis of these herbivorous mammals is still scarce and mostly based on anatomical features and tooth enamel microwear (e.g., Cassini et al, 2011;Braunn et al, 2021;Varela & Fariña, 2015). Concerning this, dental evidence (i.e., hypsodontia, which has traditionally been related to grazing habits in open grasslands, and tooth enamel microstructures that demonstrate increased resistance) suggests consumption of abrasive vegetation (Bond et al, 1995;Townsend & Croft, 2008;Cassini et al, 2011;Braunn et al, 2021); although some authors proposed that hypsodonty is not always associated with a grazing habit (e.g., Feranec, 2003;DeMiguel et al, 2008DeMiguel et al, , 2015Townsend & Croft, 2008).…”

“…Ever since the 19 th century, the representatives of Toxodontidae have been widely studied from systematic and phylogenetic perspectives (e.g., Van Frank & Simpson, 1957;Bond et al, 2006;Bonini et al, 2017;Ferrero et al, 2022); however, paleoecological analysis of these herbivorous mammals is still scarce and mostly based on anatomical features and tooth enamel microwear (e.g., Cassini et al, 2011;Braunn et al, 2021;Varela & Fariña, 2015). Concerning this, dental evidence (i.e., hypsodontia, which has traditionally been related to grazing habits in open grasslands, and tooth enamel microstructures that demonstrate increased resistance) suggests consumption of abrasive vegetation (Bond et al, 1995;Townsend & Croft, 2008;Cassini et al, 2011;Braunn et al, 2021); although some authors proposed that hypsodonty is not always associated with a grazing habit (e.g., Feranec, 2003;DeMiguel et al, 2008DeMiguel et al, , 2015Townsend & Croft, 2008). In recent decades, isotopic studies focused on the late Cenozoic fauna of South America have become increasingly frequent (e.g., Domingo et al, 2012Domingo et al, , 2020Dantas et al, 2013Dantas et al, , 2020Dantas et al, , 2022Bocherens et al, 2016;Pansani et al, 2019;Sanz-Pérez et al, 2022, 2024Varela et al, 2023), although still, many questions remain.…”