Adaptive diversity of incisor enamel microstructure in South American burrowing rodents (family Ctenomyidae, Caviomorpha)

Abstract

The aim of this study was to analyse the morphofunctional and adaptive significance of variation in the upper incisor enamel microstructure of South American burrowing ctenomyids and other octodontoid taxa. We studied the specialized subterranean tooth-digger † Eucelophorus chapalmalensis (Pliocene -Middle Pleistocene), and compared it with other fossil and living ctenomyids with disparate digging adaptations, two fossorial octodontids and one arboreal echimyid. Morphofunctionally significant enamel traits were quite similar among the species studied despite their marked differences in habits, digging behaviour and substrates occupied, suggesting a possible phylogenetic constraint for the Octodontoidea. In this context of relative similarity, the inclination of Hunter-Schreger bands, relative thickness of external index (EI) and prismless enamel zone were highest in † Eucelophorus , in agreement with its outstanding craniomandibular tooth-digging specialization. Higher inclination of Hunter-Schreger bands reinforces enamel to withstand high tension forces, while high external index provides greater resistance to wear. Results suggest increased frequency of incisor use for digging in † Eucelophorus , which could be related to a more extreme tooth-digging strategy and/or occupancy of hard soils. Higher external index values as recurring patterns in distant clades of tooth-digging rodents support an adaptive significance of this enamel trait.

Quantitative analysis of the core 2D arrangement and distribution of enamel rods in cross‐sections of mandibular mouse incisors

“…; Martin, ; Vieytes et al. ), but by far the greatest differences were reported by Moinichen et al. () relative to positional location of rows across the thickness of the inner enamel layer, that is, a decussation angle in mandibular mouse incisors of 30° near the DEJ, an angle of 60° in the middle portion of the inner enamel layer, and a decussation angle of 80° near the boundary of inner and outer enamel layers.…”

“…There has been considerable disagreement in the literature regarding the angle at which the alternating rows of rods cross one another traveling in either a mesial or lateral direction across the thickness of the inner enamel layer in rat and mouse incisors, from as little as 30° (Moinichen et al 1996) to as much as 90° (Warshawsky, 1971). As with rod incisal tilt angle, differences by species and tooth type have been noted for decussation angles in mandibular mouse incisors (Von Koenigswald, 1985;Moinichen et al 1996;Martin, 1999;Vieytes et al 2007), but by far the greatest differences were reported by Moinichen et al (1996) relative to positional location of rows across the thickness of the inner enamel layer, that is, a decussation angle in mandibular mouse incisors of 30°near the DEJ, an angle of 60°in the middle portion of the inner enamel layer, and a decussation angle of 80°near the boundary of inner and outer enamel layers. The average decussation angle observed in this study based on grand means for row tilts (Figs 4B and 8A) was comparable to a grand mean that can be computed from the range of decussation angles reported by Moinichen et al (1996): 104°À 49°= 55°vs.…”

“…This subtle bioengineering modification of forming incisally tilted sheets of enamel rods having alternated side‐to‐side angulations (laminated) achieves two clear purposes: (1) it provides a partial fracture plane along the outer enamel portions of the enamel rods that keeps the incisal tip edges sharp for gnawing and (2) it provides considerable abrasion and especially fracture resistance across the sheeted inner enamel portions and radially oriented outer enamel portions as the enamel layer is worn away by attrition at the incisal edges (Warshawsky & Smith, ; Von Koenigswald, ; Martin, ; Vieytes et al. ; Habelitz, ; Yilmaz et al. ).…”

Another one bites the dust: Bite force and ecology in three caviomorph rodents (Rodentia, Hystricognathi)

“…On the other hand, the mandibular apparatus of these rodents has been able to promote an equivalent evolutionary diversification as well. Many published analyzes on Caviomorpha support our findings, especially, by describing the morphological diversification of cranial, mandibular, and dental traits (e.g., angular process' size, zygomatic width, dental procumbency, molariforms' microstructure), and linking them to their different ecological niches, diets, social and locomotor skills, and functional potentialities, for example, for the subterranean lifestyle (see, e.g., Vassallo, '98;Fernández et al, 2000;Bacigalupe et al, 2002;Verzi, 2002;Mora et al, 2003;Mardegan Issa et al, 2007;Vieytes et al, 2007;Lessa et al, 2008;Álvarez et al, 2011). So, changes in the performance of biting force would allow the mandibular apparatus to be involved in some other behaviors (e.g., digging) beyond the strictly trophic one, which in the case of Ctenomys is closely linked to the occupation of a distinctive ecological niche.…”

The oldest South American tuco-tuco (late Pliocene, northwestern Argentina) and the boundaries of the genus Ctenomys (Rodentia, Ctenomyidae)

“…Numbers within circles internal to each node indicate relative Bremer support values resulting from evolution of 800 trees up to 31 steps longer than the optimum. (Fernández et al 2000;Verzi and Olivares 2006;Vieytes et al 2007;Lessa et al 2008;Verzi 2008).…”