Evolutionary Implications of Dental Eruption in Dasypus (Xenarthra)

Abstract

Late eruption of the permanent dentition was recently proposed as a shared anatomical feature of endemic African mammals (Afrotheria), with anecdotal reports indicating that it is also present in dasypodids (armadillos). In order to clarify this question, and address the possiblity that late eruption is shared by afrotherians and dasypodids, we quantified the eruption of permanent teeth in Dasypus, focusing on growth series of D. hybridus and D. novemcinctus. This genus is the only known xenarthran that retains two functional generations of teeth. Its adult dentition typically consists of eight upper and eight lower ever-growing (or euhypsodont) molariforms, with no premaxillary teeth. All but the posterior-most tooth are replaced, consistent with the identification of a single molar locus in each series. Comparison of dental replacement and skull metrics reveals that most specimens reach adult size with none or few erupted permanent teeth. This pattern of growth occurring prior to the full eruption of the dentition is similar to that observed in most afrotherians. The condition observed in Dasypus and many afrotherians differs from that of most other mammals, in which the permanent dentition erupts during (not after) growth, and is complete at or near the attainment of sexual maturity and adult body size. The suture closure sequence of basicranial and postcranial epiphyses does not correlate well with dental eruption. The basal phylogenetic position of the taxon within dasypodids suggests that diphyodonty and late dental replacement represent the condition of early xenarthrans. Additionally, the inferred reduction in the number of molars to a single locus and the multiplication of premolars represent rare features for any living mammal, but may represent apomorphic characters for Dasypus.

High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths

“…This phenomenon might parallel that of the inner ear in sloths and, together with previous evidence, suggests that sloths could show high variation for many parts of their phenotype. The nesting of sloths within Xenarthra and Atlantogenata might already endow them with a relaxed level of constraint in patterning several characters as notably illustrated by the high variation of axial skeleton and dental eruption pattern reported in these clades [44,48,49]. However, as mentioned earlier, sloths (especially three-toed sloths) clearly constitute outliers within this clade for showing high level of intraspecific phenotypic variability of their SC.…”

Beyond the carapace: skull shape variation and morphological systematics of long-nosed armadillos (genusDasypus)

“…Juvenile, subadult, and adult specimens were considered in order to take into account the effect of age, size, and differential growth on the dataset. Several studies (Hensel, 1872; Russell, 1953; Ciancio et al, 2012) showed that long-nosed armadillos possess tooth replacement, as is typical for mammals, and that the eruption of permanent teeth occurs relatively late, as observed in afrotherians (Asher & Lehmann, 2008). Accordingly, we used eruption of the teeth, suture closure, and size (i.e., skull length) as criteria to identify adult specimens in our dataset.…”

“…These landmarks were inspired by previous studies performed on different mammalian taxa (Hautier, Lebrun & Cox, 2012; Hautier et al, 2014). Considering the tendency toward the reduction of the number of teeth in Dasypus (Allen, 1911), specimens often lack the last dental locus, which corresponds to a molar (Ciancio et al, 2012). In order to avoid producing an artificial shortening of the entire tooth row, we decided not to place a landmark at the end of the tooth row, as it is commonly the case, but after the last premolar locus (in both mandible and maxilla).…”

Deciduous dentition and dental eruption of Hyainailouroidea (Hyaenodonta, “Creodonta,” Placentalia, Mammalia)

“…The replacement of deciduous teeth, eruption of adult dentition, and subsequent wear contribute information on the rate of maturation across lineages (e.g., Slaughter et al, 1974, Smith, 2000Asher and Lehmann, 2008;Ciancio et al, 2012;Veitschegger and Sánchez-Villagra, 2016). Dental eruption sequences can be compared with other proxies for developmental maturity, such as long bone epiphyseal fusion (Coutinho et al, 1993;Uhen, 2000) and cranial suture fusion (Sánchez-Villagra, 2010b;Rager et al, 2013), to identify key developmental phases in extinct taxa that can be used to reconstruct life histories and phylogenetic relationships (Fink, 1982;Smith, 1989;O'Leary et al, 2013).…”