Emma C. Vieytes

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

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.

Late Miocene capybaras from Argentina: Skull anatomy, taxonomy, evolution, and biochronology

Fossil capybaras are morphologically extremely varied, but previous studies have disagreed on whether this diversity reflects intraspecific variation or the existence of multiple species. Here, we review the capybaras from the classic Argentinian Late Miocene localities of Paraná River cliffs (“conglomerado osífero” of the Ituzaingó Formation, Entre Ríos), and Chillhué and Guatraché shallow lakes (Cerro Azul Formation, La Pampa), and perform a morphometric analysis of their upper cheek teeth and the posterior portion of the rostrum. Our results confirm that all of the specimens from the “conglomerado osífero” belong to the single species Cardiatherium paranense. In addition, we refer a specimen from Tupungato (Río de los Pozos Formation, Mendoza) to C. paranense, thus expanding its geographical range. The material from La Pampa represents a different taxon, and is here preliminary referred to Cardiatherium aff. orientalis. Our systematic interpretation of Late Miocene capybaras suggests that the early radiation of this group was not as explosive as previously thought, and was likely constrained by the early acquisition of large size, increasing complexity of the cheek teeth, and probably semi-aquatic habits.

Incisor Enamel Microstructure of Paleogene Caviomorph Rodents from Contamana and Shapaja (Peruvian Amazonia)

We investigate the enamel microstructure of 37 isolated rodent incisors from several late middle Eocene and late Oligocene localities of Contamana (Loreto Department, Peruvian Amazonia), and from the early Oligocene TAR-01 locality (Shapaja, San Martín Department, Peruvian Amazonia). All incisors show an enamel internal portion with multiserial Hunter-Schreger Bands (HSB). The late middle Eocene localities of Contamana yield incisors with subtypes 1, 1–2, and 2 of multiserial HSB; TAR-01 yielded incisors with subtypes 1–2, 2, 2–3, and 3 of multiserial HSB; and the late Oligocene localities of Contamana, incisors with subtypes 1–2, 2, and 2–3 of multiserial HSB. Based on our current knowledge of the South American and African rodent fossil records and given the primitiveness of the Eocene caviomorph faunas, it may be expected that the hystricognath pioneer(s) who have colonized South America from Africa sometime during the middle Eocene, most probably had incisors that displayed a multiserial enamel with an interprismatic matrix arrangement characterizing the subtype 1 (or subtype 1 + the subtype 2 and/or the transitional 1–2) of multiserial HSB. In contrast, the derived subtypes 2–3 and 3 conditions were subsequently achieved but likely rapidly, as evidenced by its record as early as the ?late Eocene/early Oligocene (e.g., Santa Rosa, Shapaja, and La Cantera), and seemingly evolved iteratively but only in the Octodontoidea clade.