Morphological Variation of the Baka Pygmies’ Deciduous Second Molars

The Baka pygmies are known for their short stature resulting from a reduced growth rate during infancy. They are peculiar also for their teeth erupt earlier than in any other African population, and their posterior dentition is larger than in non-pygmies. However, the Baka’s dental morphology, as several other aspects of their biology, is still understudied. In this study we explore the Baka’s upper and lower second deciduous molars variation in comparison to a human sample including Africans, Asians, Europeans, and South Americans by means of 3D geometric morphometrics. We analyzed both the outer and inner aspect of the dental crowns, as well as their general size based on the natural logarithm of centroid sizes. Our results show largely overlapping morphologies between populations, especially for the lower molars. The distal region of the second molars is the most variable, with the Baka’s lower second molars showing the most extreme range of variation for this aspect and in terms of crown height. Upper and lower second molars showed high morphological covariation (RV=0.82). The Baka and South Americans’ second molars were confirmed among the largest in our sample. Despite of the Baka’s particular growth pattern, long-lasting isolation and extreme dental variation, it is not possible to distinguish them from other populations on the basis of their second molar morphology only.

The cryptic case of Otomys sloggetti (Sloggett’s vlei rat): Interpreting murid molar morphology in the fossil record

Vlei rats (Family: Muridae; Subfamily: Otomyinae) have a widespread distribution in southern Africa. They are favoured prey of barn and spotted eagle owls, and frequently become associated with archaeological deposits when the owls roost in cave sites. The phylogeny of several Otomyinae species is enigmatic, and Otomys sloggetti (Sloggett’s vlei rat) is no exception. This species has been referred to as the ‘ice rat’ and present distribution ranges are seemingly limited to mountainous areas, at high altitude, in Lesotho Drakensberg and the Karoo. It was thus surprising and unexpected when specimens closely resembling Otomys sloggetti (identification was based on molar morphology) were found in several archaeological sites on the south and west coasts of South Africa, and also in modern owl pellet assemblages – all extralimital to the current reported distribution. However, further examination of and comparison between these specimens, as well as extensive differences observed between comparative Otomys sloggetti specimens from museum collections, highlighted potential problems associated with the common practice of using tooth morphology to identify fossil murid species. We identified six molar morphotypes from the fossil and modern material, all of which bore a morphological resemblance to O. sloggetti. The material discussed in this paper suggests that cryptic, undescribed vlei rat species, or subspecies, have been in the past, and may yet be, co-occurring with modern populations of O. karoensis and O. irroratus. Phylogenetic studies need to be done in conjunction with morphological studies, as, currently, the relationship between the huge variation seen in interspecific morphology with genetics is little understood, different Otomys species are not always distinguishable morphologically, and considerable chromosomal polytypes have been found. Our findings highlight the need for extensive cladistic and genetic research on the Otomyinae.

Using three-dimensional geometric morphometric and dental topographic analyses to infer the systematics and paleoecology of fossil treeshrews (Mammalia, Scandentia)

Treeshrews are small, Indomalayan mammals closely related to primates. Previously, three-dimensional geometric morphometric analyses were used to assess patterns of treeshrew lower second molar morphology, which showed that the positions of molar landmarks covary with intraordinal systematics. Another analysis used dental topographic metrics to test patterns of functional dental morphology and found that molar curvature, complexity, and relief were an effective means for examining patterns of variation in treeshrew dietary ecology. Here, we build on these analyses by adding two fossil taxa, Prodendrogale yunnanica Qiu, 1986 from the Miocene of China and Ptilocercus kylin Li and Ni, 2016 from the Oligocene of China. Our results show that Pr. yunnanica had a dental bauplan more like that of a tupaiid than that of a ptilocercid, but that the extant tupaiids, including Tupaia and Dendrogale, are more similar to one another in this regard than any are to Prodendrogale. This is contrary to our expectations as Prodendrogale is hypothesized to be most closely related to Dendrogale. Ptilocercus kylin, which has been proposed to be the sister taxon of Pt. lowii Gray, 1848, is characterized by dental morphology like that of Pt. lowii in crest and cuspal position but is interpreted to have been more frugivorous. It has been claimed that Ptilocercus has undergone little morphological change through time. Our results suggest that Pt. kylin was more ecologically distinct from Pt. lowii than previously proposed, providing a glimpse into a more complex evolutionary history of the group than had been inferred.

Mesial hyperdontia in Sigmodontinae (Rodentia: Cricetidae), with comments on the evolution of the anteroconid in Myomorpha

Supernumerary teeth are common dental anomalies reported in rodents, mainly occurring distally to molars. We report the first case of mesial hyperdontia in wild-caught sigmodontine: a simplified tooth anterior to the right first lower molar in Neacomys amoenus. It affected the first molar morphology, which exhibits an underdeveloped mesial region with a reduced anterior conulid, a similar pattern observed in early known myomorph fossils, including lineages that still possess the last premolar. However, only lineages without premolar display an elongated first lower molar with a large anteroconid, as observed in extant Myomorpha. During the odontogenesis in myomorphs, the posteriormost vestigial diastemal tooth bud, located at the same locus of the last lower premolar, has its development arrested and merges with the cap of the first molar. This process might have contributed to the development of an increased anteroconid in this lineage. The abnormal Neacomys’ atavistic phenotype corroborates the hypothesis that the absorption of the primordium of the last lower premolar had played an important role in the development of first molar’s mesial region. Additionally, it also might have promoted the evolutionary transition from a reduced conulid into an enlarged anteroconid, as deduced from the fossil record and developmental evidence

Anatomic features of mandibular second molars root canals in a Yemeni population: cone-beam computed tomography

Background: This study aimed to identify and characterize root and canal morphology and the prevalence of C-shaped canals of mandibular second molars (MSMs) in a Yemeni population using cone-beam computed tomography (CBCT). Methods: CBCT images of 500 right and left MSMs with mature roots were taken from 250 Yemeni individuals and analyzed for the following features: number of roots, total number of canals orifices within the tooth and number of canals orifices in each root, type of roots, either fused or separated, shape of root in cross section, type of canal configuration in each root, presence of C-shaped canal, and primary variations in the morphology of root and canal systems. Results: Molars with separated two roots predominated (89.6%), fused two roots were 9%, separated three roots were 0.6% and one root MSMs were 0.8%. Molars with three canals orifices were the most common. Mesial root was mainly ribbon-shaped (60.5%) and distal root was mainly kidney-shaped (50.7%). Type II and Type I canal configurations were the most frequent in mesial root (56.9%) and distal root (91.3%), respectively. C-shaped canals were found in 9%. Six variants were found with variant 3 was the most common (71%). Conclusions: Yemeni permanent MSMs have mainly two separated roots. Mesial root is mostly ribbon-shaped and distal root is mainly kidney-shaped. Vertucci type II and I canal configurations showed the higher incidence in mesial and distal roots, respectively. C-shaped canals were found in 9%. Keywords: Cone-beam computed tomography, C-shaped canal, mandibular second molar, morphology, root canal, Yemen

Primitive Old World monkey from the earliest Miocene of Kenya and the evolution of cercopithecoid bilophodonty

Old World monkeys (Cercopithecoidea) are a highly successful primate radiation, with more than 130 living species and the broadest geographic range of any extant group except humans. Although cercopithecoids are highly variable in habitat use, social behavior, and diet, a signature dental feature unites all of its extant members: bilophodonty (bi: two, loph: crest, dont: tooth), or the presence of two cross-lophs on the molars. This feature offers an adaptable Bauplan that, with small changes to its individual components, permits its members to process vastly different kinds of food. Old World monkeys diverged from apes perhaps 30 million years ago (Ma) according to molecular estimates, and the molar lophs are sometimes incompletely developed in fossil species, suggesting a mosaic origin for this key adaptation. However, critical aspects of the group’s earliest evolution remain unknown because the cercopithecoid fossil record before ∼18 Ma consists of only two isolated teeth, one from Uganda and one from Tanzania. Here we describe a primitive Old World monkey from Nakwai, Kenya, dated at ∼22 Ma, that offers direct evidence for the initial key steps in the evolution of the cercopithecoid dentition. The simple dentition and absence of bilophodonty in the Nakwai monkey indicate that the initial radiation of Old World monkeys was first characterized by a reorganization of basic molar morphology, and a reliance on cusps rather than lophs suggests frugivorous diets and perhaps hard object feeding. Bilophodonty evolved later, likely in response to the inclusion of leaves in the diet.

The role of dietary competition in the origination and early diversification of North American euprimates

The conditions under which early euprimates (adapids and omomyids) originated and evolved is an area of longstanding debate. The leading hypotheses of euprimate origins promulgate diet as a core component of the early evolution of this group, despite the role of dietary competition in euprimate originations never being tested directly. This study compared three competition models (non-competition, competitive displacement, competitive coexistence) with observed patterns of dietary niche overlap, reconstructed from three-dimensional molar morphology, at the time of the euprimate radiation in North America (at the Paleocene–Eocene boundary). Overlap of reconstructed multidimensional dietary niches between euprimates and members of their guild were analysed using a modified MANOVA to establish the nature of the competitive environment surrounding euprimate origins in North America (an immigration event). Results indicated that adapids entered the mammalian guild in the absence of competition, suggesting dietary adaptations that were unique within the community. Conversely, omomyids experienced strong, but transitory, competition with nyctitheriids, suggesting that omomyids possessed the ability to out-compete this group. These results show that adapids and omomyids experienced different competitive scenarios upon their arrival (origination) in North America and confirm the significance of diet (and dietary adaptations) in euprimate origination and early diversification in mammalian communities.