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1. Genetic correlations in the dental dimensions of Saguinus fuscicollis

   https://doi.org/10.1002/ajpa.23861  

   Hardin, Anna_M ( May 2019 , American Journal of Physical Anthropology)

   The objectives of this study are to describe genetic correlations between dental dimensions in a platyrrhine primate, to assess whether the brown‐mantled tamarin dentition exhibits genetic modularity by tooth type, and to discuss the relationship between body size reduction and the genetic architecture of dental traits.

   Genetic correlations were estimated for linear dental measurements, estimated crown areas, and measures of relative premolar and molar size from 302 individuals, using a pedigree of 386 individuals, with maximum likelihood variance decomposition in SOLAR.

   Genetic correlation estimates indicate strong genetic integration in the dentition of brown‐mantled tamarins, with little evidence of modularity by tooth type, within and between the maxilla and mandible. The relative molar size variable hypothesized to be genetically patterned in baboons is not significantly heritable, and relative premolar size does not meet the criteria to be considered genetically patterned in this population.

   These results demonstrate variation in the pattern of genetic correlations between dental dimensions in primates, providing evidence of evolution of the genetic architecture in the callitrichine lineage. Genetic integration of dental dimensions without modularity by tooth type, as demonstrated here, is expected to constrain dental evolution in ways that modularity would not. The role of body size reduction in the callitrichine lineage in the evolution of the genetic architecture of the dentition is discussed. Quantitative genetic analyses of dental dimensions in more primate populations will provide greater evidence of variation and evolution in the genetic architecture underlying primate dental morphology.

    

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2. Comparative dental anatomy in newborn primates: Cusp mineralization

   https://doi.org/10.1002/ar.24326  

   Paddock, Kelsey ; Zeigler, Larissa ; Harvey, Brianna ; Prufrock, Kristen A. ; Liptak, Jordan M. ; Ficorilli, Courtney M. ; Hogg, Russell T. ; Bonar, Christopher J. ; Evans, Sian ; Williams, Lawrence ; et al ( January 2020 , The Anatomical Record)

   Previous descriptive work on deciduous dentition of primates has focused disproportionately on great apes and humans. To address this bias in the literature, we studied 131 subadult nonhominoid specimens (including 110 newborns) describing deciduous tooth morphology and assessing maximum hydroxyapatite density (MHD). All specimens were CT scanned at 70 kVp and reconstructed at 20.5–39 μm voxels. Grayscale intensity from scans was converted to hydroxyapatite (HA) density (mg HA/cm³) using a linear conversion of grayscale values to calibration standards of known HA density (R²= .99). Using Amira software, mineralized dental tissues were captured by segmenting the tooth cusps first and then capturing the remainder of the teeth at descending thresholds of gray levels. We assessed the relationship of MHD of selected teeth to cranial length using Pearson correlation coefficients. In monkeys, anterior teeth are more mineralized than postcanine teeth. In tarsiers and most lemurs and lorises, postcanine teeth are the most highly mineralized. This suggests that monkeys have a more prolonged process of dental mineralization that begins with incisors and canines, while mineralization of postcanine teeth is delayed. This may in part be a result of relatively late weaning in most anthropoid primates. Results also reveal that in lemurs and lorises, MHD of the mandibular first permanent molar (M₁) negatively correlates with cranial length. In contrast, the MHD of M₁positively correlates with cranial length in monkeys. This supports the hypothesis that natural selection acts independently on dental growth as opposed to mineralization and indicates clear phylogenetic differences among primates.

    

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3. Evolutionary Integration and Modularity in the Archosaur Cranium

   https://doi.org/10.1093/icb/icz052  

   Felice, Ryan N. ; Watanabe, Akinobu ; Cuff, Andrew R. ; Noirault, Eve ; Pol, Diego ; Witmer, Lawrence M. ; Norell, Mark A. ; O'Connor, Patrick M. ; Goswami, Anjali ( May 2019 , Integrative and Comparative Biology)

   Complex structures, like the vertebrate skull, are composed of numerous elements or traits that must develop and evolve in a coordinated manner to achieve multiple functions. The strength of association among phenotypic traits (i.e., integration), and their organization into highly-correlated, semi-independent subunits termed modules, is a result of the pleiotropic and genetic correlations that generate traits. As such, patterns of integration and modularity are thought to be key factors constraining or facilitating the evolution of phenotypic disparity by influencing the patterns of variation upon which selection can act. It is often hypothesized that selection can reshape patterns of integration, parceling single structures into multiple modules or merging ancestrally semi-independent traits into a strongly correlated unit. However, evolutionary shifts in patterns of trait integration are seldom assessed in a unified quantitative framework. Here, we quantify patterns of evolutionary integration among regions of the archosaur skull to investigate whether patterns of cranial integration are conserved or variable across this diverse group. Using high-dimensional geometric morphometric data from 3D surface scans and computed tomography scans of modern birds (n = 352), fossil non-avian dinosaurs (n = 27), and modern and fossil mesoeucrocodylians (n = 38), we demonstrate that some aspects of cranial integration are conserved across these taxonomic groups, despite their major differences in cranial form, function, and development. All three groups are highly modular and consistently exhibit high integration within the occipital region. However, there are also substantial divergences in correlation patterns. Birds uniquely exhibit high correlation between the pterygoid and quadrate, components of the cranial kinesis apparatus, whereas the non-avian dinosaur quadrate is more closely associated with the jugal and quadratojugal. Mesoeucrocodylians exhibit a slightly more integrated facial skeleton overall than the other grades. Overall, patterns of trait integration are shown to be stable among archosaurs, which is surprising given the cranial diversity exhibited by the clade. At the same time, evolutionary innovations such as cranial kinesis that reorganize the structure and function of complex traits can result in modifications of trait correlations and modularity.

    

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4. THE JUVENILE CRANIUM AND UPPER DECIDUOUS DENTITION OF MEGALADAPIS MADAGASCARIENSIS (LEMUROIDEA, STRESIRRHINE, PRIMATES)

   Collins, Regan ; Borths, Matthew R ; Borrelli, Chiara ( October 2023 , Society of Vertebrate Paleontology)

   The living diversity of lemurs includes over 100 species spread across the diverse ecoregions of Madagascar. The late Pleistocene and Holocene subfossil record from Madagascar expands this diversity to include 17 extinct species, all larger than any extant lemur species. Numerous studies have explored this diversity by focusing on variation in life-history strategies in lemurs and other strepsirrhines, comparing them to haplorhine primates. In general, strepsirrhines gestate, wean, and reach sexual maturity more rapidly than haplorhines, but differences in relative brain size, body size, and ecology complicate these comparisons. Megaladapis madagascariensis is an extinct, large bodied (~46 kg) folivorous lemur that can provide an important point of reference in these studies, but early phases of M. madagascariensis ontogeny are necessary to develop these comparisons. Here, we describe a complete juvenile cranium of M. madagascariensis from Anjohibe cave in northern Madagascar. The specimen preserves the complete deciduous premolar row. MicroCT scans reveal the developing paracones of the canine, P2, and P3 are present in the crypts between the roots of the deciduous dentition. The crypt of M1 is preserved, though the crown is absent and the M1 alveoli are not fully developed. Using growth rate data collected from M. edwardsi dentition, the state of dental development in this specimen of M. madagascariensis suggests that it was less than one year-old at time of death. The entire cranium is about 55% the total length of an adult cranium from Anjohibe cave. As expected in such an immature individual, the rostrum is relatively short, post-orbital constriction is limited, and the nuchal crest is relatively small compared to the adult. This specimen also preserves an intact braincase, facilitating comparisons between the juvenile and adult endocasts from Megaladapis and extant lemurs at comparable developmental stages. Overall, the juvenile cranium supports previous observations that – adjusted for its large body mass – Megaladapis life history was consistent with the relatively accelerated life-history of other strepsirrhines. These observations provide important context and model parameters for exploring the impact of the very recent extinction of Megaladapis and other large bodied lemurs in Madagascar. 

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5. Sexual dimorphism in postcranial skeletal shape suggests male‐biased specialization for physical competition in anthropoid primates

   https://doi.org/10.1002/jmor.20980  

   Morris, Jeremy S. ; Cunningham, Christopher B. ; Carrier, David R. ( March 2019 , Journal of Morphology)

   Sexual dimorphism often arises as a response to selection on traits that improve a male's ability to physically compete for access to mates. In primates, sexual dimorphism in body mass and canine size is more common in species with intense male–male competition. However, in addition to these traits, other musculoskeletal adaptations may improve male fighting performance. Postcranial traits that increase strength, agility, and maneuverability may also be under selection. To test the hypothesis that males, as compared to females, are more specialized for physical competition in their postcranial anatomy, we compared sex‐specific skeletal shape using a set of functional indices predicted to improve fighting performance. Across species, we found significant sexual dimorphism in a subset of these indices, indicating the presence of skeletal shape sexual dimorphism in our sample of anthropoid primates. Mean skeletal shape sexual dimorphism was positively correlated with sexual dimorphism in body size, an indicator of the intensity of male–male competition, even when controlling for both body mass and phylogenetic relatedness. These results suggest that selection on male fighting ability has played a role in the evolution of postcranial sexual dimorphism in primates.

    

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