Deciphering the mandibular shape variation in a group of Malagasy primates using Fourier outline analysis

OBJECTIVES

Among living Malagasy primates, the family Lemuridae has previously been recognized as presenting a higher mandibular morphological variation than other families. We conducted a quantitative analysis of mandibular size and shape within the five genera (Lemur, Eulemur, Hapalemur, Prolemur, and Varecia) associated with a set of covariables that could explain this variation.

MATERIALS AND METHODS

We used Fourier outline analysis on the left hemimandible of 182 specimens covering the Lemuridae family. The influence of the phylogeny but also seven covariables (genus, diet, sex, sexual behavior, mating system, ecoregion, and forest type) on mandibular variation was examined using multivariate statistics and model selection.

RESULTS

Our results indicate that the high level of morphological variation within the family, associated with a phylogenetic effect and differences in diet, is due to a strong distinction between the genera Prolemur and Hapalemur and the other genera of the family. A second analysis, correcting this strong effect, indicates that mandibular shape variation is influenced not only by the phylogeny and the diet but by a combination of all the covariables.

DISCUSSION

The analysis of morphological variation is a powerful tool with major applications, both for the estimation of biological diversity and for the understanding of the fundamental parameters of species' ecology. Our work indicates that, if mandibular shape variation is mainly driven by dietary adaptation, other variables describing ecology and habitat should be considered and taken into account for an integrative understanding of species resources and the establishment of conservation measures.

The evolution of palate shape in the Lepilemur-Cheirogaleidae clade (Primates: Strepsirrhini)

OBJECTIVES

Phylogenies consistently group the folivorous Lepilemur species with the small-bodied insectivorous-frugivorous cheirogaleids. Juvenile lepilemurs and adult cheirogaleids share allometries in most aspects of skull morphology, except the palate. We investigated potential influences on palate shape in these taxa and several outgroups using geometric morphometrics.

MATERIALS AND METHODS

Our sample included representatives of four extant strepsirrhine families, Cheirogaleidae (including Lepilemurinae), Lemuridae, Indriidae, and Galagidae, and one subfossil Megaladapis. Our dataset comprised 32 landmarks collected from 397 specimens representing 15 genera and 28 species, and was analyzed using generalized procrustes analyses and between group principal component analysis. We explored the influence of size, phylogeny, diet, and the propagation of loud vocalizations on palate shape.

RESULTS

While congeneric species clustered within the morphospace, the phylomorphospace did not mirror molecular phylogenetic hypotheses of higher-order relationships. Four palate forms were distinguished within the Cheirogaleidae. Diet, strongly linked to body size, had the single greatest influence on palate shape. The production of long-distance advertisement calls was most often associated with positive scores on the PC1 axis.

DISCUSSION

Our results suggest that the extensive variation in palate shape among Cheirogaleidae is related to dietary shifts that accompanied changes in body size during the clade's radiation. Molecular phylogenies indicate that cheirogaleid diversification involved repeated dwarfing events, which in turn drove dietary shifts from ancestral folivory-frugivory to frugivory, gummivory, and faunivory in the descendant species. The elongated Lepilemur palate is probably related to accelerated eruption of the cheek teeth to render juveniles competent to shear leaves upon weaning.

The relative efficacy of the cranium and os coxa for taxonomic assessment in macaques

OBJECTIVES

The cranium is generally considered more reliable than the postcranium for assessing primate taxonomy, although recent research suggests that pelvic shape may be equally reliable. However, little research has focused on intrageneric taxonomic discrimination. Here, we test the relative taxonomic efficacy of the cranium and os coxa for differentiating two macaque species, with and without considering sexual dimorphism.

MATERIALS AND METHODS

Geometric morphometric analyses were performed on cranial and os coxa landmarks for 119 adult Macaca fascicularis, M. mulatta, and Chlorocebus pygerythrus. Among-group shape variation was examined using canonical variates analyses. Cross-validated discriminant function analysis provided rates of correct group classification. Additionally, average morphological distances were compared with neutral genetic distances.

RESULTS

Macaque species were clearly differentiated, both cranially and pelvically, when sex was not considered. Males were more often correctly classified based on the os coxa, while female classification rates were high for both morphologies. Female crania and male os coxa were differentiated approximately the same as genetic distance, while male crania were more similar (convergent), and female os coxa were more divergent than expected based on genetic distance.

DISCUSSION

The hypothesis that cranial and os coxal shape can be used to discriminate among macaque species was supported. The cranium was better at differentiating females, while the os coxa was better at differentiating male macaques. Hence, there is no a priori reason for preferring the cranium when assessing intragenetic taxonomic relationships, but the effects of high levels of sexual dimorphism must be corrected for to accurately assess taxonomic signatures.

The Evolutionary Radiation of Hominids: a Phylogenetic Comparative Study

Over the last 150 years the diversity and phylogenetic relationships of the hominoids have been one of the main focuses in biological and anthropological research. Despite this, the study of factors involved in their evolutionary radiation and the origin of the hominin clade, a key subject for the further understanding of human evolution, remained mostly unexplored. Here we quantitatively approach these events using phylogenetic comparative methods and craniofacial morphometric data from extant and fossil hominoid species. Specifically, we explore alternative evolutionary models that allow us to gain new insights into this clade diversification process. Our results show a complex and variable scenario involving different evolutionary regimes through the hominid evolutionary radiation –modeled by Ornstein-Uhlenbeck multi-selective regime and Brownian motion multi-rate scenarios–. These different evolutionary regimes might relate to distinct ecological and cultural factors previously suggested to explain hominid evolution at different evolutionary scales along the last 10 million years.

Morphological integration affects the evolution of midline cranial base, lateral basicranium, and face across primates

OBJECTIVES

The basicranium and face are two integrated bony structures displaying great morphological diversity across primates. Previous studies in hominids determined that the basicranium is composed of two independent modules: the midline basicranium, mostly influenced by brain size, and the lateral basicranium, predominantly associated with facial shape. To better assess how morphological integration impacts the evolution of primate cranial shape diversity, we test to determine whether the relationships found in hominids are retained across the order.

MATERIALS AND METHODS

Three-dimensional landmarks (29) were placed on 143 computed tomography scans of six major clades of extant primate crania. We assessed the covariation between midline basicranium, lateral basicranium, face, and endocranial volume using phylogenetically informed partial least squares analyses and phylogenetic generalized least squares models.

RESULTS

We found significant integration between lateral basicranium and face and between midline basicranium and face. We also described a significant correlation between midline basicranium and endocranial volume but not between lateral basicranium and endocranial volume.

DISCUSSION

Our findings demonstrate a significant and pervasive integration in the craniofacial structures across primates, differing from previous results in hominids. The uniqueness of module organization in hominids may explain this distinction. We found that endocranial volume is significantly integrated to the midline basicranium but not to the lateral basicranium. This finding underlines the significant effect of brain size on the shape of the midline structures of the cranial base in primates. With the covariations linking the studied features defined here, we suggest that future studies should focus on determining the causal links between them.

Cranial and endocranial diversity in extant and fossil atelids (Platyrrhini: Atelidae): A geometric morphometric study

OBJECTIVES

Platyrrhines constitute a diverse clade, with the modern Atelidae exhibiting the most variation in cranial and endocast morphology. The processes responsible for this diversification are not well understood. Here, we present a geometric morphometric study describing variation in cranial and endocranial shape of 14 species of Alouatta, Ateles, Brachyteles, and Lagothrix and two extinct taxa, Cartelles and Caipora.

METHODS

We examined cranial and endocranial shape variation among species using images reconstructed from CT scans and geometric morphometric techniques based on three-dimensional landmarks and semilandmarks. Principal components analyses were used to explore variation, including the Procrustes shape coordinates, summing the logarithm of the Centroid Size, the common allometric component, and residual shape components.

RESULTS

Differences in endocranial shape are related to a relative increase or decrease in the volume of the neocortex region with respect to brainstem and cerebellum regions. The relative position of the brainstem varies from a posterior position in Alouatta to a more ventral position in Ateles. The shape of both the cranium and endocast of Caipora is within the observed variation of Brachyteles. Cartelles occupies the most differentiated position relative to the extant taxa, especially in regards to its endocranial shape.

CONCLUSIONS

The pattern of variation in the extant species in endocranial shape is similar to the variation observed in previous cranial studies, with Alouatta as an outlier. The similarities between Caipora and Brachyteles were unexpected and intriguing given the frugivorous adaptations inferred from the fossil's dentition. Our study shows the importance of considering both extant and fossil species when studying diversification of complex traits.

Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Mammals (class Mammalia) have evolved diverse craniofacial morphology to adapt to a wide range of ecological niches. However, the genetic and developmental mechanisms underlying the diversification of mammalian craniofacial morphology remain largely unknown. In this paper, we focus on the facial length and orofacial clefts of mammals and deduce potential mechanisms that produced diversity in mammalian facial morphology. Small-scale changes in facial morphology from the common ancestor, such as slight changes in facial length and the evolution of the midline cleft in some lineages of bats, could be attributed to heterochrony in facial bone ossification. In contrast, large-scale changes of facial morphology from the common ancestor, such as a truncated, widened face as well as the evolution of the bilateral cleft possessed by some bat species, could be brought about by changes in growth and patterning of the facial primordium (the facial processes) at the early stages of embryogenesis.