Category Archives: Morphometrics

Skulls and violins

The sound quality of string instruments depends on many factors, including the thickness of the wood that forms the resonance box. Because of the complicated (and delicate) architecture of a musical instrument, measuring that thickness can be tricky. The Hacklinger caliper is a device able to measure a distance by virtue of a magnetic field, and it is used by luthiers to check the thickness of violins and guitars. Definitely useful to musicians and, of course, to anthropologists too. Computed tomography is wonderful to measure cranial thickness, but it is expensive, time-consuming, and not always easy to employ in many museum collections. The Hacklinger caliper is cheap and portable. Irene del Olmo has coordinated this study in which we use the Hacklinger caliper to measure the distribution of cranial thickness in archaeological samples. Conclusion: it works!

Advertisements

New World Monkey skull shape

Platyrrhines (or New World Monkeys – NWM) inhabit South America and there are currently 5 families and 151 species, possessing traits not found in Catarrhines (Apes and Old World Monkeys of Africa and Asia). The NWM fossil record is fragmentary, with the earliest fossil specimens found in Argentina and dated to the middle Miocene (~ 22 million years ago), and more recent fossil remains found on the Caribbean islands and dated to the Late Pleistocene or early Holocene (~ 20-5 thousand years ago). The evolutionary relationships among living NWM and fossil species remain highly speculative. However,  Woods et al. (2018) reported the successful recovery of ancient DNA from a Jamaican fossil species Xenothrix, closely related to living species of the Callicebinae, the Titi monkeys. The continuing uncertainty surrounding NWM evolutionary history has resulted in several Caribbean fossil NWM assigned as tentative ancestral species to living howler monkeys (genus Alouatta) based on similarities of highly prognathic faces, robust crania and smaller than expected brain size or endocranial volume (ECV).

A recent study by Halenar-Price & Tallman (2019) examined cranial shape and potential correlation with ECV in three Caribbean fossil and four living NWM genera. Patterns of cranial shape were determined for each living NWM species using geometric morphometrics and, once controlling for absolute size and phylogeny, the correlation with ECV was investigated using an encephalization quotient (EQ). Results from statistical tests for a correlation between cranial shape and brain size indicated no strong support for common trend for cranial shape describing the entire NWM clade, with the overall effect of cranial shape change in living NWM only slightly associated with brain size or ECV (less than 10%). Instead, cranial shape change was very species-specific, with species often differing in cranial width, cranial base flexion and globularity of the cranial vault. The howler monkeys had the lowest association between ECV and cranial shape, while the saki monkeys (genus Pithecia), showed greater links between ECV and cranial shape change associated with seed-eating diet and presence of cranial crests.

To examine fossil NWM and the role of encephalization on cranial shape, phylogeny was accommodated and fossil NWM added to the analyzes.  Results indicated that Dominican Republic fossil NWM Antillothrix had a higher encephalization quotient (EQ) than living howler monkeys and was instead within range of titi monkeys (genus Callicebus), while Brazilian fossil NWM Cartelles was within the range of living howler monkeys. However, the Cuban fossil NWM Paralouatta was below the range of living howler monkeys. This study highlighted that the combined presence of facial prognathism, robust cranial form and smaller than expected brain size in NWM was strongly influenced by species-specific patterns related to diet, physiological and ecological adaptations, where, in very generalized terms, similarities between fossil and living new world monkeys do not necessarily indicate shared evolutionary associations.

Alannah Pearson

***

Here a recent study on the cranial morphology of howler monkeys, and an article on atelids and ethnozoology.


Face and brain

The brain is a soft-tissue organ surrounded by the bony structure of the skull, where changes in one require changes in the other. From infancy, the bones of the skull are separated by membranous sutures and with rapid brain expansion, these membranous regions of the skull are replaced by bone, fusing the skull into a protective structure around the adult brain. Ontogeny describes changes in the same anatomical structure throughout the life cycle, including the differences between age groups, within a species and across species, while allometry can explain size-related changes to skull shape, particularly between species. The individual bones of the skull join at sutures to form modules which include the facial block, the cranial vault and the cranial base.

A new paper by Scott et al. (2018) examined allometry and ontogeny in the hominid skull. The skulls from three hominid (great ape) species included the Bornean orangutan, the Western lowland gorilla and the common chimpanzee from several age groups were analyzed, and geometric morphometrics was used to capture shape change and allometry in the facial block and endocranium (as an indirect proxy for brain form). Covariation between the facial block and endocranium was tested using 2-block partial least-squares analysis. Results for ontogeny suggested endocranial change was lesser in younger age groups but with increasing age, orangutans separated from gorillas and chimpanzees, showing the greatest difference in face-to-brain shape. Results for allometry indicated that changes in facial shape were mostly related to size differences. However, the endocranium was not entirely influenced by changes in size, suggesting shape change in the endocranium is somewhat independent.

Ultimately, Scott and colleagues have shown the covariation between the facial block and the endocranium was more conserved in all three ape species in younger age groups, but the facial block continued to change shape into adulthood even after the brain growth had stopped. This suggests the endocranium is driven by changes to brain form during earlier stages of life before the cranial vault exerts a greater influence in late adolescence. However, the greatest change to skull morphology occurred during adulthood in facial shape.

Alannah Pearson


Human craniofacial evolution

In evolutionary biology, microevolution and macroevolution impact on the variation and covariation between genotype and phenotype. A related concept is the biological ability of an organism to adapt and evolve, or its evolvability, which is of keen interest to evolutionary biologists. The quantification of genetic change is analysed via the genetic variance-covariance matrix (G-matrix) while phenotypic change is analysed via the phenotypic variance-covariance (P-Matrix). Under the assumption of a neutral evolutionary model with the absence of genetic drift, the G-matrix should be proportional to a P-matrix. Although there is potential for theoretical complications arising from organisms with higher evolvability biasing the rates of evolutionary change, this is not fully investigated and seems to warrant further empirical studies.

The diversity of craniofacial form observed in fossil species of genus Homo and modern humans has been examined in terms of craniofacial adaptation to various biomechanical and environmental stressors. The absence of recovered genomes from species of fossil Homo beyond Homo neanderthalensis and fossil Homo sapiens has required studies of fossil human phylogenetics to rely on high uncertainties in the estimation of fossil hominin phylogeny and further restricted by small sample sizes.

In a recent study, Baab (2018) used the rate of evolutionary change in populations of modern Homo sapiens to estimate evolutionary rates in species of fossil Homo, analyzing craniofacial shape change, diversification and evolvability in the genus Homo. Results were consistent with independent conclusions that a neutral evolutionary model was adequate to generate the diversity in craniofacial form observed in the genus Homo. Once accounting for the small fossil sample size and the degree of evolutionary rate being higher than chance, there was no statistically significant support for higher rates of evolvability generating more rapid rates of evolutionary change across the entire genus Homo.

In contrast, the more recent lineages showed some evidence for selection acting at a greater magnitude in H. neanderthalensis and early H. sapiens, generating a more rapid rate of evolutionary change.  Baab (2018) suggests brain expansion may be a likely contributor influencing the more rapid evolutionary rate change in craniofacial shape as observed in early H. sapiens and H. neanderthalensis and why only the more recent lineages of the genus Homo were affected by such rapid changes in craniofacial form.

Alannah Pearson

 


Pulling faces

Two different papers have been published this month on the evolution of the supraorbital anatomy in humans. The first article is on Neanderthal facial morphology, and it was coordinated by Stephen Wroe, of the FEAR lab. Here a comment on the Daily Mail. The second article, by Ricardo Miguel Godinho and coauthors, links supraorbital morphology and social dynamics, and it was commented in a News and Views by Markus Bastir.


Hystrix

The journal of mammal zoology Hystrix has a new format and webpage. Here that good old 2013 volume on
Virtual Morphology and Evolutionary Morphometrics


Digital Endocasts

[Book]   [Post]


Sliding brains

Bruner E. & Ogihara N. 2018. Surfin’ endocasts: the good and the bad on brain form. Palaentologia Electronica 21.1.1A: 1-10.

[article]    [post]


Primate Cranial Complexes

The primate skull is comprised of complexes including the cranial base, vault and facial region. How these complexes respond to different developmental and growth processes as well as varied selective pressures like diet, locomotion and sexual selection have been investigated in terms of modularity and integration. The concepts of modularity and integration concern the co-variance or independence of these complexes.

Profico et al. used several recent statistical methods to test previous research conclusions suggesting the primate cranial base and facial complex are strongly integrated. The cranium from 11 extant species of the Cercopithecoidea and Hominoidea were studied utilizing geometric morphometrics to investigate shape variation, the presence of evolutionary allometry and modularity or integration.

Shape variation of the primate cranial base and facial complex was assessed by Principal Component Analysis. Among taxa, shape variation of the cranial base reflected patterns in locomotion, cranial base flexion and the size of the foramen magnum. The shape variation of the facial complex reflected size-related and sex-linked morphology, the degree of lower and mid-facial prognathism and associated changes to narrowing of the nasal-orbital regions. Evolutionary allometry was tested by multivariate regression of size on shape and indicated the facial complex but not the cranial base was influenced by evolutionary allometry. Modularity and integration was analyzed using Partial Least Squares to test the degree of co-variation between the facial complex and cranial base which proved to be low. These combined results suggested the cranial base and facial complex complied with the concept of modularity rather than integration contrasting with previous studies.

An important reminder that although a pattern of similarity was found between Pongo pygmaeus and Hylobates lar this does not imply a close biological relationship, rather these taxa share similar cranial base and facial block morphology, potentially as a by-product of orthograde posture and the absence of quadrupedalism found in the other primate taxa with the exception of modern humans which are obligate bipeds. In light of the current findings, a more comprehensive reconsideration may be necessary of the effects from variation in the facial complex and cranial base morphology throughout primate evolution.

Alannah Pearson


Howlers

Fiorenza L., Bruner E. 2017. Cranial shape variation in adult howler monkeys (Alouatta seniculus). Am. J. Primatol. [link]