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!
Category Archives: Skull
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.
Attendants from the Royal College of Surgeons packing up human skulls to send to the Natural History Museum in London, England, on the 1st of July, 1948.
[from The Olduvai Gorge]
Cranial foramina are small openings of the cranial bones that allow the passage of nerves and blood vessels. During ontogeny, they develop in specific locations and usually remain open throughout the whole life of an individual. Recently McGonnell and Akbareian (2018) published a review on what is currently known about these anatomical elements. The authors observed the development of the foramina in chick embryos. They discuss the possible influence of different origins of the bone tissue (mesoderm or neural crest) and type of ossification (endochondral or intramembranous) in the formation and maintenance of the foramina. They also discuss the role of nerves and blood vessels in the formation of “zones of inhibition” which probably direct the foramen development. The authors also considered the impact of some diseases and syndromes on malformation or closing of the foramina. Specifically, they mention craniosynostosis, achondroplasia, hyperostosis, sclerosis, Chiari malformation, and osteopetrosis as conditions which may lead to significant alteration of their functions. Malformation can negatively affect the functions of major nerves or blood vessels, and even cause blindness, deafness and high intracranial pressure which can be fatal. In anthropology, cranial foramina together with other intracranial traits are considered in research of biological distances, population studies, paleoanthropology, paleopathology and forensic sciences (Píšová et al. 2017). Size and location of cranial foramina are used as proxies for their respective nerves and blood vessels. However, McGonnell and Akbareian also question the reliability and validity of this approach in case of fossil samples, considering our limited knowledge on the foramina development, and the fact that they might contain more than just one element (nerve or vessel). More comprehensive studies exploring human cranial foramina and their association with vascular and neural structures are needed, as well as better understanding of their interaction within the cephalic system.
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.
Precise computational modelling of the brain-skull interface is necessary for the prediction, prevention and treatment of acquired brain-injuries. The brain-skull interface comprises complex layers including the osseous cranial tissues, meninges, sub-arachnoidal space and tissues, cerebrospinal fluid (CSF), pia mater and the gray and white cerebral matter. While the tissue properties of the brain-skull interface are known, there is no consensus on how these layers interact during head impact. To generate computational models of the brain-skull interface with greater accuracy, knowing the boundary conditions or constraints is necessary. Previous experimental studies have relied on modelling the deformation of the brain-skull interface using neural density targets (NDTs) implanted into the cadaver brain, collecting information on tissue displacement during front and rear impact in motor vehicle crash-tests.
Wang et al. (2018) utilized computational bio-mechanics and finite element analysis (FEA), placing nodes in the 3D model in close approximation to the position of the experimental NDTs. Four hypotheses of the brain-skull interface were modeled, each approach placing different boundary conditions to model deformation during simulated head impact. All analyses were validated against previous experimental studies. Results showed that how the brain-skull interface was modeled appreciably affected the results. The 3D model showing the closest agreement with the experimental data, included all tissues of the brain-skull interface, allowed for displacement without separation of the skull and brain tissues, and strongly corresponded with known neuroanatomy. This 3D model indicated that non-linear stress-strain associations between brain and skull tissues best matched experimental results. Further, this 3D model could be closely predicted using an Ogden Hyperviscoelastic Constitutive model which did not over- or under-estimate deformations during head impact. The risks of over- and under-estimating head impact during motor vehicle accidents has implications for vehicle construction and prevention of serious brain trauma during accidents. Ultimately, a better understanding of the interaction between layers of the brain-skull interface can produce more accurate predictions of the likely impact during motor vehicle accidents and prevent violent head injury. Extrapolation of this research into paleoneurology could allow investigations into the structural interaction between the brain and braincase, testing if the resistence of brain-skull tissues during deformation evolved in human species as primary adaptations or secondary adjustments such as allometric responses.
In a recent paper, Beaudet and colleagues analyze the cranial vault thickness of StW 578, a partial cranium of Australopithecus not yet assigned to a species. The authors explore the utility of cranial vault thickness and of the organization of the diploe and cortical tables as potential diagnostic criteria for hominin species. For that, they also analyze a comparative sample including other South African Late Pliocene-Early Pleistocene fossils, extant humans, and chimpanzee specimens. Fossils include specimens of Australopithecus and Paranthropus recovered from Sterkfontein, Swartkrans, and Makapansgat sites. Based on cranial landmarks, the authors defined homologous parasagittal and coronal sections on the CT scans, preferentially on the right hemisphere, which is better preserved in StW 578. The thickness of the diploe, the thickness of the inner and outer cortical tables, and the total thickness were measured automatically in various points sampled throughout the length of the sections. The proportion of each layer was computed by dividing the thickness by the surface area calculated between two successive points. Specimens that preserved only the left side were used for qualitative comparison. Results emphasize differences between Australopithecus and Paranthropus. The former genus tends to have thicker vaults, with a larger proportion of the diploic layer, while the latter tends to have thinner vaults, with a larger proportion of the inner and outer tables. The distribution of thickness also differs, as StW 578 and other Australopithecus crania from Sterkfontein display disproportionately thicker frontal and posterosuperior parietal regions, while Paranthropus (SK 46) and extant chimpanzees have thickest regions on cranial superstructures (supraorbital and occipital tori). As the authors suggest, thickening of the cranial vault in frontal and parietal regions needs further investigation, as to unveil a possible correlation between bone thickness and brain anatomy. Moreover, as the increase in thickness is associated with an increase in diploe proportions, variation in this layer might indicate physiological (thermoregulation) or biomechanical differences between Australopithecus and Paranthropus. In sum, cranial vault thickness patterns of StW 578 are equivalent to those of other specimens from Sterkfontein (StW 505 and Sts 71). The presence of a Paranthropus-like pattern in two of the three Mangapansgat specimens further indicates the presence of different morphs or species of Australopithecus in this site. This methodology and results provide a fine base for further studies on the taxonomic significance of the cranial vault thickness. The authors suggest beginning by including more Paranthropus specimens, and by evaluating chronological, geographic, and taxonomic variation.