Tag Archives: Geometric morphometrics

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

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

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Eye-brain spatial relationship

We have just published a new study on the spatial relationship between visual and endocranial structures in adult modern humans, chimpanzees, and fossil humans. The survey was conducted in collaboration with Michael Masters from Montana Tech (USA), who previously hypothesized that, in modern humans, the positioning of the orbits below the frontal lobes coupled with a reduced face could result in spatial conflict among ocular, cerebral, and craniofacial structures. This could lead to vision problems, such as myopia. In addition, another study evidenced that eye and orbit dimensions have a stronger correlation with the frontal lobes, rather than with the occipital lobes, indicating that the ocular structures can be more constrained by spatial (physical) than by functional (vision) relationships. In this study we used geometric morphometrics to investigate the longitudinal (antero-posterior) spatial relationships between orbito-ocular and endocranial structures. First, we addressed the the position of the eye relatively to the frontal and temporal cortex, on a sample of 63 modern humans’ MRIs. Second, we addressed the spatial relationship between orbital and endocranial structures on a CT sample comprising 30 modern humans, 3 chimpanzees, and 3 fossil humans (Bodo, Broken Hill 1, Gibraltar 1).

The results of the MRI analysis show that in adult modern humans the main pattern of shape variation deals with the antero-posterior position of the eye relative to the temporal lobes. Individuals which eyes are closer to the temporal lobes exhibit rounder frontal outline and antero-posterior shorter eyes, indicating a possible physical constraint associated with the spatial contiguity between the eye and the middle cranial fossa. A second pattern describes the supero-inferior position of the eye, relatively to the frontal lobe. Also in this case, proximity is apparently associated with slight changes in eye form. Individuals with larger volumes of the frontal and temporal lobes tend to have eyes located more posteriorly, closer to the temporal lobe, although with no apparent change in the shape of the eye. These results partially support Master’s hypothesis, suggesting reciprocal spatial patterns influencing brain and eye form.

When analyzing orbits and braincase through CT data, the main intra-specific variation among modern humans concerns the orientation of the orbit, not the position. Nonetheless, analyzing humans, apes, and fossil hominids all together, the main differences deal with the distance between orbits and braincase: they are separated in chimps, overlapped in modern humans, and in intermediate position in fossils. In this case, fossils belong to the hypodigm of Homo heidelbergensis. Modern humans are characterized by larger temporal lobes when compared with other living primates, and longer middle cranial fossa. The proximity with the eyeballs due to face reduction can stress further a morphogenetic spatial conflict between orbits and brain. Next step: 3D analyses, ontogenetic series, and vision impairment.

Sofia Pedro


Interpolating skulls

Hideki Amano 2014During fossilization, crania are often fractured and not all the fragments are recovered. To restore the antemortem appearance of a fossil skull, different steps are required. The first process is assembling the fragments, the second process is eliminating distortions, and third process is compensating for missing parts. In this article, we consider the third step: interpolation of the missing areas. Conventionally, such interpolation is created manually with plaster as filler, and based on the knowledge and experience of skilled anthropologists. However, in order to interpolate more precisely missing surfaces in fossil crania, recently researchers have attempted to digitally interpolate missing parts in a virtual space using a composite technique involving X-ray computed tomography (CT) and computer-assisted morphology. Two methods have been proposed for such interpolation: geometric interpolation using a spline function and statistical interpolation using multivariate regression (Gunz et al., 2009). Geometric interpolation interpolates a missing part based on data mapped from a complete reference specimen using a thin-plate spline function (TPS). Specifically, the existing portion of the specimen to be interpolated is used to define a mapping function, and the corresponding portion of the complete specimen is mapped to the partial specimen in order to reconstruct the missing parts. This method yields anatomically natural and morphologically consistent interpolations of the missing parts, but the results may vary depending on the reference specimen used. On the other hand, statistical interpolation based on multivariate regression is a method to estimate missing coordinates (positions of landmarks) based on a sample of complete specimens as reference dataset. Multivariate regressions are calculated with the missing coordinates as dependent variables and the other coordinates as independent variables. These equations are then applied to predict the missing coordinates. This method can estimate the position of missing landmarks more precisely than geometric interpolation if we have enough number of reference dataset. Therefore, the number of reference dataset is the limit for this method. To compute a multivariate regression analysis, the number of the samples in the reference dataset has to exceed the number of landmark coordinates used as independent variables. However, the number of the reference specimens that can be used for analysis is often limited, although more landmark coordinates are necessary to precisely capture morphological characteristics of each cranial specimen, hindering the use of the multivariate regression analysis for the interpolation of missing parts in fossil crania. Both methods have been applied in paleoanthropology, and they succeeded to get good results (Gunz et al., 2009  Weaver and Hublin, 2009), despite some limitations. The first limitation is that the result of interpolation is always influenced by reference data. For example, if we try to interpolate missing parts of Neanderthals by using modern humans as reference sample, the interpolation will be influenced by modern human characters . At present, it is difficult to avoid this problem. A second limitation is that both methods are based on landmarks. Locating landmarks is a useful way to quantify the morphology of cranium. However, it is difficult to locate enough number of landmark on the whole skull to capture every morphological characteristics. Some methods to distribute landmark and semilandmark have been proposed (ex: sliding methods, equal spaced points), but the debate is still open. In conclusion, the existing interpolation methods are very useful and effective, but there are still some limitations which merit attention. We have to keep on developing more and more methods and techniques … I wrote a brief article about statistical interpolating methods  in this book.

Hideki Amano