Timing of Ectocranial Suture Activity in Pan troglodytes as Related to Cranial Volume and Dental Eruption

Divergent patterns of cranial suture fusion in marsupial and placental mammals

Cranial sutures, both open and closed, support a myriad of skull functions, including redistributing strain, accommodating brain expansion, supporting cranial bone growth, and protecting the brain. Thus, variation in the degree, timing, and pattern of suture fusion has functional implications. Using a comparative ontogenetic framework across Mammalia, we quantified degree and pattern of suture fusion through ontogeny for 22 mammalian species (N = 165). Suture closure was scored on a discrete scale for 31 cranial sutures and used to calculate closure scores for individual sutures and specimens. Ancestral state estimations found the degree of ancestral marsupial fusion to be more derived, differing from both the ancestral placental and ancestral therian. The average placental pattern followed the Krogman pattern of suture fusion (cranial vault, cranial base, circum-meatal, palatal, facial, and cranio-facial), whereas marsupials showed a distinct pattern. We propose a new pattern of suture fusion for marsupials: vault, cranio-facial, facial, circum-meatal, palate, cranial base. Delayed fusion of the marsupial cranial base is hypothesized here to support prolonged postnatal growth of the marsupial brain. Collectively, our study has identified a clear marsupial-placental dichotomy in the degree, timing, and pattern of suture fusion, with implications for understanding skull function and ontogeny.

New high-resolution computed tomography data of the Taung partial cranium and endocast and their bearing on metopism and hominin brain evolution

Falk and colleagues [Falk D, Zollikofer CP, Morimoto N, Ponce de León MS (2012) Proc Natl Acad Sci U S A 109(22):8467-8470] hypothesized that selective pressures favored late persistence of a metopic suture and open anterior fontanelle early in hominin evolution, and they put an emphasis on the Taung Child (Australopithecus africanus) as evidence for the antiquity of these adaptive features. They suggested three mutually nonexclusive pressures: an "obstetric dilemma," high early postnatal brain growth rates, and neural reorganization in the frontal cortex. To test this hypothesis, we obtained the first high-resolution computed tomography (CT) data from the Taung hominin. These high-resolution image data and an examination of the hominin fossil record do not support the metopic and fontanelle features proposed by Falk and colleagues. Although a possible remnant of the metopic suture is observed in the nasion-glabella region of the Taung partial cranium (but not along the frontal crest), this character state is incongruent with the zipper model of metopic closure described by Falk and colleagues. Nor do chimpanzee and bonobo endocast data support the assertion that delayed metopic closure in Taung is necessary because of widening (reorganization) of the prefrontal or frontal cortex. These results call into question the adaptive value of delaying metopic closure, and particularly its antiquity in hominin evolution. Further data from hominoids and hominins are required to support the proposed adaptive arguments, particularly an obstetric dilemma placing constraints on neural and cranial development in Australopithecus.

Ectocranial suture fusion in primates: as related to cranial volume and dental eruption

BACKGROUND

Timing of calvarial suture fusion is important in primate ontogeny. Ages at death are difficult to assess especially for museum collections.

METHODS

1550 skulls of Hominoid, Hylobatidae, Macaca and Papio were observed for fusion. Calvarial expansion (early) and dental eruption (late) were utilized as indicators of ontogeny. Homogeneity of slopes and ANOVA were used to determine differences in timing of fusion.

RESULTS

For calvarial growth the great apes all showed small levels of calvarial suture remodeling prior to full calvarial expansion. For dental eruption, Homo and Macaca share a common pattern of fusion in late adulthood. The other species show early remodeling. Papio was observed to have distinct patterns for suture fusion progression.

CONCLUSIONS

Thus, suture fusion progression although influenced by evolutionary changes in the robusticity of the craniofacial skeleton can be modeled by the phylogeny among this group. Overall, Homo appears to have a distinct pattern of delayed suture fusion progression.

Timing of ectocranial suture activity in Gorilla gorilla as related to cranial volume and dental eruption

Research has shown that Pan and Homo have similar ectocranial suture synostosis patterns and a similar suture ontogeny (relative timing of suture fusion during the species ontogeny). This ontogeny includes patency during and after neurocranial expansion with a delayed bony response associated with adaptation to biomechanical forces generated by mastication. Here we investigate these relationships for Gorilla by examining the association among ectocranial suture morphology, cranial volume (as a proxy for neurocranial expansion) and dental development (as a proxy for the length of time that it has been masticating hard foods and exerting such strains on the cranial vault) in a large sample of Gorilla gorilla skulls. Two-hundred and fifty-five Gorilla gorilla skulls were examined for ectocranial suture closure status, cranial volume and dental eruption. Regression models were calculated for cranial volumes by suture activity, and Kendall's tau (a non-parametric measure of association) was calculated for dental eruption status by suture activity. Results suggest that, as reported for Pan and Homo, neurocranial expansion precedes suture synostosis activity. Here, Gorilla was shown to have a strong relationship between dental development and suture activity (synostosis). These data are suggestive of suture fusion extending further into ontogeny than brain expansion, similar to Homo and Pan. This finding allows for the possibility that masticatory forces influence ectocranial suture morphology.