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

The DNH 7 endocast of Paranthropus robustus from Drimolen, South Africa: Reconsidering the functional significance of an enlarged occipital-marginal (O/M) sinus system in robust australopithecines

This paper presents a detailed analysis of the endocast of one of the most complete Paranthropus robustus crania known, DNH 7, from the Drimolen site (South Africa), and compares it with the morphology of other australopithecine endocasts. We focus on endocranial volume, the impressions of cortical sulci, cranial sutures, and the pattern of cranial venous sinuses on the endocast. A noteworthy observation is the estimated endocranial capacity of 403 cm3, which is small for an adult Paranthropus. Fragmentary sulci identified in the frontal and temporal lobes of DNH 7 exhibit similarities with patterns observed in chimpanzees and gracile australopithecines. We observe the presence of a large remnant of an occipital-marginal sinus on DNH 7 and provide an updated table of 13 Paranthropus endocasts that are scorable for this trait, which reinforces the hypothesis that an enlarged occipital-marginal (O/M) sinus system was fixed across the three species of Paranthropus. In light of this, the possible functional significance of the occipital-marginal sinus system is reevaluated considering the ontogenetic development of cranial venous blood flow in human children. This leads us to hypothesize that the ontogenetic development of cranial blood flow in Paranthropus and Australopithecus africanus infants were different and to suggest that Taung 1 was the only A. africanus specimen known to have exhibited an enlarged O/M sinus system because it was an immature individual.

Beyond the functional matrix hypothesis: a network null model of human skull growth for the formation of bone articulations

Craniofacial sutures and synchondroses form the boundaries among bones in the human skull, providing functional, developmental and evolutionary information. Bone articulations in the skull arise due to interactions between genetic regulatory mechanisms and epigenetic factors such as functional matrices (soft tissues and cranial cavities), which mediate bone growth. These matrices are largely acknowledged for their influence on shaping the bones of the skull; however, it is not fully understood to what extent functional matrices mediate the formation of bone articulations. Aiming to identify whether or not functional matrices are key developmental factors guiding the formation of bone articulations, we have built a network null model of the skull that simulates unconstrained bone growth. This null model predicts bone articulations that arise due to a process of bone growth that is uniform in rate, direction and timing. By comparing predicted articulations with the actual bone articulations of the human skull, we have identified which boundaries specifically need the presence of functional matrices for their formation. We show that functional matrices are necessary to connect facial bones, whereas an unconstrained bone growth is sufficient to connect non-facial bones. This finding challenges the role of the brain in the formation of boundaries between bones in the braincase without neglecting its effect on skull shape. Ultimately, our null model suggests where to look for modified developmental mechanisms promoting changes in bone growth patterns that could affect the development and evolution of the head skeleton.

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.

Developmental constraints on behavioural flexibility

We suggest that variation in mammalian behavioural flexibility not accounted for by current socioecological models may be explained in part by developmental constraints. From our own work, we provide examples of constraints affecting variation in behavioural flexibility, not only among individuals, but also among species and higher taxonomic units. We first implicate organizational maternal effects of androgens in shaping individual differences in aggressive behaviour emitted by female spotted hyaenas throughout the lifespan. We then compare carnivores and primates with respect to their locomotor and craniofacial adaptations. We inquire whether antagonistic selection pressures on the skull might impose differential functional constraints on evolvability of skulls and brains in these two orders, thus ultimately affecting behavioural flexibility in each group. We suggest that, even when carnivores and primates would theoretically benefit from the same adaptations with respect to behavioural flexibility, carnivores may nevertheless exhibit less behavioural flexibility than primates because of constraints imposed by past adaptations in the morphology of the limbs and skull. Phylogenetic analysis consistent with this idea suggests greater evolutionary lability in relative brain size within families of primates than carnivores. Thus, consideration of developmental constraints may help elucidate variation in mammalian behavioural flexibility.