Basicranium

Understanding variation in the basicranium is of central importance to paleoanthropology because of its fundamental structural role in skull development and evolution. At the beginning of the 20th century it was suggested that encephalisation plays a role in producing flexion between midline basicranial elements. It has been proposed that basicranial flexion is also influenced by the size or shape of the face. This hypothesis was further refined during the 1950s by the Swiss anatomist Biegert, who suggested that brain size and facial size act as antagonists on basicranial lexion. Biegert’s model is particularly relevant for understanding aspects of Neanderthal skull evolution because one important and unresolved problem s that these large-brained hominins have slightly less flexed basicrania than equally large-brained modern humans. We addressed this hypothesis by applying geometric morphometrics to a large comparative dataset of radiographic and/or CT images of adult nonhuman primates, hominin fossils and humans (29 species, 142 individuals). Multiple, multivariate regression and thin plate splines analyses suggest that basicranial evolution is highly significantly influenced by both brain size and face size. Our data show that in addition to brain size, the prime factor of basicranial evolution in Homo, facial size importantly influences basicranial morphology and orientation. These interactions can explain why, despite their similar brain sizes, eanderthals as well as some Mid-Pleistocene humans have less flexed cranial bases than modern humans. To gain a detailed understanding of the multifactorial inputs into basicranial flexion, future studies should also focus on the underlying factors of facial size evolution. Grant information: CGL-2006-02131 (Spanish Ministry of Science), MRTN-CT-2005-019564-EVAN, NSF: BNS-8813220.

The Toumaï cranium TM 266 is the first known from any Late Miocene African hominoid clade, and is one of the best preserved crania of any Miocene hominoid. Since its publication there has been debate in the scientific literature and discussion in the popular press over the assertion that this cranium is significant because it is the earliest known hominid 1 . The basis of the hominid assessment rests on two interpretations of the anatomy: a hominid-like, small, flat-wearing canine; and, cranial features reflecting an upright stance and bipedal locomotion. In fact, it is widely reported that the specimen is an upright hominid biped , although this is yet to be verified by independent observations and study. The history of paleoanthropology may be relevant to this assessment, because there have been similar claims for other extinct primate species. Here, we evaluate the hypothesis that Sahelanthropus (the genus TM 266 is a�ributed to) is a hominid by examining features of the canine and of the cranial base that are said to reflect canine reduction and change of function, and upright posture and bipedal locomotion. These are hominid autapomorphies and their presence or absence in late Miocene hominoids has fundamental importance for identifying the hominid clade.

In the last century, the study of the cranial base has gained much importance for paleoanthropology due to its structural and functional roles during development and evolution of the skull. Most studies have been centred on the analysis of basicranial morphology, mainly focused on the different factors that may influence cranial base flexion. Consequently diverse hypotheses have been proposed, being the neural ones the most important due to the close relationship between the brain and the basicranium and also because of the importance of encephalisation during primate and human evolution. Additionally, recent studies have shown how different parts of the cranial base are tightly related with the facial block, in the way that the anterior and middle cranial fossa (ACF and MCF) are integrated with the upper face and the midface respectively. In this context, the MCF gets special interest because of its interaction with the temporal and parietal lobes, parts of the brain which are thought to increase substantially during evolution. Several authors have suggested that evolutionary transformations of the MCF (mainly in the temporal region) may influence craniofacial morphology, affecting cranial base flexion and the facial block. Despite much research about basicranial flexion in the sagittal plane, its lateral variation is still quite unknown, being studied mostly in human-hominid samples. Here we analyse shape covariation between midline and lateral basicranial elements using geometric morphometrics in a relatively wide Cercopithecid sample (23 species, 126 individuals), combining lateral and axial X-rays images of each individual. Partial least squares and thin plate spline analysis suggest that cranial base morphology is related with variation in brain and face relative sizes. Preliminary results of ongoing research also suggest that increase in relative parietal and temporal lobes size (MCF) contributes substantially to brain size increase, so that larger MCF proportions appear to be related with flexed basicrania. These results may indicate possible structural and functional relationships that could explain the high degree of flexion observed in modern human. Future research analysing more taxa of the primate lineage, including modern humans and their ancestors, is expected to give more information about complex interactions between different craniofacial elements and its evolutionary relationships. Funding: CGL-2006-02131 (Ministerio de Ciencia e Inovacion)

In spite of a long history of research in craniofacial biology the extent to which developmental cascades between the brain, the cranial base, and the face infl uences the evolution, development and variation of the skull, is not clear. is paper discusses aspects of these problems in terms a "systemsmodel" of morphological integration and modularity of human craniofacial morphology. is model proposes analysing morphological variation and evolution of the human skull by considering ontogenetic and evolutionary modifi cations of modules by local (cranial) as well as general (non-cranial) factors, which implies an organismal view on human skull evolution. For example, the size and shape of brain lobes have local eff ects on skull shape, moulding lateral basicranial morphology. ese eff ects, which are relatively independent from midline cranial base variation, interact with the position and morphology of the mandible and the postero-superior limit of the ethmo-maxillary complex. Another local factor within the face includes changes of the size and shape of nasal cavity. However, this cranial module is functionally integrated with a more general body system, such as respiratory apparatus and functionally linked with non-cranial factors such as body energetics, size and allometry. is paper suggests that a joint consideration of local and general factors likely improves the development of holistic hypotheses about human craniofacial evolution.

The higher carnivoran taxa significantly differ in the morphology of the auditory bulla, but little is known about its non-ectotympanic elements and their contribution to phylogenetically informative bullar characters. The ventral entotympanic sinus, a principal hypotympanic compartment unique to Canidae, expands in post-ossification ontogeny from a distinct portion, rather than the whole, of what is considered the ‘caudal entotympanic’. To trace the earlier development of this sinus and to clarify the potential roles of individual entotympanics in formation of the canid auditory bulla, osteological observations were made on younger skulls of Canis lupus and four additional species. The ventral entotympanic sinus was found to invariably originate at a separate bone provisionally designated the ventral entotympanic. The rest of the ‘caudal entotympanic’ is a fusion of the posterior (or the proper) caudal entotympanic ossifying near the tympanohyal, and the anterior caudal entotympanic ossifying between the ectotympanic and rostral entotympanic. Examination of the rostral entotympanic also revealed previously unknown details. In sum, the canid auditory bulla includes at least four rather than earlier recognised two (or suspected three) entotympanics. Based on these findings, the composition of the canid intrabullar septum and the homologies of the carnivoran entotympanics are discussed. Within the established phylogenetic framework, the rostral entotympanic and posterior caudal entotympanic appear as plesiomorphic for crown-group Carnivora, while the anterior caudal entotympanic is synapomorphic for Caniformia, and the ventral entotympanic is autapomorphic for Cynoidea. This hypothesis implies that the carnivoran entotympanic patterns may have emerged before complete fusions of bullar bones observed in the fossil record.

Understanding variation in the basicranium is of central importance to paleoanthropology because of its fundamental structural role in skull development and evolution. Among primates, encephalisation is well known to be associated with flexion between midline basicranial elements, although it has been proposed that the size or shape of the face influences basicranial flexion. In particular, brain size and facial size are hypothesized to act as antagonists on basicranial flexion. One important and unresolved problem in hominin skull evolution is that large-brained Neanderthals and some Mid-Pleistocene humans have slightly less flexed basicrania than equally large-brained modern humans. To determine whether or not this is a consequence of differences in facial size, geometric morphometric methods were applied to a large comparative data set of non-human primates, hominin fossils, and humans (N ¼ 142; 29 species). Multiple multivariate regression and thin plate spline analyses suggest that basicranial evolution is highly significantly influenced by both brain size and facial size. Increasing facial size rotates the basicranium away from the face and slightly increases the basicranial angle, whereas increasing brain size reduces the angles between the spheno-occipital clivus and the presphenoid plane, as well as between the latter and the cribriform plate. These interactions can explain why Neanderthals and some Mid-Pleistocene humans have less flexed cranial bases than modern humans, despite their relatively similar brain sizes. We highlight that, in addition to brain size (the prime factor implicated in basicranial evolution in Homo), facial size is an important influence on basicranial morphology and orientation. To better address the multifactorial nature of basicranial flexion, future studies should focus on the underlying factors influencing facial size evolution in hominins.