Basicranial flexion,relative brain size,and facial kyphosis in Homo sapiens and some fossil hominids

Comparative work among nonhominid primates has demonstrated that the basicranium becomes more flexed with increasing brain size relative to basicranial length and as the -upper and lower face become more ventrally deflected (Ross and Ravosa [1993] Am. J. Phys. Anthropol. 91:305–324). In order to determine whether modern humans and fossil hominids follow these trends, the cranial base angle (measure of basicranial flexion), angle of facial kyphosis, and angle of orbital axis orientation were measured from computed tomography (CT) scans of fossil hominids (Sts 5, MLD 37/38, OH9, Kabwe) and lateral radiographs of 99 extant humans. Brain size relative to basicranial length was calculated from measures of neurocranial volume and basicranial length taken from original skulls, radiographs, CT scans, and the literature. Results of bivariate correlation analyses revealed that among modern humans basicranial flexion and brain size/basicranial length are not significantly correlated, nor are the angles of orbital axis orientation and facial kyphosis. However, basicranial flexion and orbit orientation are significantly positively correlated among the humans sampled, as are basicranial flexion and the angle of facial kyphosis. Relative to the comparative sample from Ross and Ravosa (1993), all hominids have more flexed basicrania than other primates: Archaic Homo sapiens, Homo erectus, and Australopithecus africanus do not differ significantly from Modern Homo sapiens in their degree of basicranial flexion, although they differ widely in their relative brain size. Comparison of the hominid values with those predicted by the nonhominid reduced major-axis equations reveal that, for their brain size/basicranial length, Archaic and Modern Homo sapiens have less flexed basicrania than predicted. H. erectus and A. africanus have the degree of basicranial flexion predicted by the nonhominid reduced major-axis equation. Modern humans have more ventrally deflected orbits than all other primates and, for their degree of basicranial flexion, have more ventrally deflected orbits than predicted by the regression equations for hominoids. All hominoids have more ventrally deflected orbital axes relative to their palate orientation than other primates. It is argued that hominids do not strictly obey the trend for basicranial flexion to increase with increasing relative brain size because of constraints on the amount of flexion that do not allow it to decrease much below 90°. Therefore, if basicranial flexion is a mechanism for accommodating an expanding brain among non-hominid primates, other mechanisms must be at work among hominids. © 1995 Wiley-Liss, Inc.     

Yet Another Interpretation of the Coelacanthiform Basicranial Muscle and its Innervation

Abstract A compilation of newly published information on the innervation of the basicranial muscle in the living tufttail Latimeria chalumnae and data obtained from other craniates, extant as well as extinct, suggest that the coelacanthiform basicranial musculature and the nerve supply which pertains to it are trimetameric in character rather than monometameric, as usually reported. This result supports the available data on the basic composition of the head of craniate animals and gives no support to a recently suggested homology between the tufttail basicranial muscles and the tetrapod retractor bulbi musculature.     

The unusual cranial attributes of KNM-ER 1805 and their implication for studies of sexual dimorphism inHomo habilis

A principal components analysis (PCA) of basicranial measurements (Thompson 1991) isolated KNM-ER 1805 as having the highest Principal Component (PC) score on PCI of all the fossil hominids. Two measurements with high loadings on PCI were B12 and B13 and these two measurements indicate the relative positions of the foramina ovale (FO) and infratemporal crests (IT) to the tympanic bone (TP). The object of this study was to compare the two measurements of KNM-ER 1805 with those of other early fossil hominids as well as a sample of extant hominoids. The comparison involved the raw measurements, the index of the two measurements, the coefficient of variation, and a t-test. The results of this comparison showed that KNM-ER 1805 had more forwardly placed foramina ovale than any of the comparative specimens. KNM-ER 1805 possesses a number of other unique features which differentiate it from other hominids including a persistent metopic suture, the form of the premolar roots, and the form of the asterionic region. These apparent unique features mean that KNM-ER 1805 is unlikely to represent an “average” maleHomo habilis and so is an inappropriate model for the male morph of that species.     

Mosaic Evolution of the Basicranium in <Emphasis Type="Italic">Homo</Emphasis> and its Relation to Modular Development

Mosaic evolution describes different rates of evolutionary change in different body units. Morphologically these units are described by more relationships within a unit than between different units which relates mosaic evolution with morphological integration and modularity. Recent evidence suggests mosaic evolution at the human basicranium due to different evolutionary rates of midline and lateral cranial base morphology but this hypothesis has not yet been addressed explicitly. We this hypothesis and explore how mosaic evolution relates to modular development. Evolutionary data sets on midline (N = 186) and lateral (N = 86) basicranial morphology are compared with 3D data on pre- and postnatal basicranial ontogeny (N = 71). Our results support the hypothesis of mosaic evolution and suggest a modular nature of basicranial development. Different embryological basicranial units likely became differently modified during evolution, with relatively stable midline elements and more variable lateral elements. In addition, developmental data suggests that modularity patterns change throughout ontogeny. During prenatal ontogeny lateral basicranial elements (greater sphenoid wings and petrosal pyramids) change together compared with the midline base. Close to birth the greater sphenoid wings keep a spatially stable position, while the petrosal pyramids become dissociated and shifted posteriorly. After birth the greater sphenoid wings and petrosal pyramids change again jointly and with respect to midline cranial base elements. This sequential pattern of integration and modularization and re-integration describes human basicranial ontogeny in a way that is potentially important for the understanding of evolutionary change. Phylogenetic modifications of this pattern during morphogenesis, growth, and development may underlie the mosaic evolution of the hominin basicranium.     

Significance of primate petrosal from Middle Eocene fissure-fillings at Shanghuang, Jiangsu Province, People's Republic of China

An isolated petrosal bone belonging to a diminutive primate is reported from Middle Eocene fissure-fills near Shanghuang (southern Jiangsu Province, People's Republic of China), the type locality of several newly described primates (Eosimias sinensis, a basal anthropoid; Adapoides troglodytes, a basal adapinan; Tarsius eocaenus, a congener of extant tarsiers; and Macrotarsius macrorhysis, the first Asian representative of an otherwise exclusively North American genus). Because of its fragmentary condition and unique combination of characters, the Shanghuang petrosal cannot be assigned unambiguously to any of the Shanghuang primate taxa known from dental remains. However, the possibility that the petrosal represents either an adapid or a tarsiid can be dismissed because it lacks defining basicranial apomorphines of these groups. By contrast, the element does present arterial features consistent with its being haplorhine. Deciding between the likeliest candidates for its allocation—Omomyidae and Eosimiidae—is difficult, in part because it is not known what (or even whether) basicranial characters can be used to distinguish these clades. If the Shanghuang petrosal is that of an cosimiid, as both direct and indirect evidence appears to indicate, the following implications emerge: (1) as long suspected on other grounds, anthropoids share a closer evolutionary history with Omomyidae (and Tarsiiformes) than they do with Adapidae (and Strepsirhini); (2) the specialised basicranial anatomy of extant anthropoids and their immediate cladistic relatives is derived from a primitive precursor whose otic morphology was like that of omomyids in most known respects; (3) the evolution of the defining dental and basicranial apomorphies of extant Anthropoidea has been distinctly mosaic in pattern.     

Basicranial flexion,relative brain size,and facial kyphosis in nonhuman primates

Numerous hypotheses explaining interspecific differences in the degree of basicranial flexion have been presented. Several authors have argued that an increase in relative brain size results in a spatial packing problem that is resolved by flexing the basicranium. Others attribute differences in the degree of basicranial flexion to different postural behaviors, suggesting that more orthograde animals require a ventrally flexed pre-sella basicranium in order to maintain the eyes in a correct forward-facing orientation. Less specific claims are made for a relationship between the degree of basicranial flexion and facial orientation. In order to evaluate these hypotheses, the degree of basicranial flexion (cranial base angle), palate orientation, and orbital axis orientation were measured from lateral radiographs of 68 primate species and combined with linear and volumetric measures as well as data on the size of the neocortex and telencephalon. Bivariate correlation and partial correlation analyses at several taxonomic levels revealed that, within haplorhines, the cranial base angle decreases with increasing neurocranial volume relative to basicranial length and is positively correlated with angles of facial kyphosis and orbital axis orientation. Strepsirhines show no significant correlations between the cranial base angle and any of the variables examined. It is argued that prior orbital approximation in the ancestral haplorhine integrated the medial orbital walls and pre-sella basicranium into a single structural network such that changes in the orientation of one necessarily affect the other. Gould's (“Ontogeny and Phylogeny.” Cambridge: Belknap Press, 1977) hypothesis, that the highly flexed basicranium of Homo may be due to a combination of a large brain and a relatively short basicranium, is corroborated. © 1993 Wiley-Liss, Inc.     

Gobekko cretacicus (Reptilia: Squamata) and its bearing on the interpretation of gekkotan affinities

Gobekko cretacicus, a Cretaceous lizard from the Gobi Desert of Mongolia, is a key fossil for understanding gecko phylogeny. We revisit this fossil using high‐resolution X‐ray computed tomography. The application of this imaging method reveals new information about sutures, bone shape, and structural details of the palate and basicranium. These data were used to assess the phylogenetic affinities of Gobekko in the context of an existing squamate data set. The effects of character ordering, search strategy, and the addition of another putative gekkonomorph (Hoburogekko suchanovi) on inferred gekkonomorph relationships were explored. Available specimens of G. cretacicus are skeletally mature but have unfused nasals, frontals, and parietals, and (possibly) a persistent basicranial fenestra. Some putative gekkonomorphs are not consistently supported as closer to crown clade gekkotans than to autarchoglossans. In a strict consensus both Gobekko and Hoburogekko form a polytomy with extant geckos. Some of the adult character states of Gobekko are observable in embryos of extant species. The evolution of tubular frontals and dentaries in gekkotans may be structurally related to the loss of the postorbital and supratemporal bars in this lineage. The complete lack of a parietal foramen, and presumably a light‐sensitive parietal eye, in this clade is of interest and could indicate an early origin of nocturnality in geckos. © 2013 The Linnean Society of London     

A primitive nimravine skull from the Quercy fissures, France: implications for the origin and evolution of Nimravidae (Carnivora)

A carnivore skull from the Phosphorites of Quercy, France, is described herein. Its well-preserved basicranial structure, cat-like morphology, and dental reduction support the assignment of this specimen to the Nimravidae. This is the most primitive nimravid described and its overall morphology is the closest yet to the hypothetical ancestor of the Nimravinae as previously conceived. In addition, the monophyly of the Nimravinae is reinforced by the similar basicranium of the specimen described here compared to other nimravid genera. It suggests that this typical basicranial structure appeared well ahead the sabretooth features in this group. The early evolution and diversification of the Nimravinae occurred during the Late Eocene in the northern hemisphere but the precise geographic area is still unknown. A rapid evolution during the initial radiation of the group or an incomplete fossil record in Eurasia could both equally explain the rapid and worldwide distribution of the Nimravinae.     

The evolution of hominoid cranial diversity: A quantitative genetic approach

Hominoid cranial evolution is characterized by substantial phenotypic diversity, yet the cause of this variability has rarely been explored. Quantitative genetic techniques for investigating evolutionary processes underlying morphological divergence are dependent on the availability of good ancestral models, a problem in hominoids where the fossil record is fragmentary and poorly understood. Here, we use a maximum likelihood approach based on a Brownian motion model of evolutionary change to estimate nested hypothetical ancestral forms from 15 extant hominoid taxa. These ancestors were then used to calculate rates of evolution along each branch of a phylogenetic tree using Lande's generalized genetic distance. Our results show that hominoid cranial evolution is characterized by strong stabilizing selection. Only two instances of directional selection were detected; the divergence of Homo from its last common ancestor with Pan, and the divergence of the lesser apes from their last common ancestor with the great apes. In these two cases, selection gradients reconstructed to identify the specific traits undergoing selection indicated that selection on basicranial flexion, cranial vault expansion, and facial retraction characterizes the divergence of Homo, whereas the divergence of the lesser apes was defined by selection on neurocranial size reduction.     

Phylogenetic Affinities Among the Extant Malagasy Lemurs (Lemuriformes) Based on Morphology and Behavior

The difficulty in achieving a consensus on the phylogenetic relationships of lemuriform primates has been due largely to the lack of a lemur fossil record and to the lack of an appropriate outgroup that would facilitate polarization of character states. Recent findings allow us to polarize some of the bony characters, but to a large extent this problem still remains. In the past, phylogenetic analyses have focused on specialized character sets such as dentition or basicranial traits, or they have employed differential weighting schemes to a more variable set of characters. In the analysis presented here, I combined all relevant characters available in the literature into one data set but restricted my selection to those traits having discontinuous states and for which no contradictory coding schemes were published. I reduced the assumptions in this analysis by removing most external weighting and ordering effects on these data sets. The available data from the literature were supplemented with data from my own observations at the Duke University Primate Center. Data were collected for 25 characters and 20 taxa and were submitted to a cladistic analysis. Some important findings from this study include support for (1) a sister-group relationship between Lepilemur and the Indridae, (2) a sister-group relationship between the Lemuridae (except Varecia) and the Indridae/Lepilemur clade, (3) a monophyletic genus Eulemur, and (4) the exclusion of Varecia from the Lemuridae.     

The posterior border of the sphenoid greater wing and its phylogenetic usefulness in human evolution

The elucidation of patterns of cranial skeletal maturation and growth in fossil hominids is possible not only through dental studies but also by mapping different aspects of ossification in both extant African apes and humans. However, knowledge of normal skeletal development in large samples of extant great apes is flimsy. To remedy this situation, this paper offers an extensive survey and thorough discussion of the ossification of the posterior border of the sphenoid greater wing. Indeed, this area provides much information about basicranial skeletal maturation. We investigate three variants: the absence of the foramen spinosum and the position of both the foramen spinosum and the foramen ovale in relation to the sphenosquamosal suture. Providing original data about humans and 1,425 extant great ape skulls and using a sample of 64 fossil hominids, this study aimed to test whether different ossification patterns occurred during the course of human evolution. The incidence of three derived morphologies located on the posterior border of the sphenoid greater wing increases during human evolution at different geological periods. The evolutionary polarity of these three derived morphologies is assessed by outgroup comparison and ontogenetic methods. During human evolution, there is a clear trend for the foramen spinosum to be present and wholly located on the posterior area of the sphenoid greater wing. Moreover, in all the great ape species and in Australopithecus afarensis, the sphenosquamosal suture may split the foramen ovale. Inversely, the foramen ovale always lies wholly within the sphenoid greater wing in Australopithecus africanus, robust australopithecines, early Homo, H. erectus (and/or H. ergaster), and Homo sapiens. From ontogenetic studies in humans, we conclude that, during human evolution, the ossification of the posterior area of the sphenoid greater wing progressively surrounded the middle meningeal artery (passing through the foramen spinosum) and the small meningeal artery (passing through the foramen ovale). Am J Phys Anthropol 107:387–399, 1998. © 1998 Wiley-Liss, Inc.     

Effects of brain and facial size on basicranial form in human and primate evolution

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 = 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.     

The middle and inner ears of the Palaeogene golden mole Namachloris: A comparison with extant species

Many living species of golden moles (Chrysochloridae) have greatly enlarged middle ear ossicles, believed to be used in the detection of ground vibrations through inertial bone conduction. Other unusual features of chrysochlorids include internally coupled middle ear cavities and the loss of the tensor tympani muscle. Our understanding of the evolutionary history of these characteristics has been limited by the paucity of fossil evidence. In this article, we describe for the first time the exquisitely preserved middle and inner ears of Namachloris arenatans from the Palaeogene of Namibia, visualised using computed tomography, as well as ossicles attributed to this species. We compare the auditory region of this fossil golden mole, which evidently did not possess a hypertrophied malleus, to those of three extant species with similarly sized ear ossicles, Amblysomus hottentotus, Calcochloris obtusirostris, and Huetia leucorhinus. The auditory region of Namachloris shares many common features with the living species, including a pneumatized, trabeculated basicranium and lateral skull wall, arteries and nerves of the middle ear contained in bony tubes, a highly coiled cochlea, a secondary crus commune, and no identifiable canaliculus cochleae for the perilymphatic duct. However, Namachloris differs from extant golden moles in the apparent absence of a basicranial intercommunication between the right and left ears, the possession of a tensor tympani muscle and aspects of ossicular morphology. One Namachloris skull showed what may be pneumatization of some of the dorsal cranial bones, extending right around the brain. Although the ossicles are small in absolute terms, one of the Huetia leucorhinus specimens had a more prominent malleus head than the other. This potentially represents a previously unrecognised subspecific difference.     

Kinematic data on primate head and neck posture: Implications for the evolution of basicranial flexion and an evaluation of registration planes used in paleoanthropology

Kinematic data on primate head and neck posture were collected by filming 29 primate species during locomotion. These were used to test whether head and neck posture are significant influences on basicranial flexion and whether the Frankfurt plane can legitimately be employed in paleoanthropological studies. Three kinematic measurements were recorded as angles relative to the gravity vector, the inclination of the orbital plane, the inclination of the neck, and the inclination of the Frankfurt plane. A fourth kinematic measurement was calculated as the angle between the neck and the orbital plane (the head-neck angle [HNA]). The functional relationships of basicranial flexion were examined by calculating the correlations and partial correlations between HNA and craniometric measurements representing basicranial flexion, orbital kyphosis, and relative brain size (Ross and Ravosa [1993] Am. J. Phys. Anthropol. 91:305–324). Significant partial correlations were observed between relative brain size and basicranial flexion and between HNA and orbital kyphosis. This indicates that brain size, rather than head and neck posture, is the primary influence on flexion, while the degree of orbital kyphosis may act to reorient the visual field in response to variation in head and neck posture. Regarding registration planes, the Frankfurt plane was found to be horizontal in humans but inclined in all nonhuman primates. In contrast, nearly all primates (including humans) oriented their orbits such that they faced anteriorly and slightly inferiorly. These results suggest that for certain functional craniometric studies, the orbital plane may be a more suitable registration plane than Frankfurt “Horizontal.” Am J Phys Anthropol 108:205–222, 1999. © 1999 Wiley-Liss, Inc.     

Underestimating intraspecific variation: The problem with excluding Sts 19 from Australopithecus africanus

Two analyses conclude that Sts 19 cannot be accommodated within the Australopithecus africanus hypodigm (Kimbel and Rak [1993] In Kimbel and Martin [eds.]: Species, Species Concepts, and Primate Evolution. New York: Plenum, pp. 461–484; Sarmiento [1993] Am. J. Phys. Anthropol. [Suppl.] 16:173). Both studies exclude Sts 19 because it possesses synapomorphies with Homo. Furthermore, according to Kimbel and Rak (1993), including Sts 19 in A. africanus results in an unacceptably high degree of polymorphism. This study aims to refute the null hypothesis that Sts 19 belongs to A. africanus. Twelve basicranial characters, as defined and implemented in Kimbel and Rak's study, were scored for casts of seven A. africanus and seven Homo habilis basicranial specimens. These characters were also examined on specimens from a large (N = 87) sample of African pongids. Contrary to Kimbel and Rak's (1993) findings, the null hypothesis is not refuted. The degree of polymorphism among A. africanus with Sts 19 included is less than that seen in Pan troglodytes. In addition, Sts 19 shares only one apomorphy with Homo. However, when treated metrically, Sts 19's morphology for this character is not significantly divergent from other A. africanus specimens. Am J Phys Anthropol 105:461–480, 1998. © 1998 Wiley-Liss, Inc.     

Evidence of early evolution of Australidelphia (Metatheria,Mammalia) in South America: phylogenetic relationships of the metatherians from the Late Palaeocene of Itaboraí (Brazil) based on teeth and petrosal bones

New metatherian petrosal bones from the mid to Late Palaeocene of Itaboraí, belonging to three morphotypes (VI, VII, and VII), are formally described and compared to fossil and extant taxa known by their auditory region. An attempt at assigning petrosal types to tooth‐based taxa from Itaboraí was made by combining parsimony and morphometric methods. The first large scale phylogenetic analysis of the Itaboraían metatherians, involving basicranial and dental characters in a larger number of taxa, is provided here and is at the basis of a systematic revision of the metatherians from Itaboraí. The combination of morphometric and cladistic analyses helps in understanding the affinities between the petrosals and the tooth‐based taxa. The metatherians from Itaboraí were taxonomically diverse, belonging to each of the most important radiations in marsupial evolutionary history (Didelphimorphia, Paucituberculata, Eometatheria). The inclusion of Palaeocene taxa in the crown group Marsupialia and above all in the Eometatheria radiation points to an early emergence of these clades in South America and corroborates the main molecular hypotheses. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159 , 746–784.     

Some correlates of cranial and cervical morphology with predatory modes in snakes

Dissection of the cervical and basicranial regions in three species of snakes indicates that compared to Crotalus viridis and Lichanura roseofusca, Masticophis flagellum possesses relatively high numbers of compound axial muscle insertions on the atlas-axis and vertebrae numbers 3-5. It is suggested that the condition in Masticophis facilitates its vertical-neck-horizontal-head foraging posture and has allowed axial muscles inserting on the dorsocaudal braincase in this snake to generate vertical and lateral head movements more effectively.     

Basicranial axis length v. skull length in analysis of carnivore skull shape

Skull length is the measurement most commonly used as a standard against which other aspects of cranial morphology are compared to derive an index of relative size or proportions. However, skull length is composed of two different functional components, facial skull and cerebral skull, which vary independently and have different scaling relationships with body size. An analysis of carnivore skull shape with measurements standardized against basicranium length produced very different results than an analysis using skull length as the standard. For example, expressions of relative size of cranial measurements were reduced by 13% in mustelids and increased by 20% in canids, reflecting removal of jaw length (short in mustelids and long in canids) from the comparative standard (basicranial axis length). Cranial measurements scale with higher allometric exponents against basicranial axis length than against skull length.     

The nervus rarus in Coelacanthiform Phytogeny

Bjerring, H. C. (Section of Palaeozoology, Swedish Museum of Natural History, Stockholm, Sweden). The nervus rarus in coelacanthiform phylogeny. Zool. Scripta 1(2): 57–68, 1972.–In the course of craniate vertebrate evolution the somatic motor portion of the second cranionerval segment probably became divided into two nerves, viz. the trochlear nerve and the rarus nerve. The rarus nerve, allied to the second-metamere basicranial muscle, apparently migrated backwards from the middle to the posterior part of the evolving head. Devonian coelacanthiforms exhibit an endocranial passage indicating that the rarus nerve accompanied the glossopharyngeal nerve through the otic capsule; in contrast, in the Recent Latimeria, the corresponding nerve emerges from the endocranium somewhat further posteriorly in association with the vagus nerve. Within the evolutionary history of the coelacanthiforms, this backward displacement of the rarus nerve appears to be correlated with a progressive development of the second-metamere basicranial muscle as well as with a number of changes in both the head skeleton and the brain.     

Anthropoid cranial base architecture and scaling relationships

This paper examines how various measures of basicranial length and cranial base angulation affect the relationship between basicranial flexion and relative brain size in anthropoids, including Homo sapiens. Most recent studies support the "spatial packing" hypothesis, that basicranial flexion in haplorhines maximizes braincase volume relative to basicranial length. However, a few studies find the basicranium is less flexed in H. sapiens than expected for other anthropoids, suggesting that other factors contribute to variation in hominin basicranial flexion. The measure of relative brain size used to test the spatial packing hypothesis, the Index of Relative Encephalization (IRE), is calculated with basicranial length (BL) in its denominator, so that shorter BL and larger brain size potentially inflate H. sapiens IREs. To investigate this problem, the lengths of midline cranial floor sections were scaled relative to the cube root of endocranial volume in 157 specimens from 18 anthropoid species. Results indicate that the posterior cranial base and planum sphenoideum are significantly shorter in H. sapiens than in other anthropoids, accounting for higher IREs. Including the cribriform plate in BL, advisable in studies using anthropoids, affects whether H. sapiens differs from other anthropoids for basicranial flexion vs. IRE. However, despite a shorter BL and elevated IRE, H. sapiens does not deviate significantly from the anthropoid relationship between basicranial flexion and relative brain size for two cranial base angles. Because different measures of cranial base angulation change how H. sapiens falls along the anthropoid regression line, it remains equivocal whether the basicranium is less flexed in H. sapiens than in other anthropoids when compared to relative brain size.