Exploring Phylogenetic Signal in Multivariate Phenotypes by Maximizing Blomberg's K.

Due to the hierarchical structure of the tree of life, closely related species often resemble each other more than distantly related species; a pattern termed phylogenetic signal. Numerous univariate statistics have been proposed as measures of phylogenetic signal for single phenotypic traits, but the study of phylogenetic signal for multivariate data, as is common in modern biology, remains challenging. Here we introduce a new method to explore phylogenetic signal in multivariate phenotypes. Our approach decomposes the data into linear combinations with maximal (or minimal) phylogenetic signal, as measured by Blomberg's K. The loading vectors of these phylogenetic components or K-components can be biologically interpreted, and scatterplots of the scores can be used as a low-dimensional ordination of the data that maximally (or minimally) preserves phylogenetic signal. We present algebraic and statistical properties, along with two new summary statistics, KA and KG, of phylogenetic signal in multivariate data. Simulation studies showed that KA and KG have higher statistical power than the previously suggested statistic Kmult, especially if phylogenetic signal is low or concentrated in a few trait dimensions. In two empirical applications to vertebrate cranial shape (crocodyliforms and papionins), we found statistically significant phylogenetic signal concentrated in a few trait dimensions. The finding that phylogenetic signal can be highly variable across the dimensions of multivariate phenotypes has important implications for current maximum likelihood approaches to phylogenetic signal in multivariate data.

Hominoid intraspecific cranial variation mirrors neutral genetic diversity.

Natural selection, developmental constraint, and plasticity have all been invoked as explanations for intraspecific cranial variation in humans and apes. However, global patterns of human cranial variation are congruent with patterns of genetic variation, demonstrating that population history has influenced cranial variation in humans. Here we show that this finding is not unique to Homo sapiens but is also broadly evident across extant ape species. Specifically, taxa that exhibit greater intraspecific cranial shape variation also exhibit greater genetic diversity at neutral autosomal loci. Thus, cranial shape variation within hominoid taxa reflects the population history of each species. Our results suggest that neutral evolutionary processes such as mutation, gene flow, and genetic drift have played an important role in generating cranial variation within species. These findings are consistent with previous work on human cranial morphology and improve our understanding of the evolutionary processes that generate intraspecific cranial shape diversity within hominoids. This work has implications for the analysis of selective and developmental pressures on the cranium and for interpreting shape variation in fossil hominin crania.

Crocodylomorph cranial shape evolution and its relationship with body size and ecology.

Crocodylomorpha, which includes living crocodylians and their extinct relatives, has a rich fossil record, extending back for more than 200 million years. Unlike modern semi-aquatic crocodylians, extinct crocodylomorphs exhibited more varied lifestyles, ranging from marine to fully terrestrial forms. This ecological diversity was mirrored by a remarkable morphological disparity, particularly in terms of cranial morphology, which seems to be closely associated with ecological roles in the group. Here, I use geometric morphometrics to comprehensively investigate cranial shape variation and disparity in Crocodylomorpha. I quantitatively assess the relationship between cranial shape and ecology (i.e. terrestrial, aquatic, and semi-aquatic lifestyles), as well as possible allometric shape changes. I also characterize patterns of cranial shape evolution and identify regime shifts. I found a strong link between shape and size, and a significant influence of ecology on the observed shape variation. Terrestrial taxa, particularly notosuchians, have significantly higher disparity, and shifts to more longirostrine regimes are associated with large-bodied aquatic or semi-aquatic species. This demonstrates an intricate relationship between cranial shape, body size and lifestyle in crocodylomorph evolutionary history. Additionally, disparity-through-time analyses were highly sensitive to different phylogenetic hypotheses, suggesting the description of overall patterns among distinct trees. For crocodylomorphs, most results agree in an early peak during the Early Jurassic and another in the middle of the Cretaceous, followed by nearly continuous decline until today. Since only crown-group members survived through the Cenozoic, this decrease in disparity was likely the result of habitat loss, which narrowed down the range of crocodylomorph lifestyles.

Moderate climate signature in cranial anatomy of late holocene human populations from Southern South America.

OBJECTIVE: The aim of this study is to analyze the association between cranial variation and climate in order to discuss their role during the diversification of southern South American populations. Therefore, the specific objectives are: (1) to explore the spatial pattern of cranial variation with regard to the climatic diversity of the region, and (2) to evaluate the differential impact that the climatic factors may have had on the shape and size of the diverse cranial structures studied. MATERIALS AND METHODS: The variation in shape and size of 361 crania was studied, registering 62 3D landmarks that capture shape and size variation in the face, cranial vault, and base. Mean, minimum, and maximum annual temperature, as well as mean annual precipitation, but also diet and altitude, were matched for each population sample. A PCA, as well as spatial statistical techniques, including kriging, regression, and multimodel inference were employed. RESULTS: The facial skeleton size presents a latitudinal pattern which is partially associated with temperature diversity. Both diet and altitude are the variables that mainly explain the skull shape variation, although mean annual temperature also plays a role. The association between climate factors and cranial variation is low to moderate, mean annual temperature explains almost 40% of the entire skull, facial skeleton and cranial vault shape variation, while annual precipitation and minimum annual temperature only contribute to the morphological variation when considered together with maximum annual temperature. The cranial base is the structure less associated with climate diversity. CONCLUSION: These results suggest that climate factors may have had a partial impact on the facial and vault shape, and therefore contributed moderately to the diversification of southern South American populations, while diet and altitude might have had a stronger impact. Therefore, cranial variation at the southern cone has been shaped both by random and nonrandom factors. Particularly, the influence of climate on skull shape has probably been the result of directional selection. This study supports that, although cranial vault is the cranial structure more associated to mean annual temperature, the impact of climate signature on morphology decreases when populations from extreme cold environments are excluded from the analysis. Additionally, it shows that the extent of the geographical scales analyzed, as well as differential sampling may lead to different results regarding the role of ecological factors and evolutionary processes on cranial morphology.

Pterion variation in the skulls of rhesus macaques from Cayo Santiago: Inheritance, development, and pathology.

The pterion is the sutural juncture of the frontal, parietal, sphenoidal, temporal, and zygomatic bones on the lateral aspect of the cranium. As a craniometric landmark, the pterion has a taxonomic valence, in addition to a common neurosurgical entry point in medicine. Variation in the articulation patterns at the pterion have been documented between primate species yet have a high degree of uniformity within species, suggesting a genetic control for this complex region of the skull. In this study, pterion pattern variation was investigated in 1627 Rhesus macaque crania of the Cayo Santiago colony. The colony's associated skeletal collections accompany known age, sex, and maternal lineages. Pterion pattern prevalence rates were tested against matrilines, as well as cranial shape, and cranial sutural fusion ages (including individuals with prematurely fused sutures). Five patterns were identified, the most prominent being the prevailing Old World Monkey frontotemporal (FT) articulation (83.4%). The relative frequency of those not exhibiting the FT pattern was found to vary considerably between matrilineal families (p = 0.037), ranging from 5.3% to 34.2%. Mothers with the non-FT pterion pattern were three times as likely to bear non-FT offspring. Cranial shape additionally varied with pterion type. Males exhibiting zygomaticotemporal (ZT) and sphenoparietal (SP) articulations possessed a relatively longer and narrower cranium than those with the default FT type (p = < 0.001). Cranial sutural fusion ages were not found to differ between pterion types, though all individuals with craniosynostosis (6; 0.38%) exhibited the FT type. The study provided strong evidence for a genetic source for pterion pattern as well as outlining a relatively novel relationship with cranial shape and sutural fusion ages. A unifying explanation may lie in those genes involved in both sutural and craniofacial development, or in the variation of brain growth processes channeling sutural articulation at the pterion. Both may be heritable and responsible for producing observed matrilineal differences in the pterion.

The three-dimensional course of cranial development of very preterm infants during the first year of life.

BACKGROUND: Cranial measurements are crucial for evaluating preterm general development because they are a surrogate tool for evaluating brain growth. Usually, they are based on tape-measured head circumference; however, a three-dimensional (3D) approach expands the diagnostic spectrum to the evaluation of cranial volume and shape. AIMS: Very preterm (vPT) infants face multiple risks and obstacles in their early development. In this study, we analyze the risk for cranial anomalies of vPT compared with term-born (TB) infants during the first year of life. STUDY DESIGN AND SUBJECTS: In this single-centre prospective cohort study, 23 vPT and 24 TB healthy infants were assessed. At term equivalent age (TEA) of vPT and time of birth of TB, and 1, 3, 6 and 12 months of postmenstrual age, respectively, a 3D head scan was performed and cephalometrically evaluated regarding cranial growth (head circumference, cranial volume) and shape. RESULTS: Head circumference and cranial volume showed a similar course in vPT and TB. Cranial shape differed significantly between vPT and TB. At TEA, vPT showed longer and narrower heads (dolichocephaly), a difference that disappeared around the 6th month of life. Presence of plagiocephaly was initially similar in vPT and TB, with an increasing difference between both groups with a peak at six months when 34.8 % of the vPT versus none of the TB showed a moderate to severe plagiocephaly (p = 0.004). In vPT, cranial volume significantly correlated with cranial shape, whereas dolichocephaly at TEA had no influence on the further course of plagiocephaly. CONCLUSION: Cranial development of vPT follows a different course than of TB in terms of cranial shape anomalies, while cranial growth remains unaffected. German Clinical Trials Register number: DRKS00022558.

Cranial Shape Measurements Obtained Using a Caliper and Elastic Bands Are Useful for Brachycephaly and Deformational Plagiocephaly Screening.

We assessed a method for screening the cranial shape of 1-month-old infants using a simple measuring instrument instead of a three-dimensional scanner. The Mimos craniometer was used to measure cranial length, cranial width, and two diagonal lengths to calculate the cranial index (CI) and cranial asymmetry (CA). We defined a CI > 90% as brachycephaly and CA > 5 mm as deformational plagiocephaly (DP). Intra- and inter-examiner accuracy analyses were performed on a dummy doll and 1-month-old infants. The measurements of healthy 1-month-old infants were compared with previously reported three-dimensional scanner measurements. Intra- and inter-rater measurements showed good accuracy; diagnostic accuracy comparisons of brachycephaly and DP using a three-dimensional scanner showed kappa values of 1.0 and 0.8, respectively. Comparisons were made among 113 infants matched for day-age at the date of measurement; there were no significant differences in the CI (85.0% vs. 85.2%, p = 0.98) and CA (5.9 mm vs. 6.0 mm, p = 0.48) between the scanner and caliper measurements, nor in the prevalence of brachycephaly (12.4% vs. 17.7%, p = 0.35) or DP (58.4% vs. 56.6%, p = 0.89). This simple measurement method using calipers and bands was useful in screening for brachycephaly and DP in 1-month-old infants.

Evaluating Cranial Growth in Japanese Infants Using a Three-dimensional Scanner: Relationship between Growth-related Parameters and Deformational Plagiocephaly.

In this study, we aimed to evaluate the longitudinal changes in the cranial shape of healthy Japanese infants using a three-dimensional scanner and construct a normal values database for the growth process. Preterm infants (gestational age < 37 weeks), infants with neonatal asphyxia (5-minute Apgar score of <7), and patients who started helmet therapy for deformational plagiocephaly were excluded from this study. The first scan was performed at approximately 1 month of age, followed by two scans conducted at 3 and 6 months of age. The parameters considered were as follows: cranial length, width, height, circumference, volume, cranial vault asymmetry index, and cephalic index. A cranial vault asymmetry index >5% was defined as deformational plagiocephaly. Changes in each parameter were examined using repeated-measures analysis of variance classified by sex and deformational plagiocephaly status. The rate of increase in each parameter was also examined. In total, 88 infants (45 boys and 43 girls) were included in this study. All growth-related parameters were noted to increase linearly with time. Sex differences were observed in all parameters except cranial length. Deformational plagiocephaly was found to have no effect on growth-related parameters. Cranial volume increased by 60% from 1 to 6 months of age. The growth almost uniformly influenced the rate of increase in volume in each coordinate axis direction. Overall, the mean trends in three-dimensional parameters in infants up to 6 months of age were obtained using a three-dimensional scanner. These trends could be used as a guide by medical professionals involved in cranioplasty.

Cranial ecomorphology of turtles and neck retraction as a possible trigger of ecological diversification.

Turtles have a highly modified body plan, including a rigid shell that constrains postcranial anatomy. Skull morphology and neck mobility may therefore be key to ecological specialization in turtles. However, the ecological signal of turtle skull morphologies has not been rigorously evaluated, leaving uncertainties about the roles of ecological adaptation and convergence. We evaluate turtle cranial ecomorphology using three-dimensional geometric morphometrics and phylogenetic comparative methods. Skull shape correlates with allometry, neck retraction capability, and different aquatic feeding ecologies. We find that ecological variables influence skull shape only, whereas a key functional variable (the capacity for neck retraction) influences both shape and size. Ecology and functional predictions from three-dimensional shape are validated by high success rates for extant species, outperforming previous two-dimensional approaches. We use this to infer ecological and functional traits of extinct species. Neck retraction evolved among crownward stem-turtles by the Late Jurassic, signaling functional decoupling of the skull and neck from the shell, possibly linked to a major episode of ecomorphological diversification. We also find strong evidence for convergent ecological adaptations among marine groups. This includes parallel loss of neck retraction, evidence for active hunting, possible grazing, and suction feeding in extinct marine groups. Our large-scale assessment of dietary and functional adaptation throughout turtle evolution reveals the timing and origin of their distinct ecomorphologies, and highlights the potential for ecology and function to have distinct effects on skull form.

Tartarugas tem um plano corpóreo bastante modificado, que inclui um casco rígido que restringe sua anatomia pós-craniana. Portanto, a morfologia craniana e a mobilidade do pescoço devem ser centrais nas especializações ecológicas de tartarugas. No entanto, o sinal ecológico das diferentes morfologias de crânio de tartarugas não foi ainda rigorosamente avaliado, deixando incertezas sobre os papéis de adaptações ecológicas e convergência. Avaliamos a ecomorfologia craniana de tartarugas utilizando morfometria geométrica tridimensional e métodos filogenéticos comparativos. A forma craniana correlaciona com alometria, capacidade de retração do pescoço e diferentes ecologias alimentares aquáticas. Encontramos que variáveis ecológicas influenciam apenas a forma do crânio, enquanto uma importante variável funcional (a capacidade de retração do pescoço) influencia tanto a forma como o tamanho do crânio. Predições ecológicas e funcionais para espécies viventes a partir de formas tridimensionais são validadas com altas taxas de sucesso, superando abordagens bidimensionais. Utilizamos isso para inferir traços ecológicos e funcionais de espécies extintas. A retração do pescoço evoluiu em linhagens extintas mais próximas à origem do grupo-coronal durante o Jurássico Final, indicando uma dissociação funcional entre crânio e pescoço do casco, algo possivelmente ligado a um importante episódio de diversificação ecomorfológica. Também encontramos forte evidência para adaptações ecológicas convergentes em grupos marinhos. Isso inclui a perda paralela da retração do pescoço, evidência de caça ativa, alimentação por sucção, além de possível preferência por plantas aquáticas em grupos marinhos extintos. Nosso estudo de larga-escala sobre adaptações funcionais e relacionadas à dieta ao longo da evolução de tartarugas revela o tempo e origem de suas distintas ecomorfologias, e destaca ainda o potencial de ecologia e função terem efeitos distintos sobre a forma craniana.

Cranial shapes of Japanese preterm infants at one month of age using a three-dimensional scanner.

BACKGROUND: Recently, cranial shape measurements of preterm infants have been performed using handheld three-dimensional (3D) scanners and can now be objectively quantified. AIMS: To measure the cranial shapes of Japanese preterm infants at one month of age using a 3D scanner, compare these values with those of healthy term infants, and examine the risk factors for dolichocephaly. STUDY DESIGN: A multicenter, retrospective cohort study. SUBJECTS: Preterm infants born at <37 weeks of gestation and staying in the neonatal intensive care unit or visiting an outpatient clinic for a one-month checkup between April 2020 and March 2022. OUTCOME MEASURES: A 3D scanner was used to quantify cranial shape. Comparison was made with full-term, one-month-old infants. RESULTS: Ninety-four preterm infants (42 boys) and 165 full-term infants were enrolled. Preterm infants had a significantly lower cephalic index (77.9% and 85.0%, p < 0.01) and a higher incidence of dolichocephaly (54.3% and 13.3%, p < 0.01) compared to term infants. No significant difference in incidence of deformational plagiocephaly was found between the groups (41.5% vs. 47.3%, p = 0.44). The risk of dolichocephaly was significantly higher for female sex (odds ratio [OR], 3.32; 95% confidence interval, 1.30-8.50), cesarean section (OR, 4.07; 95% confidence interval, 1.23-13.5), and use of mechanical ventilation (OR, 4.66; 95% confidence interval, 1.09-20.0). CONCLUSIONS: Japanese preterm infants at the first month of life had longer heads than full-term infants; the risk factors identified were female sex, cesarean section, and use of mechanical ventilation.

Cranial Shape in Infants Aged One Month Can Predict the Severity of Deformational Plagiocephaly at the Age of Six Months.

In this study, we aimed to monitor changes in cranial shape using three-dimensional (3D) scanning to determine whether the severity of deformational plagiocephaly (DP) at the age of 6 months could be predicted at the age of 1 month. The cranial shape was measured at the ages of 1, 3, and 6 months (T1, T2, and T3, respectively) in 92 infants. We excluded those who received helmet treatment before T3. The cranial vault asymmetry index (CVAI) using 3D scanning was evaluated in all infants. DP was defined as a CVAI > 5.0% with mild (CVAI ≤ 6.25%) or moderate/severe severity (CVAI > 6.25%). The CVAI cut-off value at T1 for severe DP at T3 was determined using receiver operating characteristic (ROC) curves. At T1, T2, and T3, the respective CVAI median values were 5.0%, 5.8%, and 4.7% and the DP incidence was 50.0%, 56.8%, and 43.2%, respectively. The DP severity temporarily worsened from T1 to T2 but then improved at T3. Among the infants, 73.9% had a similar DP severity at T1 and T3 (p = 1.0). A ROC curve analysis revealed a CVAI cut-off value of 7.8% at T1 predicted severe DP. It was concluded that later DP severity could be predicted using 3D scanning at T1 with properly defined cut-off values.

Selection for increased tibia length in mice alters skull shape through parallel changes in developmental mechanisms.

Bones in the vertebrate cranial base and limb skeleton grow by endochondral ossification, under the control of growth plates. Mechanisms of endochondral ossification are conserved across growth plates, which increases covariation in size and shape among bones, and in turn may lead to correlated changes in skeletal traits not under direct selection. We used micro-CT and geometric morphometrics to characterize shape changes in the cranium of the Longshanks mouse, which was selectively bred for longer tibiae. We show that Longshanks skulls became longer, flatter, and narrower in a stepwise process. Moreover, we show that these morphological changes likely resulted from developmental changes in the growth plates of the Longshanks cranial base, mirroring changes observed in its tibia. Thus, indirect and non-adaptive morphological changes can occur due to developmental overlap among distant skeletal elements, with important implications for interpreting the evolutionary history of vertebrate skeletal form.

Cranial index in a modern people of Thai ancestry.

The present research aims to examine the cranial index in a modern people of Thai ancestry. Ultimately, this study will help to create a databank containing a cranial index for the classifications of the people from Asia. In this study, 185 modern crania of people of supposed Thai ancestry were examined. They were collected from the Department of Anatomy at Chulalongkorn University in Bangkok, Thailand. The maximum cranial length and breadth were measured using standard anthropometric instruments based on Martin's methods. The cranial index was calculated using the equation ([maximum cranial breadth/maximum cranial length]×100). The mean cranial indices for the male and female skulls examined were 81.81±4.23 and 82.99±4.37, respectively. The most common type of skull in the modern Thai people in this study was the brachycranic type with a frequency of 42.7%, followed by the mesocranic (27.03%) and hyperbrachycranic types (25.59%). The rarest type observed in this study was the dolichocranic type (4.32%). The present study provides valuable data pertaining to the cranial index in a modern Thai population and reveals that modern Thai males and females belong to the brachycranic group. The results of this study will be of forensic anthropological importance to populations in close proximity to the location where the skulls studied here were sourced.

Carnivory maintains cranial dimorphism between males and females: Evidence for niche divergence in extant Musteloidea.

The evolution and maintenance of sexual dimorphism has long been attributed to sexual selection. Niche divergence, however, serves as an alternative but rarely tested selective pressure also hypothesized to drive phenotypic disparity between males and females. We reconstructed ancestral social systems and diet and used Ornstein-Uhlenbeck (OU) modeling approaches to test whether niche divergence is stronger than sexual selection in driving the evolution of sexual dimorphism in cranial size and bite force across extant Musteloidea. We found that multipeak OU models favored different dietary regimes over social behavior and that the greatest degree of cranial size and bite force dimorphism were found in terrestrial carnivores. Because competition for terrestrial vertebrate prey is greater than other dietary groups, increased cranial size and bite force dimorphism reduces dietary competition between the sexes. In contrast, neither dietary regime nor social system influenced the evolution of sexual dimorphism in cranial shape. Furthermore, we found that the evolution of sexual dimorphism in bite force is influenced by the evolution of sexual dimorphism in cranial size rather than cranial shape. Overall, our results highlight niche divergence as an important mechanism that maintains the evolution of sexual dimorphism in musteloids.

Gender and Population Variation in Craniometry and Freehand Pass Ventriculostomy.

BACKGROUND: Ventriculostomy using the freehand pass method is subject to complications arising from misplacement of the catheter tip. This method may require multiple passes for successful catheterization. Methods of determining the burr-hole location (known as the Kocher point) were derived historically from European patients and may not be appropriate for other populations with different cranial shapes. This study examines the possibility that anatomic variation in interpopulation variation together with sexual dimorphism in cranial size and shape may contribute to this problem. METHODS: Sagittal and parasagittal measurements of the frontal bone were taken of 300 Thai (150 female, 150 male) crania and 300 American white (150 female, 150 male) crania. These measurements were compared to determine sexual dimorphism and interpopulation variation in size and shape. RESULTS: The measurements were statistically significantly larger in males than in females and on the right side than the left in both sexes in both Thai and American white samples. The frontal bone is significantly longer at the sagittal plane in Europeans of both sexes than in Thai, but in the parasagittal plane, there is no difference. This finding indicates a difference in frontal bone shape between the 2 populations and between males and females. CONCLUSIONS: The dimensions of the frontal bone vary between males and females and can vary among populations. The optimal location for the burr hole in freehand pass ventriculostomy may depend on both the gender and the ancestry of the patient.

Skull ontogeny of extant caimans: a three-dimensional geometric morphometric approach.

Ontogenetic variation of cranial characters used in crocodylian phylogenetic systematics has never been studied. Furthermore, the relationship between diet and skull morphological transformation during ontogeny has not been properly explored yet. We quantify the inter- and intraspecific skull morphological variation in extant caiman species focusing on those areas relevant to systematics and, also investigate the relation between diet and morphological changes during ontogeny. We applied a three-dimensional approach of geometric morphometrics on post-hatching ontogenetic cranial series of Caiman latirostris and C. yacare. In order to incorporate incomplete material, we additionally tested four different methods of missing landmark estimation and apply the thin-plate spline interpolation. We detected morphological changes between species and during ontogeny (snout and pterygoid flanges increase their proportions and, orbits, temporal fenestrae, skull roof and foramen magnum decrease their relative size) that constitutes part of a general morphological change in the cranial ontogeny of crocodylians. Moreover, the negative allometry of the fenestrae and neurocranium and the positive allometry of the splanchnocranium in both caiman species are the plesiomorphic condition, at least, for tetrapods. Shape changes during growth were found to be related to ontogenetic changes in the diet. Dissimilarities between species seem to be related to different mechanical requirements and different use of the habitat. We found inter- and intraspecific variation in some morphological characters with systematic implications (the contact of nasals with naris, the contact of prefrontals in the midline, and the bones that border the suborbital fenestra and the proportion in which one of them participates) that are not currently considered in phylogenetic analyses.

Evolutionary population history of early Paleoamerican cranial morphology.

The nature and timing of the peopling of the Americas is a subject of intense debate. In particular, it is unclear whether high levels of between-group craniometric diversity in South America result from multiple migrations or from local diversification processes. Previous attempts to explain this diversity have largely focused on testing alternative dispersal or gene flow models, reaching conflicting or inconclusive results. Here, a novel analytical framework is applied to three-dimensional geometric morphometric data to partition the effects of population divergence from geographically mediated gene flow to understand the ancestry of the early South Americans in the context of global human history. The results show that Paleoamericans share a last common ancestor with contemporary Native American groups outside, rather than inside, the Americas. Therefore, and in accordance with some recent genomic studies, craniometric data suggest that the New World was populated by multiple waves of dispersion from northeast Asia throughout the late Pleistocene and early Holocene.

Ontogenetic allometry constrains cranial shape of the head-first burrowing worm lizard Cynisca leucura (Squamata: Amphisbaenidae).

Amphisbaenians are fossorial, predominantly limbless squamate reptiles with distinct cranial shapes corresponding to specific burrowing behaviors. Due to their cryptic lifestyles and the scarcity of museum specimens, little is known of their intraspecific variation, particularly regarding cranial osteology. This represents a critical lack of information, because the majority of morphological investigations of squamate relationships are based on cranial characters. We investigated cranial variation in the West African Coast Worm Lizard Cynisca leucura, a round-headed member of the Amphisbaenidae. Using geometric morphometric analyses of three-dimensional computed tomographic scans, we found that cranial osteology of C. leucura is highly conserved, with the majority of shape changes occurring during growth as the cranium becomes more slender and elongate, accompanied by increasing interdigitation among the dermal roofing bones. Elements of the ventral portion of the cranium remain loosely connected in adults, possibly as a protective mechanism against repeated compression and torsion during burrow excavation. Intraspecific variation was strongly correlated with size change from juveniles to adults, indicating a dominant role of ontogenetic allometry in determining cranial shape. We found no evidence of sexual dimorphism, either during growth or among adults. Given the fossorial habits of C. leucura, we hypothesize that cranial allometry is under strong stabilizing selection to maintain adequate proportions for head-first digging, thereby constraining the ability of individuals to respond to differing selection pressures, including sexual selection and variation in diet or microhabitat. For species in which digging imposes less mechanical stress (e.g., in softer sand), allometric associations during growth may be weakened, allowing changes to the ontogenetic trajectory and subsequent morphological traits. Such developmental dissociation between size and shape, known as heterochrony, may also be implicit in the evolution of the other amphisbaenian cranial shapes (shovel, spade, and keel), which may themselves be functionally adapted for their respective burrowing techniques. J. Morphol. 277:1159-1167, 2016. © 2016 Wiley Periodicals, Inc.