Allometric and Metameric Shape Variation in Pan Mandibular Molars: A Digital Morphometric Analysis

The predominance of molar teeth in fossil hominin assemblages makes the patterning of molar shape variation a topic of bioanthropological interest. Extant models are the principal basis for understanding dental variation in the fossil record. As the sister taxon to the hominin clade, Pan is one such model and the only widely accepted extant hominid model for both interspecific and intraspecific variation. To explore the contributions of allometric scaling and meristic variation to molar variation in Pan, we applied geometric shape analysis to 3D landmarks collected from virtual replicas of chimpanzee and bonobo mandibular molars. Multivariate statistical analysis and 3D visualization of metameric and allometric shape vectors were used to characterize shape differences and test the hypothesis that species of Pan share patterns of metameric variation and molar shape allometry. Procrustes‐based shape variables were found to effectively characterize crown shape, sorting molars into species and tooth‐row positions with ≥95% accuracy. Chimpanzees and bonobos share a common pattern of M₁–M₂ metameric variation, which is defined by differences in the relative position of the metaconid, size of the hypoconulid, curvature of the buccal wall, and proportions of the basins and foveae. Allometric scaling of molar shape is homogeneous for M₁ and M₂ within species, but bonobo and chimpanzee allometric vectors are significantly different. Nevertheless, the common allometric shape trend explains most molar‐shape differences between P. paniscus and P. troglodytes. When allometric effects are factored out, chimpanzee and bonobo molars are not morphometrically distinguishable. Implications for hominid taxonomy and dietary reconstruction are discussed. Anat Rec, 2011. © 2010 Wiley‐Liss, Inc.     

Ontogenetic divergence generates novel phenotypes in hybrid cichlids

Hybridization is suggested to contribute to ecomorphological and taxonomic diversity in lacustrine East African cichlids. This is supported by studies demonstrating that genetic diversity within lake radiations has been influenced by hybridization events, leading to extensive phenotypic differentiation of genetically closely related species. Hybrid persistence and speciation in sympatry with gene flow can be explained by pleiotropy in traits involved in reproductive isolation; however, little attention has been given to how trait differentiation is established during hybrid ontogeny, and how this may relate to trophic and locomotor specialization. This study compares body shape changes in a Lake Victoria cichlid hybrid throughout its post-hatch ontogeny to those of its parental species. Across the considered age/size categories, hybrids occupy a distinct and intermediate morphological space, yet where several transgressive traits emerge. A between-group principal component analysis on body shapes across size categories reveals axes of shape variation exclusive to the hybrids in the youngest/smallest size categories. Shape differences in the hybrids involved morphological traits known to be implicated in trophic and locomotor specializations in the parental species. Combined, our findings suggest that phenotypic divergence in the hybrid can lead to functional differences that may potentially release them to some degree from competition with the parental species. These findings agree with recent literature that addresses the potential importance of hybridization for the unusually recent origin of the Lake Victoria cichlid super-species flock.     

Ontogenetic Patterns of Morphological Variation in the Ectocranial Human Vault

The skull is considered a modular structure in which different parts are influenced by different factors and, as a result, achieve adult shape at different ages. Previous studies have suggested that the basicranium presents a modular pattern that distinguishes sagittal and lateral parts, probably affected by the brain and masticatory structures, respectively. The vault of modern humans, in contrast, has been considered as a highly integrated system mainly influenced by brain growth. Here, we explored developmental shape variation in sagittal and lateral ectocranial vault in humans in order to assess if both regions are ontogenetically dissociated. We used a sample of 135 cranial computed tomography images from 0 to 31 ages. Landmarks and semilandmarks were collected on sagittal and lateral regions and geometric morphometric techniques were applied separately for each region. On the shape coordinates, we used Goodall's F‐test in order to assess the age when the adult configuration is attained. Principal component analysis enabled us to evaluate shape variation during ontogeny. Results indicated that both sagittal and lateral structures attain adult shape at early adolescence. Both regions express coordinated shape modifications probably due to shared developmental factors. It is concluded that masticatory muscles may not exert a strong enough influence to produce independent variation in the lateral traits. Thus, it is likely that the brain integrates sagittal and lateral parts of the vault across human ontogeny. Anat Rec, 296:1008–1015, 2013. © 2013 Wiley Periodicals, Inc.     

Seasonal and Ontogenetic Variation in Subcutaneous Adipose Of the Bowhead Whale (Balaena mysticetus)

Cetacean evolution was shaped by an extraordinary land‐to‐sea transition in which the ancestors of whales became fully aquatic. As part of this transition, these mammals evolved unusually thick blubber which acts as a metabolic reservoir as well as an insulator and provides buoyancy and streamlining. This study describes blubber stratification and correlates it to seasonal variation, feeding patterns, and ontogeny in an arctic‐adapted mysticete, the bowhead whale (Balaena mysticetus). Bowheads are unique among mammals for possessing the largest known blubber stores. We found that adipocyte numbers in bowheads, like other mammals, do not vary with season or feeding pattern but that adipocyte size and structural fiber densities do vary with blubber depth. Anat Rec, 298:1416–1423, 2015. © 2015 Wiley Periodicals, Inc.     

Ontogenetic allometry and scaling in catarrhine crania

In studies of ontogenetic allometry, ontogenetic scaling has often been invoked to explain cranial morphological differences between smaller and larger forms of closely related taxa. These scaled variants in shape have been hypothesized to be the result of the extension or truncation of common growth allometries. In this scenario, change in size is the determining factor, perhaps under direct selection, and changes in cranial shapes are byproducts, not under direct selection themselves. However, many of these conclusions are based on studies that used bivariate generalizations of shape. Even among multivariate analyses of growth allometries, there are discrepancies as to the prevalence of ontogenetic scaling among primates, how shared the trajectories need to be, and which taxa evince properties of scaled variants. In this investigation, we use a large, comparative ontogenetic sample, geometric morphometric methods, and multivariate statistical tests to examine ontogenetic allometry and evaluate if differences in cranial shape among closely related catarrhines of varying sizes are primarily driven by size divergence, that is, ontogenetic scaling. We then evaluate the hypothesis of size as a line of least evolutionary resistance in catarrhine cranial evolution. We found that patterns of ontogenetic allometry vary among taxa, indicating that ontogenetic scaling sensu stricto does not often account for most morphological differences and that large and small taxa within clades are generally not scaled variants. The presence of a variety of ontogenetic pathways for the evolution of cranial shapes provides indirect evidence for selection acting directly on the cranial shape, rather than on size alone.     

Ontogenetic Variation in the Bony Labyrinth of Monodelphis domestica (Mammalia: Marsupialia) Following Ossification of the Inner Ear Cavities

Ontogeny, or the development of an individual from conception to death, is a major source of variation in vertebrate morphology. All anatomical systems are affected by ontogeny, and knowledge of the ontogenetic history of these systems is important to understand when formulating biological interpretations of evolutionary history and physiology. The present study is focused on how variation affects the bony labyrinth across a growth series of an extant mammal after ossification of the inner ear chambers. Digital endocasts of the bony labyrinth were constructed using CT data across an ontogenetic sequence of Monodelphis domestica, an important experimental animal. Various aspects of the labyrinth were measured, including angles between the semicircular canals, number of turns of the cochlea, volumes of inner ear constituents, as well as linear dimensions of semicircular canals. There is a strong correlation between skull length and age, but from 27 days after birth onward, there is no correlation with age among most of the inner ear measurements. Exceptions are the height of the arc of the lateral semicircular canal, the angular deviation of the lateral canal from planarity, the length of the slender portion of the posterior semicircular canal, and the length of the canaliculus cochleae. Adult dimensions of several of the inner ear structures, such as the arcs of the semicircular canals, are achieved before the inner ear is functional, and the non‐ontogenetic variation in the bony labyrinth serves as an important source for behavioral, physiological, and possibly phylogenetic information. Anat Rec, 2010. © 2010 Wiley‐Liss, Inc.     

Allometric growth in mass by the brain of mammals

I re-examined published data for ontogenetic change in relative mass of the brain in six species of mammal (i.e., sheep, pig, cow, horse, rat, cat) to illustrate an insidious problem with conventional analyses of brain–body allometry. Graphical displays of logarithmic transformations of the original data for each species give the appearance of two discrete mathematical distributions, but untransformed observations nonetheless conform to a single distribution that is well described by a single, nonlinear equation. The concept of biphasic, allometric growth by the brain consequently is an artifact of transformation. The notion of Rapid and Slow phases in relative growth by the brain also is an artifact, because the notion is based explicitly on the concept of biphasic growth allometry. Relative growth by the brain in sheep, pigs, cows, and horses follows the path of a power curve with an exponent less than 1, so relative growth declines progressively as animals grow to their maximum size, at which point growth effectively ends for both brain and body. Relative growth by the brain in rats and cats follows the path of an exponential curve and consequently is more like relative growth by the brain of odontocoete cetaceans and primates, with the brain growing rapidly relative to the body early in ontogeny and attaining maximum (cats) or near-maximum (rats) mass well before the body reaches its maximum. An exponential pattern of relative growth by the brain appears to have evolved independently in rodents, carnivores, odontocoetes, and primates.     

Limb bone allometry during postnatal ontogeny in non-avian dinosaurs

Although the interspecific scaling of tetrapods is well understood, remarkably little work has been done on the ontogenetic scaling within tetrapod species, whether fossil or recent. Here the ontogenetic allometry of the femur, humerus, and tibia was determined for 23 species of non‐avian dinosaur by regressing log‐transformed length against log‐transformed circumference for each bone using reduced major axis bivariate regression. The femora of large theropod species became more robust during ontogeny, whereas growth in the femora of sauropodomorphs and most ornithischians was not significantly different from isometry. Hadrosaur hindlimb elements became significantly more gracile during ontogeny. Scaling constants were higher in all theropods than in any non‐theropod taxa. Such clear taxonomically correlated divisions were not evident in the ontogenetic allometry of the tibia and hindlimb bones did not scale uniformly within larger taxonomic groups. For taxa in which the ontogenetic allometry of the humerus was studied, only Riojasaurus incertus exhibited a significant departure from isometry. Using independent contrasts, the regression of femoral allometry against the log of adult body mass was found to have a significant negative correlation but such a relationship could not be established for other limb elements or growth parameters, mainly due to the small sample size. The intraspecific scaling patterns observed in dinosaurs and other amniotes do not support earlier hypotheses that intraspecific scaling differs between endothermic and ectothermic taxa.     

Ontogenetic changes in the internal and external morphology of the ilium in modern humans

Trabecular architecture forms an important structural component of bone and, depending on the loading conditions encountered during life, is organised in a systematic, bone- and species-specific manner. However, recent studies suggested that gross trabecular arrangement (e.g. density distribution), like overall bone shape, is predetermined and/or affected by factors other than loading and perhaps less plastic than commonly assumed. To explore this issue further, the present cross-sectional ontogenetic study investigated morphological changes in external bone shape in relation to changes in trabecular bundle orientation and anisotropy. Radiographs of 73 modern human ilia were assessed using radiographic and Geometric Morphometric techniques. The study confirmed the apparently strong predetermination of trabecular bundle development, i.e. prior to external loading, although loading clearly also had an effect on overall morphology. For example, the sacro-pubic bundle, which follows the path of load transmission from the auricular surface to the acetabulum, is well defined and shows relatively high levels of anisotropy from early stages of development; the situation for the ischio-iliac strut is similar. However, while the sacro-pubic strut retains a constant relationship with the external landmarks defining the joint surfaces, the ischio-iliac bundle changes its relationship with the external landmarks and becomes aligned with the iliac tubercle only during late adolescence/early adulthood. It is tentatively proposed that the rearrangement of the ischio-iliac strut may reflect a change in locomotor pattern and/or a shift in positional behavior with increasing mass after growth of external bone dimensions has slowed/ceased.     

Variation in accessory mental foramen frequency and number in extant hominoids

Nerves providing sensation to the lower face and jaw exit the mandibular canal via the mental foramen. In humans, there are many documented occurrences of additional foramina (accessory mental foramina, AMFs) on the lateral mandibular surface that may also contain nervous structures. There are large discrepancies in the literature regarding how often AMFs occur in humans, and investigations of non-human hominoid AMFs are rare. Consequently, the causes of interspecific diversity in this variable have not been explored. This project seeks to compare the frequency and number of AMFs between males and females, and among human regional groups and hominoid subspecies and species, and to investigate possible causal factors for any differences identified. No significant differences were found between males and females in any group. Gorillas and orangutans had the highest percentages of individuals with AMFs and the highest mean number of foramina, while modern humans and siamangs had the lowest figures for these variables. Significant differences (p < .05) were found for the mean number of foramina between most pairs of species. The results also showed that species with mandibles that are larger overall, have a larger area anterior to mental foramen, and a longer mandibular canal typically present more AMFs. The strongest correlation was found between the mean number of mental foramina and mandibular canal length. We suggest that these results provide preliminary support for the hypothesis that increasing mandibular canal length increases the likelihood that that nerves will ramify, leading to greater frequencies of accessory mental foramina.     

Foramen Magnum Ontogeny in Homo sapiens: A Functional Matrix Perspective

Historically, the foramen magnum (FM) has been an integral component of studies on skull ontogeny and evolutionary transformations of cranial form. Although this foramen has been considered a single entity, we hypothesize that it comprises two functional matrices, a ventral matrix and a dorsal matrix. In general, the ventral matrix is related to locomotor functions, whereas the dorsal matrix is related to neurological functions and fluid flow dynamics. To test our hypothesis, we used a large ontogenetic sample of modern human crania (seventh fetal month to adult) to (1) delineate bony size and shape ontogeny for both the foramen and its dorsal and ventral units; (2) delineate the role of synchondroses in the observed growth patterns and rates; and (3) explore the relationship between FM and cranial size, shape, and growth. Detailed growth patterns and rates are established for the bony FM and its ventral and dorsal skeletal units. These data are supplemented by literature and observational data on embryonic and fetal FM ontogeny, soft tissue relationships, anomalous/pathological extremes of size, and craniocervical anatomy and locomotor functions. The hypothesis that the FM is composed of a ventral and a dorsal functional matrix is supported by observed ontogenetic differences between ventral and dorsal skeletal units, as well as by the soft tissue anatomy of these matrices. Further documentation of these matrices has the potential to significantly enhance our understanding of the ontogenetic and evolutionary transformations of skull base morphology. Anat Rec, 2011. © 2010 Wiley‐Liss, Inc.     

Variability in Premolar and Molar Root Number in a Modern Population of Pan troglodytes verus

While teeth are the most common fossil remains for hominoids, little is known of the tooth root morphology in Primates. With the exception of modern humans, the variability of the number of roots within a species is scarcely documented and not conclusively quantified. This lack of knowledge hinders the interpretation of observed evolutionary trends, such as the reduction of the number of roots of premolars within the hominins. Here, we present the first quantification of the variability of the number of roots in a nonhuman ape population including 405 specimens. Our sample is made of a single biological population of Pan troglodytes verus from Liberia, which is compared to other extant hominoids. Both permanent and deciduous teeth were analyzed and comprise premolars and molars from maxillaries and mandibles. The estimated variability is very low for each tooth position; more than 97% of the specimens displaying the same number of roots except for P₄ (94%), M₁ (89%), and P⁴ (57%). No variability at all was observed for lacteal teeth. Males and females are statistically identical, and no difference linked to the tooth size (estimated by the occlusal surface) was observed. When compared to the observation in other hominoid species, these results emphasize that the difference of the number of roots observed between modern humans and apes is significant, and suggests that the evolution of premolar root number is mosaic, with the common ancestor of Pan and Homo probably displaying a reduced number of roots for P⁴ and maybe P³, but a plesiomorphic morphology of the roots of lower premolars. Anat Rec, 297:1927–1934, 2014. © 2014 Wiley Periodicals, Inc.     

太行山猕猴下颌骨的异速生长分析

目的：探讨太行山猕猴下颌骨随年龄变化的形态学改变,为太行山猕猴的体质学、医学实验、进一步确定其分类地位提供可靠的依据。方法：30例太行山猕猴下颌骨标本,按其齿序及牙齿磨损程度的不同分成6组,并对其18项变量进行了测量、统计和分析。结果：下颌骨的有关变量与齿序和牙齿磨损程度成异速生长,即有关变量在生长、发育过程中的变化与其年龄相关,并且不同变量在不同年龄段呈现不同的生长模式。结论：下颌体各变量的变化趋势与其功能是相适应的,与猕猴的食物组成和其取食习性亦具有相关关系。     

Scaling of cardiac morphology is interrupted by birth in the developing sheep Ovis aries

Scaling of the heart across development can reveal the degree to which variation in cardiac morphology depends on body mass. In this study, we assessed the scaling of heart mass, left and right ventricular masses, and ventricular mass ratio, as a function of eviscerated body mass across fetal and postnatal development in Horro sheep Ovis aries (~50‐fold body mass range; N = 21). Whole hearts were extracted from carcasses, cleaned, dissected into chambers and weighed. We found a biphasic relationship when heart mass was scaled against body mass, with a conspicuous ‘breakpoint’ around the time of birth, manifest not by a change in the scaling exponent (slope), but rather a jump in the elevation. Fetal heart mass (g) increased with eviscerated body mass (M_b, kg) according to the power equation 4.90 M_b^{0.88 ± 0.26 (± 95% CI)}, whereas postnatal heart mass increased according to 10.0 M_b^{0.88 ± 0.10}. While the fetal and postnatal scaling exponents are identical (0.88) and reveal a clear dependence of heart mass on body mass, only the postnatal exponent is significantly less than 1.0, indicating the postnatal heart becomes a smaller component of body mass as the body grows, which is a pattern found frequently with postnatal cardiac development among mammals. The rapid doubling in heart mass around the time of birth is independent of any increase in body mass and is consistent with the normalization of wall stress in response to abrupt changes in volume loading and pressure loading at parturition. We discuss variation in scaling patterns of heart mass across development among mammals, and suggest that the variation results from a complex interplay between hard‐wired genetics and epigenetic influences.     

Ontogenetic Allometry and Cranial Shape Diversification Among Human Populations From South America

Modifications of ontogenetic allometries play an important role in patterning the shape differentiation among populations. This study evaluates the influence of size variation on craniofacial shape disparity among human populations from South America and assesses whether the morphological disparity observed at the interpopulation level resulted from a variable extension of the same ontogenetic allometry, or whether it arose as a result of divergences in the pattern of size‐related shape changes. The size and shape of 282 adult and subadult crania were described by geometric morphometric‐based techniques. Multivariate regressions were used to evaluate the influence of size on shape differentiation between and within populations, and phylogenetic comparative methods were used to take into account the shared evolutionary history among populations. The phylogenetic generalized least‐squares models showed that size accounts for a significant amount of shape variation among populations for the vault and face but not for the base, suggesting that the three modules did not exhibit a uniform response to changes in overall growth. The common slope test indicated that patterns of evolutionary and ontogenetic allometry for the vault and face were similar and characterized by a heightening of the face and a lengthening of the vault with increasing size. The conservation of the same pattern of shape changes with size suggests that differences in the extent of growth contributed to the interpopulation cranial shape variation and that certain directions of morphological change were favored by the trait covariation along ontogeny. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.     

Variation in shape of the lingula in the adult human mandible

The lingulae of both sides of 165 dry adult human mandibles, 131 males and 34 females of Indian origin, were classified by their shape into 4 types: 1, triangular; 2, truncated; 3, nodular; and 4, assimilated. Triangular lingulae were found in 226 (68.5%) sides, truncated in 52 (15.8%), nodular in 36 (10.9%) and assimilated in 16 (4.8%) sides. Triangular lingulae were found bilaterally in 110, truncated in 23, nodular in 17 and assimilated in 7 mandibles. Of the remaining 8 mandibles with different appearances on the 2 sides, 6 had a combination of triangular and truncated and 2 had nodular and assimilated. The incidence of triangular and assimilated types in the male and female mandibles are almost equal. In the truncated type it was double in the male mandibles while the nodular type was a little less than double in the female mandibles.     

Ontogenetic differences in retention of spatial learning tested with Morris water maze

Two experiments examined retention of spatial learning in rats using a Morris water maze. Retention was scored in terms of probe trial performance when the platform was removed. Latency to reach the platform location, percent of time in the quadrant that had contained the platform, and relative frequency of visits to the platform location were analyzed. Results of the first experiment showed that preweanlings and juveniles exhibited substantial forgetting at 3- and 7-day retention intervals. Forgetting in adults was much lower than that found in the younger animals, and no differences in amount of forgetting appeared between the 3- and 7-day retention intervals at any age. The second experiment showed that forgetting in juveniles was alleviated by a single training trial administered just prior to the probe trial. These results are discussed in terms of ontogenetic differences in memory processing and measurement issues pertinent to the Morris water maze test procedure. © 1997 John Wiley & Sons, Inc. Dev Psychobiol 30: 329–341, 1997