Ontogenetic Allometry Research Articles

Hybridization and its impact on the ontogenetic allometry of skulls in macaques.

The role of hybridization in morphological diversification is a fundamental topic in evolutionary biology. However, despite the accumulated knowledge on adult hybrid variation, how hybridization affects ontogenetic allometry is less well understood. Here, we investigated the effects of hybridization on postnatal ontogenetic allometry in the skulls of a putative hybrid population of introduced Taiwanese macaques (Macaca cyclopis) and native Japanese macaques (Macaca fuscata). Genomic analyses indicated that the population consisted of individuals with varying degrees of admixture, formed by male migration from Japanese to Taiwanese macaques. For overall skull shape, ontogenetic trajectories were shifted by hybridization in a nearly additive manner, with moderate transgressive variation observed throughout development. In contrast, for the maxillary sinus (hollow space in the face), hybrids grew as fast as Taiwanese macaques, diverging from Japanese macaques, which showed slow growth. Consequently, adult hybrids showed a mosaic pattern, that is, the maxillary sinus is as large as that of Taiwanese macaques, while the overall skull shape is intermediate. Our findings suggest that the transgressive variation can be caused by prenatal shape modification and nonadditive inheritance on regional growth rates, highlighting the complex genetic and ontogenetic bases underlying hybridization-induced morphological diversification.

Ontogenetic skull variation in a shovel-headed amphisbaenian species.

Leposternon microcephalum is a species belonging to the Amphisbaenia, a group of burrowing reptiles. Amphisbaenia present various morphological and physiological adaptations that allow them to penetrate the ground and live underground, through a system of galleries and permanent chambers that they build themselves. Among the morphological adaptations in this group, those of the skull stand out as it serves as the main excavation tool. Four basic skull shapes are recognized: rounded, keeled, shovel-shaped, and spade-shaped. The skull of L. microcephalum belongs to this last type, which is considered the most specialized. The species inhabits soils that are highly compacted and difficult to penetrate. Among the species of Leposternon present in South America, L. microcephalum has the widest distribution, being found in all Brazilian biomes and neighboring countries such as Bolivia, Argentina, Paraguay, and Uruguay. The analysis of the skull of this species was carried out using three-dimensional geometric morphometrics (3D-GMM), a technique that allows comparative analysis, through robust statistical methods, of shape and its variations, using Cartesian coordinate data from a configuration of homologous landmarks. The technique allows the size and shape components of a structure to be analyzed separately. From an ontogenetic point of view, this methodology had also been used to investigate variations in Cynisca leucura, a member of the Amphisbaenidae with a rounded head. Our hypothesis is that the patterns of morphological differentiation in the skull, mainly in the intermediate and occipital regions, are similar in different Amphisbaenia species. Therefore, the objective of this study was to analyze cranial morphological variations in an ontogenetic series of L. microcephalum using 3D-GMM. Computed Tomographic scans of 13 specimens were analyzed: juveniles (N = 8) and adults (N = 5), based on 20 landmarks that characterize the skull. Principal components and regression analyses between shape (dependent variable) and size (independent variable) showed a clear difference between the cranial morphological pattern of juvenile individuals and that of adults. For instance, young specimens tend to have a dorsoventrally tall neurocranium, with the tip of the snout more anteriorly oriented and its dorsal border subtly curved. Dorsally, the parietal region is thicker and smoothly dome-shaped in juveniles. As in C. leucura, the variation was strongly correlated with the size change from juvenile to adult, indicating a dominant role for ontogenetic allometry in determining skull shape.

Sexual dimorphism and ontogenetic variation in the head shape of two neighbouring populations of the common lancehead Bothrops jararaca (Serpentes: Viperidae)

Abstract The head is the most important element in trophic ecology among snakes. Head shape is affected by several factors, both intrinsic, such as size and sex, and extrinsic, such as geographical variation. Often, different populations are subject to different environmental conditions that are reflected in different phenotypes adapted to local conditions. Here, we investigate sexual dimorphism, ontogenetic allometry and environmental effects on head shape in two populations of Bothrops jararaca. We have found a significant effect of sex and population, with females from the highland population having a large braincase region and a more arrow-shaped head. Size increase accounted for a considerable amount of variation in shape, but the coefficient of correlation diverged between populations. Sexual dimorphism in head shape was not found in juveniles, but the ontogenetic trajectory was non-parallel between populations. Rainfall was most associated with a larger snout and reduced braincase. In the genus Bothrops, except in early stages, females often grow faster and attain larger sizes than males, which might affect head shape. Also, possible differences in diet, behaviour and ontogenetic variations between populations might be a reasonable cause of specific allometric trajectories.

Geometric morphometric assessment of the fossil bears of Namur, Belgium: Allometry and ecomorphology

The Namur area in Belgium is useful to study brown (Ursus arctos) and cave bears (Ursus spelaeus) as the assemblage contains little temporal and no geographical variation. Here, we aim to assess ontogenetic allometry within cave bears, as well as ecomorphological differences between adult brown bears (n = 9), adult cave bears (n = 5) and juvenile cave bears (n = 3). Landmarks for 3D digitization of the mandible were chosen based on the taphonomical damage of the specimens. Extant brown bears and extinct Pleistocene brown and cave bears were digitized with a Microscribe G2. Generalized Procrustes superimposition was performed on the coordinates. Allometry was studied using regression analysis. Principal component analysis (PCA) was conducted to assess ecomorphological differences between the groups. 61% of the shape variance within juvenile and adult cave bears was predicted by size (n = 8, p < 0.01). The juvenile cave bears have relatively deep horizontal rami. In adult cave bears, the horizontal ramus is much narrower dorsoventrally. Juvenile cave bears have a small masseteric fossa and a short coronoid process, whereas both are larger, relative to mandible size, in adult cave bears. This made juvenile cave bears likely less effective masticators than fully grown cave bears. In the PCA, principal component (PC) 1 accounts for 45.0% of the total variance and PC2 accounts for 27.6%. Fossil U. arctos from Namur fall within the 95% confidence interval of modern North American U. arctos on both PCs, but are more similar to cave bears than the average extant brown bear. From the similarity of fossil and modern brown bears, it can be deduced that the diet of fossil brown bears was probably also within the range of their modern North American conspecifics, although they might have been more efficient at masticating plant matter.

Morphology, variation, and systematics of the late Cambrian Laurentian dikelocephalid trilobiteWalcottaspis vanhornei(Walcott, 1914)

AbstractWalcottaspis vanhornei(Walcott, 1914) is a large, late Cambrian trilobite with a unique pygidial morphology known only from a narrow outcrop belt of the St. Lawrence Formation in the Upper Mississippi Valley. Found in carbonate-rich layers within heterolithic facies that represent the toesets of a prograding shoreface, it is restricted to a single or small number of parasequences. Only four specimens of any of its sclerites have been illustrated previously. Here holaspid examples of all its biomineralized sclerites are described and illustrated, along with a morphometric analysis of cranidial landmarks and landmarks plus semilandmarks for the pygidium. Ontogenetic allometry accounts for 29% of the variance among holaspid cranidia and includes a relative shortening and narrowing of the palpebral lobe and a reduction in the relative length and width of the frontal area. Notable pygidial phenotypic variation occurs in the extent of the postaxial region and in the proportion of the structure occupied by the axis. Phylogenetic analysis suggests thatW. vanhorneiis sister taxon toDikelocephalus minnesotensisOwen, 1852, which also occurs in the St. Lawrence Formation and has a broadly similar cephalon but distinctive trunk. The holaspid pygidium ofW. vanhorneiis uniquely characterized by the interpleural furrows of the first two segments becoming abruptly obsolete on approaching the axial furrow. Some pygidia show narrow, shallow, flat-bottomed grooves etched into the internal sides of the dorsal surface or doublure that apparently represent infestation of the live trilobite.

Ontogenetic progression of individual head size in the larvae of the beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae): catch-up growth within stages and per-stage growth rate changes across stages.

The ontogenetic progression of insect larval head size has received much attention due to its fundamental and practical importance. However, although previous studies have analyzed the population mean head size, such an approach may not be appropriate for developmental studies of larval head sizes when the trajectory of individual head size growth is correlated with pre-molt head size and developmental stage. Additionally, there is covariation between the head and body sizes; however, few studies have compared the ontogenetic progression of individual head sizes with that of individual body sizes. In this investigation, the per-stage growth rates (PSGRs) for head width (HW) and cubic-rooted body mass at the beginning of each instar (body size, BS) were assessed in Trypoxylus dichotomus. Linear models were used to test the size- and instar-dependence of the ontogenetic progression of individual HW and BS. The individual PSGRs of the HW (iPSGRH) and BS (iPSGRB) were then compared. In addition, the allometric relationship between HW and BS was examined. The iPSGRH was negatively correlated with the pre-molt HW at every instar (i.e., head catch-up growth). Furthermore, the mean iPSGRH at L2 was relatively higher than that at L1 when the pre-molt HW was used as covariate in the analysis (i.e., instar-effect), whereas the mean iPSGRH decreased ontogenetically. The iPSGRB showed catch-up growth and instar-effects similar to those of iPSGRH; however, iPSGRH was found to be lower than iPSGRB. Due to the differences between the PSGRs for the larval head and body, the larval head size showed negative ontogenetic allometry against body size.

Patterns of ontogenetic evolution across extant marsupials reflect different allometric pathways to ecomorphological diversity

The relatively high level of morphological diversity in Australasian marsupials compared to that observed among American marsupials remains poorly understood. We undertake a comprehensive macroevolutionary analysis of ontogenetic allometry of American and Australasian marsupials to examine whether the contrasting levels of morphological diversity in these groups are reflected in their patterns of allometric evolution. We collate ontogenetic series for 62 species and 18 families of marsupials (n = 2091 specimens), spanning across extant marsupial diversity. Our results demonstrate significant lability of ontogenetic allometric trajectories among American and Australasian marsupials, yet a phylogenetically structured pattern of allometric evolution is preserved. Here we show that species diverging more than 65 million years ago converge in their patterns of ontogenetic allometry under animalivorous and herbivorous diets, and that Australasian marsupials do not show significantly greater variation in patterns of ontogenetic allometry than their American counterparts, despite displaying greater magnitudes of extant ecomorphological diversity.

Ontogenetic allometry reveals the imprint of myrmecophagy in the skull of the numbat, Myrmecobius fasciatus Waterhouse, 1836 (Marsupialia: Myrmecobiidae)

Ontogenetic allometry, the covariation of shape with size over the course of development, represents a fundamental component of morphological diversification that can vary across species and in association with ecological factors. Ontogenetic allometry patterns for the skull have been described for species from several marsupial lineages, associated with common dietary niches, e.g., carnivory. We here conduct a novel detailed examination of cranial ontogeny in the numbat (Myrmecobius fasciatus), unique among marsupials in being myrmecophagous and adopting a strictly diurnal habit. We investigated ontogenetic allometry using multivariate analyses in a postnatal series (n = 28) of complete numbat skulls to estimate the rate of growth of its major constituent elements, described by 15 measurements. We find positive allometry for length of the nasals and palate, which relates directly to the remarkable rostral elongation in the species relative to other marsupials, in addition to several other ontogenetic changes. These comprise negative allometry of palate breadth, elongation of upper and lower toothrows that produces diastemata between weakly developed teeth, and decreasing muzzle height, all of which contribute to the development of a long, tapering, narrow rostrum, with limited mastication ability, as seen in the small temporal space and poor development of crests and processes associated with jaw musculature. Our specific allometric results and comparisons, together with qualitative observations, reveal a distinct imprint of myrmecophagy on skull ontogeny in the numbat and help explain the development and evolution of specialized feeding function of this species.Norberto P. Giannini [ngiannini@amnh.org], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Facultad de Ciencias Naturales, Instituto Miguel Lillo, Universidad Nacional de Tucumán, Tucumán, Argentina, Department of Mammalogy, American Museum of Natural History, New York, NY, USA;Fernando Abdala [nestor.abdala@wits.ac.za], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa;David A. Flores [dflores@macn.gov.ar], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Instituto de Vertebrados, Fundación Miguel Lillo, Tucumán, Argentina;Laura A. B. Wilson [laura.wilson@anu.edu.au], School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia, School of Archaeology and Anthropology, The Australian National University, Canberra, ACT 2601, Australia.

The effects of body size and training environment on the physical performance of adolescent basketball players: the INEX study

Background Although adolescent basketballers differ in body size, shape, and composition, less is known about how these factors interact during physical development. Aim We used ontogenetic allometry to identify the optimal body size and shape characteristics associated with physical performance in adolescent basketball players, and investigated the effects of training experience, training volume, maturity status, and club characteristics on physical performance development. Subjects and methods Two hundred and sixty-four male basketballers, from five age-cohorts (11–15 years of age), were followed consecutively over three years. Three physical performance components, anthropometrics, training information, and biological maturation were assessed bi-annually. Longitudinal multiplicative allometric models were developed. Results Players with a physique that had a dominant ectomorphic component performed better in all physical performance components. When adjusting for confounders other than size, the development of running speed was independent of body size. Players advanced in maturation were physically fitter. Training data had no significant effect on developmental trajectories of running speed or lower body explosive strength. Club characteristics had no significant association with any physical performance trajectories. Conclusion Leaner players have advantages in physical performance and individual characteristics play an important role, over and beyond club structure, in developing physical performance.

Cephalopod retinal development shows vertebrate-like mechanisms of neurogenesis

Coleoid cephalopods, including squid, cuttlefish, and octopus, have large and complex nervous systems and high-acuity, camera-type eyes. These traits are comparable only to features that are independently evolved in the vertebrate lineage. The size of animal nervous systems and the diversity of their constituent cell types is a result of the tight regulation of cellular proliferation and differentiation in development. Changes in the process of development during evolution that result in a diversity of neural cell types and variable nervous system size are not well understood. Here, we have pioneered live-imaging techniques and performed functional interrogation to show that the squid Doryteuthis pealeii utilizes mechanisms during retinal neurogenesis that are hallmarks of vertebrate processes. We find that retinal progenitor cells in the squid undergo nuclear migration until they exit the cell cycle. We identify retinal organization corresponding to progenitor, post-mitotic, and differentiated cells. Finally, we find that Notch signaling may regulate both retinal cell cycle and cell fate. Given the convergent evolution of elaborate visual systems in cephalopods and vertebrates, these results reveal common mechanisms that underlie the growth of highly proliferative neurogenic primordia. This work highlights mechanisms that may alter ontogenetic allometry and contribute to the evolution of complexity and growth in animal nervous systems.

Temperature-driven heterochrony as a main evolutionary response to climate changes in conodonts.

Can we predict the evolutionary response of organisms to climate changes? The direction of greatest intraspecific phenotypic variance is thought to correspond to an 'evolutionary line of least resistance', i.e. a taxon's phenotype is expected to evolve along that general direction, if not constrained otherwise. In particular, heterochrony, whereby the timing or rate of developmental processes are modified, has often been invoked to describe evolutionary trajectories and it may be advantageous to organisms when rapid adaptation is critical. Yet, to date, little is known empirically as to which covariation patterns, whether static allometry, as measured in adult forms only, or ontogenetic allometry, the basis for heterochrony, may be prevalent in what circumstances. Here, we quantify the morphology of segminiplanate conodont elements during two distinct time intervals separated by more than 130 Myr: the Devonian-Carboniferous boundary and the Carnian-Norian boundary (Late Triassic). We evidence that the corresponding species share similar patterns of intraspecific static allometry. Yet, during both crises, conodont evolution was decoupled from this common evolutionary line of least resistance. Instead, it followed heterochrony-like trajectories that furthermore appear as driven by ocean temperature. This may have implications for our interpretation of conodonts' and past marine ecosystems' response to environmental perturbations.

An alternative size variable for allometric investigations in subadults

AbstractOBJECTIVESEffective allometry research relies on appropriate size variables; however, two of the largest obstacles in subadult (ontogenetic) allometry research is small sample sizes and unknown dimensions. This study overcomes a barrier of ontogenetic allometry research by proposing alternative size variables that do not require additional calculations for use in subadult allometry research and retain general patterns among long bones when stature is used for size.MATERIALS AND METHODSDiaphyseal measurements, stature, and age were collected from computed tomography (CT) and full‐body radiographic images for a sample of subadults between birth and 13 years from the United States (U.S., n = 308) and South Africa (Z.A., n = 25). Nineteen alternative size variables were evaluated using reduced‐major‐axis regression to identify the closest one‐to‐one relationship to stature. The applicability across samples was then evaluated using the selected alternative size variables.RESULTSRadius midshaft breadth (RMSB), femur midshaft breadth (FMSB), and the geometric mean of midshaft breadths (GM midshaft) yielded the closest isometric relationships to stature. Allometric relationships among long bones are maintained when using stature, FMSB, and GM midshaft as size variables for both the U.S. and Z.A. samples.DISCUSSIONA large, modern dataset facilitated an investigation into alternative size variables that can be used for single‐bone ontogenetic allometry. Generalizability of the model suggests FMSB and GM midshaft are persistent across populations. This methodology identifies alternative size variables appropriate for other allometry research and offers a robust approach even when historically relied upon size variables are unknown.

Heterochronies and allometries in the evolution of the hominid cranium: a morphometric approach using classical anthropometric variables.

This article studies the evolutionary change of allometries in the relative size of the two main cranial modules (neurocranium and splanchnocranium) in the five living hominid species and a diverse sample of extinct hominins. We use six standard craniometric variables as proxies for the length, width and height of each cranial module. Factor analysis and two-block partial least squares (2B-PLS) show that the great apes and modern humans share a pervasive negative ontogenetic allometry in the neurocranium and a positive one in the splanchnocranium. This developmental constraint makes it possible to interpret the cranial heterochronies in terms of ontogenetic scaling processes (i.e., extensions or truncations of the ancestral ontogenetic trajectory) and lateral transpositions (i.e., parallel translations of the entire trajectory starting from a different shape for a given cranial size). We hypothesize that ontogenetic scaling is the main evolutionary modality in the australopithecines while in the species of Homo it is also necessary to apply transpositions. Both types of processes are coordinated in Homo, which result in an evolutionary trend toward an increase in brain size and in the degree of paedomorphosis from the earliest habilines.

The roles of growth regulation and appendage patterning genes in the morphogenesis of treehopper pronota.

Treehoppers of the insect family Membracidae have evolved enlarged and elaborate pronotal structures, which is hypothesized to involve co-opted expression of genes that are shared with the wings. Here, we investigate the similarity between the pronotum and wings in relation to growth. Our study reveals that the ontogenetic allometry of the pronotum is similar to that of wings in Membracidae, but not the outgroup. Using transcriptomics, we identify genes related to translation and protein synthesis, which are mutually upregulated. These genes are implicated in the eIF2, eIF4/p70S6K and mTOR pathways, and have known roles in regulating cell growth and proliferation. We find that species-specific differential growth patterning of the pronotum begins as early as the third instar, which suggests that expression of appendage patterning genes occurs long before the metamorphic molt. We propose that a network related to growth and size determination is the more likely mechanism shared with wings. However, regulators upstream of the shared genes in pronotum and wings need to be elucidated to substantiate whether co-option has occurred. Finally, we believe it will be helpful to distinguish the mechanisms leading to pronotal size from those regulating pronotal shape as we make sense of this spectacular evolutionary innovation.

Methods for studying allometry in geometric morphometrics: a comparison of performance

Allometry has been the focus of growing interest in studies using geometric morphometric methods to address a wide range of research questions at the interface of ecology and evolution. This study uses computer simulations to compare four methods for estimating allometric vectors from landmark data: the multivariate regression of shape on a measure of size, the first principal component (PC1) of shape, the PC1 in conformation space, and a recently proposed method, the PC1 of Boas coordinates. Simulations with no residual variation around the allometric relationship showed that all four methods are logically consistent with one another, up to minor nonlinearities in the mapping between conformation space and shape tangent space. In simulations that included residual variation, either isotropic or with a pattern independent of allometry, regression of shape on size performed consistently better than the PC1 of shape. The PC1s of conformation and of Boas coordinates were very similar and very close to the simulated allometric vectors under all conditions. An extra series of simulations to elucidate the relation between conformation and Boas coordinates indicated that they are almost identical, with a marginal advantage for conformation. Empirical examples of ontogenetic allometry in rat skulls and rockfish body shape illustrate simple biological applications of the methods. The paper concludes with recommendations how these methods for estimating allometry can be used in studies of evolution and ecology.

Ontogenetic allometry underlies trophic diversity in sea turtles (Chelonioidea)

Despite only comprising seven species, extant sea turtles (Cheloniidae and Dermochelyidae) display great ecological diversity, with most species inhabiting a unique dietary niche as adults. This adult diversity is remarkable given that all species share the same dietary niche as juveniles. These ontogenetic shifts in diet, as well as a dramatic increase in body size, make sea turtles an excellent group to examine how morphological diversity arises by allometric processes and life habit specialisation. Using three-dimensional geometric morphometrics, we characterise ontogenetic allometry in the skulls of all seven species and evaluate variation in the context of phylogenetic history and diet. Among the sample, the olive ridley (Lepidochelys olivacea) has a seemingly average sea turtle skull shape and generalised diet, whereas the green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) show different extremes of snout shape associated with their modes of food gathering (grazing vs. grasping, respectively). Our ontogenetic findings corroborate previous suggestions that the skull of the leatherback (Dermochelys coriacea) is paedomorphic, having similar skull proportions to hatchlings of other sea turtle species and retaining a hatchling-like diet of relatively soft bodied organisms. The flatback sea turtle (Natator depressus) shows a similar but less extreme pattern. By contrast, the loggerhead sea turtle (Caretta caretta) shows a peramorphic signal associated with increased jaw muscle volumes that allow predation on hard shelled prey. The Kemp’s ridley (Lepidochelys kempii) has a peramorphic skull shape compared to its sister species the olive ridley, and a diet that includes harder prey items such as crabs. We suggest that diet may be a significant factor in driving skull shape differences among species. Although the small number of species limits statistical power, differences among skull shape, size, and diet are consistent with the hypothesis that shifts in allometric trajectory facilitated diversification in skull shape as observed in an increasing number of vertebrate groups.

Ontogenetic dietary shifts in North American hadrosaurids (Dinosauria: Ornithischia)

Ontogenetic niche shifts, the phenomenon whereby animals change their resource use with growth, were probably widespread in dinosaurs, but most studies of duck-billed dinosaur ontogeny have so far focused mainly on the development of the cranial ornamentation. Here, we quantify allometry of 13 ecomorphological variables of the skull and examine tooth microwear in a sample of North American hadrosaurids to better understand their ecological functioning with growth. Our results indicate that, consistent with the Jarman-Bell principle relating body size to fibre intake and feeding selectivity, juvenile hadrosaurids were relatively more selective than their adult counterparts and subsisted on softer, low-growing browse cropped using lateral rotations of the neck. Chewing movements of the jaw probably did not differ greatly between growth stages. Our findings invite further investigation relating to cranial ontogenetic allometry in hadrosauromorphs more broadly, and to the possible role of ontogenetic niche shifts in the size structuring of Late Cretaceous herbivore communities.

Ontogenetic drivers of morphological evolution in monitor lizards and allies (Squamata: Paleoanguimorpha), a clade with extreme body size disparity

BackgroundHeterochrony, change in the rate or timing of development, is thought to be one of the main drivers of morphological evolution, and allometry, trait scaling patterns imposed by size, is traditionally thought to represent an evolutionary constraint. However, recent studies suggest that the ontogenetic allometric trajectories describing how organisms change as they grow may be labile and adaptive. Here we investigated the role of postnatal ontogenetic development in the morphological diversification of Paleoanguimorpha, the monitor lizards and allies, a clade with extreme body size disparity. We obtained linear and geometric morphometric data for more than 1,600 specimens belonging to three families and 60 species, representing ~ 72% of extant paleoanguimorph diversity. We used these data to undertake one of the largest comparative studies of ontogenetic allometry to date.ResultsHeterochrony is likely dictating morphological divergence at shallow evolutionary scales, while changes in the magnitude and direction of ontogenetic change are found mainly between major clades. Some patterns of ontogenetic variation and morphological disparity appear to reflect ontogenetic transitions in habitat use. Generally, juveniles are more similar to each other than adults, possibly because species that differ in ecology as adults are arboreal as juveniles. The magnitude of ontogenetic change follows evolutionary models where variation is constrained around an optimal value. Conversely, the direction of ontogenetic change may follow models with different adaptive optima per habitat use category or models where interspecific interactions influence its evolution. Finally, we found that the evolutionary rates of the ontogenetic allometric trajectories are phylogenetically variable.ConclusionsThe attributes of ontogenetic allometric trajectories and their evolutionary rates are phylogenetically heterogeneous in Paleoanguimorpha. Both allometric constraints and ecological factors have shaped ontogeny in the group. Our study highlights the evolutionary lability and adaptability of postnatal ontogeny, and teases apart how different evolutionary shifts in ontogeny contribute to the generation of morphological diversity at different evolutionary scales.

Geometric morphometric analysis of morphologic disparity, intraspecific variation and ontogenetic allometry of beyrichitine ammonoids.

Beyrichitine ammonoids of NV Nevada reveal a high taxonomic diversity of Anisian (Middle Triassic). This diversity is, however, in contrast to their relatively low morphologic disparity. Depending on the exact definition, morphologic disparity of a data set is a direct consequence of the sum of all ontogenetic changes. In the past, however, the interplay of both morphological processes has only rarely been addressed. Using geometric morphometric methods, this study aims at a quantification of allometric processes and the morphologic disparity of beyrichitine ammonoids. The multivariate statistical analysis revealed that morphologic disparity, intraspecific variation respectively, within and between the studied species seems to be the result of deviations in the ontogenetic allometric growth pattern (i.e. heterochrony). During deposition of the studied stratigraphic sequence, a general progressive pedomorphism (juvenilization) was observed. The intraspecific variability pattern coincides with the total morphologic disparity of the analyzed species, which suggests that intraspecific variability facilitated morphologic disparity. The comparison of ontogenetic allometric patterns and changes in intraspecific variation and morphologic disparity are likely to refine our understanding of the intrinsic factors influencing the speciation of this group.

Allometry between Suture Line Length and Phragmocone Volume in Some Cretaceous Ammonoids

The allometric relationship between suture line length and phragmocone volume was examined in 28 specimens belonging to six ammonoid species recovered from the Upper Cretaceous Yezo Group of Hokkaido, Japan. The suture perimeter was calculated by summing the distances between digitized points along the suture line in a photograph. The phragmocone volume was calculated using a modified Raup's model. The relationship between suture perimeter and phragmocone volume in each species was expressed as an allometric equation on a logarithmic scale. The exponent of ontogenetic allometry estimated for each species ranged from 0.462 to 0.626. The exponents estimated were much greater than the exponent in the case of isometry (0.333) and the value previously obtained through interspecific comparison. Calculation using a theoretical morphologic model indicates that allometric exponents for the suture perimeter of approximately 0.5 reflect positive allometry of the septal surface area with respect to body size. The present results suggest that strong positive allometry of the suture perimeter leads to a considerable positive allometry of the septal surface area resulting in satisfying Kleiber's scaling law, although its functional significance remains unclear.