What's luck got to do with it? A generative model for examining the role of stochasticity in age-at-death, with implications for bioarchaeology.

INTRODUCTION: The role of "luck" in determining individual exposure to health insults is a critical component of the processes that shape age-at-death distributions in mortality samples but is difficult to address using traditional bioarcheological analysis of skeletal materials. The present study introduces a computer simulation approach to modeling stochasticity's contribution to the mortality schedule of a simulated cohort. METHODS: The present study employs an agent-based model of 15,100 individuals across a 120 year period to examine the predictive value of birth frailty on age-at-death when varying the likelihood of exposure to health insults. RESULTS: Birth frailty, when accounting for varying exposure likelihood scenarios, was found to account for 18.7% of the observed variation in individual age-at-death. Analysis stratified by exposure likelihood demonstrated that birth frailty alone explains 10.2%-12.1% of the variation observed across exposure likelihood scenarios, with the stochasticity associated with exposure to health insults (i.e., severity of health insult) and mortality likelihood driving the majority of variation observed. CONCLUSIONS: Stochasticity of stressor exposure and intrinsic stressor severity are underappreciated but powerful drivers of mortality in this simulation. This study demonstrates the potential value of simulation modeling for bioarchaeological research.

Shared reproductive disruption, not neural crest or tameness, explains the domestication syndrome.

Altered neural crest cell (NCC) behaviour is an increasingly cited explanation for the domestication syndrome in animals. However, recent authors have questioned this explanation, while others cast doubt on whether domestication syndrome even exists. Here, we review published literature concerning this syndrome and the NCC hypothesis, together with recent critiques of both. We synthesize these contributions and propose a novel interpretation, arguing shared trait changes under ancient domestication resulted primarily from shared disruption of wild reproductive regimes. We detail four primary selective pathways for 'reproductive disruption' under domestication and contrast these succinct and demonstrable mechanisms with cryptic genetic associations posited by the NCC hypothesis. In support of our perspective, we illustrate numerous important ways in which NCCs contribute to vertebrate reproductive phenotypes, and argue it is not surprising that features derived from these cells would be coincidentally altered under major selective regime changes, as occur in domestication. We then illustrate several pertinent examples of Darwin's 'unconscious selection' in action, and compare applied selection and phenotypic responses in each case. Lastly, we explore the ramifications of reproductive disruption for wider evolutionary discourse, including links to wild 'self-domestication' and 'island effect', and discuss outstanding questions.

Developmental instability in domesticated mammals.

Measures of fluctuating asymmetry (FA) have been adopted widely as an estimate of developmental instability. Arising from various sources of stress, developmental instability is associated with an organism's capacity to maintain fitness. The process of domestication has been framed as an environmental stress with human-specified parameters, suggesting that FA may manifest to a larger degree among domesticates compared to their wild relatives. This study used three-dimensional geometric morphometric landmark data to (a) quantify the amount of FA in the cranium of six domestic mammal species and their wild relatives and, (b) provide novel assessment of the commonalities and differences across domestic/wild pairs concerning the extent to which random variation arising from the developmental system assimilates into within-population variation. The majority of domestic mammals showed greater disparity for asymmetric shape, however, only two forms (Pig, Dog) showed significantly higher disparity as well as a higher degree of asymmetry compared to their wild counterparts (Wild Boar, Wolf). Contra to predictions, most domestic and wild forms did not show a statistically significant correspondence between symmetric shape variation and FA, however, a moderate correlation value was recorded for most pairs (r-partial least squares >0.5). Within pairs, domestic and wild forms showed similar correlation magnitudes for the relationship between the asymmetric and symmetric components. In domesticates, new variation may therefore retain a general, conserved pattern in the gross structuring of the cranium, whilst also being a source for response to selection on specific features.

Phylogeny and foraging behaviour shape modular morphological variation in bat humeri.

Bats show a remarkable ecological diversity that is reflected both in dietary and foraging guilds (FGs). Cranial ecomorphological adaptations linked to diet have been widely studied in bats, using a variety of anatomical, computational and mathematical approaches. However, foraging-related ecomorphological adaptations and the concordance between cranial and postcranial morphological adaptations remain unexamined in bats and limited to the interpretation of traditional aerodynamic properties of the wing (e.g. wing loading [WL] and aspect ratio [AR]). For this reason, the postcranial ecomorphological diversity in bats and its drivers remain understudied. Using 3D virtual modelling and geometric morphometrics (GMM), we explored the phylogenetic, ecological and biological drivers of humeral morphology in bats, evaluating the presence and magnitude of modularity and integration. To explore decoupled patterns of variation across the bone, we analysed whole-bone shape, diaphyseal and epiphyseal shape. We also tested whether traditional aerodynamic wing traits correlate with humeral shape. By studying 37 species from 20 families (covering all FGs and 85% of dietary guilds), we found similar patterns of variation in whole-bone and diaphyseal shape and unique variation patterns in epiphyseal shape. Phylogeny, diet and FG significantly correlated with shape variation at all levels, whereas size only had a significant effect on epiphyseal morphology. We found a significant phylogenetic signal in all levels of humeral shape. Epiphyseal shape significantly correlated with wing AR. Statistical support for a diaphyseal-epiphyseal modular partition of the humerus suggests a functional partition of shape variability. Our study is the first to show within-structure modular morphological variation in the appendicular skeleton of any living tetrapod. Our results suggest that diaphyseal shape correlates more with phylogeny, whereas epiphyseal shape correlates with diet and FG.

Ontogenetic changes in femoral cross-sectional geometry during childhood locomotor development.

OBJECTIVES: The femur is a major weight-bearing bone that is variably loaded throughout growth as children transition through locomotory states prior to the attainment of a mature bipedal gait. Here, we document ontogenetic trends in femoral cross-sectional geometry (CSG) and explore how changes in loading regime may impact the structural arrangement of cortical bone along the length of the developing diaphysis. MATERIALS AND METHODS: Micro-CT scans of 110 immature femora were generated from a documented archaeological sample ranging in age from birth to 8.5 years old. CSG properties indicative of relative bone strength and bending rigidity were analyzed from cross-sections extracted at 35%, 50% and 65% of total intermetaphyseal length. RESULTS: Infants experience a marked redistribution of cortical bone between birth and 7 months facilitating a more advantageous mechanical structure for early load bearing behaviors as bone is displaced further from the section centroid. Early walkers are characterized by a mediolaterally reinforced cross-section that becomes more circular as gait continues to develop. DISCUSSION: During ontogeny the femur undergoes distinct morphological phases, which correspond with changes in loading regime. This study illustrates the importance of loading conditions in shaping immature bone morphology. Nonmechanical factors such as changes in hormonal environmental can also impact on this dynamic.

Sex differences in the patterning of age-related bone loss in the human hallucal metatarsal in rural and urban populations.

OBJECTIVES: Age-degenerative features of the metatarsals are poorly known despite the importance of metatarsal bone properties for investigating mobility patterns. We assessed the role of habitual activity in shaping the patterning and magnitude of sexual dimorphism in age-related bone loss in the hallucal metatarsal. MATERIALS AND METHODS: Cross-sections were extracted at midshaft from micro-computed tomography scan models of individuals from medieval rural (Abingdon Vineyard) and early industrial urban (Spitalfields) settings (n = 71). A suite of cross-sectional geometry dimensions and biomechanical properties were compared between populations. RESULTS: The rural group display generally stronger and larger metatarsals that show a greater capacity to resist torsion and that have comparatively greater bending strength along the medio-lateral plane. Men in both groups show greater values of cortical area than women, but only in the urban group do men show lower magnitudes of age-related decline compared to females. Women in rural and urban populations show different patterns of age-related decline in bone mass, particularly old women in the urban group show a marked decline in cortical area that is absent for women in the rural group. DISCUSSION: Lifetime exposure to hard, physical activity in an agricultural setting has contributed to the attainment of greater bone mass and stronger bones in young adults. Furthermore, over the life-course, less of this greater amount of bone is lost, such that sustained activity levels may have acted to buffer against age-related decline, and this is most pronounced for women, who are expected to experience greater bone loss later in life than men.

Postnatal development of the largest subterranean mammal (Bathyergus suillus): Morphology, osteogenesis, and modularity of the appendicular skeleton.

BACKGROUND: Subterranean mammals show a suite of musculoskeletal adaptations that enables efficient digging. However, little is known about their development. We assessed ontogenetic changes in functionally relevant skeletal traits, and ossification patterns (periosteal and endochondral bone modules) in a truly subterranean scratch-digging rodent, Bathyergus. We studied 52 individuals (202 long bones) from a wild population by using a multiscale approach involving internal and external morphology. RESULTS: Multivariate analysis showed significant morphological changes during ontogeny. A specialized phenotype is expressed perinatally (eg, greater external robustness and developed olecranon, teres major, and deltoid processes), whereas adults presented slender bones with significantly thicker cross-sections. Ossification modules scaled mostly isometrically with body size parameters. Periosteal modules showed high variability and tended to grow faster than endochondral modules. CONCLUSIONS: Scratch-digging adaptations appear at perinatal age and then specialize in subadults. Early development of agonistic and digging behaviors and onset of sexual maturation seems to contribute to its development, although genetic factors also seem to play an important role. Ontogenetic differences are probably a trade-off to counteract weaker cortical bone properties and poor muscle development in juveniles, whereas slender but thicker cortical bones maximize bone resistance during burrow construction without compromising locomotor performance in adults.

Postcranial heterochrony, modularity, integration and disparity in the prenatal ossification in bats (Chiroptera).

BACKGROUND: Self-powered flight is one of the most energy-intensive types of locomotion found in vertebrates. It is also associated with a range of extreme morpho-physiological adaptations that evolved independently in three different vertebrate groups. Considering that development acts as a bridge between the genotype and phenotype on which selection acts, studying the ossification of the postcranium can potentially illuminate our understanding of bat flight evolution. However, the ontogenetic basis of vertebrate flight remains largely understudied. Advances in quantitative analysis of sequence heterochrony and morphogenetic growth have created novel approaches to study the developmental basis of diversification and the evolvability of skeletal morphogenesis. Assessing the presence of ontogenetic disparity, integration and modularity from an evolutionary approach allows assessing whether flight may have resulted in evolutionary differences in the magnitude and mode of development in bats. RESULTS: We quantitatively compared the prenatal ossification of the postcranium (24 bones) between bats (14 species), non-volant mammals (11 species) and birds (14 species), combining for the first time prenatal sequence heterochrony and developmental growth data. Sequence heterochrony was found across groups, showing that bat postcranial development shares patterns found in other flying vertebrates but also those in non-volant mammals. In bats, modularity was found as an axial-appendicular partition, resembling a mammalian pattern of developmental modularity and suggesting flight did not repattern prenatal postcranial covariance in bats. CONCLUSIONS: Combining prenatal data from 14 bat species, this study represents the most comprehensive quantitative analysis of chiropteran ossification to date. Heterochrony between the wing and leg in bats could reflect functional needs of the newborn, rather than ecological aspects of the adult. Bats share similarities with birds in the development of structures involved in flight (i.e. handwing and sternum), suggesting that flight altriciality and early ossification of pedal phalanges and sternum are common across flying vertebrates. These results indicate that the developmental modularity found in bats facilitates intramodular phenotypic diversification of the skeleton. Integration and disparity increased across developmental time in bats. We also found a delay in the ossification of highly adaptable and evolvable regions (e.g. handwing and sternum) that are directly associated with flight performance.

Prenatal allometric trajectories and the developmental basis of postcranial phenotypic diversity in bats (Chiroptera).

Most morphological and physiological adaptations associated with bat flight are concentrated in the postcranium, reflecting strong functional demands for flight performance. Despite an association between locomotory diversity and trophic differentiation, postcranial morphological diversity in bats remains largely unexplored. Evolutionary developmental biology is a novel approach providing a link between the analysis of genotypic and phenotypic variation resulting from selective pressures. To quantify the morphological diversity of the postcranium in bats and to explore its developmental basis, we reconstructed the postcranial allometric trajectories of nine bat species from different prenatal developmental series, representing five families and both suborders. We tested for allometric growth in Chiroptera and also quantified levels of allometric disparity and inter-trajectory distances. Using a phylogenetic scaffold, we assessed whether ontogenetic differences reflect evolutionary relationships. We found significant allometric growth trajectories in almost all species. Interspecific trajectory distances showed lower variance within Yinpterochiroptera than within Yangochiroptera and between suborders. Each suborder occupied nonoverlapping sections of allometric space, showing changes in the growth rates of specific bones for each suborder. The allometry-corrected disparity was significantly higher in larger species. Statistically significant phylogenetic signal in our results suggests that there is an ontogenetic basis for the postcranial morphological diversity in modern bats. Ancestral state reconstruction also showed an increase in the amount of change in shape with size in the larger species studied. We hypothesize that differences in allometric patterns among bat taxa may reflect a size-dependent evolutionary constraint, whereby variability in body size and allometric patterns are associated.

The Australian dingo: untamed or feral?

BACKGROUND: The Australian dingo continues to cause debate amongst Aboriginal people, pastoralists, scientists and the government in Australia. A lingering controversy is whether the dingo has been tamed and has now reverted to its ancestral wild state or whether its ancestors were domesticated and it now resides on the continent as a feral dog. The goal of this article is to place the discussion onto a theoretical framework, highlight what is currently known about dingo origins and taxonomy and then make a series of experimentally testable organismal, cellular and biochemical predictions that we propose can focus future research. DISCUSSION: We consider a canid that has been unconsciously selected as a tamed animal and the endpoint of methodical or what we now call artificial selection as a domesticated animal. We consider wild animals that were formerly tamed as untamed and those wild animals that were formerly domesticated as feralized. Untamed canids are predicted to be marked by a signature of unconscious selection whereas feral animals are hypothesized to be marked by signatures of both unconscious and artificial selection. First, we review the movement of dingo ancestors into Australia. We then discuss how differences between taming and domestication may influence the organismal traits of skull morphometrics, brain and size, seasonal breeding, and sociability. Finally, we consider cellular and molecular level traits including hypotheses concerning the phylogenetic position of dingoes, metabolic genes that appear to be under positive selection and the potential for micronutrient compensation by the gut microbiome. CONCLUSIONS: Western Australian Government policy is currently being revised to allow the widespread killing of the Australian dingo. These policies are based on an incomplete understanding of the evolutionary history of the canid and assume the dingo is feralized. However, accumulated evidence does not definitively show that the dingo was ever domesticated and additional focused research is required. We suggest that incorporating ancient DNA data into the debate concerning dingo origins will be pivotal to understanding the evolutionary history of the canid. Further, we advocate that future morphological, behavioural and genetic studies should focus on including genetically pure Alpine and Desert dingoes and not dingo-dog hybrids. Finally, we propose that future studies critically examine genes under selection in the dingo and employ the genome from a wild canid for comparison.

Shape variation and modularity of skull and teeth in domesticated horses and wild equids.

BACKGROUND: In horses, the morphological changes induced by the process of domestication are reportedly less pronounced than in other species, such as dogs or pigs - although the horses' disparity has rarely been empirically tested. We investigated shape differences and modularity of domesticated horses, Przewalski's horses, donkeys and zebras. Mandibular and tooth shape have been shown to be valuable features for differentiating wild and domesticated forms in some mammals. RESULTS: Both mandible and teeth, show a pattern of shape space occupation analogous to that of the cranium, with domesticated horses occupying a similar extension in shape space to that of wild equids. Only cranial shape data exhibit a tendency to separate domesticated horses and Przewalski's horses from donkeys and zebras. Maximum likelihood model-based tests confirm the horse cranium is composed of six developmental modules, as reported for placental mammals in general. The magnitude of integration in domesticated horse skull was lower than in wild equids across all six cranial modules, and lower values of integration were associated with higher disparity values across all modules. CONCLUSION: This is the first study that combines different skeletal features for the description and comparison of shape changes in all living equid groups using geometric morphometrics. We support Darwin's hypothesis that the shape variation in the skull of domesticated horses is similar to the shape variation of all wild equid species existing today. Lower magnitudes of module integration are recovered in domesticated horses compared to their wild relatives.

Quantification of 3D curvature in the iliac crest: Ontogeny and implications for sex determination in juveniles.

OBJECTIVES: Sex-specific differences in the shape of the iliac crest have been reported based on quantification of simplified curvature using two-dimensional variables or qualitative identification of faint/marked S-shaped curvature. An objective and quantitative approach for the evaluation of iliac crest curvature in juveniles was developed. Using three-dimensional data, ontogenetic variation in iliac curvature was investigated in relation to size and chronological age and its accuracy for sex estimation among juveniles was evaluated. MATERIALS AND METHODS: Three-dimensional geometric morphometric landmarks were collected from virtual models of 53 surface-scanned ilia and used to: (1) quantify differences between male and female shape, and (2) to calculate a measure of sinuosity by extracting linear measurements relative to a plane constructed in virtual space. RESULTS: Males were found to exhibit on average more pronounced curvature of the iliac crest than females in equivalent age groups, and the difference between sexes was more marked on the posterior section of the bone than on the anterior section. Classification accuracy was higher for measurements of sinuosity relative to a three-dimensional plane (62-87%) than for landmarks describing crest shape (64-74%). CONCLUSIONS: An increased likelihood of correct male identification (reduced rate of false positive identification) was observed in older children (>5 years). This reflects divergence of male and female iliac crest shape with size (or age), and the development of a unique, more curved iliac crest shape in males. The results suggest more conspicuous male trait expression than female trait expression.

Moa diet fits the bill: virtual reconstruction incorporating mummified remains and prediction of biomechanical performance in avian giants.

The moa (Dinornithiformes) are large to gigantic extinct terrestrial birds of New Zealand. Knowledge about niche partitioning, feeding mode and preference among moa species is limited, hampering palaeoecological reconstruction and evaluation of the impacts of their extinction on remnant native biota, or the viability of exotic species as proposed ecological 'surrogates'. Here we apply three-dimensional finite-element analysis to compare the biomechanical performance of skulls from five of the six moa genera, and two extant ratites, to predict the range of moa feeding behaviours relative to each other and to living relatives. Mechanical performance during biting was compared using simulations of the birds clipping twigs based on muscle reconstruction of mummified moa remains. Other simulated food acquisition strategies included lateral shaking, pullback and dorsoventral movement of the skull. We found evidence for limited overlap in biomechanical performance between the extant emu (Dromaius novaehollandiae) and extinct upland moa (Megalapteryx didinus) based on similarities in mandibular stress distribution in two loading cases, but overall our findings suggest that moa species exploited their habitats in different ways, relative to both each other and extant ratites. The broad range of feeding strategies used by moa, as inferred from interspecific differences in biomechanical performance of the skull, provides insight into mechanisms that facilitated high diversities of these avian herbivores in prehistoric New Zealand.

Evolution of neck vertebral shape and neck retraction at the transition to modern turtles: an integrated geometric morphometric approach.

The unique ability of modern turtles to retract their head and neck into the shell through a side-necked (pleurodiran) or hidden-necked (cryptodiran) motion is thought to have evolved independently in crown turtles. The anatomical changes that led to the vertebral shapes of modern turtles, however, are still poorly understood. Here we present comprehensive geometric morphometric analyses that trace turtle vertebral evolution and reconstruct disparity across phylogeny. Disparity of vertebral shape was high at the dawn of turtle evolution and decreased after the modern groups evolved, reflecting a stabilization of morphotypes that correspond to the two retraction modes. Stem turtles, which had a very simple mode of retraction, the lateral head tuck, show increasing flexibility of the neck through evolution towards a pleurodiran-like morphotype. The latter was the precondition for evolving pleurodiran and cryptodiran vertebrae. There is no correlation between the construction of formed articulations in the cervical centra and neck mobility. An increasing mobility between vertebrae, associated with changes in vertebral shape, resulted in a more advanced ability to retract the neck. In this regard, we hypothesize that the lateral tucking retraction of stem turtles was not only the precondition for pleurodiran but also of cryptodiran retraction. For the former, a kink in the middle third of the neck needed to be acquired, whereas for the latter modification was necessary between the eighth cervical vertebra and first thoracic vertebra. Our paper highlights the utility of 3D shape data, analyzed in a phylogenetic framework, to examine the magnitude and mode of evolutionary modifications to vertebral morphology. By reconstructing and visualizing ancestral anatomical shapes, we provide insight into the anatomical features underlying neck retraction mode, which is a salient component of extant turtle classification.

A virtual geometric morphometric approach to the quantification of long bone bilateral asymmetry and cross-sectional shape.

OBJECTIVE: The widespread use of three-dimensional digitization means that models of whole bone geometry are routinely captured for many applications in virtual anthropology. In this article, we test whether a geometric morphometric (GMM) approach can be used to accurately quantify directional bilateral asymmetry in cross-sectional shape using virtual bone models. METHODS: We introduce a method to (1) orient virtual long bone models and extract cross sections and (2) collect GMM landmarks to analyze cross-sectional shape. Landmark data, captured using polar radii, were analyzed for an example study set of paired humeri from Andaman Islanders sampled at 35%, 50%, and 65% locations. RESULTS: The GMM method can (1) detect significant differences in left and right cross-sectional shape and (2) allow the directionality of shape change (distribution of bone from centroid) to be evaluated in the context of whole outline shape and in relation to the axis of maximum bending rigidity. The protocol may be used with models created from laser (surface) scan or computed tomography (CT) scan data, and applied to cross-sectional images that were collected using periosteal molding techniques. CONCLUSIONS: The results demonstrate directional bilateral asymmetry in shape, but do not recover the same signal for size measurements. Our method offers a pathway to quantify both the pattern of variation in shape and the relationship between size and shape variation, opening new questions about how those patterns manifest over ontogeny, change temporally or differ in relation to the nature and intensity of the activity, and bone loading conditions.

Shape, size, and maturity trajectories of the human ilium.

Morphological traits of the ilium have consistently been more successful for juvenile sex determination than have techniques applied to other skeletal elements, however relatively little is known about the ontogeny and maturation of size and shape dimorphism in the ilium. We use a geometric morphometric approach to quantitatively separate the ontogeny of size and shape of the ilium, and analyze interpopulation differences in the onset, rate and patterning of sexual dimorphism. We captured the shape of three traits for a total of 191 ilia from Lisbon (Portugal) and London (UK) samples of known age and sex (0-17 years). Our results indicate that a) there is a clear dissociation between the ontogeny of size and shape in males and females, b) the ontogeny of size and shape are each defined by non-linear trajectories that differ between the sexes, c) there are interpopulation differences in ontogenetic shape trajectories, which point to population-specific patterning in the attainment of sexual dimorphism, and d) the rate of shape maturation and size maturation is typically higher for females than males. Male and female shape differences in the ilium are brought about by trajectory divergence. Differences in size and shape maturation between the sexes suggest that maturity may confound our ability to discriminate between the sexes by introducing variation not accounted for in age-based groupings. The accuracy of sex determination methods using the ilium may be improved by the use of different traits for particular age groups, to capture the ontogenetic development of shape in both sexes.

Quantifying evolutionary development using non-model organisms: integrating morphology, metrical frameworks, and gene expression.

Keeping pace with the forward progression of evolutionary developmental studies and their trajectory toward ever-more integrative and broad-scale study presents a challenge for researchers from diverse disciplines. Increasing the capacity for discourse and opening opportunity to further interdisciplinary work is highly desirable, and one way that activities can be hindered is through a lack of communication between those developing new methods and those applying methods to new data sets. The goal of this special issue, which brings together contributions from a recent symposium at the 10(th) International Congress for Vertebrate Morphology (ICVM 10, Barcelona, July 2013) along with select additions, was to integrate methodological developments with molecular and morphological data to present a broad spectrum of avenues for investigating ontogeny in land vertebrates. A balance between methods-focused papers and papers presenting novel data and perspectives from molecular and morphological approaches in evo-devo was sought with the hope of promoting greater interchange between each side, and drawing attention to new opportunities for future research on non-model organisms.

Regionalization of the axial skeleton in the 'ambush predator' guild--are there developmental rules underlying body shape evolution in ray-finned fishes?

BACKGROUND: A long, slender body plan characterized by an elongate antorbital region and posterior displacement of the unpaired fins has evolved multiple times within ray-finned fishes, and is associated with ambush predation. The axial skeleton of ray-finned fishes is divided into abdominal and caudal regions, considered to be evolutionary modules. In this study, we test whether the convergent evolution of the ambush predator body plan is associated with predictable, regional changes in the axial skeleton, specifically whether the abdominal region is preferentially lengthened relative to the caudal region through the addition of vertebrae. We test this hypothesis in seven clades showing convergent evolution of this body plan, examining abdominal and caudal vertebral counts in over 300 living and fossil species. In four of these clades, we also examined the relationship between the fineness ratio and vertebral regionalization using phylogenetic independent contrasts. RESULTS: We report that in five of the clades surveyed, Lepisosteidae, Esocidae, Belonidae, Sphyraenidae and Fistulariidae, vertebrae are added preferentially to the abdominal region. In Lepisosteidae, Esocidae, and Belonidae, increasing abdominal vertebral count was also significantly related to increasing fineness ratio, a measure of elongation. Two clades did not preferentially add abdominal vertebrae: Saurichthyidae and Aulostomidae. Both of these groups show the development of a novel caudal region anterior to the insertion of the anal fin, morphologically differentiated from more posterior caudal vertebrae. CONCLUSIONS: The preferential addition of abdominal vertebrae in fishes with an elongate body shape is consistent with the existence of a conservative positioning module formed by the boundary between the abdominal and caudal vertebral regions and the anterior insertion of the anal fin. Dissociation of this module is possible, although less probable than changes in the independently evolving abdominal region. Dissociation of the axial skeleton-median fin module leads to increased regionalization within the caudal vertebral column, something that has evolved several times in bony fishes, and may be homologous with the sacral region of tetrapods. These results suggest that modularity of the axial skeleton may result in somewhat predictable evolutionary outcomes in bony fishes.

Evolution: Mislabeling marsupial development as primitive.

Compared to placentals, marsupial mammals have previously been considered primitive in terms of their reproductive biology. A new study suggests that, rather, marsupials represent a derived state of mammalian development, and the ancestral therian mammal developed like placentals do today.

Assessing the association of skeletal indicators of stress with mean age-at-death in sub-adults.

OBJECTIVES: The present study investigated the association of skeletal indicator of stress presence with mean age-at-death as a means of understanding whether commonly studied indicators are indeed indicative of increased frailty. MATERIALS AND METHODS: Using a medieval Gaelic population from Ballyhanna (Co. Donegal), the present study assessed the association between skeletal indicators of stress and mean age-at-death using the Kaplan-Meier survival function with log rank test to determine whether these indicators were associated with younger age-at-death, and therefore increased frailty, in sub-adults only (0 to 18 years, N = 139) and through comparison to an all-ages cohort (N = 318). RESULTS: Only linear enamel hypoplasia was found to be associated with significantly decreased survivorship across the all-ages cohort but, conversely, was associated with increased survivorship when analysis was restricted to sub-adults. All other indicators assessed were associated with increased age-at-death for both all-age cohorts and sub-adult cohorts (cribra orbitalia), increased age-at-death when assessing all ages only (porotic hyperostosis and healed periosteal lesions); or were sufficiently rare in adults to prevent comparative analysis (stunting and micronutrient deficiency). Increased survivorship in individuals with higher numbers of co-morbid skeletal indicators was observed for both sub-adults alone and all age cohort. DISCUSSION: These findings suggest that these commonly recorded skeletal indicators may be more accurately viewed simply as records of stressor exposure and subsequent survival only, rather than providing evidence that these sub-adults are frailer than their similarly aged-at-death peers. Thus, the demographic and sociocultural context is essential to the interpretation of observed skeletal indicators of stress.