Heterochrony and post-natal growth in mammals - an examination of growth plates in limbs

From slenderness to robustness: Understanding long bone shape in sigmodontine rodents

The morphological evolution of the appendicular skeleton may reflect the selective pressures specific to different environments, phylogenetic inheritance, or allometry. Covariation in bone shapes enhances morphological integration in response to ecological specializations. In contrast to previous multivariate studies using classical linear morphometry, we use a geometric morphometric approach to explore the morphological diversity of long bones and examine relationships between ecological categories and morphological characters in a species-rich and ecomorphologically diverse group of rodents. We examined the humerus, ulna, femur, and tibiofibula of 19 sigmodontine species with different locomotor types (ambulatory, quadrupedal-saltatorial, natatorial, semifossorial and scansorial) to investigate the influence of locomotor type and phylogeny on limb bone shape and morphological integration of the appendicular skeleton. This study represents the most detailed examination of the morphological diversity of long bones in sigmodontines, employing geometric morphometrics within an ecomorphological framework. Our results indicate that functional demands and evolutionary history jointly influence the shape of forelimb and hindlimb bones. The main variation in bone shape is associated with a slenderness-robustness gradient observed across all ecological categories. Quadrupedal-saltatorial species, with their need for agility, possess slender and elongated limbs, while natatorial and semifossorial species exhibit shorter and more robust bone shapes, suited for their respective environments. This gradient also influences bone covariation within limbs, demonstrating interconnectedness between elements. We found functional covariation between the ulna-tibiofibula and humerus-tibiofibula, likely important for propulsion, and anatomical covariation between the humerus-ulna and femur-tibiofibula, potentially reflecting overall limb structure. This study demonstrates that the versatile morphology of long bones in sigmodontines plays a critical role in their remarkable ecological and phylogenetic diversification.

Transcriptome-based comparison reveals key genes regulating allometry growth of forelimb and hindlimb bone in duck embryos

Allometric growth of the forelimb and hindlimb is a widespread phenomenon observed in vertebrates. As a typical precocial bird, ducks exhibit more advanced development of their hindlimbs compared to their forelimbs, enabling them to walk shortly after hatching. This phenomenon is closely associated with the development of long bones in the embryonic stage. However, the molecular mechanism governing the allometric growth of duck forelimb and hindlimb bones is remains elusive. In this study, we employed phenotypic, histological, and gene expression analyses to investigate developmental differences between the humerus (forelimb bone) and tibia/femur (hindlimb bones) in duck embryos. Our results revealed a gradual increase in weight and length disparity between the tibia and humerus from E12 to E28 (embryo age). At E12, endochondral ossification was observed solely in the tibia but not in the humerus. The number of differentially expressed genes (DEGs) gradually increased at H12 vs. T12, H20 vs. T20, and H28 vs. T28 stages consistent with phenotypic variations. A total of 38 DEGs were found across all 3 stages. Protein-protein interaction network analysis demonstrated strong interactions among members of HOXD gene family (HOXD3/8/9/10/11/12), HOXB gene family (HOXB8/9), TBX gene family (TBX4/5/20), HOXA11, SHOX2, and MEIS2. Gene expression profiling indicated higher expression levels for all HOXD genes in the humerus compared to tibia while opposite trends were observed for HOXA/HOXB genes with low or no expression detected in the humerus. These findings suggest distinct roles played by different clusters within HOX gene family during skeletal development regulation of duck embryo's forelimbs versus hind limbs. Notably, TBX4 exhibited high expression levels specifically in tibia whereas TBX5 showed similar patterns exclusively within humerus as seen previously across other species' studies. In summary, this study identified key regulatory genes involved in allometric growth of duck forelimb and hindlimb bones during embryonic development. Skeletal development is a complex physiological process, and further research is needed to elucidate the regulatory role of candidate genes in endochondral ossification.

A new confuciusornithid bird with a secondary epiphyseal ossification reveals phylogenetic changes in confuciusornithid flight mode

The confuciusornithids are the earliest known beaked birds, and constitute the only species-rich clade of Early Cretaceous pygostylian birds that existed prior to the cladogenesis of Ornithothoraces. Here, we report a new confuciusornithid species from the Lower Cretaceous of western Liaoning, northeastern China. Compared to other confuciusornithids, this new species and the recently reported Yangavis confucii both show evidence of stronger flight capability, although the wings of the two taxa differ from one another in many respects. Our aerodynamic analyses under phylogeny indicate that varying modes of flight adaptation emerged across the diversity of confuciusornithids, and to a lesser degree over the course of their ontogeny, and specifically suggest that both a trend towards improved flight capability and a change in flight strategy occurred in confuciusornithid evolution. The new confuciusornithid differs most saliently from other Mesozoic birds in having an extra cushion-like bone in the first digit of the wing, a highly unusual feature that may have helped to meet the functional demands of flight at a stage when skeletal growth was still incomplete. The new find strikingly exemplifies the morphological, developmental and functional diversity of the first beaked birds.

Development of male-larger sexual size dimorphism in a lizard: IGF1 peak long after sexual maturity overlaps with pronounced growth in males

Squamate reptiles have been considered to be indeterminate growers for a long time. However, recent studies demonstrate that bone prolongation is stopped in many lizards by the closure of bone growth plates. This shift in the paradigm of lizard growth has important consequences for questions concerning the proximate causes of sexual size dimorphism. The traditional model of highly plastic and indeterminate growth would correspond more to a long-term action of a sex-specific growth regulator. On the other hand, determinate growth would be more consistent with a regulator acting in a sex-specific manner on the activity of bone growth plates operating during the phase when a dimorphism in size develops. We followed the growth of males and females of the male-larger Madagascar ground gecko (Paroedura picta) and monitored the activity of bone growth plates, gonad size, levels of steroids, expression of their receptors (AR, ESR1), and expression of genes from the insulin-like growth factor network (IGF1, IGF2, IGF1R, and IGF2R) in livers. Specifically, we measured gene expression before the onset of dimorphic growth, at the time when males have more active bone growth plates and sexual size dimorphism was clearly visible, and after a period of pronounced growth in both sexes. We found a significant spike in the expression of IGF1 in males around the time when dimorphism develops. This overexpression in males comes long after an increase in circulating testosterone levels and sexual maturation in males, and it might be suppressed by ovarian hormones in females. The results suggest that sexual size dimorphism in male-larger lizards can be caused by a positive effect of high levels of IGF1 on bone growth. The peak in IGF1 resembles the situation during the pubertal growth spurt in humans, but in lizards, it seems to be sex-specific and disconnected from sexual maturation.

Sex-specific growth arrest in a lizard

(1) In contrast to mammals and birds, reptiles have been considered as indeterminate growers, whose growth reflects differential allocation of resources to growth versus other energetically demanding processes such as reproduction. (2) We monitored the growth and activity of bone growth plates, hormonal profiles, and reproductive activity in males and females of the male-larger gecko Paroedura picta. We show that growth plates fuse in this species in a sex-specific manner. The more abrupt epiphyseal closure and more pronounced growth deceleration in females coincide with the increased activity of their reproductive organs. (3) We conclude that at least some lizards are determinate growers whose sexual size dimorphism is potentially driven by ovarian hormones. The major difference in growth between endothermic and ectothermic amniotes appears to be in the magnitude of growth before and after the first reproduction, not in the mechanistic processes such as senescence of growth plate cells

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We monitored activity of bone growth plates in a male-larger gecko

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Growth plates fused in a sex-specific manner

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At least some lizards are determinate growers

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Their sexual size dimorphism seems to be driven by ovarian hormones

Bone remodeling in the longest living rodent, the naked mole-rat: Interelement variation and the effects of reproduction

The pattern of bone remodeling of one of the most peculiar mammals in the world, the naked mole-rat (NMR), was assessed. NMRs are known for their long lifespans among rodents and for having low metabolic rates. We assessed long-term in vivo bone labeling of subordinate individuals, as well as the patterns of bone resorption and bone remodeling in a large sample including reproductive and non-reproductive individuals (n = 70). Over 268 undecalcified thin cross-sections from the midshaft of humerus, ulna, femur and tibia were analyzed with confocal fluorescence and polarized light microscopy. Fluorochrome analysis revealed low osteogenesis, scarce bone resorption and infrequent formation of secondary osteons (Haversian systems) (i.e., slow bone turnover), thus most likely reflecting the low metabolic rates of this species. Secondary osteons occurred regardless of reproductive status. However, considerable differences in the degree of bone remodeling were found between breeders and non-breeders. Pre-reproductive stages (subordinates) exhibited quite stable skeletal homeostasis and bone structure, although the attainment of sexual maturity and beginning of reproductive cycles in female breeders triggered a series of anabolic and catabolic processes that up-regulate bone turnover, most likely associated with the increased metabolic rates of reproduction. Furthermore, bone remodeling was more frequently found in stylopodial elements compared to zeugopodial elements. Despite the limited bone remodeling observed in NMRs, the variation in the pattern of skeletal homeostasis (interelement variation) reported here represents an important aspect to understand the skeletal dynamics of a small mammal with low metabolic rates. Given the relevance of the remodeling process among mammals, this study also permitted the comparison of such process with the well-documented histomorphology of extinct therapsids (i.e., mammalian precursors), thus evidencing that bone remodeling and its endocortical compartmentalization represent ancestral features among the lineage that gave rise to mammals. It is concluded that other factors associated with development (and not uniquely related to biomechanical loading) can also have an important role in the development of bone remodeling.

The epiphyseal secondary ossification center: Evolution, development and function

Bone age is used widely by pediatricians to assess the skeletal maturity of a child and predict growth potential. This entails measuring the size of secondary ossification centers (SOCs), which develop with age in the ends of long bones, which are initially cartilaginous. However, little is presently known about the developmental biology, evolution and functional role of these skeletal elements. Here, we summarize the knowledge currently available in this area and discuss potential primary functions of the SOC.

Eco-evolutionary adaptations of ochotonids (Mammalia: Lagomorpha) to islands: new insights into Late Miocene pikas from the Gargano palaeo-archipelago (Italy)

Climate change strongly affects the range of ochotonids (Order Lagomorpha), fragmenting their habitats and restricting them to ecological islands. The present paper discusses the adaptations of extinct ochotonids to insular stressors, providing baseline data for the management and conservation of extant species. For this purpose, the body mass (BM) and locomotion of the endemic Prolagus apricenicus and Prolagus imperialis from the Gargano palaeo archipelago (Late Miocene) were assessed. P. apricenicus was a small-sized ochotonid (BM 150–250 g) and P. imperialis was probably the largest Prolagus that ever lived (BM 500–750 g). The eco-evolutionary BM dynamics suggest a targeted ecological niche for P. apricenicus, whereas the BM of P. imperialis rose abruptly as a result of growth-rate increase. In both species, the locomotion was stable and less cursorial, with leaping skills, resembling extant rocky ochotonids. Convergent eco-evolutionary patterns are observed in extinct insular ochotonids, concerning an increase of BM (giants), more efficient chewing, less cursorial and more stable locomotion, leaping skills, as well as a slower life history (longer lifespan). Such adaptations are triggered by the specific selective pressures of insular regimes. The present results point to the long-lasting insular Prolagus species as reference taxa for addressing the management of extant rocky ochotonids.

Ontogenetic growth and the development of a unique fibrocartilage entheses in Macropus fuliginosus

Here we examine the bone histology of the femora and humeri of the Western Grey Kangaroo, Macropus fuliginosus. Our results reveal that bone modelling in response to ontogenetic growth and the development of tuberosities on the femur, and especially in the humerus, lead to a highly complex histology. We propose that the alternating fast and slow rates of bone deposition are seasonal, and are likely correlated with heterothermy related to ecological constraints during the summer months. In females, after the fourth growth mark in the femur, there is a distinctive change to a more lamellar textured bone deposition with sparse vascularisation, directly indicating a slowdown in growth. However, in males, the zones remain woven textured and well vascularised, which is indicative of continued fast growth. Here we also report the novel occurrence of a fibrocartilaginous entheses for the attachment of the m. quadratus femoris to the caudal femoral tuberosity. Using a combination of methodologies, we show that perimeter measurements of growth marks provide a reasonable estimation of the age of kangaroos. Additionally, we observed large individuals that have ceased diaphyseal appositional growth of the femur and the humerus, as well as fusion of the distal epiphyses of both bones, though the proximal epiphyses may remain unfused.

Secondary ossification center induces and protects growth plate structure

Growth plate and articular cartilage constitute a single anatomical entity early in development but later separate into two distinct structures by the secondary ossification center (SOC). The reason for such separation remains unknown. We found that evolutionarily SOC appears in animals conquering the land - amniotes. Analysis of the ossification pattern in mammals with specialized extremities (whales, bats, jerboa) revealed that SOC development correlates with the extent of mechanical loads. Mathematical modeling revealed that SOC reduces mechanical stress within the growth plate. Functional experiments revealed the high vulnerability of hypertrophic chondrocytes to mechanical stress and showed that SOC protects these cells from apoptosis caused by extensive loading. Atomic force microscopy showed that hypertrophic chondrocytes are the least mechanically stiff cells within the growth plate. Altogether, these findings suggest that SOC has evolved to protect the hypertrophic chondrocytes from the high mechanical stress encountered in the terrestrial environment.

Bilateral radial and ulnar fractures in a red kangaroo (Macropus rufus)

CASE DESCRIPTION

An 11-month-old sexually intact male red kangaroo (Macropus rufus) was examined because of bilateral radial and ulnar fractures.

CLINICAL FINDINGS

Radiography of the forelimbs revealed bilateral, short oblique fractures in the proximal to mid diaphyses of the radii and ulnae. Fractures were overriding and moderately displaced. Wider than expected gaps were evident in the humeroulnar and humeroradial joints bilaterally. Although several physes remained open, no proximal radial physis was radiographically evident.

TREATMENT AND OUTCOME

Dual bone fixation was performed bilaterally, and dynamic luxation of the left radial head was identified and stabilized intraoperatively. Although satisfactory function of both forelimbs was evident at 8 weeks and 26 months after surgery, a persistent gait abnormality affecting the right forelimb was noted. Twenty-six months after surgery, radiography revealed bilateral proximal radial physes and resolution of the abnormally wide gaps in the humeroradial and humeroulnar joints. Despite dual bone fixation, synostoses formed bilaterally and may have contributed to the persistent lameness in the kangaroo's right forelimb.

CLINICAL RELEVANCE

Veterinarians treating kangaroos should be aware of difficulties in determining skeletal maturity and planning fracture stabilization because of potential differences in skeletal growth and fracture healing, compared with other species. We described critical issues observed in the treatment and outcome of the kangaroo of the present report and provided lessons learned as well as potential explanations of these issues to facilitate future treatment of kangaroos with forelimb fractures.

Endochondral ossification and the evolution of limb proportions

Mammals have remarkably diverse limb proportions hypothesized to have evolved adaptively in the context of locomotion and other behaviors. Mechanistically, evolutionary diversity in limb proportions is the result of differential limb bone growth. Longitudinal limb bone growth is driven by the process of endochondral ossification, under the control of the growth plates. In growth plates, chondrocytes undergo a tightly orchestrated life cycle of proliferation, matrix production, hypertrophy, and cell death/transdifferentiation. This life cycle is highly conserved, both among the long bones of an individual, and among homologous bones of distantly related taxa, leading to a finite number of complementary cell mechanisms that can generate heritable phenotype variation in limb bone size and shape. The most important of these mechanisms are chondrocyte population size in chondrogenesis and in individual growth plates, proliferation rates, and hypertrophic chondrocyte size. Comparative evidence in mammals and birds suggests the existence of developmental biases that favor evolutionary changes in some of these cellular mechanisms over others in driving limb allometry. Specifically, chondrocyte population size may evolve more readily in response to selection than hypertrophic chondrocyte size, and extreme hypertrophy may be a rarer evolutionary phenomenon associated with highly specialized modes of locomotion in mammals (e.g., powered flight, ricochetal bipedal hopping). Physical and physiological constraints at multiple levels of biological organization may also have influenced the cell developmental mechanisms that have evolved to produce the highly diverse limb proportions in extant mammals. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Organ System Comparisons Between Species.

Island rule and bone metabolism in fossil murines from Timor

Skeletal growth rates reconstructed from bone histology in extinct insular hippopotamids, elephants, bovids and sauropods have been used to infer dwarfism as a response to island conditions. Limited published records of osteocyte lacunae densities (Ot.Dn), a proxy for living osteocyte proliferation, have suggested a slower rate of bone metabolism in giant mammals. Here, we test whether insularity might have affected bone metabolism in a series of small to giant murine rodents from Timor. Ten adult femora were selected from a fossil assemblage dated to the Late Quaternary (~5000–18 000 years old). Femur morphometric data were used in computing phylogenetically informed body mass regressions, although the phylogenetic signal was very low (Pagel’s λ = 0.03). Estimates of body weight calculated from these femora ranged from 75 to 1188 g. Osteocyte lacunae densities from histological sections of the midshaft femur were evaluated against bone size and estimated body weight. Statistically significant (P < 0.05) and strongly negative relationships between Ot.Dn, femur size and estimated weight were found. Larger specimens were characterized by lower Ot.Dn, indicating that giant murines from Timor might have had a relatively slow pace of bone metabolic activity, consistent with predictions made by the island rule.

Marsupial versus placental: assessing the evolutionary changes in the scapula of didelphids and sigmodontines

It is not a new concept that marsupials and placentals are distant and distinct clades among mammals. In South America, these animals coexist, occupy similar niches and, in some cases, are similar in appearance. This is especially true with respect to the locomotor categories of smaller rodents belonging to the family Cricetidae or, more specifically, the subfamily Sigmodontinae, compared with the marsupials of the Didelphidae family. In this study, we have investigated both the similarities and the differences between the two clades by examining locomotion-dependent adaptation, a crucial survival mechanism that has affected the morphology of both clades. We applied geometric morphometrics to quantify the shape of the scapula, which is a very adaptable structure. We found similar morphological adaptations between the clades, especially with respect to adaptation to life in trees. Moreover, Didelphidae are influenced by phylogenetic history to a greater extent than Sigmodontinae with regard to variation of scapula shape and allometry. These differences can be explained by the greater degree of body size variation that exists within the Didelphidae. Didelphidae have an ancient evolutionary history in South America compared with the Sigmodontinae, which have undergone a very successful and rapid diversification more recently.

Developmental differences between two marine turtle species and potential consequences for their survival at hatching

Anatomical variation is a major source for natural selection. Marine turtles are endangered and survival predictions are of important biological, ecological, social, and political value. Here, we perform a preliminary study illustrating how comparative embryology permits understanding of ontogenetic variation as a contributor for evolutionary fitness. To that end, we studied samples of Chelonia mydas and Caretta caretta embryos relative to a standardized staging system from the literature. We examined external anatomy using discrete characters in order to document interspecific variation during development of these species. We employed the 'Standard Event System to Study Vertebrate Embryos' to examine fitness-relevant structures. These include the limb paddles and elbows of Ch. mydas, which differentiate relatively late in ontogeny. We detected interspecific variation in the timing of trait differentiation - such as the egg tooth, closure of skull vault, carapace formation, and scale covering - and propose that these differences might be functionally and ecologically relevant for marine turtles.

The impact of bipedal mechanical loading history on longitudinal long bone growth

Longitudinal bone growth is accomplished through a process where proliferating chondrocytes produce cartilage in the growth plate, which ultimately ossifies. Environmental influences, like mechanical loading, can moderate the growth of this cartilage, which can alter bone length. However, little is known about how specific behaviors like bipedalism, which is characterized by a shift in body mass (mechanical load), to the lower limbs, may impact bone growth. This study uses an experimental approach to induce bipedal behaviors in a rodent model (Rattus norvegicus) over a 12-week period using a treadmill-mounted harness system to test how rat hindlimbs respond to the following loading conditions: 1) fully loaded bipedal walking, 2) partially loaded bipedal walking, 3) standing, 4) quadrupedal walking, and 5) no exercise control. These experimental conditions test whether mechanical loading from 1) locomotor or postural behaviors, and 2) a change in the magnitude of load can moderate longitudinal bone growth in the femur and tibia, relative to controls. The results demonstrate that fully loaded bipedal walking and bipedal standing groups showed significant differences in the percentage change in length for the tibia and femur. When comparing the change from baseline, which control for body mass, all bipedal groups showed significant differences in tibia length compared to control groups. However, there were no absolute differences in bone length, which suggests that mechanical loads from bipedal behaviors may instead be moderating changes in growth velocity. Implications for the relationship between bipedal behaviors and longitudinal bone growth are discussed.

Bone Toolbox: Biomarkers, Imaging Tools, Biomechanics, and Histomorphometry

Bone is a unique tissue with turnover, metabolic, and cellular activities that vary through development to aging and with a mineralized matrix in which the current state and the history of a bone coexist. Qualitative histopathology often lacks sensitivity to detect changes in bone formation, mineralization and resorption, which often requires chronic dosing to result in structural changes such as variation in bone mass and geometry. A large panel of modalities can be used to fully analyze the health of the skeleton, including biomarker evaluation in serum or urine, imaging techniques ranging from radiology to computed tomography, biomechanical testing, and undecalcified tissue processing with bone histomorphometry. The use of clinically relevant biomarkers provides an important noninvasive, sensitive, rapid, and real-time tool to monitor bone activity at the whole skeleton level when conducting safety assessments in a preclinical setting. Imaging modalities also allow in vivo longitudinal assessments with a powerful, noninvasive and clinically translatable tools to monitor drug effects. Different imaging modalities are used in the preclinical studies to evaluate the bone tissues: standard radiography, dual-energy X-ray absorptiometry, peripheral quantitative computed tomography (pQCT), micro-computed tomography, and high-resolution pQCT. Bone histomorphometry is an important tool that provides sensitive evaluation to detect effects of test articles on bone resorption, formation, mineralization, remodeling rates and growth to address a potential target- or class-related theoretical bone liability. Ultimately, the measurement of bone mechanical properties in pharmaceutical testing is critical to understand the potential effects of that pharmaceutical on bone health and fracture risk. Important considerations are required for including these different techniques in toxicology rodents and nonrodent studies, to actually integrate these into safety assessment.

Morphological integration in the appendicular skeleton of two domestic taxa: the horse and donkey

Organisms are organized into suites of anatomical structures that typically covary when developmentally or functionally related, and this morphological integration plays a determinant role in evolutionary processes. Artificial selection on domestic species causes strong morphological changes over short time spans, frequently resulting in a wide and exaggerated phenotypic diversity. This raises the question of whether integration constrains the morphological diversification of domestic species and how natural and artificial selection may impact integration patterns. Here, we study the morphological integration in the appendicular skeleton of domestic horses and donkeys, using three-dimensional geometric morphometrics on 75 skeletons. Our results indicate that a strong integration is inherited from developmental mechanisms which interact with functional factors. This strong integration reveals a specialization in the locomotion of domestic equids, partly for running abilities. We show that the integration is stronger in horses than in donkeys, probably because of a greater degree of specialization and predictability of their locomotion. Thus, the constraints imposed by integration are weak enough to allow important morphological changes and the phenotypic diversification of domestic species.

Artificial selection sheds light on developmental mechanisms of limb elongation

Species diversity in limb lengths and proportions is thought to have evolved adaptively in the context of locomotor and habitat specialization, but the heritable cellular processes that drove this evolution within species are poorly understood. In this study, we take a novel "micro-evo-devo" approach, using artificial selection on relative limb length to amplify phenotypic variation in a population of mice, known as Longshanks, to examine the cellular mechanisms of postnatal limb development that contribute to intraspecific limb length variation. Cross-sectional growth data indicate that differences in bone length between Longshanks and random-bred controls are not due to prolonged growth, but to accelerated growth rates. Histomorphometric and cell proliferation assays on proximal tibial growth plates show that Longshanks' increased limb bone length is associated with an increased number of proliferative chondrocytes. In contrast, we find no differences in other growth plate cellular features known to underlie interspecific differences in limb bone size and shape, such as the rates of chondrocyte proliferation or the size and number of hypertrophic cells in the growth plate. These data suggest that small differences among individuals in the number of proliferating chondrocytes are a potentially important determinant of selectable intraspecific variation in individual limb bone lengths, independent of body size.

Skeleton of an unusual, cat-sized marsupial relative (Metatheria: Marsupialiformes) from the middle Eocene (Lutetian: 44-43 million years ago) of Turkey

We describe a near-complete, three-dimensionally preserved skeleton of a metatherian (relative of modern marsupials) from the middle Eocene (Lutetian: 44–43 million years ago) Lülük member of the Uzunçarşıdere Formation, central Turkey. With an estimated body mass of 3–4 kg, about the size of a domestic cat (Felis catus) or spotted quoll (Dasyurus maculatus), it is an order of magnitude larger than the largest fossil metatherians previously known from the Cenozoic of the northern hemisphere. This new taxon is characterised by large, broad third premolars that probably represent adaptations for hard object feeding (durophagy), and its craniodental morphology suggests the capacity to generate high bite forces. Qualitative and quantitative functional analyses of its postcranial skeleton indicate that it was probably scansorial and relatively agile, perhaps broadly similar in locomotor mode to the spotted quoll, but with a greater capacity for climbing and grasping. Bayesian phylogenetic analysis of a total evidence dataset comprising 259 morphological characters and 9kb of DNA sequence data from five nuclear protein-coding genes, using both undated and “tip-and-node dating” approaches, place the new taxon outside the marsupial crown-clade, but within the clade Marsupialiformes. It demonstrates that at least one metatherian lineage evolved to occupy the small-medium, meso- or hypo-carnivore niche in the northern hemisphere during the early Cenozoic, at a time when there were numerous eutherians (placentals and their fossil relatives) filling similar niches. However, the known mammal fauna from Uzunçarşıdere Formation appears highly endemic, and geological evidence suggests that this region of Turkey was an island for at least part of the early Cenozoic, and so the new taxon may have evolved in isolation from potential eutherian competitors. Nevertheless, the new taxon reveals previously unsuspected ecomorphological disparity among northern hemisphere metatherians during the first half of the Cenozoic.

Evolution of organogenesis and the origin of altriciality in mammals

Mammals feature not only great phenotypic disparity, but also diverse growth and life history patterns, especially in maturity level at birth, ranging from altriciality to precocity. Gestation length, morphology at birth, and other markers of life history are fundamental to our understanding of mammalian evolution. Based on the first synthesis of embryological data and the study of new ontogenetic series, we reconstructed estimates of the ancestral chronology of organogenesis and life-history modes in placental mammals. We found that the ancestor of marsupial and placental mammals was placental-like at birth but had a long, marsupial-like infancy. We hypothesize that mammalian viviparity might have evolved in association with the extension of growth after birth, enabled through lactation, and that mammalian altriciality is inherited from the earliest amniotes. The precocial lifestyle of extant sauropsids and that of many placental mammals were acquired secondarily. We base our conclusions on the best estimates and provide a comprehensive discussion on the methods used and the limitations of our dataset. We provide the most comprehensive embryological dataset ever published, "rescue" old literature sources, and apply available methods and illustrate thus an approach on how to investigate comparatively organogenesis in macroevolution.

Out on a limb: bandicoot limb co-variation suggests complex impacts of development and adaptation on marsupial forelimb evolution

Marsupials display far less forelimb diversity than placentals, possibly because of the laborious forelimb-powered climb to the pouch performed by most marsupial neonates. This is thought to result in stronger morphological integration (i.e., higher co-variance) within the marsupial forelimb skeleton, and lower integration between marsupial fore- and hind limbs, compared to other mammals. Possible mechanisms for this constraint are a fundamental developmental change in marsupial limb patterning, or alternatively more immediate perinatal biomechanical and metabolic requirements. In the latter case, peramelid marsupials (bandicoots), which have neonates that climb very little, should show lower within-limb and higher between-limb integration, compared to other marsupials. We tested this in four peramelid species and the related bilby, using partial correlation analyses of between-landmark linear measurements of limb bones, and Procrustes-based two-block partial least-squares analysis (2B-PLS) of limb bone shapes using the same landmarks. We find extensive between-limb integration in partial correlation analyses of only bone lengths, consistent with a reduction of a short-term biomechanical/allocation constraint in peramelid forelimbs. However, partial correlations of bone proportions and 2B-PLS reveal extensive shape divergence between correlated bone pairs. This result contradicts expectations of developmental constraints or serial homology, instead suggesting a function-driven integration pattern. Comparing visualizations from cross-species principal components analysis and 2B-PLS, we tentatively identify selection for digging and half-bounding as the main driver of bandicoot limb integration patterning. This calls for further assessments of functional versus developmental limb integration in marsupials with a more strenuous neonatal climb to the pouch.

Aggregations and parental care in the Early Triassic basal cynodonts Galesaurus planiceps and Thrinaxodon liorhinus

Non-mammaliaform cynodonts gave rise to mammals but the reproductive biology of this extinct group is still poorly known. Two exceptional fossils of Galesaurus planiceps and Thrinaxodon liorhinus, consisting of juveniles closely associated with an adult, were briefly described more than 50 years ago as examples of parental care in non-mammaliaform cynodonts. However, these two Early Triassic fossils have largely been excluded from recent discussions of parental care in the fossil record. Here we re-analyse these fossils in the context of an extensive survey of other aggregations found in these two basal cynodont taxa. Our analysis revealed six other unequivocal cases of aggregations in Thrinaxodon, with examples of same-age aggregations among immature or adult individuals as well as mixed-age aggregations between subadult and adult individuals. In contrast, only one additional aggregation of Galesauruswas identified. Taking this comprehensive survey into account, the two previously described cases of parental care in Galesaurus and Thrinaxodon are substantiated. The juveniles are the smallest specimens known for each taxon, and the size difference between the adult and the two associated juveniles is the largest found for any of the aggregations. The juveniles of Thrinaxodon are approximately only 37% of the associated adult size; whereas in Galesaurus, the young are at least 60% of the associated adult size. In each case, the two juvenile individuals are similar in size, suggesting they were from the same clutch. Even though parental care was present in both Galesaurus and Thrinaxodon, intraspecific aggregations were much more common in Thrinaxodon, suggesting it regularly lived in aggregations consisting of both similar and different aged individuals.

Unaltered sequence of dental, skeletal, and sexual maturity in domestic dogs compared to the wolf

BACKGROUND

It has been hypothesised that domestication altered the sequence of dental, skeletal, and sexual maturity of dogs when compared to their wolf ancestor. To test this we investigated a comprehensive sample of domestic dogs.

METHODS

We documented the timing of completed eruption of permanent dentition into occlusion (dental maturity) and the timing of growth plate closure at the proximal humerus (skeletal maturity) in ontogenetic series of wolves and 15 domestic dog breeds. Data for 137 domestic dog and 64 wolf individuals were collected based on radiographs and examination of macerated bones.

RESULTS

Our analyses show that domestic dogs exhibit a similar sequence of dental and skeletal maturity as the ancestral wolf. Although the absolute change of the age at attainment of sexual maturity is great in domestic dogs as compared to the wolf, the sequence of dental, skeletal, and sexual maturity is not altered as extensively, contradicting one previous hypothesis. Moreover, our data suggest that the chondrodystrophic dachshund attains skeletal maturity earlier than the non-chondrodystrophic breeds examined here.

CONCLUSIONS

Domestic dogs are more wolf-like in terms of the sequence of dental, skeletal, and sexual maturation than previously hypothesised. This implies that the domestication process and/or breed formation did not have a major impact on this sequence, although the absolute values of life history variables do have a greater range of variation than in the wild wolf.

SURGICAL CORRECTION OF A CERVICAL SPINAL FRACTURE IN A BENNETT'S WALLABY (MACROPUS RUFOGRISEUS)

Macropods are susceptible to trauma, and fractures of the cervical vertebrae due to collisions are relatively common. A 4-yr-old, intact male Bennett's wallaby (Macropus rufogriseus) was presented acutely nonambulatory and tetraparetic but with motor function present in all limbs. Cervical radiographs revealed a C4 vertebral fracture. Surgical stabilization was obtained through a ventral midline approach whereby the vertebral bodies were distracted and then secured with screws and polymethylmethacrylate (PMMA). Postoperative radiographs revealed restoration of the spinal canal and appropriate implant placement. The patient was discharged 4 days postoperatively and gradually recovered full neurologic function per the owner over the ensuing 10 wk. Subsequent radiographs obtained 10.5 mo postoperatively revealed a healed fracture with stable implants. The ventral spinal distraction and stabilization technique using screws and PMMA, as are used in the domestic dog, was successful in this wallaby despite its smaller vertebral size and comparable lack of soft-tissue support.

Heterochrony repolarized: a phylogenetic analysis of developmental timing in plethodontid salamanders

Background

Disentangling evolutionary shifts in developmental timing (heterochony) is dependent upon accurate estimates of ancestral patterns. However, many classic assessments of heterochronic patterns predate robust phylogenetic hypotheses and methods for trait reconstruction, and therefore may have been polarized with untested ‘primitive’ conditions. Here we revisit the heterochronic modes of development that underlie the evolution of metamorphosis, maturation, and paedomorphosis in plethodontid salamanders. We focus on the tribe Spelerpini, which is a diverse clade that exhibits tremendous variation in timing of metamorphosis and maturation, as well as multiple independent instances of larval form paedomorphosis. Based on morphology and biogeography, early investigators concluded that the most recent common ancestors of plethodontids, and also spelerpines, were large salamanders, with very long larval periods and late maturation times. This prevailing assumption influenced subsequent heterochronic assessments, which concluded that most modern spelerpines (with shorter larval periods) were derived through multiple independent accelerations in larval development. It was also concluded that most occurrences of larval form paedomorphosis in this clade resulted from progenesis (acceleration of gonadal development relative to metamorphosis).

Results

By reconstructing the time to metamorphosis on a molecular-based phylogeny of plethodontids, we find that ancestral spelerpines likely had relatively shorter larval periods than previously proposed. Taken together with the credibility interval from our ancestral state estimation we show that very long larval periods are likely derived decelerations, only a few lineages have undergone appreciable accelerations in metamorphic timing, and the remaining taxa have lower probabilities of being different than the ancestral condition (possibly due to stasis). Reconstructing maturation age across nodes concomitant with the evolution of larval form paedomorphosis in one large radiation does not show clear evidence of progenesis, but more likely indicates a case of neoteny (delayed metamorphosis).

Conclusions

This study demonstrates cases in plethodontid salamanders where phylogenetic-based character reconstructions reject previously hypothesized ancestral life history conditions. As a result, several prior hypotheses of heterochronic evolution in this family are reversed.