Limb bone histology records birth in mammals

Intraskeletal histovariability and skeletochronology in an ornithopod dinosaur from the Maestrazgo Basin (Teruel, Spain)

Ornithopods are an extinct group of dinosaurs that were particularly abundant and diverse in the Cretaceous of the Iberian Peninsula, and whose abundance in the Maestrazgo Basin has allowed numerous taxa to be identified over the last decade. Many of these fossil remains are still taxonomically indeterminate and require a more detailed study on both a macroscopic and microscopic scale. In this contribution, an osteohistological analysis is carried out on a partial skeleton-composed of five incomplete vertebrae, two dorsal ribs, an ischium, a fibula, and a tibia-found in the province of Aliaga (Teruel, NE Spain). We identified a progressive slowdown in tissue apposition and a variation in the type of growth marks generated in every bone, allowing a more precise identification of the ontogenetic stage of the specimen as a subadult individual. The skeletochronological correlation between the different elements also suggests that the specimen reached sexual maturity at around seven years of age and died between nine and twelve years of age. Likewise, the usefulness of the three-front model is proven for the first time in an ornithopod dinosaur, as a tool for analysing the histology expressed by the different bone elements of a single specimen and inferring their skeletochronological potential. Comparison with other ornithopod taxa reveals the great variability that each bone element shows depending on the taxon analysed, which prevents us from determining a single element suitable for studying the skeletochronology of any ornithopod taxon.

Analyzing the Life History of Caimans: The Growth Dynamics of Caiman latirostris From an Osteohistological Approach

Skeletochronology and growth dynamics are intensively investigated in vertebrate osteohistology. These techniques are particularly important for interpreting the life history of long-lived species, such as crocodilians. To understand the longevity, growth dynamics, sexual maturity, and sexual dimorphism of caimans we studied an almost complete ontogenetic series of captive and wild specimens of Caiman latirostris from different localities of Argentina. We identified both cyclical and noncyclical growth marks in juvenile caimans, and we suggest that the latter are associated with environmental stress. By overlapping the growth marks of different individuals, we were able to estimate the minimum age of each specimen. Variations in growth rate are evident in different bones, with the femur and scapula having the highest growth rates, while the fibula and pubis have much slower growth rates. We were able to determine the approximate age of sexual maturity from growth curves deduced from osteohistology, which concurred with those assessed in ecological studies. Additionally based on the growth curves we were able to document different growth dynamics which may be related to sexual dimorphism. This study provides valuable insights into the life history and ecological dynamics of crocodilians, shedding light on their growth patterns, attainment of sexual maturity, and the influence of environmental factors on growth. Furthermore it documents the intraspecific and interelemental osteohistological variation in crocodilians and has direct implications for studies that assess the life history of extinct archosaurs and other sauropsids.

Growth and life history of freshwater chelydrid turtles (Testudines: Cryptodira): A bone histological approach

The current study examines the growth pattern and lifestyle habits of the freshwater snapping turtles Chelydra and Macrochelys based on limb bone histology. Femora, humeri, and tibiae of 25 individuals selected from a range of ontogenetic stages were assessed to determine inter-element and intraskeletal histological variation. Osteohistological assessment of multiple elements is consistent with overall moderate growth rates as revealed by the dominance of parallel-fibered bone. However, the growth was cyclical as shown by deposition of multiple lines of arrested growths in the compacta. It appears that the bone tissue of C. serpentina is more variable through ontogeny with intermittent higher growth rates. M. temminckii appears to grow more slowly than C. serpentina possessing compact and thick cortices in accordance with their larger size. Overall, vascularization decreases through ontogeny with humeri and femora being well-vascularized in both species. Contrarily, epipodials are poorly vascularized, though simple longitudinal and radial canals are present, suggesting differences in growth patterns when compared with associated diaphyseal sections. The tibiae were found to be the least remodeled of the limb bones and therefore better suited for skeletochronology for snapping turtles. Intra-elementally, femora and humeri preserved higher cortical vascularity ventrally, suggestive of faster relative growth. We hypothesize that the differential growth pattern in limb bones of snapping turtles may relate to differential functional constraints, where forelimbs are operational in swimming while the hindlimbs provide stability.

Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries

Dinosauria debuted on Earth's stage in the aftermath of the Permo-Triassic Mass Extinction Event, and survived two other Triassic extinction intervals to eventually dominate terrestrial ecosystems. More than 231 million years ago, in the Upper Triassic Ischigualasto Formation of west-central Argentina, dinosaurs were just getting warmed up. At this time, dinosaurs represented a minor fraction of ecosystem diversity. Members of other tetrapod clades, including synapsids and pseudosuchians, shared convergently evolved features related to locomotion, feeding, respiration, and metabolism and could have risen to later dominance. However, it was Dinosauria that radiated in the later Mesozoic most significantly in terms of body size, diversity, and global distribution. Elevated growth rates are one of the adaptations that set later Mesozoic dinosaurs apart, particularly from their contemporary crocodilian and mammalian compatriots. When did the elevated growth rates of dinosaurs first evolve? How did the growth strategies of the earliest known dinosaurs compare with those of other tetrapods in their ecosystems? We studied femoral bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation in order to test whether the oldest known dinosaurs exhibited novel growth strategies. Our results indicate that the Ischigualasto vertebrate fauna collectively exhibits relatively high growth rates. Dinosaurs are among the fastest growing taxa in the sample, but they occupied this niche alongside crocodylomorphs, archosauriformes, and large-bodied pseudosuchians. Interestingly, these dinosaurs grew at least as quickly, but more continuously than sauropodomorph and theropod dinosaurs of the later Mesozoic. These data suggest that, while elevated growth rates were ancestral for Dinosauria and likely played a significant role in dinosaurs' ascent within Mesozoic ecosystems, they did not set them apart from their contemporaries.

Ontogenetic and interelemental study of appendicular bones of Caiman latirostris Daudin, 1802 sheds light on osteohistological variability in crocodylians

The osteohistology of vertebrates provides a reliable source to deduce biological information, particularly regarding growth and development. Although osteohistological studies in Neosuchia (Crocodyliformes, Mesoeucrocodylia) are relatively numerous, the number of species studied within the group is still small. Extant crocodilians are known to exhibit intraspecific variability linked to environmental conditions, habitat, feeding, and other intrapopulation factors. Here, we analyzed the osteohistology of the living South American Caiman latirostris throughout posthatching ontogeny. The histology of several appendicular bones of 13 different-sized captive and wild individuals were examined. Although some thin sections revealed the classic lamellar, parallel-fibered, or woven bone matrices, others showed a variation and a mix between the organization of the bone tissue. These histological differences are likely related to variability in the growth dynamics of caimans. In some bones of the juveniles studied, remnants of embryonic bone were observed. Osteohistological variation related to prevailing environmental conditions is documented. Furthermore, our results show ontogenetic variation in the type of bone tissues deposited throughout the development of C. latirostris. This study offers a broad framework for life history interpretations for C. latirostris and provides insight into the evolutionary history and ontogenetic growth of extinct crocodylian lineages.

A new oviraptorosaur (Dinosauria: Theropoda) from the end-Maastrichtian Hell Creek Formation of North America

Caenagnathidae is a clade of derived, Late Cretaceous oviraptorosaurian theropods from Asia and North America. Because their remains are rare and often fragmentary, caenagnathid diversity is poorly understood. Anzu wyliei is the only caenagnathid species currently described from the late Maastrichtian Hell Creek Formation of the USA and is also among the largest and most completely preserved North American caenagnathids. Smaller, less complete caenagnathid material has long been known from the Hell Creek Formation, but it is unclear whether these are juvenile representatives of Anzu or if they represent distinct, unnamed taxa. Here, we describe a relatively small caenagnathid hindlimb from the Hell Creek Formation, and conduct osteohistological analysis to assess its maturity. Histological data and morphological differences from Anzu wyliei and other caenagnathids allow us to conclude that this specimen represents a new species of caenagnathid from the Hell Creek Formation, with a smaller adult body size than Anzu. This new taxon is also distinct from other small caenagnathid material previously described from the area, potentially indicating the coexistence of three distinct caenagnathid species in the Hell Creek Formation. These results show that caenagnathid diversity in the Hell Creek ecosystem has been underestimated.

Insular giant leporid matured later than predicted by scaling

The island syndrome describes morphological, behavioral, and life history traits that evolve in parallel in endemic insular organisms. A basic axiom of the island syndrome is that insular endemics slow down their pace of life. Although this is already confirmed for insular dwarfs, a slow life history in giants may not be adaptive, but merely a consequence of increasing body size. We tested this question in the fossil insular giant leporid Nuralagus rex. Using bone histology, we constructed both a continental extant taxon model derived from experimentally fluorochrome-labeled Lepus europaeus to calibrate life history events, and a growth model for the insular taxon. N. rex grew extremely slowly and delayed maturity well beyond predictions from continental phylogenetically corrected scaling models. Our results support the life history axiom of the island syndrome as generality for insular mammals, regardless of whether they have evolved into dwarfs or giants.

Palaeoecological deductions from osteohistology

Palaeoecological deductions are vital for understanding the evolution and diversification of species within prehistoric environments. This review highlights the multitude of ways in which the microanatomy and microscopic structure of bones enables palaeoecological deductions. The occurrence of growth marks in bones is discussed, and their usefulness in deducing the ontogenetic status and age of individuals is considered, as well as how such marks in bones permit the assessment of the growth dynamics of individuals and species. Here osteohistology is shown to provide insight into the structure of past populations, as well as ecological relationships between individuals. In addition, the response of bones to trauma, disease and moulting is considered. Finally, I explore how osteohistology can give insight into ecomorphological adaptations, such as filter feeding, probe feeding and saltatorial locomotion. Methodological advances in three-dimensional microtomography and synchrotron scanning bodes well for future studies in osteohistology and despite some compromises in terms of tissue identity, circumvents the crucial issue of destructive analyses.

Bone histology of Neogene angulate tortoises (Testudines: Testudinidae) from South Africa: palaeobiological and skeletochronological implications

Here we examine the tibial microstructure of modern and fossil angulate tortoises to assess the histology and growth from the late Miocene-early Pliocene, Pleistocene through to modern forms. The cross-sections of all the tibiae sampled revealed highly vascularized, uninterrupted, fibrolamellar bone tissue during early ontogeny, which suggests that early growth was fast. However, later in ontogeny, growth was slower, as indicated by the deposition of parallel-fibred bone tissue in the outer cortex, and even ceased periodically, as indicated by lines of arrested growth. Comparative analyses of the growth rates of the tortoises from different time periods showed that the tortoises from the late Miocene-early Pliocene Langebaanweg locality and from Diepkloof Rock Shelter had relatively slower growth rates under less optimal growth conditions. Additionally, these prehistoric specimens show extensive remodelling, and several generations of secondary osteons further suggest functional and/or metabolic stresses on the skeleton. Palaeoenvironmental reconstructions suggest that it was mostly cooler and drier with seasonal fluctuations in late Miocene-early Pliocene, and it is likely that Chersina responded to these conditions by having a lower rate of growth as compared with their modern counterparts, which thrive in the current prevailing more favourable Mediterranean type of climate.

Osteohistology and palaeobiology of giraffids from the Mio-Pliocene Langebaanweg (South Africa)

The reconstruction of life history traits, such as growth rate, age at maturity and age at death can be estimated from the histological analysis of long bones. Here, we studied 20 long bones (metapodials, tibia and femora) of Sivatherium hendeyi and Giraffa cf. Giraffa jumae recovered from the Miocene-Pliocene locality of Langebaanweg on the West Coast of South Africa. We analysed the long bone histology and growth marks of juvenile and adult specimens of these taxa. Our results show that bone tissue types and vascular canal orientation varies during ontogeny, as well as between the different skeletal elements, and also across single cross sections of bones. Majority of our specimens appear to be still growing, with only an adult metacarpal of S. hendeyi being skeletally mature as indicated by the presence of an outer circumferential layer. We propose that the growth marks preserved in the cortices of the bones studied are most likely related to multiple catastrophic events as opposed to being annual/seasonal.

The origin of placental mammal life histories

After the end-Cretaceous extinction, placental mammals quickly diversified¹, occupied key ecological niches^2,3 and increased in size^4,5, but this last was not true of other therians⁶. The uniquely extended gestation of placental young⁷ may have factored into their success and size increase⁸, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping^9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30-75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.

First osteohistological and histotaphonomic approach of Equus occidentalis Leidy, 1865 (Mammalia, Equidae) from the late Pleistocene of Rancho La Brea (California, USA)

Rancho La Brea (California, USA) is the most emblematic Quaternary fossiliferous locality in the world, since both the high number and diversity of the specimens recovered and their excellent preservational quality. In the last decades, paleobiological and paleoecological knowledge of the different groups of mammals from this site has increased notably; however, some aspects have not yet been inquired or there is little information. In this work we provide information on one of the most abundant mammals of this site, the equid Equus occidentalis, based on the study, from osteohistological and histotaphonomic perspectives, of thin sections of different limb bones. On the one hand, from an osteohistological viewpoint, we observe that the distribution and characterization of bone tissues in the different skeletal elements are, in general lines, similar to that mentioned for other extant and extinct equids. Cyclical growth marks allowed us to propose preliminary skeletochronological interpretations. On the other hand, from a taphonomic viewpoint, we note that all the samples reflect an excellent preservation of the bone microstructure, slightly altered by different pre- and post-burial processes. The variations recorded evidence different taphonomic history and preservation conditions among pits. This is the first study including fossil material from Rancho La Brea exclusively based on the analysis of the bone microstructure features.

Palaeohistology reveals a slow pace of life for the dwarfed Sicilian elephant

The 1-m-tall dwarf elephant Palaeoloxodon falconeri from the Pleistocene of Sicily (Italy) is an extreme example of insular dwarfism and epitomizes the Island Rule. Based on scaling of life-history (LH) traits with body mass, P. falconeri is widely considered to be ‘r-selected’ by truncation of the growth period, associated with an early onset of reproduction and an abbreviated lifespan. These conjectures are, however, at odds with predictions from LH models for adaptive shifts in body size on islands. To settle the LH strategy of P. falconeri, we used bone, molar, and tusk histology to infer growth rates, age at first reproduction, and longevity. Our results from all approaches are congruent and provide evidence that the insular dwarf elephant grew at very slow rates over an extended period; attained maturity at the age of 15 years; and had a minimum lifespan of 68 years. This surpasses not only the values predicted from body mass but even those of both its giant sister taxon (P. antiquus) and its large mainland cousin (L. africana). The suite of LH traits of P. falconeri is consistent with the LH data hitherto inferred for other dwarfed insular mammals. P. falconeri, thus, not only epitomizes the Island Rule but it can also be viewed as a paradigm of evolutionary change towards a slow LH that accompanies the process of dwarfing in insular mammals.

Living fast in the Triassic: New data on life history in Lystrosaurus (Therapsida: Dicynodontia) from northeastern Pangea

Lystrosaurus was one of the few tetrapods to survive the Permo-Triassic mass extinction, the most profound biotic crisis in Earth’s history. The wide paleolatitudinal range and high abundance of Lystrosaurus during the Early Triassic provide a unique opportunity to investigate changes in growth dynamics and longevity following the mass extinction, yet most studies have focused only on species that lived in the southern hemisphere. Here, we present the long bone histology from twenty Lystrosaurus skeletal elements spanning a range of sizes that were collected in the Jiucaiyuan Formation of northwestern China. In addition, we compare the average body size of northern and southern Pangean Triassic-aged species and conduct cranial geometric morphometric analyses of southern and northern taxa to begin investigating whether specimens from China are likely to be taxonomically distinct from South African specimens. We demonstrate that Lystrosaurus from China have larger average body sizes than their southern Pangean relatives and that their cranial morphologies are distinctive. The osteohistological examination revealed sustained, rapid osteogenesis punctuated by growth marks in some, but not all, immature individuals from China. We find that the osteohistology of Chinese Lystrosaurus shares a similar growth pattern with South African species that show sustained growth until death. However, bone growth arrests more frequently in the Chinese sample. Nevertheless, none of the long bones sampled here indicate that maximum or asymptotic size was reached, suggesting that the maximum size of Lystrosaurus from the Jiucaiyuan Formation remains unknown.

Intraskeletal bone growth patterns in the North Island Brown Kiwi (Apteryx mantelli): Growth mark discrepancy and implications for extinct taxa

Osteohistology, the study of bone microstructure, provides an important avenue for assessing extinct and extant vertebrate growth and life history. Cortical vascularity and collagen fibre organization are direct reflections of growth rate, while bone growth marks are indicative of absolute age. However, each skeletal element has its own ontogenetic trajectory and microstructure of certain bones may not be a true representation of whole body growth. Extensive comparative study of modern taxa is required to resolve intraskeletal discrepancies among age, vascularity and tissue organization in extinct vertebrates. Despite their comparative utility, studies of bone microstructure in modern taxa are severely lacking. Here, we add to a growing comparative osteohistological database by describing (1) bone tissue organization, (2) growth mark count, (3) sexually dimorphic bone (e.g. medullary bone) and (4) secondary cortical reconstruction in the bone microstructure of a 14-year-old male and 5-year-old female North Island Brown Kiwi (Apteryx mantelli). Transverse and longitudinal histological ground sections were processed and described for femora, tibiotarsi, tarsometatarsi, humeri, ulnae and radii in both kiwis. Cortical bone can generally be described as parallel-fibered tissue, interrupted by cyclical growth marks, with vascular canals oriented longitudinally within primary and secondary osteons. Tissue morphologically resembling medullary bone is present in the hindlimbs of the female, and coarse compacted cancellous bone (CCCB) is found sporadically in the male and female hindlimbs. Lines of arrested growth (LAGs) are present in all hindlimb bones of both kiwi, but remodelling has obliterated all LAGs in the male ulnae and radii. LAG count varies intraskeletally, but large weight bearing elements such as femora and tibiotarsi have less remodelling and, thus, higher number of LAGs. LAG count did not match absolute age in any skeletal element; a maximum of seven LAGs are present in the male kiwi and a maximum of seven LAGs in the female kiwi. The tissue organization within the forelimbs and hindlimbs is reflective of the protracted growth strategy of the North Island Brown Kiwi and congruent with previous studies of the kiwi. LAGs were highly variable throughout the skeleton of the kiwi and a decoupling of age and LAG deposition is apparent from the male kiwi samples. Excess LAGs in the 5-year-old female kiwi may be a product of hatching, egg laying or captivity. Regardless, LAG count variation in the kiwi stresses the importance of intraskeletal sampling when assessing growth patterns of extinct taxa. An extensive ontogenetic sampling of kiwi is necessary for future investigations of bone growth patterns, CCCB formation, medullary bone and LAG deposition and obliteration in these elusive birds.

Mineralized-tissue histology reveals protracted life history in the Pliocene three-toed horse from Langebaanweg (South Africa)

We studied the bone and dental histology of the tri-dactyl equid Eurygnathohippus hooijeri, one of the most iconic mammals found at the world-renowned Pliocene site of Langebaanweg, South Africa, to reconstruct important features of its life history. Our results show that key life-history events, such as weaning, skeletal maturity and reproductive maturity, occurred later in this African hipparionine compared with European three-toed equids and several extant Equus. Its late life-history schedule agrees with an ecological context of low adult extrinsic mortality and low juvenile survival rates. We also observed high rates of bone growth in Eu. hooijeri that were probably achieved through a high-quality diet and plentiful available water. Our research highlights the significance of combining bone and dental histology in the same taxon to obtain refined palaeobiological information about extinct vertebrates.

Labelling experiments in red deer provide a general model for early bone growth dynamics in ruminants

Growth rates importantly determine developmental time and are, therefore, a key variable of a species' life history. A widely used method to reconstruct growth rates and to estimate age at death in extant and particularly in fossil vertebrates is the analysis of bone tissue apposition rates. Lines of arrested growth (LAGs) are of special interest here, as they indicate a halt in bone growth. However, although of great importance, the time intervals between, and particularly the reason of growth arrests remains unknown. Therefore, experiments are increasingly called for to calibrate growth rates with tissue types and life history events, and to provide reliable measurements of the time involved in the formation of LAGs. Based on in vivo bone labelling, we calibrated periods of bone tissue apposition, growth arrest, drift and resorption over the period from birth to post-weaning in a large mammal, the red deer. We found that bone growth rates tightly matched the daily weight gain curve, i.e. decreased with age, with two discrete periods of growth rate disruption that coincided with the life history events birth and weaning, that were visually recognisable in bone tissue as either partial LAGs or annuli. Our study identified for the first time in a large mammal a general pattern for juvenile bone growth rates, including periods of growth arrest. The tight correlation between daily weight gain and bone tissue apposition suggests that the red deer bone growth model is valid for ruminants in general where the daily weight gain curve is comparable.

A first comparison of bone histomorphometry in extant domestic horses (Equus caballus) and a Pleistocene Indian wild horse (Equus namadicus)

The microstructural features of the tissue of long bones subjected to different biomechanical stresses could be a helpful tool for a better understanding of locomotor behavior in extant and extinct mammals, including equids. However, few researches have attempted to describe the bone tissue of extinct horses. In our study, we analyze and compare the histomorphometric features of the bone tissue in extant modern horses, Equus caballus, and Equus namadicus, a Pleistocene Indian extinct wild horse. The number, position, and size of the osteons and Haversian canals of the bone tissue, classifiable as dense Haversian tissue, were considered for the comparison. The results obtained highlight some differences between the analyzed species, E. caballus having fewer and bigger osteons than E. namadicus. The microstructural differences may depend on the different lifestyles and environmental conditions characterizing the two species. The results obtained suggest that comparing the biomechanical properties of extinct and modern horse species may provide indirect information on their paleoenvironment.

Ontogenetic changes of tissue compartmentalization and bone type distribution in the humerus of Soay sheep

We studied ontogenetic changes of histomorphological features and bone type distribution in the humeral midshaft region of Soay sheep from three postnatal age classes (13, 25, and 33 months). Our study demonstrated a marked change of bone type distribution in the humeri with age. In the cortical midshaft region of 13-month-old individuals, periosteal fibrolamellar bone was the dominating bone type. This indicates a rapid bone growth during the first year of life, which was only interrupted by a seasonal growth arrest in the animals' first winter. In individuals from the two older age classes, periosteal lamellar-zonal bone and intermediate fibrolamellar bone had been formed at the periosteal surface, and endosteal lamellar-zonal bone at the endosteal surface. These bone types are indicative of a reduced bone growth rate. A marked reduction in radial growth was already recorded in the 25-month-old individuals. Distribution and extent of secondary bone showed a marked bilateral symmetry in the humeri of individual sheep. The presence of secondary bone was largely restricted to the anterior (cranial) and the medial cortical areas. This characteristic distribution of remodeling activity within the humeral cortex of sheep is consistent with the view that remodeling activity is largely caused by compressive stress. Our study further demonstrated the presence of a considerable cortical drift in the sheep humeri over the study period, with endosteal resorption occurring predominantly in the posterior (caudal) quadrant and formation of a prominent endosteal lamellar pocket in the anterior (cranial) and medial cortical quadrants.

Calibration of life history traits with epiphyseal closure, dental eruption and bone histology in captive and wild red deer

The study of skeletochronology and bone tissue as a record of information on ontogenetic stages and events is widely used for improving the knowledge about life histories (LHs) of extinct and extant vertebrates. Compared with dinosaurs and extant reptiles, mammalian bone histology has received little attention. Here, we calibrate for the first time bone and dental age with histological bone characteristics and LH stages in ontogenetic series of red deer. We rely on known LHs of different aged individuals of captive Cervus elaphus hippelaphus from Austria to correlate epiphyseal closure, dental eruption pattern, bone growth marks and bone tissue patterns in femora and tibiae, and of wild Cervus elaphus hispanicus from Spain. Our data show that females (of both subspecies) attain skeletal maturity earlier than males. At this moment, epiphyseal closure (in femora and tibiae) and dental eruption are complete and long bones start to deposit an external fundamental system. The results also show that the attainment of reproductive maturity in red deer occurs slightly before skeletal maturity.

Bone histology provides insights into the life history mechanisms underlying dwarfing in hipparionins

Size shifts may be a by-product of alterations in life history traits driven by natural selection. Although this approach has been proposed for islands, it has not yet been explored in continental faunas. The trends towards size decrease experienced by some hipparionins constitute a good case study for the application of a life history framework to understand the size shifts on the continent. Here, we analysed bone microstructure to reconstruct the growth of some different-sized hipparionins from Greece and Spain. The two dwarfed lineages studied show different growth strategies. The Greek hipparions ceased growth early at a small size thus advancing maturity, whilst the slower-growing Spanish hipparion matured later at a small size. Based on predictive life history models, we suggest that high adult mortality was the likely selective force behind early maturity and associated size decrease in the Greek lineage. Conversely, we infer that resource limitation accompanied by high juvenile mortality triggered decrease in growth rate and a relative late maturity in the Spanish lineage. Our results provide evidence that different selective pressures can precipitate different changes in life history that lead to similar size shifts.