Placental Clades Research Articles

In vivo strains in the femur of the nine-banded armadillo (Dasypus novemcinctus).

The capacity of limb bones to resist the locomotor loads they encounter depends on both the pattern of those loads and the material properties of the skeletal elements. Among mammals, understanding of the interplay between these two factors has been based primarily on evidence from locomotor behaviors in upright placentals, which show limb bones that are loaded predominantly in anteroposterior bending with minimal amounts of torsion. However, loading patterns from the femora of opossums, marsupials using crouched limb posture, show appreciable torsion while the bone experiences mediolateral (ML) bending. These data indicated greater loading diversity in mammals than was previously recognized, and suggested the possibility that ancestral loading patterns found in sprawling lineages (e.g., reptilian sauropsids) might have been retained among basal mammals. To further test this hypothesis, we recorded in vivo locomotor strains from the femur of the nine-banded armadillo (Dasypus novemcinctus), a member of the basal xenarthran clade of placental mammals that also uses crouched limb posture. Orientations of principal strains and magnitudes of shear strains indicate that armadillo femora are exposed to only limited torsion; however, bending is essentially ML, placing the medial aspect of the femur in compression and the lateral aspect in tension. This orientation of bending is similar to that found in opossums, but planar strain analyses indicate much more of the armadillo femur experiences tension during bending, potentially due to muscles pulling on the large, laterally positioned third trochanter. Limb bone safety factors were estimated between 3.3 and 4.3 in bending, similar to other placental mammals, but lower than opossums and most sprawling taxa. Thus, femoral loading patterns in armadillos show a mixture of similarities to both opossums (ML bending) and other placentals (limited torsion and low safety factors), along with unique features (high axial tension) that likely relate to their distinctive hindlimb anatomy.

High-level systematics of placental mammals: Current status of the problem

According to molecular data, the modern clades of placental mammals can be grouped into four supraordinal taxa (Xenarthra, Afrotheria, Euarchontoglires, and Laurasiatheria) whose phylogenetic interrelationships have been interpreted inconsistently. Among these taxa, the group Afrotheria has no morphological support. Within this group, there are orders that are clearly related to “condylarthrans” (“afroungulates”) and to insectivores (“afrosoricids”). The radiation of placental mammals (the crown clade Placentalia) started before the K/T boundary on the northern continents. Laurasiatheria are likely to be the oldest clade of placentals: the diversification of Lipotyphla and Euungulata started ∼80 Mya (Campanian) and ∼70 Mya (Maastrichtian), respectively. It is shown that the fossil record is the only reliable method to test the phylogenetic hypotheses based on the material of the molecular and morphological studies of recent taxa.

Reassessing the relationship between brain size, life history, and metabolism at the marsupial/placental dichotomy.

A vigorous discussion surrounds the question as to what enables some mammals--including primates and cetaceans--to evolve large brains. We recently published a study suggesting that the radiation of marsupial mammals is highly relevant to this question because of the unique reproductive and metabolic traits within this clade. In particular, we controversially suggested that marsupial brain sizes are not systematically smaller than those of placentals, and that elevated basal metabolic rates (BMR) are not linked to larger marsupial brains. As our dataset was found to contain some erroneous body size data, derived from a published source, we here use an updated and corrected dataset and employ standard as well as phylogenetically corrected analyses to re-assess and elaborate on our original conclusions. Our proposal that marsupials are not systematically smaller-brained than placentals remains supported, particularly when the unusually large-brained placental clade, Primates, is excluded. Use of the new dataset not only confirms that high metabolic rates are not associated with larger brain size in marsupials, but we additionally find some support for a striking negative correlation between BMR and brain size. The best supported correlates of large brain size remain the reproductive traits of weaning age and litter size. These results support our suggestion that mammalian brain sizes (including, by inference, those of monotremes) are predominantly constrained by the ability of females to fuel the growth of their offspring's large brains, rather than by the maintenance requirements of the adult brain.

Discovery of prosimian and afrotherian foamy viruses and potential cross species transmissions amidst stable and ancient mammalian co-evolution.

BackgroundFoamy viruses (FVs) are a unique subfamily of retroviruses that are widely distributed in mammals. Owing to the availability of sequences from diverse mammals coupled with their pattern of codivergence with their hosts, FVs have one of the best-understood viral evolutionary histories ever documented, estimated to have an ancient origin. Nonetheless, our knowledge of some parts of FV evolution, notably that of prosimian and afrotherian FVs, is far from complete due to the lack of sequence data.ResultsHere, we report the complete genome of the first extant prosimian FV (PSFV) isolated from a lorisiforme galago (PSFVgal), and a novel partial endogenous viral element with high sequence similarity to FVs, present in the afrotherian Cape golden mole genome (ChrEFV). We also further characterize a previously discovered endogenous PSFV present in the aye-aye genome (PSFVaye). Using phylogenetic methods and available FV sequence data, we show a deep divergence and stable co-evolution of FVs in eutherian mammals over 100 million years. Nonetheless, we found that the evolutionary histories of bat, aye-aye, and New World monkey FVs conflict with the evolutionary histories of their hosts. By combining sequence analysis and biogeographical knowledge, we propose explanations for these mismatches in FV-host evolutionary history.ConclusionOur discovery of ChrEFV has expanded the FV host range to cover the whole eutherian clade, and our evolutionary analyses suggest a stable mammalian FV-host co-speciation pattern which extends as deep as the exafroplacentalian basal diversification. Nonetheless, two possible cases of host switching were observed. One was among New World monkey FVs, and the other involves PSFVaye and a bat FV which may involve cross-species transmission at the level of mammalian orders. Our results highlight the value of integrating multiple sources of information to elucidate the evolutionary history of viruses, including continental and geographical histories, ancestral host locations, in addition to the natural history of host and virus.Electronic supplementary materialThe online version of this article (doi:10.1186/1742-4690-11-61) contains supplementary material, which is available to authorized users.

Endogenous Viral Sequences from the Cape Golden Mole (Chrysochloris asiatica) Reveal the Presence of Foamy Viruses in All Major Placental Mammal Clades

Endogenous retroviruses provide important insights into the deep history of this viral lineage. Endogenous foamy viruses are thought to be very rare and only a few cases have been identified to date. Here we report a novel endogenous foamy virus (CaEFV) within the genome of the Cape golden mole (Chrysochloris asiatica). The identification of CaEFV reveals the presence of foamy virus in the placental mammal superorder Afrotheria. Phylogenetic analyses place CaEFV basal to other foamy viruses of Eutherian origin, suggesting an ancient codivergence between foamy virus and placental mammals. These findings have implications for understanding the long-term evolution, diversity, and biology of retroviruses.

Evolution of hantaviruses: Co-speciation with reservoir hosts for more than 100 MYR

The most recent (9th) Report of the International Committee on Taxonomy of Viruses (ICTV) lists 23 established and 30 provisional species in the genus Hantavirus (family Bunyaviridae) (Plyusnin et al., 2012). These virus species are harbored by altogether 51 species of rodents, shrews and moles and thus in most cases it is a relationship of “one hantavirus-one host”. Such a tight bond between the two, in combination with the observed association between whole groups of hantaviruses and (sub)families of rodents, helped to develop the widely accepted view of a long-term co-evolution (co-speciation) of these viruses with their hosts. Accumulating evidence of host-switching events, both recent and ancient, however challenged some of the earlier views on hantavirus evolution. In this paper we discuss the concept of hantavirus-host co-speciation and propose a scenario of hantavirus evolution based on the currently available genetic information. This scenario is based on the hypothesis that hantaviruses are very ancient viruses which already existed at the estimated diversification point of major placental clades, of which one includes the ancestors of the order Rodentia and another the ancestors of both orders Eulipotyphla and Chiroptera; the diversification occurred approximately at 90–100MYA. We also speculate that the evolutionary history of hantaviruses extents even deeper in the past, beyond this time-point, and included the transmission of a (pre)bunyavirus from an insect host to a mammal host.

New eutherian mammals from the Late Cretaceous of Aix-en-Provence Basin, south-eastern France

In Europe, the fossil record of Late Cretaceous eutherian mammals is very poor, being limited to only three genera (Labes, Lainodon, and Valentinella). Labes and Lainodon are well-supported members of Zhelestidae, a stem eutherian clade, whereas Valentinella is more problematic, being recently considered as a nomen dubium. Based on X-ray computed microtomography scan analysis of the holotype and thanks to the discovery of new specimens from the type locality (Vitrolles-La Plaine, south-eastern France, late Maastrichtian), we reassessed Valentinella. This genus is unique by the association of an enlarged and rounded jaw angle with an assumed relatively elevated angular process, a bulbous protoconid and an unbasined heel on p4, a p5 with a wide molariform talonid and a hypoflexid, a robust molar morphology with a potential specialized crushing-grinding function (bulbously constructed cusps, large talonid, and horizontal apical wear facet of the hypocone), a somewhat reduced m3 relative to m2, a premolariform ?P3 or ?P4 lacking a metacone, and a relatively large hypocone on upper molars. These characters reinstate Valentinella as a valid genus. We also describe Mistralestes arcensis gen. et sp. nov. from a newly discovered locality (La Cairanne-Highway, south-eastern France, late Campanian). Mistralestes is defined by a robust premolariform p5 with no cingulid, paraconid, or metaconid; molars with a transverse protocristid, a gradual compression of the trigonid from m1 to m3, and paracristid and protocristid probably confluent on m3. Based on comparisons and phylogenetic analyses, Valentinella and Mistralestes may belong to Zhelestidae but this systematic attribution remains poorly supported. © 2013 The Linnean Society of London

Timing of cranial suture closure in placental mammals: Phylogenetic patterns, intraspecific variation, and comparison with marsupials

Used as markers of postnatal growth closure sequences of 22 ectocranial sutures and synchondroses were recorded in a sample of 1161 skulls belonging to 38 species from all major placental clades: Afrotheria, Xenarthra, Laurasiatheria and Euarchontoglires (Boreoeutheria). The maximum closure level, which is not significantly correlated to body mass, is higher in Afrotheria and Xenarthra than in Boreoeutheria. Only the basioccipito-basisphenoid and the basioccipito-exoccipital synchondroses close in all species sampled, the supraoccipito-exoccipital and the inter-parietal sutures do in most species. Parsimov retrieved more heterochronic shifts for Afrotheria and Xenarthra than for Boreoeutheria. The amount of intraspecific variation differs among the species sampled being high among xenarthran species and low among afrotherians. Specimens (162) representing 12 marsupial genera were also analysed. Placentals exhibit a larger number of suture closures than marsupials and in both groups the sutures at the base of the skull are the first to fuse starting with the basioccipito-exoccipital.

PATTERNS OF OSSIFICATION IN SOUTHERN VERSUS NORTHERN PLACENTAL MAMMALS

Consensus on placental mammal phylogeny is fairly recent compared to that for vertebrates as a whole. A stable phylogenetic hypothesis enables investigation into the possibility that placental clades differ from one another in terms of their development. Here, we focus on the sequence of skeletal ossification as a possible source of developmental distinctiveness in "northern" (Laurasiatheria and Euarchontoglires) versus "southern" (Afrotheria and Xenarthra) placental clades. We contribute data on cranial and postcranial ossification events during growth in Afrotheria, including elephants, hyraxes, golden moles, tenrecs, sengis, and aardvarks. We use three different techniques to quantify sequence heterochrony: continuous method, sequence-ANOVA (analysis of variance) and event-paring/Parsimov. We show that afrotherians significantly differ from other placentals by an early ossification of the orbitosphenoid and caudal vertebrae. Our analysis also suggests that both southern placental groups show a greater degree of developmental variability; however, they rarely seem to vary in the same direction, especially regarding the shifts that differ statistically. The latter observation is inconsistent with the Atlantogenata hypothesis in which afrotherians are considered as the sister clade of xenarthrans. Interestingly, ancestral nodes for Laurasiatheria and Euarchontoglires show very similar trends and our results suggest that developmental homogeneity in some ossification sequences may be restricted to northern placental mammals (Boreoeutheria).

Why are There Fewer Marsupials than Placentals? On the Relevance of Geography and Physiology to Evolutionary Patterns of Mammalian Diversity and Disparity

Placental mammals occupy a larger morphospace and are taxonomically more diverse than marsupials by an order of magnitude, as shown by quantitative and phylogenetic studies of several character complexes and clades. Many have suggested that life history acts as a constraint on the evolution of marsupial morphology. However, the frequent circumvention of constraints suggests that the pattern of morphospace occupation in marsupials is more a reflection of lack of ecological opportunity than one of biases in the production of variants during development. Features of marsupial physiology are a potential source of biases in the evolution of the group; these could be coupled with past macroevolutionary patterns that followed conditions imposed by global temperature changes. This is evident at the K/Pg boundary and at the Eocene/Oligocene boundary. The geographic pattern of taxonomic and morphological diversity in placental clades mirrors that of extant placentals as a whole versus marsupials: placentals of northern origin are more diverse those of southern one and include the clades that are outliers in taxonomic (rodents and bats) and ecomorphological (whales and bats) richness.

Stratigraphy and Geochronology of Vertebrate Fossil‐Bearing Jurassic Strata from Linglongta, Jianchang County, Western Liaoning, Northeastern China

Abstract:Northeastern China contains widely distributed Jurassic terrestrial strata that have yielded many spectacular mammal and pterosaur fossils, in addition to feathered dinosaur fossils and more recent discoveries from Jianchang, particularly from western Liaoning. However, the fossil‐bearing stratigraphic succession, regional correlation, and age estimates of the fossils found in Jianchang County and nearby areas have been contentious. Here, we report on the vertebrate fossil‐bearing Jurassic stratigraphy from Linglongta, Jianchang County, western Liaoning, including a SHRIMP U‐Pb zircon date unambiguously associated with the fossil horizons. The primary goal was to determine the vertebrate fossil‐bearing succession. A further aim was to provide age estimations for the fossil‐bearing horizon as well as the earliest appearance of feathered dinosaurs, the eutherian–placental clade, and transitional pterosaurs. Field investigations showed that the vertebrate fossil‐bearing stratigraphic succession in Jianchang County mainly consists of basal andesites overlain by rhythmic tuffs and tuffaceous lacustrine sediments, with the upper intermediate or acidic lavas interbedded with laminated more or less tuffaceous lacustrine deposits. This sequence correlates well with the Middle Jurassic Lanqi/Tiaojishan Formation in northeastern China. Detailed and accurate field observations showed that the well‐preserved vertebrate fossils were buried in either the middle or the upper fine‐grained laminated lacustrine deposits. Previous and current SHRIMP U‐Pb zircon dates provide an age estimation of 161–159 Myr for the fossil‐bearing horizon and vertebrates. This indicates that the earliest appearance of feathered dinosaurs here was more than 159 Myr ago and unquestionably older than Archaeopteryx from Germany, making these the earliest known feathered dinosaurs in the world. Furthermore, the eutherian–placental clade and the known transitional pterosaurs first emerged no later than 161 Myr. The vertebrate assemblage unearthed recently from Linglongta and neighboring areas in Jianchang County belongs to the Daohugou Biota. In addition to feathered dinosaurs, this biota was characterized by mammals, primitive pterosaurs, insects, and plants and was present in Inner Mongolia, western Liaoning, and northern Hebei in northeastern China during the Middle–Late Jurassic.

The link between dental microwear and feeding ecology in tree sloths and armadillos (Mammalia: Xenarthra)

The Xenarthra represents an enigmatic clade of placental mammals that includes living tree sloths, armadillos, and their extinct relatives, yet certain aspects of the biology of this group remains poorly understood. Here, we use scanning electron microscopy to test the hypothesis that orthodentine microwear patterns in extant xenarthrans are significantly different among different dietary groups. In a blind analysis, microwear patterns were quantified at a magnification of 500× by two independent observers for extant species from four dietary groups (carnivore–omnivores, folivores, frugivore–folivores, and insectivores). Independent observers recovered the same relative between-group differences in microwear patterns. Insectivores and folivores have a significantly lower numbers of scratches and greater scar widths than frugivore–folivores and carnivore–omnivores, yet we were neither able to statistically distinguish insectivores from folivores, nor differentiate frugivore–folivores from carnivore–omnivores. Nevertheless, a clear distinction exists between taxa from the same trophic level and habitat, which suggests that orthodentine microwear reflects niche partitioning and habitat more than diet among related forms. We suggest that bite force and chewing mechanics have a strong influence on the formation of orthodentine microwear, which may explain some of the observed overlap between distinct groups (e.g. frugivore–folivores versus carnivore–omnivores). This study serves as a positive step forwards in our understanding of the ecological role of living xenarthrans, and serves as a foundation for using orthodentine microwear to reconstruct palaeoecology in extinct ground sloths, glyptodonts, and pampatheres. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

Phylogenetic analysis, taxonomic revision, and dental ontogeny of the Cretaceous Zhelestidae (Mammalia: Eutheria)

The eutherian, family-level clade Zhelestidae is consistently although weakly supported in five phylogenetic analyses that we performed on all Cretaceous eutherians. Additionally in the fifth analysis, which included some placentals, Zhelestidae is placed as a stem eutherian clade rather than grouping within the crown clade Placentalia as argued in some previous studies but not others. The subfamily Zhelestinae, Dzharakuduk (Turonian –?Coniacian ages), Kyzylkum Desert, Uzbekistan includes Zhelestes temirkayzk, Aspanlestes aptap, Parazhelestes mynbulakensis (= Sorlestes budan), Parazhelestes robustus, Eoungulatum kudukensis. Additional taxa for the time being recognized as Zhelestidae incertae sedis are: Sheikhdzheilia rezvyii (Cenomanian, Uzbekistan), Borisodon kara gen. nov. (=‘Sorlestes’kara) (Turonian, Kazakhstan), Lainodon orueetxebarriai (Campanian or Maastrichtian, Spain), Labes quintanillensis (Maastrictian, Spain), Labes garimondi (Campanian, France), Gallolestes pachymandibularis (Campanian, Mexico), Gallolestes agujaensis (Campanian, USA), and Avitotherium utahensis (Campanian, USA). Eozhelestes mangit (Cenomanian, Uzbekistan) is a questionable zhelestid (?Zhelestidae), possibly stem to Zhelestidae. Paranyctoides (Asia and North America) is often linked to Zhelestidae. Alostera, previously referred to Zhelestidae, is a eutherian of unknown affinities. Associated skull fragments permitted the first reconstruction of a zhelestid (Aspanlestes) skull. Abundant dentulous and edentulous dentaries allowed examination of dental replacement from the canine posteriorly in Dzharakuduk zhelestids as follows: [dc, p1, dp2, p3, dp4, dp5]-> m1-> p2-> c, p4, m2-> p5-> m3. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164, 361–426.

A radiation of arboreal basal eutherian mammals beginning in the Late Cretaceous of India

India's Late Cretaceous fossil mammals include the only undisputed pre-Tertiary Gondwanan eutherians, such as Deccanolestes. Recent studies have suggested a relationship between Deccanolestes and African and European Paleocene adapisoriculids, which have been variably identified as stem euarchontans, stem primates, lipotyphlan insectivores, or afrosoricids. Support for a close relationship between Deccanolestes and any of these placental mammal clades would be unique in representing a confirmed Mesozoic record of a placental mammal. However, some paleogeographic reconstructions place India at its peak isolation from all other continents during the latest Cretaceous, complicating reconstructions of the biogeographic history of the placental radiation. Recent fieldwork in India has recovered dozens of better-preserved specimens of Cretaceous eutherians, including several new species. Here, we incorporate these new specimens into an extensive phylogenetic analysis that includes every clade with a previously hypothesized relationship to Deccanolestes. Our results support a robust relationship between Deccanolestes and Paleocene adapisoriculids, but do not support a close affinity between these taxa and any placental clade, demonstrating that Deccanolestes is not a Cretaceous placental mammal and reinforcing the sizeable gap between molecular and fossil divergence time estimates for the placental mammal radiation. Instead, our expanded data push Adapisoriculidae, including Deccanolestes, into a much more basal position than in earlier analyses, strengthening hypotheses that scansoriality and arboreality were prevalent early in eutherian evolution. This comprehensive phylogeny indicates that faunal exchange occurred between India, Africa, and Europe in the Late Cretaceous-Early Paleocene, and suggests a previously unrecognized ∼30 to 45 Myr "ghost lineage" for these Gondwanan eutherians.

Reconstruction of karyotype evolution in core Glires. I. The genome homology revealed by comparative chromosome painting

Glires represent a eutherian clade consisting of rodents and lagomorphs (hares, rabbits, and pikas). Chromosome evolution of Glires is known to have variable rates in different groups: from slowly evolving lagomorphs and squirrels to extremely rapidly evolving muroids. Previous interordinal homology maps between slowly evolving Glires were based on comparison with humans. Here, we used sets of chromosome-specific probes from Tamias sibiricus (Sciuridae), Castor fiber (Castoridae) and humans to study karyotypes of six ground squirrels (genera Marmota and Spermophilus) and one tree squirrel (genus Sciurus), mountain hare (genus Lepus), and rabbit (genus Oryctolagus). These data supplemented with GTG banding comparisons allowed us to build comparative chromosome maps. Our data showed the absence of previously found squirrel associations HSA 1/8 and 2/17 in the Eurasian ground squirrels--sousliks and woodchucks, and disruptions of squirrel HSA 10/13 and HSA 8/4/8/12/22 syntenies in the four Spermophilus species studied here. We found that the karyotypes of Sciuridae and Leporidae are highly conserved and close to the Rodentia ancestral karyotype, while Castoridae chromosomes underwent many more changes. We suggest that Lagomorpha and Sciuridae (in contrast to all other rodent families) should be considered as core Glires lineages, characterized by cytogenetically conserved karyotypes which contain chromosomal elements inherent to karyotype of common Glires ancestor. Our data allowed us to further refine the putative ancestral karyotypes of Rodentia. We also describe here the putative ancestral karyotypes of Glires and lagomorphs.

Flat latitudinal gradient in Paleocene mammal richness suggests decoupling of climate and biodiversity

Mammal faunas from western North America exhibit no significant change in species richness with latitude during the Torrejonian (ca. 63–60 Ma) and Tiffanian (ca. 60–58 Ma) North American Land Mammal Ages, in contrast to a strong richness gradient in modern mammalian faunas of the same region today. The latitudinal gradient in oxygen isotope composition of mammalian bioapatite from the Paleocene faunas is similar to that of modern meteoric and surface waters, suggesting that the temperature gradient in the Paleocene was similar to the modern one. The flat richness gradient in the middle Paleocene indicates either different responses to climatic gradients of faunas dominated by extinct clades of placental mammals or distinct ecological processes during the Paleocene diversification of mammals following the end-Cretaceous mass extinction.

Biting through constraints: cranial morphology, disparity and convergence across living and fossil carnivorous mammals

Carnivory has evolved independently several times in eutherian (including placental) and metatherian (including marsupial) mammals. We used geometric morphometrics to assess convergences associated with the evolution of carnivory across a broad suite of mammals, including the eutherian clades Carnivora and Creodonta and the metatherian clades Thylacoleonidae, Dasyuromorphia, Didelphidae and Borhyaenoidea. We further quantified cranial disparity across eutherians and metatherians to test the hypothesis that the marsupial mode of reproduction has constrained their morphological evolution. This study, to our knowledge the first to extensively sample pre-Pleistocene taxa, analysed 30 three-dimensional landmarks, focused mainly on the facial region, which were digitized on 130 specimens, including 36 fossil taxa. Data were analysed with principal components (PC) analysis, and three measures of disparity were compared between eutherians and metatherians. PC1 showed a shift from short to long faces and seemed to represent diet and ecology. PC2 was dominated by the unique features of sabre-toothed forms: dramatic expansion of the maxilla at the expense of the frontal bones. PC3, in combination with PC1, distinguished metatherians and eutherians. Metatherians, despite common comparisons with felids, were more similar to caniforms, which was unexpected for taxa such as the sabre-toothed marsupial Thylacosmilus. Contrary to previous studies, metatherian carnivores consistently exhibited disparity which exceeded that of the much more speciose eutherian carnivore radiations, refuting the hypothesis that developmental constraints have limited the morphological evolution of the marsupial cranium.

Brain size, life history, and metabolism at the marsupial/placental dichotomy

The evolution of mammalian brain size is directly linked with the evolution of the brain's unique structure and performance. Both maternal life history investment traits and basal metabolic rate (BMR) correlate with relative brain size, but current hypotheses regarding the details of these relationships are based largely on placental mammals. Using encephalization quotients, partial correlation analyses, and bivariate regressions relating brain size to maternal investment times and BMR, we provide a direct quantitative comparison of brain size evolution in marsupials and placentals, whose reproduction and metabolism differ extensively. Our results show that the misconception that marsupials are systematically smaller-brained than placentals is driven by the inclusion of one large-brained placental clade, Primates. Marsupial and placental brain size partial correlations differ in that marsupials lack a partial correlation of BMR with brain size. This contradicts hypotheses stating that the maintenance of relatively larger brains requires higher BMRs. We suggest that a positive BMR-brain size correlation is a placental trait related to the intimate physiological contact between mother and offspring during gestation. Marsupials instead achieve brain sizes comparable to placentals through extended lactation. Comparison with avian brain evolution suggests that placental brain size should be constrained due to placentals' relative precociality, as has been hypothesized for precocial bird hatchlings. We propose that placentals circumvent this constraint because of their focus on gestation, as opposed to the marsupial emphasis on lactation. Marsupials represent a less constrained condition, demonstrating that hypotheses regarding placental brain size evolution cannot be generalized to all mammals.

The influence of modularity on cranial morphological disparity in Carnivora and Primates (Mammalia).

BackgroundAlthough variation provides the raw material for natural selection and evolution, few empirical data exist about the factors controlling morphological variation. Because developmental constraints on variation are expected to act by influencing trait correlations, studies of modularity offer promising approaches that quantify and summarize patterns of trait relationships. Modules, highly-correlated and semi-autonomous sets of traits, are observed at many levels of biological organization, from genes to colonies. The evolutionary significance of modularity is considerable, with potential effects including constraining the variation of individual traits, circumventing pleiotropy and canalization, and facilitating the transformation of functional structures. Despite these important consequences, there has been little empirical study of how modularity influences morphological evolution on a macroevolutionary scale. Here, we conduct the first morphometric analysis of modularity and disparity in two clades of placental mammals, Primates and Carnivora, and test if trait integration within modules constrains or facilitates morphological evolution.Principal FindingsWe used both randomization methods and direct comparisons of landmark variance to compare disparity in the six cranial modules identified in previous studies. The cranial base, a highly-integrated module, showed significantly low disparity in Primates and low landmark variance in both Primates and Carnivora. The vault, zygomatic-pterygoid and orbit modules, characterized by low trait integration, displayed significantly high disparity within Carnivora. 14 of 24 results from analyses of disparity show no significant relationship between module integration and morphological disparity. Of the ten significant or marginally significant results, eight support the hypothesis that integration within modules constrains morphological evolution in the placental skull. Only the molar module, a highly-integrated and functionally important module, showed significantly high disparity in Carnivora, in support of the facilitation hypothesis.ConclusionsThis analysis of within-module disparity suggested that strong integration of traits had little influence on morphological evolution over large time scales. However, where significant results were found, the primary effect of strong integration of traits was to constrain morphological variation. Thus, within Primates and Carnivora, there was some support for the hypothesis that integration of traits within cranial modules limits morphological evolution, presumably by limiting the variation of individual traits.

Physical mapping of the elephant X chromosome: conservation of gene order over 105 million years

All therian mammals (eutherians and marsupials) have an XX female/XY male sex chromosome system or some variant of it. The X and Y evolved from a homologous pair of autosomes over the 166 million years since therian mammals diverged from monotremes. Comparing the sex chromosomes of eutherians and marsupials defined an ancient X conserved region that is shared between species of these mammalian clades. However, the eutherian X (and the Y) was augmented by a recent addition (XAR) that is autosomal in marsupials. XAR is part of the X in primates, rodents, and artiodactyls (which belong to the eutherian clade Boreoeutheria), but it is uncertain whether XAR is part of the X chromosome in more distantly related eutherian mammals. Here we report on the gene content and order on the X of the elephant (Loxodonta africana)-a representative of Afrotheria, a basal endemic clade of African mammals-and compare these findings to those of other documented eutherian species. A total of 17 genes were mapped to the elephant X chromosome. Our results support the hypothesis that the eutherian X and Y chromosomes were augmented by the addition of autosomal material prior to eutherian radiation. Not only does the elephant X bear the same suite of genes as other eutherian X chromosomes, but gene order appears to have been maintained across 105 million years of evolution, perhaps reflecting strong constraints posed by the eutherian X inactivation system.