The Uto‐Aztecan premolar among North and South Amerindians: Geographic variation and genetics

The Uto‐Aztecan premolar (UAP) is a dental polymorphism characterized by an exaggerated distobuccal rotation of the paracone in combination with the presence of a fossa at the intersection of the distal occlusal ridge and distal marginal ridge of upper first premolars. This trait is important because, unlike other dental variants, it has been found exclusively in Native American populations. However, the trait's temporal and geographic variation has never been fully documented. The discovery of a Uto‐Aztecan premolar in a prehistoric skeletal series from northern South America calls into question the presumed linguistic and geographic limits of this trait. We examined published and unpublished data for this rare but highly distinctive trait in samples representing over 5,000 Native Americans from North and South America. Our findings in living Southwest Amerindian populations corroborate the notion that the variable goes beyond the bounds of the Uto‐Aztecan language family. It is found in prehistoric Native Americans from South America, eastern North America, Northern and Central Mexico, and in living and prehistoric populations in the American Southwest that are not members of the Uto‐Aztecan language stock. The chronology of samples, its geographic distribution, and trait frequencies suggests a North American origin (Southwest) for UAP perhaps between 15,000 BP and 4,000 BP and a rapid and widespread dispersal into South America during the late Holocene. Family data indicate that it may represent an autosomal recessive mutation that occurred after the peopling of the Americas as its geographic range appears to be limited to North and South Amerindian populations. Am J Phys Anthropol, 2010. © 2010 Wiley‐Liss, Inc.     

Nucleotide variation of the Est-6 gene region in natural populations of Drosophila melanogaster

We have investigated nucleotide polymorphism in the Est-6 gene region in four samples of Drosophila melanogaster derived from natural populations of East Africa (Zimbabwe), Europe (Spain), North America (California), and South America (Venezuela). There are two divergent sequence types in the North and South American samples, which are not perfectly (North America) or not at all (South America) associated with the Est-6 allozyme variation. Less pronounced or no sequence dimorphism occurs in the European and African samples, respectively. The level of nucleotide diversity is highest in the African sample, lower (and similar to each other) in the samples from Europe and North America, and lowest in the sample from South America. The extent of linkage disequilibrium is low in Africa (1.23% significant associations), but much higher in non-African populations (22.59, 21.45, and 37.68% in Europe, North America, and South America, respectively). Tests of neutrality with recombination are significant in non-African samples but not significant in the African sample. We propose that demographic history (bottleneck and admixture of genetically different populations) is the major factor shaping the nucleotide patterns in the Est-6 gene region. However, positive selection modifies the pattern: balanced selection creates elevated levels of nucleotide variation around functionally important (target) polymorphic sites (RsaI-/RsaI+ in the promoter region and F/S in the coding region) in both African and non-African samples; and directional selection, acting during the geographic expansion phase of D. melanogaster, creates an excess of very similar sequences (RsaI- and S allelic lineages, in the promoter and coding regions, respectively) in the non-African samples.     

The Applicability of North American Growth Standards in Developing Countries

For the purpose of developmental evaluation of growing children and the nutritional assessment of individuals or populations, reliance on current North American standards for size, maturational timing, body fatness or osseous development may be unwise not only with respect to various African, Asiatic and Pacific populations but often with respect to adults and children in North America. In particular, the dental development and early ossification progress of most human groups are in advance of contemporary North American standards. The use of stature as a reference standard and the simple ratio of weight to height are both unsatisfactory measures except within clearly defined population samples. At the very least, age-size tables based on parental statures would be more realistic and useful for children of known parentage, whereas present-day averages that neglect parental size are most applicable to foundlings and illegitimate children.     

Genotype-by-environment interactions and adaptation to local temperature affect immunity and fecundity in Drosophila melanogaster

Natural populations of most organisms harbor substantial genetic variation for resistance to infection. The continued existence of such variation is unexpected under simple evolutionary models that either posit direct and continuous natural selection on the immune system or an evolved life history "balance" between immunity and other fitness traits in a constant environment. However, both local adaptation to heterogeneous environments and genotype-by-environment interactions can maintain genetic variation in a species. In this study, we test Drosophila melanogaster genotypes sampled from tropical Africa, temperate northeastern North America, and semi-tropical southeastern North America for resistance to bacterial infection and fecundity at three different environmental temperatures. Environmental temperature had absolute effects on all traits, but there were also marked genotype-by-environment interactions that may limit the global efficiency of natural selection on both traits. African flies performed more poorly than North American flies in both immunity and fecundity at the lowest temperature, but not at the higher temperatures, suggesting that the African population is maladapted to low temperature. In contrast, there was no evidence for clinal variation driven by thermal adaptation within North America for either trait. Resistance to infection and reproductive success were generally uncorrelated across genotypes, so this study finds no evidence for a fitness tradeoff between immunity and fecundity under the conditions tested. Both local adaptation to geographically heterogeneous environments and genotype-by-environment interactions may explain the persistence of genetic variation for resistance to infection in natural populations.     

Continent-Wide Decoupling of Y-Chromosomal Genetic Variation from Language and Geography in Native South Americans

Numerous studies of human populations in Europe and Asia have revealed a concordance between their extant genetic structure and the prevailing regional pattern of geography and language. For native South Americans, however, such evidence has been lacking so far. Therefore, we examined the relationship between Y-chromosomal genotype on the one hand, and male geographic origin and linguistic affiliation on the other, in the largest study of South American natives to date in terms of sampled individuals and populations. A total of 1,011 individuals, representing 50 tribal populations from 81 settlements, were genotyped for up to 17 short tandem repeat (STR) markers and 16 single nucleotide polymorphisms (Y-SNPs), the latter resolving phylogenetic lineages Q and C. Virtually no structure became apparent for the extant Y-chromosomal genetic variation of South American males that could sensibly be related to their inter-tribal geographic and linguistic relationships. This continent-wide decoupling is consistent with a rapid peopling of the continent followed by long periods of isolation in small groups. Furthermore, for the first time, we identified a distinct geographical cluster of Y-SNP lineages C-M217 (C3*) in South America. Such haplotypes are virtually absent from North and Central America, but occur at high frequency in Asia. Together with the locally confined Y-STR autocorrelation observed in our study as a whole, the available data therefore suggest a late introduction of C3* into South America no more than 6,000 years ago, perhaps via coastal or trans-Pacific routes. Extensive simulations revealed that the observed lack of haplogroup C3* among extant North and Central American natives is only compatible with low levels of migration between the ancestor populations of C3* carriers and non-carriers. In summary, our data highlight the fact that a pronounced correlation between genetic and geographic/cultural structure can only be expected under very specific conditions, most of which are likely not to have been met by the ancestors of native South Americans.     

Quantitative trait analysis and geographic variability of natural populations of Zaprionus indianus, a recent invader in Brazil

Five natural samples of a recent South America invader, the drosophilid Zaprionus indianus, were investigated with the isofemale line technique. These samples were compared to five African mainland populations, investigated with the same method. The results were also compared to data obtained on mass cultures of other populations from Africa and India. Three quantitative traits were measured on both sexes, wing and thorax length and sternopleural bristle number. We did not find any latitudinal trend among the American samples, while a significant increase in body size with latitude was observed in the Indian and, to a lesser degree, in the African populations. American populations were also characterized by their bigger size. Genetic variability, estimated by the intraclass correlation among isofemale lines, was similar in American and African populations. The intraline, nongenetic variability was significantly less in the American samples, suggesting a better developmental stability, the origin of which is unclear. A positive relationship was evident between intraline variability of size traits and the wing-thorax length correlation. Altogether, our data suggest that the colonizing propagule introduced to Brazil had a fairly large size, preventing any bottleneck effect being detected. The big body size of American flies suggests that they came from a high-latitude African country. The lack of a latitudinal dine in America seems to be related to the short time elapsed since introduction. The very rapid spread of Z. indianus all over South America suggests that it might rapidly invade North America.     

Recent colonization and radiation of North American Lycaeides (Plebejus) inferred from mtDNA

North American Lycaeides populations exhibit remarkable variation in ecological, morphological, and behavioral characters, as well as an established history of introgressive hybridization. We examined mitochondrial DNA variation from 55 Eurasian and North American Lycaeides populations using molecular phylogenetics and coalescent-based methods in order to clarify the evolutionary and demographic history of this polytypic group. Specifically we addressed the following questions: (1) Do mitochondrial alleles sampled from North America form a monophyletic group, which would be expected if North American Lycaeides were descended from a single Eurasian ancestor? (2) When did Lycaeides colonize North America? and (3) What is the demographic history of North American Lycaeides since their colonization? Bayesian maximum likelihood methods identified three major mitochondrial lineages for Lycaeides; each lineage contained haplotypes sampled from both Eurasia and North America. This suggests a complex colonization history for Lycaeides, which likely involved multiple founding lineages. Coalescent-based analyses placed the colonization of North America by Eurasian Lycaeides sometime during or after the late Pliocene. This was followed by a sudden increase in population size of more than an order of magnitude for the North American population of Lycaeides approximately 100,000-150,000 years before the present. These mitochondrial data, in conjunction with data from previous ecological, morphological, and behavioral studies, suggest that the diversity observed in Lycaeides in North America is the result of a recent evolutionary radiation, which may have been facilitated, in part, by hybridization.     

“South American” Marsupials from the Late Cretaceous of North America and the Origin of Marsupial Cohorts

Newly described marsupial specimens of Judithian (late Campanian) and Lancian (Maastrichtian) age in the western interior of North America (Wyoming to Alberta) have dental morphologies consistent with those expected in comparably aged sediments in South America (yet to be found). Three new Lancian species are referable to the didelphimorphian Herpetotheriidae, which suggests that the ameridelphian radiation was well under way by this time. The presence of a polydolopimorphian from Lancian deposits with a relatively plesiomorphic dental morphology and an additional polydolopimorphian taxon from Judithian deposits with a more derived molar form indicate that this lineage of typically South American marsupials was diversifying in the Late Cretaceous of North America. This study indicates that typical South American lineages (e.g. didelphimorphians and polydolopimorphians) are not the result of North American peradectian progenitors dispersing into South America at the end of the Cretaceous (Lancian), or at the beginning of the Paleocene (Puercan), and giving rise to the ameridelphian marsupials. Instead, these lineages, and predictably others as well, had their origins in North America (probably in more southerly latitudes) and then dispersed into South America by the end of the Cretaceous. Geophysical evidence concerning the connections between North and South America in the Late Cretaceous is summarized as to the potential for overland mammalian dispersal between these places at those times. Paleoclimatic reconstructions are considered, as is the dispersal history of hadrosaurine dinosaurs and boid snakes, as to their contribution to an appraisal of mammalian dispersals in the Late Cretaceous. In addition, we present a revision of the South American component of the Marsupialia. One major outcome of this process is that the Polydolopimorphia is placed as Supercohort Marsupialia incertae sedis because no characteristics currently known from this clade securely place it within one of the three named marsupial cohorts.     

Convergence and contingency in production–precipitation relationships in North American and South African C4 grasslands

Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C₄ grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m² respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.     

Parallel flowering time clines in native and introduced ragweed populations are likely due to adaptation

As introduced species expand their ranges, they often encounter differences in climate which are often correlated with geography. For introduced species, encountering a geographically variable climate sometimes leads to the re‐establishment of clines seen in the native range. However, clines can also be caused by neutral processes, and so it is important to gather additional evidence that population differentiation is the result of selection as opposed to nonadaptive processes. Here, we examine phenotypic and genetic differences in ragweed from the native (North America) and introduced (European) ranges. We used a common garden to assess phenotypic differentiation in size and flowering time in ragweed populations. We found significant parallel clines in flowering time in both North America and Europe. Height and branch number had significant clines in North America, and, while not statistically significant, the patterns in Europe were the same. We used SNP data to assess population structure in both ranges and to compare phenotypic differentiation to neutral genetic variation. We failed to detect significant patterns of isolation by distance, geographic patterns in population structure, or correlations between the major axes of SNP variation and phenotypes or latitude of origin. We conclude that the North American clines in size and the parallel clines seen for flowering time are most likely the result of adaptation.     

Discrete morphological variants of human cervical vertebrae: Exploring pattern of distribution and biological significance

Studies on discrete traits of the human cervical vertebrae, appearing at certain intervals during the last century, posed some questions regarding evolutionary processes that human cervical spine underwent during phylogenesis. To address questions of significance of these morphological traits we need first a good knowledge of the extent of their variation in modern humans. The aim of the current work was to integrate available data on the occurrence of various non-metric traits in the human cervical spine and search for the pattern of their distribution on intra- and inter-population levels. The study was based on data from five osteological samples from North America (Terry and Grant collections) and Russia (mid 20th c. and 18th c.); and data taken from literature. Traits were categorized into rare (<3%), low frequency (up to 10%), often encountered (10–30%) and characteristic for modern humans (>50% on average). Several traits showed mild to strong association with each other indicating interrelation between various spine characteristics. Of the traits analyzed, the following had consistent pattern of sex-related variability: complete dorsal ponticle, bifid spinous processes and cervical ribs; and ancestry-related variability: dorsal ponticle and bifid spinous processes. Each ancestry group (European, African, Asian and North American) had its specifics regarding the latter two traits which might be related to genetic isolation. Most of the traits, however, showed relatively similar pattern of distribution among various populations, including the pattern of within-spine variability. This suggests a common intraspecific pattern and a possible link to some fundamental characteristics of the human vertebral column.     

Clonal Expansion of the Pseudogymnoascus destructans Genotype in North America Is Accompanied by Significant Variation in Phenotypic Expression

Pseudogymnoascus destructans is the causative agent of an emerging infectious disease that threatens populations of several North American bat species. The fungal disease was first observed in 2006 and has since caused the death of nearly six million bats. The disease, commonly known as white-nose syndrome, is characterized by a cutaneous infection with P. destructans causing erosions and ulcers in the skin of nose, ears and/or wings of bats. Previous studies based on sequences from eight loci have found that isolates of P. destructans from bats in the US all belong to one multilocus genotype. Using the same multilocus sequence typing method, we found that isolates from eastern and central Canada also had the same genotype as those from the US, consistent with the clonal expansion of P. destructans into Canada. However, our PCR fingerprinting revealed that among the 112 North American isolates we analyzed, three, all from Canada, showed minor genetic variation. Furthermore, we found significant variations among isolates in mycelial growth rate; the production of mycelial exudates; and pigment production and diffusion into agar media. These phenotypic differences were influenced by culture medium and incubation temperature, indicating significant variation in environmental condition - dependent phenotypic expression among isolates of the clonal P. destructans genotype in North America.     

Phylogeny and taxonomy of the damselfly genus Enallagma and related taxa (Odonata: Zygoptera: Coenagrionidae)

Abstract The zygopteran genus Enallagma has been the subject of numerous behavioural and ecological studies, but phylogenetic relationships among species have been examined only within eastern North America, and even the composition and diagnosis of the genus are unclear on a world-wide basis. Most authorities currently recognize about seventy species within Enallagma , comprising two major radiations, in North America and Africa. This study, using morphological data, demonstrates that the North American and a few related Palaearctic species form a monophyletic group that is quite distinct from the African species. The latter are themselves divided into at least three, and probably four, separate clades, one of which may be related to E. parvum of India. Consequently, three of Kennedy's long disused genera, Africallagma , Amphiallagma and Proischnura ( Kennedy, 1920 ) are resurrected and two new genera, Azuragrion gen.n. and Pinheyagrion gen.n. are established for the remaining African taxa. Finally, Enallagma is divided into two subgenera, Enallagma s.s ., the typical 'bluets', including many North American, Holarctic and Palaearctic species, and Chromatallagma subgen.n., comprising a group of species of more variable colour that is confined to North America, the Caribbean and northernmost South America.     

Linguistic and maternal genetic diversity are not correlated in Native Mexicans

Mesoamerica, defined as the broad linguistic and cultural area from middle southern Mexico to Costa Rica, might have played a pivotal role during the colonization of the American continent. The Mesoamerican isthmus has constituted an important geographic barrier that has severely restricted gene flow between North and South America in pre-historical times. Although the Native American component has been already described in admixed Mexican populations, few studies have been carried out in native Mexican populations. In this study, we present mitochondrial DNA (mtDNA) sequence data for the first hypervariable region (HVR-I) in 477 unrelated individuals belonging to 11 different native populations from Mexico. Almost all of the Native Mexican mtDNAs could be classified into the four pan-Amerindian haplogroups (A2, B2, C1, and D1); only two of them could be allocated to the rare Native American lineage D4h3. Their haplogroup phylogenies are clearly star-like, as expected from relatively young populations that have experienced diverse episodes of genetic drift (e.g., extensive isolation, genetic drift, and founder effects) and posterior population expansions. In agreement with this observation, Native Mexican populations show a high degree of heterogeneity in their patterns of haplogroup frequencies. Haplogroup X2a was absent in our samples, supporting previous observations where this clade was only detected in the American northernmost areas. The search for identical sequences in the American continent shows that, although Native Mexican populations seem to show a closer relationship to North American populations, they cannot be related to a single geographical region within the continent. Finally, we did not find significant population structure in the maternal lineages when considering the four main and distinct linguistic groups represented in our Mexican samples (Oto-Manguean, Uto-Aztecan, Tarascan, and Mayan), suggesting that genetic divergence predates linguistic diversification in Mexico.     

“South American” Marsupials from the Late Cretaceous of North America and the Origin of Marsupial Cohorts

Newly described marsupial specimens of Judithian (late Campanian) and Lancian (Maastrichtian) age in the western interior of North America (Wyoming to Alberta) have dental morphologies consistent with those expected in comparably aged sediments in South America (yet to be found). Three new Lancian species are referable to the didelphimorphian Herpetotheriidae, which suggests that the ameridelphian radiation was well under way by this time. The presence of a polydolopimorphian from Lancian deposits with a relatively plesiomorphic dental morphology and an additional polydolopimorphian taxon from Judithian deposits with a more derived molar form indicate that this lineage of typically South American marsupials was diversifying in the Late Cretaceous of North America. This study indicates that typical South American lineages (e.g. didelphimorphians and polydolopimorphians) are not the result of North American peradectian progenitors dispersing into South America at the end of the Cretaceous (Lancian), or at the beginning of the Paleocene (Puercan), and giving rise to the ameridelphian marsupials. Instead, these lineages, and predictably others as well, had their origins in North America (probably in more southerly latitudes) and then dispersed into South America by the end of the Cretaceous. Geophysical evidence concerning the connections between North and South America in the Late Cretaceous is summarized as to the potential for overland mammalian dispersal between these places at those times. Paleoclimatic reconstructions are considered, as is the dispersal history of hadrosaurine dinosaurs and boid snakes, as to their contribution to an appraisal of mammalian dispersals in the Late Cretaceous. In addition, we present a revision of the South American component of the Marsupialia. One major outcome of this process is that the Polydolopimorphia is placed as Supercohort Marsupialia incertae sedis because no characteristics currently known from this clade securely place it within one of the three named marsupial cohorts. This article contains corrections to the text and a new Figure 11 not incorporated in the originally published version in Vol. 11, Nos. 3/4. For purposes of future citation, the present version (Vol. 12 and Nos. 3/4) should be used.     

Genomic ancestry of North Africans supports back-to-Africa migrations

North African populations are distinct from sub-Saharan Africans based on cultural, linguistic, and phenotypic attributes; however, the time and the extent of genetic divergence between populations north and south of the Sahara remain poorly understood. Here, we interrogate the multilayered history of North Africa by characterizing the effect of hypothesized migrations from the Near East, Europe, and sub-Saharan Africa on current genetic diversity. We present dense, genome-wide SNP genotyping array data (730,000 sites) from seven North African populations, spanning from Egypt to Morocco, and one Spanish population. We identify a gradient of likely autochthonous Maghrebi ancestry that increases from east to west across northern Africa; this ancestry is likely derived from "back-to-Africa" gene flow more than 12,000 years ago (ya), prior to the Holocene. The indigenous North African ancestry is more frequent in populations with historical Berber ethnicity. In most North African populations we also see substantial shared ancestry with the Near East, and to a lesser extent sub-Saharan Africa and Europe. To estimate the time of migration from sub-Saharan populations into North Africa, we implement a maximum likelihood dating method based on the distribution of migrant tracts. In order to first identify migrant tracts, we assign local ancestry to haplotypes using a novel, principal component-based analysis of three ancestral populations. We estimate that a migration of western African origin into Morocco began about 40 generations ago (approximately 1,200 ya); a migration of individuals with Nilotic ancestry into Egypt occurred about 25 generations ago (approximately 750 ya). Our genomic data reveal an extraordinarily complex history of migrations, involving at least five ancestral populations, into North Africa.     

Spatial and temporal stability of mtDNA haplogroup frequencies in native North America

Mitochondrial DNA lineage frequencies in prehistoric Aleut, eastern Utah Fremont, Southwestern Anasazi, Pyramid Lake, and Stillwater Marsh skeletal samples from northwest Nevada and the Oneota of western Illinois are compared with those in 41 contemporary aboriginal populations of North America. The ancient samples range in age from 300 years to over 6,000 years. The results indicate that the prehistoric inhabitants of North America exhibit the same level of mtDNA variability as contemporary populations of the continent. Variation in modern mtDNA haplogroup frequencies is highly geographically structured, and the prehistoric samples exhibit the same geographic pattern of variation. This indicates that differentiation of regional patterns of mtDNA lineage variation occurred early in North American prehistory (much more than 2,000 years B.P.), has remained relatively stable since its origin, and was little influenced by the disruptions hypothesized for other genetic systems as a result of population declines and relocations at contact.     

Genetic evidence of intercontinental movement of avian influenza in a migratory bird: the northern pintail (Anas acuta)

The role of migratory birds in the movement of the highly pathogenic (HP) avian influenza H5N1 remains a subject of debate. Testing hypotheses regarding intercontinental movement of low pathogenic avian influenza (LPAI) viruses will help evaluate the potential that wild birds could carry Asian-origin strains of HP avian influenza to North America during migration. Previous North American assessments of LPAI genetic variation have found few Asian reassortment events. Here, we present results from whole-genome analyses of LPAI isolates collected in Alaska from the northern pintail (Anas acuta), a species that migrates between North America and Asia. Phylogenetic analyses confirmed the genetic divergence between Asian and North American strains of LPAI, but also suggested inter-continental virus exchange and at a higher frequency than previously documented. In 38 isolates from Alaska, nearly half (44.7%) had at least one gene segment more closely related to Asian than to North American strains of LPAI. Additionally, sequences of several Asian LPAI isolates from GenBank clustered more closely with North American northern pintail isolates than with other Asian origin viruses. Our data support the role of wild birds in the intercontinental transfer of influenza viruses, and reveal a higher degree of transfer in Alaska than elsewhere in North America.     

Genetic variation and differentiation at microsatellite loci in Drosophila simulans. Evidence for founder effects in new world populations.

Drosophila simulans isofemale lines from Africa, South America, and two locations in North America were surveyed for variation at 16 microsatellite loci on the X, second, and third chromosomes, and 18 microsatellites, which are unmapped. D. simulans is thought to have colonized New World habitats only relatively recently (within the last few hundred years). Consistent with a founder effect occurring as colonizers moved into these New World habitats, we find less microsatellite variability in North and South American D. simulans populations than for an African population. Population subdivision as measured at microsatellites is moderate when averaged across all loci (FST = 0.136), but contrasts sharply with previous studies of allozyme variation, which have showed significantly less differentiation in D. simulans than in D. melanogaster. There are substantially fewer private alleles observed in New World populations of D. simulans than seen in a similar survey of D. melanogaster. In addition to possible differences in population size during their evolutionary histories, varying colonization histories or other demographic events may be necessary to explain discrepancies in the patterns of variation observed at various genetic markers between these closely related species.     

A re-evaluation of the ‘mid-Cretaceous sauropod hiatus’ and the impact of uneven sampling of the fossil record on patterns of regional dinosaur extinction

The mid-Cretaceous of North America and Europe has long been noted for the absence of sauropod dinosaurs, leading several authors to suggest that this depauperate interval is a consequence of an end-Albian sauropod extinction. This time period has become known as the ‘mid-Cretaceous sauropod hiatus’, with the subsequent presence of titanosaurian sauropods in the latest Cretaceous of North America and Europe interpreted as the result of dispersal of taxa from South America and Africa, respectively. However, several lines of evidence indicate that this hiatus is probably a sampling artefact. New fossil and trackway discoveries have considerably shortened the hiatus, reducing it to the Turonian–early Campanian in North America, and to just two short intervals in the late Cenomanian–early Turonian and late Coniacian–Santonian of Europe. Palaeoenvironmental analyses of sauropods demonstrate an inland terrestrial preference for titanosaurs, the dominant Late Cretaceous sauropods; however, during the hiatus there was a decline in inland deposits and increase in coastal sediments in Europe and North America, which would have greatly reduced the probability of preserving titanosaurs. Neither the decline in inland deposits, nor the ‘sauropod hiatus’, occurred elsewhere in the world. Statistical comparisons also demonstrate a significant positive correlation between fluctuations in inland deposits and sauropod occurrences during the mid–Late Cretaceous in Europe and North and South America. Lastly, cladistic analyses do not place latest Cretaceous North American and European titanosaurs within South American and African clades, contradicting the predictions of the ‘austral immigrant’ hypothesis. The latter hypothesis also receives little support from biogeographical analysis of dispersal among titanosaurs. Thus, the ‘sauropod hiatus’ of North America and Europe is most plausibly interpreted as the product of a sampling bias pertaining to the rarity of inland sediments and dominance of coastal deposits preserved in these two regions during the mid-Cretaceous. The presence of titanosaurs in these areas during the latest Cretaceous can be explained by dispersal from Southern Hemisphere continents, but this is no more probable than descent from Early Cretaceous incumbent faunas or dispersal from Asia.