Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths.

Living sloths represent two distinct lineages of small-sized mammals that independently evolved arboreality from terrestrial ancestors. The six extant species are the survivors of an evolutionary radiation marked by the extinction of large terrestrial forms at the end of the Quaternary. Until now, sloth evolutionary history has mainly been reconstructed from phylogenetic analyses of morphological characters. Here, we used ancient DNA methods to successfully sequence 10 extinct sloth mitogenomes encompassing all major lineages. This includes the iconic continental ground sloths Megatherium, Megalonyx, Mylodon, and Nothrotheriops and the smaller endemic Caribbean sloths Parocnus and Acratocnus. Phylogenetic analyses identify eight distinct lineages grouped in three well-supported clades, whose interrelationships are markedly incongruent with the currently accepted morphological topology. We show that recently extinct Caribbean sloths have a single origin but comprise two highly divergent lineages that are not directly related to living two-fingered sloths, which instead group with Mylodon. Moreover, living three-fingered sloths do not represent the sister group to all other sloths but are nested within a clade of extinct ground sloths including Megatherium, Megalonyx, and Nothrotheriops. Molecular dating also reveals that the eight newly recognized sloth families all originated between 36 and 28 million years ago (mya). The early divergence of recently extinct Caribbean sloths around 35 mya is consistent with the debated GAARlandia hypothesis postulating the existence at that time of a biogeographic connection between northern South America and the Greater Antilles. This new molecular phylogeny has major implications for reinterpreting sloth morphological evolution, biogeography, and diversification history.

Phylogeny, Macroevolutionary Trends and Historical Biogeography of Sloths: Insights From a Bayesian Morphological Clock Analysis.

Sloths, like other xenarthrans, are an extremely interesting group of mammals that, after a long history of evolution and diversification in South America, became established on islands in the Caribbean and later reached North America during the Great American Biotic Interchange. In all three regions, they were part of the impressive Pleistocene megafauna. Most taxa became extinct and only two small, distantly related tree-dwelling genera survived. Here, we incorporate several recently described genera of sloths into an assembled morphological data supermatrix and apply Bayesian inference, using phylogenetic and morphological clock methods, to 64 sloth genera. Thus, we investigate the evolution of the group in terms of the timing of divergence of different lineages and their diversity, morphological disparity and biogeographical history. The phylogeny obtained supports the existence of the commonly recognized clades for the group. Our results provide divergence time estimates for the major clades within Folivora that could not be dated with molecular methods. Lineage diversity shows an early increase, reaching a peak in the Early Miocene followed by a major drop at the end of the Santacrucian (Early Miocene). A second peak in the Late Miocene was also followed by a major drop at the end of the Huayquerian (Late Miocene). Both events show differential impact at the family level. After that, a slight Plio-Pleistocene decline was observed before the marked drop with the extinction at the end of the Pleistocene. Phenotypic evolutionary rates were high during the early history of the clade, mainly associated with Mylodontidae, but rapidly decreased to lower values around 25 Ma, whereas Megalonychidae had lower rates at the beginning followed by a steady increase, peaking during the Late Miocene and the Pliocene. Morphological disparity showed a similar trend, with an early increase, followed by a slowly increasing phase through the Late Oligocene and Early Miocene, and ending with another increase beginning at the middle of the Miocene. Biogeographic analysis showed southern South America as the most probable area of origin of the clade and the main region in which the early diversification events took place. Both Megatheriinae and Nothrotheriinae basal nodes were strongly correlated with Andean uplift events, whereas the early history of Mylodontidae is closely associated with southern South America and also shows an early occupation of the northern regions. Within Megalonychidae, our results show Choloepus as a descendant of an island dispersing ancestor and a probable re-ingression to South America by a clade that originated in Central or North America.

Resolving the phylogenetic position of Darwin's extinct ground sloth (Mylodon darwinii) using mitogenomic and nuclear exon data.

Mylodon darwinii is the extinct giant ground sloth named after Charles Darwin, who first collected its remains in South America. We have successfully obtained a high-quality mitochondrial genome at 99-fold coverage using an Illumina shotgun sequencing of a 12 880-year-old bone fragment from Mylodon Cave in Chile. Low level of DNA damage showed that this sample was exceptionally well preserved for an ancient subfossil, probably the result of the dry and cold conditions prevailing within the cave. Accordingly, taxonomic assessment of our shotgun metagenomic data showed a very high percentage of endogenous DNA with 22% of the assembled metagenomic contigs assigned to Xenarthra. Additionally, we enriched over 15 kb of sequence data from seven nuclear exons, using target sequence capture designed against a wide xenarthran dataset. Phylogenetic and dating analyses of the mitogenomic dataset including all extant species of xenarthrans and the assembled nuclear supermatrix unambiguously place Mylodon darwinii as the sister-group of modern two-fingered sloths, from which it diverged around 22 million years ago. These congruent results from both the mitochondrial and nuclear data support the diphyly of the two modern sloth lineages, implying the convergent evolution of their unique suspensory behaviour as an adaption to arboreality. Our results offer promising perspectives for whole-genome sequencing of this emblematic extinct taxon.

Mitogenomics phylogenetic relationships of the current sloth's genera and species (Bradypodidae and Megalonychidae).

We sequenced the complete mitogenome of 39 sloths (19 Bradypus variegatus, 4 B. tridactylus, 1 B. pygmaeus, 1 B. torquatus, 4 Choloepus didactylus, and 10 C. hoffmanni). A Bayesian tree (BI) indicated a temporal split between Bradypus and Choloepus around 31 million years ago (MYA, Oligocene) and the other major splits within each genera during the Miocene and Pliocene. A haplotype network (MJN) estimated a lower temporal split between the sloth genera (around 23.5 MYA). Both methods detected the ancestor of B. torquatus as the first to diverge within Bradypus (21 for BI and 19 MJN), followed by that of the ancestor of B. tridactylus. The split of B. pygmaeus from the common ancestor with B. variegatus was around 12 MYA (BI) or 4.3 MYA (MJN). The splits among the previous populations of B. variegatus began around 8 MYA (BI) or 3.6 MYA (MJN). The trans-Andean population was the first to diverge from the remaining cis-Andean populations of B. variegatus. The genetic differentiation of the trans-Andean B. variegatus population relative to the cis-Andean B. variegatus is similar to that found for different species of sloths. The mitogenomic analysis resolved the differentiation of C. hoffmanni from the C. didactylus individuals of the Guiana Shield. However, one C. didactylus from the Colombian Amazon specimen was inside the C. hoffmanni clade. This could be the first example of possible natural hybridization in the Amazon of both Choloepus taxa or the existence of un-differentiable phenotypes of these two species in some Amazonian areas.

Cophylogenetics and biogeography reveal a coevolved relationship between sloths and their symbiont algae.

Specialized species, like arboreal folivores, often develop beneficial relationships with symbionts to exploit ecologically constrained lifestyles. Although coevolution can drive speciation by specialization of a symbiont to a host, a symbiotic relationship is not indicative of coevolution between host and symbiont. We tested for coevolved relationships between highly specialized two- and three-toed sloths (Choloepus spp. and Bradypus spp., respectively) and their symbiotic algae using cophylogenies and phylogeography. Our phylogeographic analysis showed a biogeographic pattern for the sloth distribution that was not found in the algal phylogeny. We found support for congruence between the sloth and algae phylogenies, implying cospeciation, only in the Bradypus lineage. Algae host-switching occurred from Bradypus spp. to Choloepus spp. Our results support a previously hypothesized symbiotic relationship between sloths and the algae in their fur and indicate that coevolution may have played a role in algae diversification. More broadly, convergent evolution may facilitate host switching between deeply diverged host lineages.

Was Frozen Mammoth or Giant Ground Sloth Served for Dinner at The Explorers Club?

Accounts of woolly mammoths (Mammuthus primigenius) preserved so well in ice that their meat is still edible have a long history of intriguing the public and influencing paleontological thought on Quaternary extinctions and climate, with some scientists resorting to catastrophism to explain the instantaneous freezing necessary to preserve edible meat. Famously, members of The Explorers Club purportedly dined on frozen mammoth from Alaska, USA, in 1951. This event, well received by the press and general public, became an enduring legend for the Club and popularized the notorious annual tradition of serving rare and exotic food at Club dinners that continues to this day. The Yale Peabody Museum holds a sample of meat preserved from the 1951 meal, interestingly labeled as a South American giant ground sloth (Megatherium), not mammoth. We sequenced a fragment of the mitochondrial cytochrome-b gene and studied archival material to verify its identity, which if genuine, would extend the range of Megatherium over 600% and alter our views on ground sloth evolution. Our results indicate that the meat was not mammoth or Megatherium but green sea turtle (Chelonia mydas). The prehistoric dinner was likely an elaborate publicity stunt. Our study emphasizes the value of museums collecting and curating voucher specimens, particularly those used for evidence of extraordinary claims.

Complex body size trends in the evolution of sloths (Xenarthra: Pilosa).

BACKGROUND: Extant sloths present an evolutionary conundrum in that the two living genera are superficially similar (small-bodied, folivorous, arboreal) but diverged from one another approximately 30 million years ago and are phylogenetically separated by a radiation of medium to massive, mainly ground-dwelling, taxa. Indeed, the species in the two living genera are among the smallest, and perhaps most unusual, of the 50+ known sloth species, and must have independently and convergently evolved small size and arboreality. In order to accurately reconstruct sloth evolution, it is critical to incorporate their extinct diversity in analyses. Here, we used a dataset of 57 species of living and fossil sloths to examine changes in body mass mean and variance through their evolution, employing a general time-variable model that allows for analysis of evolutionary trends in continuous characters within clades lacking fully-resolved phylogenies, such as sloths. RESULTS: Our analyses supported eight models, all of which partition sloths into multiple subgroups, suggesting distinct modes of body size evolution among the major sloth lineages. Model-averaged parameter values supported trended walks in most clades, with estimated rates of body mass change ranging as high as 126 kg/million years for the giant ground sloth clades Megatheriidae and Nothrotheriidae. Inclusion of living sloth species in the analyses weakened reconstructed rates for their respective groups, with estimated rates for Megalonychidae (large to giant ground sloths and the extant two-toed sloth) were four times higher when the extant genus Choloepus was excluded. CONCLUSIONS: Analyses based on extant taxa alone have the potential to oversimplify or misidentify macroevolutionary patterns. This study demonstrates the impact that integration of data from the fossil record can have on reconstructions of character evolution and establishes that body size evolution in sloths was complex, but dominated by trended walks towards the enormous sizes exhibited in some recently extinct forms.

Towards a standard framework to describe behaviours in the common-sloth (Bradypus variegatus Schinz, 1825): novel interactions data observed in distinct fragments of the Atlantic forest, Brazil.

The common three-toed sloth is a widespread species, but the location and the observation of its individuals are greatly hindered by its biological features. Their camouflaged pelage, its slow and quiet movements, and the strictly arboreal habits resulted in the publication of sparse, fragmented and not patterned information on the common sloth behaviour. Thus, herein we propose an updated standardized behavioural categories' framework to the study of the species. Furthermore we describe two never reported interaction behaviours: a probable mating / courtship ritual between male and female; and apparent recognition behaviour between two males. Finally we highlight the contribution of small-duration field works in this elusive species ethological study.

Towards a standard framework to describe behaviours in the common-sloth (Bradypus variegatus Schinz, 1825): novel interactions data observed in distinct fragments of the Atlantic forest, Brazil / Contribuição para a padronização da descrição de comportamentos de preguiça-comum (Bradypus variegatus Schinz, 1825): novos dados de interação observados em distintos fragmentos de Mata Atlântica, Brasil

The common three-toed sloth is a widespread species, but the location and the observation of its individuals are greatly hindered by its biological features. Their camouflaged pelage, its slow and quiet movements, and the strictly arboreal habits resulted in the publication of sparse, fragmented and not patterned information on the common sloth behaviour. Thus, herein we propose an updated standardized behavioural categories' framework to the study of the species. Furthermore we describe two never reported interaction behaviours: a probable mating / courtship ritual between male and female; and apparent recognition behaviour between two males. Finally we highlight the contribution of small-duration field works in this elusive species ethological study.

A preguiça-de-três-dedos comum é uma espécie amplamente distribuída. No entanto, a biologia da espécie dificulta a sua deteção e observação na natureza. A camuflagem da sua pelagem, seus movimentos lentos e silenciosos e seus hábitos estritamente arbóreos resultaram na publicação de dados comportamentais esparsos, fragmentados e não padronizados. Assim, no presente trabalho propomos uma tabela atualizada e padronizada de comportamentos de preguiça-comum. Além disso, descrevemos dois novos comportamentos de interação para a espécie: um provável evento de corte ou acasalamento entre macho e fêmea e um aparente comportamento de reconhecimento entre dois machos. Finalmente, discutimos a contribuição de trabalhos de campo de curta-duração no estudo etológico desta espécie tão elusiva.

Species identification and chromosome variation of captive two-toed sloths.

Two-toed sloth species, Linnaeus's and Hoffmman's, are frequent residents of zoo collections in North America. However, species identification has always been problematic because of their large overlap in external morphology, which represents an obstacle to the captive breeding program. We describe here a PCR-based technique that allows species identification of two-toed sloths without requiring sequencing, by using a mitochondrial marker (COI gene) and restriction enzyme assay. We also report intra- and inter-specific patterns of chromosome variation in captive two-toed sloths. Molecularly, we identified 22 samples of Linnaeus's and Hoffmman's two-toed sloths corresponding to 14 and 8 individuals, respectively. One animal was identified as a hybrid using the nuclear gene Enam having alleles derived from both species. The chromosome number in Hoffman's two-toed sloths showed low variation ranging only between 50 and 51. In contrast, Linnaeus's two-toed sloths appeared to vary widely, with diploid numbers ranging from 53 to 67, suggesting distinct geographic groups. The species identification method presented here represents a low-cost easy-to-use tool that will help to improve management of the captive population of two-toed sloths.

Citologia vaginal de preguiça-de-coleira (bradypus torquatus) / Vaginal cytology of maned sloth (bradypus torquatus)

As preguiças-de-coleira (Bradypus torquatus) são mamíferos arborícolas da família Bradypodidae. Podem ser encontradas nos trechos de Mata Atlântica do Brasil e a maior diversidade genética de suas populações ocorre em matas do sul da Bahia. A observação desses animais na natureza é muito difícil, pois passam a maior parte da vida escondidos no denso emaranhado das copas, por isso, dados sobre aspectos reprodutivos são escassos e não existem informações sobre ciclo estral dessa espécie. Este trabalho teve por objetivo identificar as células do epitélio vaginal da preguiça-de-coleira (Bradypus torquatus) como forma de viabilizar o uso dessa técnica para estudar as fases do ciclo estral desses animais. As amostras para citologia vaginal foram obtidas de quatro preguiças de coleira que habitavam áreas de Mata Atlântica do sul da Bahia. Após captura manual do animal, procedeu-se a coleta de material biológico, introduzindo uma escova ginecológica estéril, na comissura dorsal da vulva. Para cada amostra foram feitos dois esfregaços rotacionando a extremidade da escova sobre cada lâmina de vidro, fazendo-se em geral três impressões lineares. O esfregaço foi imediatamente corado pelo método Panótico rápido (Laborclin®). Nas preguiças BT033, BT065 e BT042 foi possível identificar respectivamente 30%, 33% e 7% de células parabasais (PB); 56%, 22% e 10% de células intermediárias pequenas (IP); 6%, 18% e 6% de células intermediárias grandes (IG); 2%, 13% e 24% de células superficiais nucleadas (SN); 6%, 14% e 53% de células superficiais anucleadas (SA). Na preguiça BT464 foi possível fazer duas coletas com intervalo de 13 meses. Os dados da primeira e segunda coleta foram, respectivamente: 6% e 17,5 de células PB, 5% e 25% de células IP, 11% e 15,5% de células IG, 8% e 19,5% de células SN e 70% e 22,5% de células SA. Enfatiza-se que as porcentagens de células do epitélio vaginal variaram entre indivíduos e também na mesma preguiça. Isto sugere que a citologia vaginal possa ser uma ferramenta de avaliação do ciclo estral em preguiça-de-coleira.

Maned sloths (Bradypus torquatus) are arboreal mammals of the family Bradypodidae. They can be only found in the Atlantic coast forest of Brazil and its most genetically diverse populations occur in forests of southern Bahia. The observation of these animals in the wild is very difficult as they spend most of their lifetime hidden in the dense forest canopy. Data on their reproductive aspects are scarce, and there is none information about their estrous cycle. This research aimed at identifying the vaginal epithelial cells of maned sloths (Bradypus torquatus) as a possible way to study the phases of the estrous cycle of this animal. The samples for vaginal cytology were obtained from four free ranging maned sloths living in a protected area of coastal forest in the South of Bahia. The sterile gynecological brush was inserted up to the necessary distance to reach the pelvic channel. For each sample two smears were made by rotating the tip of the brush onto each glass slide, producing in general three linear impressions. Staining was performed using rapid Panotic Kit (Laborclin®). Maned sloths BT033, BT065, and BT042 presented, respectively, 30%, 33%, and 7% of parabasal epithelial cells (PB); 56%, 22%, and 10% of small intermediate cells (IP); 6%, 18%, and 6% of large intermediate cells (IG); 2%, 13%, and 24% of superficial epithelial cell with a nucleus (SN); 6%, 14%, and 53% of anucleated superficial epithelial cell (AS). Two cell samples were collected for maned sloth BT464 with a 13 months interval. Cytological differences were observed between the two samples (1st and 2nd): 6% and 17,5% of PB cells, 5% and 25% of IP cells, 11% and 15,5% of IG cells, 8% and 19,5% of SN cells and 70% and 22,5% of AS cells, respectively. It’s interesting to remark that the percentage of vaginal epithelial cells varied among sloths and also for the same animal. This result suggests that vaginal cytology of maned sloth can be used as a tool to evaluate of estrous cycle.

Macroevolution of complex retroviruses.

Retroviruses can leave a "fossil record" in their hosts' genomes in the form of endogenous retroviruses. Foamy viruses, complex retroviruses that infect mammals, have been notably absent from this record. We have found an endogenous foamy virus within the genomes of sloths and show that foamy viruses were infecting mammals more than 100 million years ago and codiverged with their hosts across an entire geological era. Our analysis highlights the role of evolutionary constraint in maintaining viral genome structure and indicates that accessory genes and mammalian mechanisms of innate immunity are the products of macroevolutionary conflict played out over a geological time scale.

Identifying management units in non-endangered species: the example of the sloth Bradypus variegatus Schinz, 1825

In this study we propose the analysis of genetic diversity of the common three-toed sloth, Bradypus variegatus, in an attempt to understand population structure, identify divergent intraspecific units, and contribute to the knowledge of biodiversity in the neotropical forests. We analyzed a 387 bp segment of the mitochondrial DNA control region in 28 individuals distributed in different localities of both Atlantic and Amazon forests. Our results demonstrated that the genetic diversity of B. variegatus is distributed in six management units, MUs. The observed MUs encompass six phylogenetic lineages and represent respectively north and south regions of Atlantic forest, three regions within the Amazon forest, and a transition region between these two biomes. Considering the fact that these MUs are concordant with phylogroups and endemism areas already described for other vertebrate species, we can say that the study of B. variegatus, a widely distributed and not endangered species, can help to identify areas for conservation biology purposes in neotropical rain forests.

Neste estudo nós realizamos a análise da diversidade genética da preguiça comum, Bradypus variegatus, a fim de compreender os padrões de estrutura populacional, identificar unidades intraespecíficas divergentes e contribuir para o conhecimento da biodiversidade nas florestas da região neotropical. Nós analisamos um segmento de 387 pb da região controle do DNA mitocondrial de 28 indivíduos distribuídos em diferentes localidades da Floresta Amazônica e da Mata Atlântica. Os resultados obtidos demonstram que a diversidade genética da espécie pode ser representada em seis diferentes unidades de manejo (UM). Tais UMs englobam seis linhagens filogenéticas e estão localizadas em diferentes regiões geográficas sendo elas, as porções norte e sul da Mata Atlântica, três regiões dentro da área de Floresta Amazônica e uma área de transição entre os dois domínios de mata. As diferentes unidades intraespecíficas de B. variegatus são concordantes com grupos filogeográficos e áreas de endemismo já observadas para outras espécies de vertebrados. Levando em consideração o fato de que estas UMs concordam com filogrupos e áreas de endemismo previamente descritos para outras espécies de vertebrados, o estudo da preguiça comum, uma espécie amplamente distribuída e considerada não ameaçada de extinção, pode auxiliar na identificação de áreas destinadas à conservação biológica ao longo das florestas úmidas da região neotropical.

Identifying management units in non-endangered species: the example of the sloth Bradypus variegatus Schinz, 1825.

In this study we propose the analysis of genetic diversity of the common three-toed sloth, Bradypus variegatus, in an attempt to understand population structure, identify divergent intraspecific units, and contribute to the knowledge of biodiversity in the neotropical forests. We analyzed a 387 bp segment of the mitochondrial DNA control region in 28 individuals distributed in different localities of both Atlantic and Amazon forests. Our results demonstrated that the genetic diversity of B. variegatus is distributed in six management units, MUs. The observed MUs encompass six phylogenetic lineages and represent respectively north and south regions of Atlantic forest, three regions within the Amazon forest, and a transition region between these two biomes. Considering the fact that these MUs are concordant with phylogroups and endemism areas already described for other vertebrate species, we can say that the study of B. variegatus, a widely distributed and not endangered species, can help to identify areas for conservation biology purposes in neotropical rain forests.

Molecular phylogeny of living xenarthrans and the impact of character and taxon sampling on the placental tree rooting.

Extant xenarthrans (armadillos, anteaters and sloths) are among the most derived placental mammals ever evolved. South America was the cradle of their evolutionary history. During the Tertiary, xenarthrans experienced an extraordinary radiation, whereas South America remained isolated from other continents. The 13 living genera are relics of this earlier diversification and represent one of the four major clades of placental mammals. Sequences of the three independent protein-coding nuclear markers alpha2B adrenergic receptor (ADRA2B), breast cancer susceptibility (BRCA1), and von Willebrand Factor (VWF) were determined for 12 of the 13 living xenarthran genera. Comparative evolutionary dynamics of these nuclear exons using a likelihood framework revealed contrasting patterns of molecular evolution. All codon positions of BRCA1 were shown to evolve in a strikingly similar manner, and third codon positions appeared less saturated within placentals than those of ADRA2B and VWF. Maximum likelihood and Bayesian phylogenetic analyses of a 47 placental taxa data set rooted by three marsupial outgroups resolved the phylogeny of Xenarthra with some evidence for two radiation events in armadillos and provided a strongly supported picture of placental interordinal relationships. This topology was fully compatible with recent studies, dividing placentals into the Southern Hemisphere clades Afrotheria and Xenarthra and a monophyletic Northern Hemisphere clade (Boreoeutheria) composed of Laurasiatheria and Euarchontoglires. Partitioned likelihood statistical tests of the position of the root, under different character partition schemes, identified three almost equally likely hypotheses for early placental divergences: a basal Afrotheria, an Afrotheria + Xenarthra clade, or a basal Xenarthra (Epitheria hypothesis). We took advantage of the extensive sampling realized within Xenarthra to assess its impact on the location of the root on the placental tree. By resampling taxa within Xenarthra, the conservative Shimodaira-Hasegawa likelihood-based test of alternative topologies was shown to be sensitive to both character and taxon sampling.

Dwarfism in insular sloths: biogeography, selection, and evolutionary rate.

The islands of Bocas del Toro, Panama, were sequentially separated from the adjacent mainland by rising sea levels during the past 10,000 years. Three-toed sloths (Bradypus) from five islands are smaller than their mainland counterparts, and the insular populations themselves vary in mean body size. We first examine relationships between body size and physical characteristics of the islands, testing hypotheses regarding optimal body size, evolutionary equilibria, and the presence of dispersal in this system. To do so, we conduct linear regressions of body size onto island area, distance from the mainland, and island age. Second, we retroactively calculate two measures of the evolutionary rate of change in body size (haldanes and darwins) and the standardized linear selection differential, or selection intensity (i). We also test the observed morphological changes against models of evolution by genetic drift. The results indicate that mean body size decreases linearly with island age, explaining up to 97% of the variation among population means. Neither island area nor distance from the mainland is significant in multiple regressions that include island age. Thus, we find no evidence for differential optimal body size among islands, or for dispersal in the system. In contrast, the dependence of body size on island age suggests uniform directional selection for small body size in the insular populations. Although genetic drift cannot be discounted as the cause for this evolution in body size, the probability is small given the consistent direction of evolution (repeated dwarfism). The insular sloths show a sustained rate of evolution similar to those measured in haldanes over tens of generations, appearing to unite micro- and macroevolutionary time scales. Furthermore, the magnitude and rate of this example of rapid differentiation fall within predictions of theoretical models from population genetics. However, the linearity of the relationship between body size and island age is not predicted, suggesting that either more factors are involved than those considered here, or that theoretical advances are necessary to explain constant evolutionary rates over long time spans in new selective environments.

Brazilian distribution of Amblyomma varium Koch, 1844 (Acari: Ixodidae), a common parasite of sloths (Mammalia: Xenarthra).

Amblyomma varium, commonly known in Brazil as the "carrapato-gigante-da-pregui a" (sloth's giant tick) is found from southern Central America to Argentina. The present study adds information on the geographical distribution of A. varium, as well as on their hosts, based on material deposited in the main Brazilian collections and on the available literature. Eighty-two vials, containing 191 adult specimens, deposited in five Acari collections between 1930 and 2001, were examined. These vials included data on the host and collection localities. The biology of A. varium is unknown. However it is known that, during the adult stage, the tick presents a high host specificity and is found almost exclusively on the sloths Bradypus tridactylus, B. variegatus, B.torquatus (Bradypodidae), Choloepus hoffmanni and C. didactylus (Megalonychidae). Based on the material examined, the states of Rond nia, Amazonas, Bahia and Alagoas are newly assigned to geographic distribution of A. varium in Brazil.

A molecular phylogeny of two extinct sloths.

Xenarthra (Edentata) is an extremely diverse mammalian order whose modern representatives are the armadillos, anteaters, and sloths. The phylogeny of these groups is poorly resolved. This is particularly true for the sloths (phyllophagans), originally a large and diverse group now reduced to two genera in two different families. Both morphological analyses and molecular analyses of rDNA genes of living and extinct sloths have been used with limited success to elucidate their phylogeny. In an attempt to clarify relationships among the sloths, DNA was extracted and mitochondrial cytochrome b gene sequences were determined from representatives of two extinct groups of sloths (Mylodontidae and Megatheriidae), their two living relatives (two-toed sloths [Megalonychidae], three-toed sloths [Bradypodidae]), anteaters and armadillos. A consistent feature of the latter two species was the nuclear copies of cytochrome b gene sequences. Several methods of phylogenetic reconstruction were applied to the sequences determined, and the results were compared with 12S rDNA sequences obtained in previous studies. The cytochrome b gene exhibited a phylogenetic resolving power similar to that of the 12S rDNA sequences. When both data sets were combined, they tended to support the grouping of two-toed sloths with mylodontids and three-toed sloths with megatheriids. The results strengthen the view that the two families of living sloths adapted independently to an arboreal life-style.

Cephalic arterial pattern in New World edentates and Old World pangolins with special reference to their phylogenetic relationships and taxonomy.

The cephalic arterial pattern in edentates and pangolins is described on the basis of 9 corrosion specimens, representing all the classical superfamilies, with special reference to their phylogenetic relationship and taxonomy. In this respect, the importance of the manner in which the external carotid artery system annexes the stepedial area of supply and of the course of the internal carotid artery in relation to the tympanic cavity is emphasized. The investigation does not indicate any special relationship between the New World edentates and the Old World pangolins, whereas the marked difference in the course of the internal carotid artery in recent edentates stresses the independent development of the South American anteaters compared with that of the two other edentate groups (armadillos and tree sloths). Most probably the edentates were divided very early into two main lines which have evolved independently since the early Tertiary, i.e. one for the anteaters and one for the tree sloths and armadillos, indicating a probable subdivision of the true edentates into two suborders. This subdivision is markedly different from the classical two-fold division of the edentates.