Episodes Of Dispersal Research Articles

New Late Cretaceous macrobaenid turtle with Asian affinities from the High Canadian Arctic: Dispersal via ice-free polar routes

Research Article| February 01, 2009 New Late Cretaceous macrobaenid turtle with Asian affinities from the High Canadian Arctic: Dispersal via ice-free polar routes Deborah Vandermark; Deborah Vandermark 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar John A. Tarduno; John A. Tarduno 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA 2Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar Donald B. Brinkman; Donald B. Brinkman 3Royal Tyrrell Museum of Palaeontology, P.O. Box 7500, Drumheller, Alberta TOJ OYO, Canada Search for other works by this author on: GSW Google Scholar Rory D. Cottrell; Rory D. Cottrell 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar Stephanie Mason Stephanie Mason 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (2): 183–186. https://doi.org/10.1130/G25415A.1 Article history received: 19 Aug 2008 rev-recd: 06 Oct 2008 accepted: 13 Oct 2008 first online: 12 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Deborah Vandermark, John A. Tarduno, Donald B. Brinkman, Rory D. Cottrell, Stephanie Mason; New Late Cretaceous macrobaenid turtle with Asian affinities from the High Canadian Arctic: Dispersal via ice-free polar routes. Geology 2009;; 37 (2): 183–186. doi: https://doi.org/10.1130/G25415A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A new turtle, Aurorachelys gaffneyi gen. et sp. nov., is described on the basis of a carapace and plastron of late Turonian–Coniacian age from Axel Heiberg Island in the High Canadian Arctic. This turtle is a member of the Macrobaenidae, a group that is thought to have originated in Asia. It differs from all other Late Cretaceous macrobaenids in its nearly circular shell. The earliest record of macrobaenids in North America documented by this specimen, together with the earliest records of other turtle clades that presumably originated in Asia, indicates that turtle dispersal was episodic, with most first occurrences of Asian taxa in North America clustering around the Turonian. The high global temperatures of the Turonian are interpreted as facilitating this episode of dispersal of Asian turtle groups into North America. Islands and seamounts in the young, volcanically active Arctic Ocean during the Late Cretaceous may have acted as an alternative to an Alaskan intercontinental route, and may have allowed macrobaenids to enter the part of North America east of the interior continental seaway. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ecological niche models and patterns of richness and endemism of the southern Andean genus Eurymetopum (Coleoptera, Cleridae)

Eurymetopum is an Andean clerid genus with 22 species. We modeled the ecological niches of 19 species with Maxent and used them as potential distributional maps to identify patterns of richness and endemicity. All modeled species maps were overlapped in a single map in order to determine richness. We performed an optimality analysis with NDM/VNDM in a grid of 1o latitude-longitude in order to identify endemism. We found a highly rich area, located between 32o and 41o south latitude, where the richest pixels have 16 species. One area of endemism was identified, located in the Maule and Valdivian Forest biogeographic provinces, which extends also to the Santiago province of the Central Chilean subregion, and contains four endemic species (E. parallelum, E. prasinum, E. proteus, and E. viride), as well as 16 non-endemic species. The sympatry of these phylogenetically unrelated species might indicate ancient vicariance processes, followed by episodes of dispersal. Based on our results, we suggest a close relationship between these provinces, with the Maule representing a complex area.

AMS 14C dating of early human occupation of southern South America

The time of appearance of a persistent and demographically-viable hunter-gatherer population in late Pleistocene southern South America must be determined by evaluating evidence from as large as possible a sample of candidate archaeological sites in the region. We co-ordinated the AMS dating of multiple bone and charcoal samples from previously-excavated strata at the following sites: Arroyo Seco 2, Paso Otero 5, Piedra Museo, and Cueva Tres Tetas (all in Argentina), and Cueva del Lago Sofia 1 and Tres Arroyos (both in Chile). With one possible exception, we did not obtain new results to confirm earlier observations of pre-Clovis-age cultural activity at any of the sites considered in this study. The possible exception, Arroyo Seco 2, is considered in detail elsewhere [Politis G., Gutierrez M.A., Scabuzzo, C. (Eds), in press. Estado actual de las Investigaciones en el sitio 2 de Arroyo Seco (región pampeana, Argentina). Serie Monográfica INCUAPA 5. Olavarría]. However, our results for the samples which were the most preferred indicators of cultural events (hearth charcoal and cut-marked bone) confirm that people were in the southern cone of South America at or soon after 11,000 BP (13,000 cal BP). Considered alongside recent age estimates for the Clovis culture in North America, these results imply the contemporaneous emergence of a consistent and archaeologically-robust human occupation signal at widely-separated locations across the Western Hemisphere. Such findings suggest that Palaeoindian demic expansion may have involved more than one terminal Pleistocene dispersal episode.

Rapid evolution of seed dispersal in an urban environment in the weedCrepis sancta

Dispersal is a ubiquitous trait in living organisms. Evolutionary theory postulates that the loss or death of propagules during dispersal episodes (cost of dispersal) should select against dispersal. The cost of dispersal is expected to be a strong selective force in fragmented habitats. We analyzed patchy populations of the weed Crepis sancta occupying small patches on sidewalks, around trees planted within the city of Montpellier (South of France), to investigate the recent evolutionary consequences of the cost of dispersal. C. sancta produces both dispersing and nondispersing seeds. First, we showed that, in urban patches, dispersing seeds have a 55% lower chance of settling in their patch compared with nondispersing seeds and, thus, fall on a concrete matrix unsuitable for germination. Second, we showed that the proportion of nondispersing seeds in urban patches measured in a common environment is significantly higher than in surrounding, unfragmented populations. Third, by using a quantitative genetic model, we estimated that the pattern is consistent with short-term evolution that occurs over approximately 5-12 generations of selection, which is generated by a high cost of dispersal in urban populations. This study shows that a high cost of dispersal after recent fragmentation causes rapid evolution toward lower dispersal.

Global diversity of water mites (Acari, Hydrachnidia; Arachnida) in freshwater

The Hydrachnidia (water mites) represent the most important group of the Arachnida in fresh water. Over 6,000 species have been described worldwide, representing 57 families, 81 subfamilies and more than 400 genera. The article analyzes extant water mite diversity and biogeography. Data on distribution and species richness of water mites are substantial but still far from complete. Many parts of the world are poorly investigated, Oriental and Afrotropical regions in particular. Moreover, information among different freshwater habitats is unbalanced with springs and interstitial waters disproportionately unrepresented. Therefore, more than 10,000 species could be reasonably expected to occur in inland waters worldwide. Based on available information, the Palaearctic region represents one of the better investigated areas with the highest number of species recorded (1,642 species). More than 1,000 species have been recorded in each of the Neotropical (1,305 species) and Nearctic regions (1,025 species). Known species richness is lower in Afrotropical (787 species) and Australasian (694 species) regions, and lowest in the Oriental region (554 species). The total number of genera is not correlated with species richness and is distinctly higher in the Neotropical (164 genera); genus richness is similar in the Palaearctic, Nearctic and Australasian regions (128–131 genera) and is lower in the Afrotropical and Oriental regions with 110 and 94 genera, respectively. A mean number of about three genera per family occur in the Palaeartic, Nearctic and Oriental while an average of more than four genera characterizes the families of Australasian and Afrotropical regions and more than five genera those of the Neotropical. Australasian fauna is also characterized by the highest percentage of endemic genera (62%), followed by Neotropical (50.6%) and Afrotropical (47.2%) regions. Lower values are recorded for the Palaearctic (26.9%), Oriental (24.4%) and Nearctic (21.4%). The Palaearctic and Nearctic have the highest faunistic similarity, some minor affinities are also evident for the generic diversification of Holarctic and Oriental families. The faunas of Southern Hemisphere bioregions are more distinct and characterized by the presence of ancient Gondwanan clades with a regional diversification particularly evident in the Neotropics and Australasia. This scenario of water mite diversity and distribution reflect the basic vicariance pattern, isolation, phylogenetic diversification, recent climatic vicissitudes and episodes of dispersal between adjacent land masses together with extant ecological factors can be evoked to explain distribution patterns at a global scale.

Rapid diversification and dispersal during periods of global warming by plethodontid salamanders.

A phylogeny and timescale derived from analyses of multilocus nuclear DNA sequences for Holarctic genera of plethodontid salamanders reveal them to be an old radiation whose common ancestor diverged from sister taxa in the late Jurassic and underwent rapid diversification during the late Cretaceous. A North American origin of plethodontids was followed by a continental-wide diversification, not necessarily centered only in the Appalachian region. The colonization of Eurasia by plethodontids most likely occurred once, by dispersal during the late Cretaceous. Subsequent diversification in Asia led to the origin of Hydromantes and Karsenia, with the former then dispersing both to Europe and back to North America. Salamanders underwent rapid episodes of diversification and dispersal that coincided with major global warming events during the late Cretaceous and again during the Paleocene-Eocene thermal optimum. The major clades of plethodontids were established during these episodes, contemporaneously with similar phenomena in angiosperms, arthropods, birds, and mammals. Periods of global warming may have promoted diversification and both inter- and transcontinental dispersal in northern hemisphere salamanders by making available terrain that shortened dispersal routes and offered new opportunities for adaptive and vicariant evolution.

DIVERSIFICATION OF ASCLEPIADOIDEAE (APOCYNACEAE) IN THE NEW WORLD1

Plastid sequences of trnL-F are used to estimate the age of biogeographical events in New World Asclepiadoideae. These data suggest that the subfamily arose somewhere in the Old World in the Late Eocene (40 million years ago (Ma)) and that there were at least four independent episodes of dispersal to the New World. The MOG clade, comprising Metastelmatinae, Oxypetalinae, and Gonolobinae, probably colonized South America through transoceanic dispersal from Africa at 32 Ma. The three subtribes emerged at 24 Ma, radiating 15–16 Ma. Cynanchum L. subg. Mellichampia (A. Gray) Woodson (Cynanchinae) only arrived in the New World at 24 Ma and has since diversified gradually. Asclepias L. (Asclepiadinae) probably arrived in the New World through the Bering Strait at 20 Ma, dispersing to South America before the emergence of the Isthmus of Panama. More recently, tropical Marsdenia R. Br. (Marsdenineae) probably arrived in the New World by long-distance dispersal at 16 Ma. Most diversification in New World Asclepiadoideae is estimated to have occurred during the Miocene. The synchrony of events suggests an environmental influence on the diversification of the lineages. Nevertheless, individual rates of diversification point out the importance of intrinsic factors.

Miocene hominoid biogeography: pulses of dispersal and differentiation

AbstractAim To test the hypothesis that the ancestor of the hominines (African apes and humans) had an African origin by comparing the historical biogeographical patterns of hominoids with those of two other large land mammal clades, namely the hyaenids and proboscideans.Location Global, primarily the Old World over the last 25 Myr (Miocene to present).Methods Creation of a general area cladogram using pact, a new method for generating area cladograms, and interpretation of general and clade‐specific speciation events involving hominoids, proboscideans and hyaenids.Results The analysis of the areas using pact reveals both general patterns and clade‐specific exceptions to these patterns. All three groups share a general episode of species formation in Africa in the early Miocene, followed by ‘out of Africa’ expansion into Europe, Asia and North America, and a second general episode of species formation in Asia in the mid‐Miocene, followed by ‘out of Asia’ expansion into Africa, Europe and North America. Finally, there were two additional ‘out of Africa’ events during the late Miocene and into the Pliocene, the last one setting the stage for the emergence and spread of Homo. In addition to these shared episodes of vicariance and dispersal, each group exhibits clade‐specific within‐area and peripatric speciation events.Main conclusions The complex history of dispersal and speciation over large areas exhibited by hominoids is part of a more general history of biotic diversification by taxon pulses. Refining this scenario will require the integration of additional clades from the same areas and times, as well as more detailed palaeoclimatological, palaeoenvironmental and geological evidence.

Across Lydekker's Line - first report of mite harvestmen (Opiliones : Cyphophthalmi : Stylocellidae) from New Guinea

Opiliones (harvestmen) in the suborder Cyphophthalmi are not known to disperse across oceans and each family in the suborder is restricted to a clear biogeographic region. While undertaking a revisionary study of the South-east Asian family Stylocellidae, two collections of stylocellids from New Guinea were noted. This was a surprising find, since the island appears never to have had a land connection with Eurasia, where the rest of the family members are found. Here, 21 New Guinean specimens collected from the westernmost end of the island (Manokwari Province, Indonesia) are described and their relationships to other cyphophthalmids are analysed using molecular sequence data. The specimens represent three species, Stylocellus lydekkeri, sp. nov., S. novaguinea, sp. nov. and undescribed females of a probable third species, which are described and illustrated using scanning electron microscope and stereomicroscope photographs. Stylocellus novaguinea, sp. nov. is described from a single male and it was collected with a juvenile and the three females of the apparent third species. Molecular phylogenetic analyses indicate that the new species are indeed in the family Stylocellidae and they therefore reached western New Guinea by dispersing through Lydekker’s line – the easternmost limit of poor dispersers from Eurasia. The New Guinean species may indicate at least two episodes of oceanic dispersal by Cyphophthalmi, a phenomenon here described for the first time. Alternatively, the presence in New Guinea of poor dispersers from Eurasia may suggest novel hypotheses about the history of the island.

Seed dispersal in fens

AbstractQuestion: How does seed dispersal reduce fen isolation and contribute to biodiversity?Location: European and North American fens.Methods: This paper reviews the literature on seed dispersal to fens.Results: Landscape fragmentation may reduce dispersal opportunities thereby isolating fens and reducing genetic exchange. Species in fragmented wetlands may have lower reproductive success, which can lead to biodiversity loss. While fens may have always been relatively isolated from each other, they have become increasingly fragmented in modern times within agricultural and urban landscapes in both Europe and North America. Dispersal by water, animals and wind has been hampered by changes related to development in landscapes surrounding fens. Because the seeds of certain species are long‐lived in the seed bank, frequent episodes of dispersal are not always necessary to maintain the biodiversity of fens. However, of particular concern to restoration is that some dominant species, such as the tussock sedge Carex stricta, may not disperse readily between fens.Conclusions: Knowledge of seed dispersal can be used to maintain and restore the biodiversity of fens in fragmented landscapes. Given that development has fragmented landscapes and that this situation is not likely to change, the dispersal of seeds might be enhanced by moving hay or cattle from fens to damaged sites, or by reestablishing lost hydrological connections.

Genetic relationships among American species of the genus Prosopis (Mimosoideae, Leguminosae) inferred from ITS sequences: evidence for long‐distance dispersal

AbstractAim The genus Prosopis includes 44 species and has a pseudoamphitropical, disjunct distribution. We aimed to determine whether American Prosopis sections arose in North or South America, and to explain the current distribution of their species on the basis of their genetic relationships.Location South‐western USA, Mexico, Caribbean Antilles, Peru–Ecuador, central and northern Argentina, south‐western Argentina (Patagonia) and Cuyo, south‐western Asia and northern Africa.Methods Internal transcribed spacer fragments from 21 species of Prosopis were sequenced and the data were used to analyse the phylogenetic relationships using Microlobius and Mimosa as outgroups. Genetic distances were calculated to estimate the degree of divergence. Dispersal–vicariance (DIVA) analysis was conducted to help understand the biogeographical history of the genus.Main conclusions The sections Strombocarpa and Algarobia are not monophyletic. Prosopis argentina (section Monilicarpa) and the species of Algarobia are included in single clade. The phylogeny, DIVA analysis, and the pattern of genetic distances indicate that the ancestral area for the American species was wide, from south‐western USA to Central and northern Argentina. Successive vicariance events split this area, and long‐distance dispersal episodes (perhaps mediated by birds) led to recolonizations from North to South America, and vice versa.

El poblamiento de Europa en torno al límite Matuyama/Brunhes: su origen y su significado en la evolución humana

A review of the European Lower Palaeolithic record shows three human dispersal episodes. The second one is dated between OIS25/OIS24-OIS19/OIS18. It is recorded by lithic assemblages and human remains that point to an eastern Asian origin. This paper reviews this hominid dispersal, related to the Galerian faunal event. Also we analyse the most feasible migratory routes, and the reasons of its abrupt ending. The phylogenetic relationships of these populations are also proposed, putting their context into the human evolutionary sequences of the Lower and Middle Pleistocene.

The oldest human groups in the Levant

Between 2.5 and 0.8 Myr, at least four episodes of hominine dispersal from Africa to the Levant are known, each culturally distinct. All have settled on lakes or river banks. The oldest occurrence is Yiron, in the northern portion of the Israeli Rift, with flint artefacts in a fluviatile deposit below a basalt layer dated 2.4 Myr. Yiron was followed by the Ubeidiya group in the central Rift ca. 1.4 Myr, with a Lower Acheulean industry. Somewhat later, the Bizat Ruhama group has settled (1.0 Myr) in the eastern coastal plain, with a small, microlith-size industry. Around 0.8 Myr, newcomers have settled at Gesher Benot Yaaqov (GBY) in the northern Rift, introducing the cleaver tradition. None of the sites yielded human remains. The cultures of Bizat Ruhama and GBY have subsequently disappeared. During the Lower Palaeolithic the Levant remained largely an Acheulean province, probably evolved from Ubeidiya. The Late/Final Acheulean, with developed Levallois technology is the possible ancestor of the Middle Palaeolithic ‘Levallois–Mousterian’. To cite this article: A. Ronen, C. R. Palevol 5 (2006) .

Seed dispersal in fens

Question: How does seed dispersal reduce fen isolation and contribute to biodiversity? Location: European and North American fens. Methods: This paper reviews the literature on seed dispersal to fens. Results: Landscape fragmentation may reduce dispersal opportunities thereby isolating fens and reducing genetic exchange. Species in fragmented wetlands may have lower reproductive success, which can lead to biodiversity loss. While fens may have always been relatively isolated from each other, they have become increasingly fragmented in modern times within agricultural and urban landscapes in both Europe and North America. Dispersal by water, animals and wind has been hampered by changes related to development in landscapes surrounding fens. Because the seeds of certain species are long-lived in the seed bank, frequent episodes of dispersal are not always necessary to maintain the biodiversity of fens. However, of particular concern to restoration is that some dominant species, such as the tussock sedge Carex stricta, may not disperse readily between fens. Conclusions: Knowledge of seed dispersal can be used to maintain and restore the biodiversity of fens in fragmented landscapes. Given that development has fragmented landscapes and that this situation is not likely to change, the dispersal of seeds might be enhanced by moving hay or cattle from fens to damaged sites, or by reestablishing lost hydrological connections.

Paleobiogeography: Documenting the Ebb and Flow of Evolutionary Diversification

Historical biogeography has recently experienced a significant advancement in three integrated areas. The first is the adoption of an ontology of complexity, replacing the traditional ontology of simplicity, or a priori parsimony; simple and elegant models of the biosphere are not sufficient for explaining the geographical context of the origin of species and their post-speciation movements, producing evolutionary radiations and complex multi-species biotas. The second is the development of a powerful method for producing area cladograms from complex data, especially cases of reticulated area relationships, without loss of information. That method, called Phylogenetic Analysis for Comparing trees (PACT), is described herein. The third element is the replacement of the model of maximum vicariance with the model called the Taxon Pulse hypothesis. PACT analysis of Hominoidea, Hyaenidae, and Proboscidea beginning in the Miocene, reveals that all three groups share a general episode of species formation in Africa in the early Miocene, followed by “out of Africa” expansion into Europe, Asia and North America, a second general episode of species formation in Asia in the mid-Miocene, followed by “out of Asia” expansion into Africa, Europe and North America. Finally, there were two additional “out of Africa” events during the late Miocene and into the Pliocene, the last one setting the stage for the emergence and spread ofHomo. In addition to these shared episodes of vicariance and dispersal, each group exhibits clade-specific within-area and peripheral isolates speciation events. The complex history of dispersal and speciation over largeareas exhibited by hominoids is part of a more general historyof biotic diversification by taxon pulses.

Distribution and phylogeny of glacier ice worms (Mesenchytraeus solifugus and Mesenchytraeus solifugus rainierensis)

Glacier ice worms, Mesenchytraeus solifugus (Emery, 1898) and Mesenchytraeus solifugus rainierensis Welch, 1916 (Enchytraeidae), are the only known oligochaetes adapted to life in ice. We have collected ice worm specimens from over 100 populations throughout the Pacific northwestern region of North America. Their current range extends ~2500 km along the Pacific coastline between south-central Alaska and central Oregon, with most populations occurring on relatively low-elevation, temperate glaciers. Phylogenetic analyses utilizing partial nuclear 28S rDNA and mitochondrial cytochrome c oxidase subunit I (COI) loci revealed the presence of two geographically distinct clades (northern and southern). The northern clade comprises all Alaskan populations, while the southern clade contains British Columbia, Washington State, and Oregon State populations. No evidence of gene flow was detected between these two lineages or between noncontiguous glaciers throughout their geographic range. Our results suggest that the mechanism of ice worm dispersal is primarily active, though at least one episode of passive dispersal is noted at the southern extent of their range.

The Africanized honey bee dispersal: a mathematical zoom

A general mathematical model for population dispersal featuring long range taxis is presented and exemplified by the dispersal episode of the Africanized honey bees ( Apis mellifera adansonii) throughout the American Continent. The mathematical model is a discrete-time and nonlocal model represented by an integrodifference recursion. A new taxis concept is defined and introduced into the mathematical model by an appropriate modification of the redistribution kernel. The model is capable of predicting the natural barrier for the expansion of the Africanized honey bees in the southern part of the Continent due to low winter temperatures. It also describes a sensitive expansion velocity with respect to the quality of resources, which can explain the AHB’s astounding spread rate, by using two different kinds of population dynamics strategies, one for a resourceful environment and the other for poor regions.

Dispersal ghosts in Oceania.

Anthropological genetics helps expand our understanding of human phenotypes in the Pacific, in part because of its focus on gene genealogies to infer past episodes of dispersal and to differentiate these events from adaptations due to long-duration directional selection. Sewall Wright's 1949 seminal paper on population structure emphasized that there were two strong forces that exerted systematic and therefore determinant pressure on the gene pool: recurrent immigration and gene flow. These are important topics to all discussions of human dispersal in any region of the world. Furthermore, Wright listed five unique kinds of events that produced indeterminate or unpredictable changes that could lead to phenotypic and genotypic effects. In this category, he placed unique selective incidents, unique hybridization events, unique reductions in number, swamping by mass immigration, and mutational drive due to an allele always being favored since its origin or introduction. This discussion of human dispersal in the Pacific will touch on these topics, since they provide a second level of complexity in knowing who moved about a region of the world found already settled when rediscovered by colonial explorers during the 16-18th centuries.

Phylogenetic Reconstructions of Ostracodes – A Molecular Approach

Molecular work on ostracodes has thus far either used allozyme-based or DNA-based techniques. The application of allozyme-based methods has provided numerous data on population genetics and reproductive modes in ostracodes. With this technique, it has also been possible to construct phylogenies, although these have been restricted to distance-based methods. With the usage of DNA-based methods, a new era in ostracode research has begun. It is now possible to obtain accurate estimates for genetic diversities at very fine scales, even within individuals. The DNA-based approach is also very useful for reconstructing evolutionary histories at different taxonomic levels. Within species, for example, phylogenies reveal past episodes of dispersal and genetic exchange. Together with morphological data, DNA sequence data can test for congruence of molecular and morphological evolution and variability. At high taxonomic levels, DNA sequence data provide information on mechanisms of evolution and speciation. This allows for testing whether a certain, morphological feature has evolved once or several times independently. By applying the principle of the molecular clock, DNA sequence data provide relative age estimates that can be calibrated against fossil data and be linked to past climatic or geological events.

Neogene paleobiogeography and East African paleoenvironments: contributions from the Tugen Hills rodents and lagomorphs

A minimum of 28 genera of rodents and one genus of lagomorph were recovered from the Tugen Hills, Baringo District, Kenya, from localities dating from over 15·5 to about 4·4Ma. The middle Miocene (sites dated between 15·8 and 15·3Ma) rodent fauna recovered primarily from the Kipsaramon site complex, Muruyur Formation, includes a mixture of characteristically early Miocene taxa, and more derived forms. Composition of the African rodent fauna changes dramatically with the introduction of myocricetodontines, democricetodontines, and dendromurines, immigrants primarily from southern Asia. In the Tugen Hills, these taxa are first found in the Kabasero localities, Ngorora Formation, at sites dating from 12·5–12·33Ma. A second major change in the African rodent fauna reflects the introduction of murines, immigrants from southern Asia. In the Tugen Hills murines are first encountered at Kapcheberek, Lukeino Formation, dated to 5·9–5·7Ma. One rodent genus from the Lukeino Formation (Arvicanthis), and two from the Tabarin locality, Chemeron Formation (Heliosciurus, Paraxerus; 4·5–4·4Ma), represent the earliest records of these extant African genera. A cricetomyine from the Ngorora Formation (12·5Ma) is likely the earliest report of this exclusively African group. One of the earliest African records of porcupines (Hystricide) is from the Lukeino Formation. Lagomorphs are poorly represented, but include one of the earliest African occurrences of the family Leporidae from the Mpesida Beds (bracketed by dates of 7–6·2Ma), and possibly a new genus of leporid from the Kapcheberek locality. Analysis of the Tugen Hills small mammals in association with other African records suggests several episodes of dispersal between Africa and Eurasia during the middle and late Miocene. Rodents from Kipsaramon are indicative of forests in conjunction with more open habitats. Those from the Kapcheberek locality are suggestive of a savanna habitat. The rodents from the Tabarin locality suggest a woodland environment.