Middle Eocene habitat shifts in the North American western interior: A case study

There has been great interest in the global warming events that heralded the onset of the Eocene and particularly the response of mammalian faunas to these events. However, little information is available on the subsequent deterioration of tropical habitats in the interior of North America after these major warming episodes. The decline of tropical habitats is thought to have begun during the middle Eocene in the interior of North America, but until now, no studies have been able to document the details of this event. Recent fossil collection and stratigraphic studies from sites in southwestern Wyoming and northeastern Utah that span the middle Eocene offer a unique opportunity to evaluate changes in habitat in the western interior. Using a discriminant function analysis, habitats were reconstructed for a sequence of eight stratigraphically controlled middle Eocene assemblages. Adaptive profiles (diets, substrate use, and body masses) of fossil mammal communities were statistically compared to those of extant faunas from a variety of Neotropical habitats. Previously published magnetostratigraphic data from Utah provided a means to correlate our stratigraphic sections to the geomagnetic polarity time scale and the oxygen isotope record. The discriminant model shows that there was a significant change in the mammalian community ecology near the end of the late middle Eocene that is likely reflective of a habitat shift. When correlated to the time scale and oxygen isotope record, this key transition from forested habitats typical of the tropical early Eocene to more open woodlands began about 42 million years ago in this region of the Rocky Mountains.     

Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction

The effect of the Cretaceous–Palaeogene (K–Pg) mass extinction on the evolution of many groups, including placental mammals, has been hotly debated. The fossil record suggests a sudden adaptive radiation of placentals immediately after the event, but several recent quantitative analyses have reconstructed no significant increase in either clade origination rates or rates of character evolution in the Palaeocene. Here we use stochastic methods to date a recent phylogenetic analysis of Cretaceous and Palaeocene mammals and show that Placentalia likely originated in the Late Cretaceous, but that most intraordinal diversification occurred during the earliest Palaeocene. This analysis reconstructs fewer than 10 placental mammal lineages crossing the K–Pg boundary. Moreover, we show that rates of morphological evolution in the 5 Myr interval immediately after the K–Pg mass extinction are three times higher than background rates during the Cretaceous. These results suggest that the K–Pg mass extinction had a marked impact on placental mammal diversification, supporting the view that an evolutionary radiation occurred as placental lineages invaded new ecological niches during the Early Palaeocene.     

Patterns in the structure of Asian and North American desert small mammal communities

Aim We compared assemblages of small mammal communities from three major desert regions on two continents in the northern hemisphere. Our objective was to compare these with respect to three characteristics: (1) species richness and representation of trophic groups; (2) the degree to which these assemblages exhibit nested community structure; and (3) the extent to which competitive interactions appear to influence local community assembly. Location We studied small mammal communities from the deserts of North America (N=201 sites) and two regions in Central Asia (the Gobi Desert (N=97 sites) and the Turan Desert Region (N=36 sites), including the Kara-Kum, Kyzyl-Kum, NE Daghestan, and extreme western Kazakhstan Deserts). Method To provide baseline data we characterized each desert region in terms of alpha, beta, and gamma diversity, and in terms of the distribution of taxa across trophic and locomotory groups. We evaluated nestedness of these communities using the Nestedness Temperature Calculator developed by Atmar & Patterson (1993, 1995) , and we evaluated the role of competitive interactions in community assembly and applied a null model of local assembly under varying degrees of competitive interaction ( Kelt et al., 1995, 1996 ). Results All three desert regions have low alpha diversity and high beta diversity. The total number of species in each region varied, being highest in North America, and lowest in the Turan Desert Region. The deserts studied all present evidence of significant nestedness, but the mechanism underlying this structure appears different in North American and Asia. In North America, simulations strongly implicate interspecific competition as a dominant mechanism influencing community and assemblage structure. In contrast, data from Asian desert rodent communities suggest that these are not strongly influenced by competition; in fact, they have greater numbers of ecologically and morphologically similar species than expected. These results appear to reflect strong habitat selection, with positive associations among species that share similar habitat requirements in these communities. Our analyses support earlier reports suggesting that predation and abiotic forces may have greater influences on the assembly and organization of Asian desert rodent communities, whereas interspecific competition dominates assembly processes in North America. Additionally, we suggest that structuring mechanisms may be very different among the two Asian deserts studied. Gobi assemblages appear structured by trophic and locomotory strategies. In contrast, Turan Desert Region assemblages appear to be randomly structured with respect to locomotory strategies. When trophic and locomotory categories are combined, however, Turan species are positively and nonrandomly associated. Main conclusions Very different ecological dynamics evidently exist not only between these continents, but within them as well. These small mammal faunas differ greatly in terms of community structure, but also appear to differ in the underlying mechanisms by which communities are assembled. The underlying role of history and geography are strongly implicated as central features in understanding the evolution of mammalian faunas in different deserts of the world.     

Paleogene Land Mammal Faunas of South America; a Response to Global Climatic Changes and Indigenous Floral Diversity

An appraisal of Paleogene floral and land mammal faunal dynamics in South America suggests that both biotic elements responded at rate and extent generally comparable to that portrayed by the global climate pattern of the interval. A major difference in the South American record is the initial as well as subsequent much greater diversity of both Neotropical and Austral floras relative to North American counterparts. Conversely, the concurrent mammal faunas in South America did not match, much less exceed, the diversity seen to the north. It appears unlikely that this difference is solely due to the virtual absence of immigrants subsequent to the initial dispersal of mammals to South America, and cannot be explained solely by the different collecting histories of the two regions. Possible roles played by non-mammalian vertebrates in niche exploitation remain to be explored. The Paleogene floras of Patagonia and Chile show a climatic pattern that approximates that of North America, with an increase in both Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) from the Paleocene into the Early Eocene Climatic Optimum (EECO), although the Paleocene-Eocene Thermal Maximum (PETM) is not recognized in the available data set. Post-EECO temperatures declined in both regions, but more so in the north than the south, which also retained a higher rate of precipitation. The South American Paleogene mammal faunas developed gradual, but distinct, changes in composition and diversity as the EECO was approached, but actually declined somewhat during its peak, contrary to the record in North America. At about 40 Ma, a post-EECO decline was recovered in both hemispheres, but the South American record achieved its greatest diversity then, rather than at the peak of the EECO as in the north. This post-EECO faunal turnover apparently was a response to the changing conditions when global climate was deteriorating toward the Oligocene. Under the progressively more temperate to seasonally arid conditions in South America, this turnover reflected a major change from the more archaic, and more tropical to subtropical-adapted mammals, to the beginning of the ultimately modern South American fauna, achieved completely by the Eocene-Oligocene transition. Interestingly, hypsodonty was achieved by South American cursorial mammals about 15–20 m.y. earlier than in North America. In addition to being composed of essentially different groups of mammals, those of the South American continent seem to have responded to the climatic changes associated with the ECCO and subsequent conditions in a pattern that was initially comparable to, but subsequently different from, their North American counterparts.     

中国晚中新世陆相哺乳动物群序列:陕西蓝田的新证据(英文)

自 2 0世纪 60年代 ,陕西蓝田地区就以连续的晚新生代地层及丰富的哺乳动物化石而成为研究中国晚新生代哺乳动物与地层划分对比的经典地区之一。李传夔等 ( 1 984)根据灞河组发现的哺乳动物化石建立了灞河期 ,并与欧洲的Vallesian期对比。邱占祥等( 1 990 ,1 995)认为化石证据不充分 ,而将其与保德期合并。近年来 ,黄土高原三趾马红粘土大量的年代学资料表明 ,红粘土沉积最早可能开始于约 8Ma前。因此 ,狭义的保德期可能只代表了晚中新世晚期 ,晚中新世哺乳动物分期的再划分问题又重新被提出。自 1 997年以来 ,我们课题组在蓝田地区共发现 52个化石地点 ,其中的 2 6个含有哺乳动物化石 ,大哺乳动物化石标本共计 1 666件。经过详细野外地层测量 ,绘制了综合地层剖面 ,并将主要含哺乳动物化石的地点对比标定到综合地层剖面上。经过对各门类化石的初步研究 ,以及对已有化石材料的初步修订 ,到目前为止 ,陕西蓝田灞河组共计发现哺乳动物化石 45种 ,蓝田组 2 0种。新的化石证据表明 ,灞河组发现的哺乳动物化石组合完全不同于典型意义上的保德三趾马动物群。大哺乳动物化石以灞河三趾马与贾氏三趾马为代表 ,牛科化石组合在种级甚至属级上有明显的区别。大量的陕西转角羚羊 ,小型的Protoryx、Dorcadoryx以及Ga     

中国晚新近纪哺乳动物群落与东亚环境变化

根据已有的中国晚新近纪哺乳动物化石记录,综合新近研究成果,初步探讨中国晚新近纪哺乳动物群落演变进程及其对于气候与环境变化的响应。结果表明,自中中新世晚期至晚中新世(约13 Ma至7-8 Ma),中国北方哺乳动物群落组成上没有明显的分异;而在其后的晚中新世晚期发生了较为明显的区域性分化,分化为东部地区以适应潮湿的较为封闭的动物群落与西部地区适应干旱的开阔环境的群落,这种分异可能与东亚夏季风的加强密切相关; 青藏高原及其周边地区化石类群的扩散与交流资料也表明由青藏高原隆升而形成的地理阻隔也可能发生在该时期;上新世哺乳动物群落演化表现出对于持续的干旱化气候背景的适应,而晚上新世某些类群的分异与主导地位的确立与该时期气候环境背景的剧烈震荡相一致。由于化石资料的不完整,时段分布的不均一,以及地理分布区域的局限,全面认识中国晚新近纪哺乳动物群落组成以及与环境变化的关系尚有很长的一段历程。     

Evidence for a Late Pliocene faunal transition based on a new rodent assemblage from Oldowan locality Hadar A.L. 894, Afar Region, Ethiopia

The time interval between 3 Ma and 2 Ma marks several important transitions in human evolution, including the extinction of Australopithecus afarensis, the origin of the genus Homo, and the appearance of concentrated stone tool assemblages forming recognizable archaeological sites. The period also marks important changes in Earth’s climatic history, with the onset of northern hemisphere glaciation starting sometime between 2.8 Ma and 2.5 Ma, and it remains an unresolved question in paleoanthropology whether or not the global climatic events influenced in whole or in part, local terrestrial paleoenvironments in Africa and, through this, the course of human evolution.Changes in the terrestrial mammalian faunas of East Africa during this time interval are an important source of data about terrestrial paleoenvironments, and it has been argued that during this time period the mammalian faunas of Africa experienced a sudden pulse in the extinction and origination of taxa. The data corroborating this Turnover Pulse Hypothesis derive from both large mammal and micromammal data, though the fossil record of the former is much more abundant in this interval. New micromammal fossils recovered from ca. 2.4 Ma deposits at locality A.L. 894, low in the Busidima Formation in the Hadar study area of the Afar region, Ethiopia, reveal a significant faunal turnover when compared with previously published material from older 3.2 Ma micromammal assemblages from the Hadar Formation deposits. The results support the hypothesis of a major faunal transition, but larger sample sizes and more extensive temporal sampling are needed to refine the time and rate of change within this interval at Hadar.     

Holocene vegetation change and the mammal faunas of South America and Africa

Aim Although sharing many similarities in their vegetation types, South America and Africa harbour very dissimilar recent mammal faunas, not only taxonomically but also in terms of several faunistic patterns. However late Pleistocene and mid‐Holocene faunas, albeit taxonomically distinct, presented many convergent attributes. Here we propose that the effects of the Holocene climatic change on vegetation physiognomy has played a crucial role in shaping the extant mammalian faunistic patterns. Location South America and Africa from the late Pleistocene to the present. Methods Data presented here have been compiled from many distinct sources, including palaeontological and neontological mammalian studies, palaeoclimatology, palynology, and publications on vegetation ecology. Data on Pleistocene, Holocene and extant mammal faunas of South America and Africa allowed us to establish a number of similar and dissimilar faunistic patterns between the two continents across time. We then considered what changes in vegetation physiognomy would have occurred under the late Pleistocene last glacial maximum (LGM) and the Holocene climatic optimum (HCO) climatic regimes. We have ordained these proposed vegetation changes along rough physiognomic seral stages according to assumptions based on current botanical research. Finally, we have associated our hypothesized vegetation changes in South America and Africa with mammalian faunistic patterns, establishing a putative causal relationship between them. Results The extant mammal faunas of South America and Africa differ widely in taxonomical composition; the number of medium and large species they possess; behavioural and ecological characteristics related to herbivore herding, migration and predation; and biogeographical patterns. All such distinctions are mostly related to the open formation faunas, and have been completely established around the mid‐Holocene. Considering that the mid‐Holocene was a time of greater humidity than the late Pleistocene, vegetation cover in South America and Africa would have been dominated by forest or closed vegetation landscapes, at least for most of their lower altitude tropical regions. We attribute the loss of larger‐sized mammal lineages in South America to the decrease of open vegetation area, and their survival in Africa to the existence of vast savannas in formerly steppic or desertic areas in subtropical Africa, north and south of the equator. Alternative explanations, mostly dealing with the disappearance of South American megamammals, are then reviewed and criticized. Main conclusions The reduction of open formation areas during the HCO in South America and Africa explains most of the present distinct faunistic patterns between the two continents. While South America would have lost most of its open formations within the 30° latitudinal belt, Africa would have kept large areas suitable to the open formation mammalian fauna in areas presently occupied by desert and semi‐arid vegetation. Thus, the same general climatic events that affected South America in the late Pleistocene and Holocene also affected Africa, leading to our present day faunistic dissimilarities by maintaining the African mammalian communities almost unchanged while dramatically altering those of South America.     

Past Climate Change and Plant Evolution in Western North America: A Case Study in Rosaceae

Species in the ivesioid clade of Potentilla (Rosaceae) are endemic to western North America, an area that underwent widespread aridification during the global temperature decrease following the Mid-Miocene Climatic Optimum. Several morphological features interpreted as adaptations to drought are found in the clade, and many species occupy extremely dry habitats. Recent phylogenetic analyses have shown that the sister group of this clade is Potentilla section Rivales, a group with distinct moist habitat preferences. This has led to the hypothesis that the ivesioids (genera Ivesia, Horkelia and Horkeliella) diversified in response to the late Tertiary aridification of western North America. We used phyloclimatic modeling and a fossil-calibrated dated phylogeny of the family Rosaceae to investigate the evolution of the ivesioid clade. We have combined occurrence- and climate data from extant species, and used ancestral state reconstruction to model past climate preferences. These models have been projected into paleo-climatic scenarios in order to identify areas where the ivesioids may have occurred. Our analysis suggests a split between the ivesioids and Potentilla sect. Rivales around Late Oligocene/Early Miocene (∼23 million years ago, Ma), and that the ivesioids then diversified at a time when summer drought started to appear in the region. The clade is inferred to have originated on the western slopes of the Rocky Mountains from where a westward range expansion to the Sierra Nevada and the coast of California took place between ∼12-2 Ma. Our results support the idea that climatic changes in southwestern North America have played an important role in the evolution of the local flora, by means of in situ adaptation followed by diversification.     

Tracing the patterns of non-marine turtle richness from the Triassic to the Palaeogene: from origin to global spread

Turtles are key components of modern vertebrate faunas and their diversity and distributions are likely to be affected by anthropogenic climate change. However, there is limited baseline data on turtle taxonomic richness through time or assessment of their past responses to global environmental change. We used the extensive Triassic–Palaeogene (252–223 Ma) fossil record of terrestrial and freshwater turtles to investigate diversity patterns, finding substantial variation in richness through time and between continents. Globally, turtle richness was low from their Triassic origin until the Late Jurassic. There is strong evidence for high richness in the earliest Cretaceous of Europe, becoming especially high following the Cretaceous Thermal Maximum and declining in all continents by the end-Cretaceous. At the K–Pg boundary, South American richness levels changed little while North American richness increased, becoming very high during the earliest Palaeogene (Danian). Informative data are lacking elsewhere for this time period. However, the Selandian–Thanetian interval, approximately 5 myr after the K–Pg mass extinction, shows low turtle richness in Asia, Europe and South America, suggesting that the occurrence of exceptional turtle richness in the post-extinction Paleocene fauna of North America is not globally representative. Richness decreased over the Eocene–Oligocene boundary in North America but increased to its greatest known level for Europe, implying very different responses to dramatic climatic shifts. Time series regressions suggest number of formations sampled and palaeotemperature are the primary influencers of face-value richness counts, but additional factors not tested here may also be involved.     

Uplift of the Ou Backbone Range in Northeast Japan at around 10 Ma and its implication for the tectonic evolution of the eastern margin of Asia

The Late Cenozoic uplift history of a sedimentary basin located in the axial part of the Ou Backbone Range, Northeast Japan, was studied using detailed mapping, fission-track dating and basin analysis. The subsidence analysis of the basin clarified the more complex stepwise uplift of the Ou Backbone Range. Three stages of uplift have been recognized and are interpreted to be the result of compressional stress, possibly accompanied by basin inversion. The three stages are identified as (1) a phase of surface uplift and regional unconformity (12-9 Ma), (2) a stage of differential uplift and compression (6.5-3 Ma) and (3) an intense compression stage (∼ 3 Ma). In the first stage, the eastern sector of the Backbone Range uplifted and a notable unconformity was formed at ∼ 10 Ma. The western sector remained submerged, suggesting that the eastern sector uplifted earlier than the western sector. Although the first uplift stage has been regarded as a tectonically quiet period in Northeast Japan, this tectonic event at ∼ 10 Ma is supposed to have a regional origin because coeval tectonic events took place across all Northeast Japan, as well as on the eastern margin of Asia. This study thus provides new insights into the Neogene tectonic evolution in the eastern margin of Asia.     

Body size structure in north-western Mediterranean Plio-Pleistocene mammalian faunas

Aim We investigated the patterns of body‐size changes of the north‐western Mediterranean Plio‐Pleistocene large mammal faunas (excluding rodents, bats, lagomorphs and insectivores) in order to identify the tempo and mode of the major shifts in body size distribution, and to put them in the context of Plio‐Pleistocene environmental changes and the development of the Mediterranean climate. Location We analysed fossil faunas of Spain, France and Italy. A set of recent regional faunas from several macroclimatic regions was selected to serve as elements for comparison of the size distribution of past faunas, consisting of: Spain, France and Italy together, Florida, California, Central Chile, Indochina, India, Korea‐Manchuria, Malawi, The Cape, North Africa, Turkey and Australia. Methods Mammal species were grouped into five body size categories for carnivores and four categories for noncarnivore species. The number of species in each size category was computed and the resulting matrix of body weight classes × regions and time intervals was used as an input matrix in a Correspondence Analysis. Results Recent and fossil faunas strongly differ in body size structure. The distribution of recent faunas within the CA seems to reflect both ecological and historic factors, intertwined in a complex fashion. No clear relationship has been observed between body size structure and environmental factors. During the late Pliocene to early Pleistocene there were only minor changes in the pattern of size distribution, although plant communities were in a transition process from subtropical forests to Mediterranean woodlands and steppes. The major change in body size structure of the north‐western Mediterranean fauna occurred at the Galerian, around 1 Ma ago. This marked the beginning of the modern fauna, and a general trend towards a larger body size, reduction in the number of medium sized herbivores, and an increase of large herbivores and megaherbivores. Main conclusions The Plio‐Pleistocene faunas lack modern analogues. The body size structure of mammalian regional faunas appears to be strongly dependent on historical factors. The only major shift in body size distribution occurred during the Plio‐Pleistocene, in the late Villafranchian‐Galerian transition, coincident with the onset of the Pleistocene high intensity glacial cycles.     

Tangled up in two: a burst of genome duplications at the end of the Cretaceous and the consequences for plant evolution

Genome sequencing has demonstrated that besides frequent small-scale duplications, large-scale duplication events such as whole genome duplications (WGDs) are found on many branches of the evolutionary tree of life. Especially in the plant lineage, there is evidence for recurrent WGDs, and the ancestor of all angiosperms was in fact most likely a polyploid species. The number of WGDs found in sequenced plant genomes allows us to investigate questions about the roles of WGDs that were hitherto impossible to address. An intriguing observation is that many plant WGDs seem associated with periods of increased environmental stress and/or fluctuations, a trend that is evident for both present-day polyploids and palaeopolyploids formed around the Cretaceous–Palaeogene (K–Pg) extinction at 66 Ma. Here, we revisit the WGDs in plants that mark the K–Pg boundary, and discuss some specific examples of biological innovations and/or diversifications that may be linked to these WGDs. We review evidence for the processes that could have contributed to increased polyploid establishment at the K–Pg boundary, and discuss the implications on subsequent plant evolution in the Cenozoic.     

A New Fishfly Species (Megaloptera: Corydalidae: Neohermes Banks) Discovered from North America by a Systematic Revision,with Phylogenetic and Biogeographic Implications

The taxonomy of Megaloptera from the Nearctic region is fairly well known and their faunal diversity has been largely surveyed, even in relatively remote regions. However, the evolutionary history of Nearctic Megaloptera is still poorly known with phylogenetic and biogeographic studies lacking. In this paper, we report a new fishfly species of the endemic North American genus Neohermes Banks, 1908, increasing the total number known of species to six. This new species (Neohermes inexpectatus sp. nov.) is currently known to occur only in California (USA) and is apparently confined to the Northern Coastal Range. The new species resembles the three Neohermes species from eastern North America based on the relatively small body size and the presence of female gonostyli 9. However, our phylogenetic analysis using adult morphological data recovered the new species as the sister species to the remaining Neohermes, which includes two species from western North America and three from eastern North America. According to the present interspecific phylogeny of Neohermes, with reconstructed ancestral areas, the initial divergence within the genus was found to take place in western North America, with a subsequent eastward dispersal. This likely lead to the modern distribution of Neohermes in eastern North America with the closure of the Mid-Continental Seaway, which separated western and eastern North America in the Mid-Late Cretaceous (100–80 MYA) and finally disappeared at the end of the Cretaceous (70 MYA). The uplift of the Cordilleran System probably accounted for the divergence between the eastern and two western Neohermes species.     

Late Cretaceous Mammal Tracks from North America

Mammal tracks from the latest Cretaceous (Campanian-Maastrichtian) of Colorado are described as Schadipes crypticus ichnogen. et ichnosp. nov., on the basis of material from the Laramie Formation in Golden, eastern Colorado. This ichnospecies, and a closely related form (Schadipes sp.) from the Mesaverde Group of western Colorado, represent the only mammal tracks so far identified from the Upper Cretaceous. A possible mammal track from North Africa (Agadirichnus elegans) was originally attributed to a lizard/lacertilian. Other purported Lower Cretaceous mammal tracks are based on isolated specimens of materials that are dubious or as yet undescribed in detail. Morphologically, Late Cretaceous mammal tracks resemble those of some modern rodents. However, based on the dominant mammalian elements of faunas at that time they are probably of marsupial or multituberculate affinity.     

Diversification of Lupinus (Leguminosae) in the western New World: derived evolution of perennial life history and colonization of montane habitats

Previous phylogenetic studies of Lupinus (Leguminosae) based on nuclear DNA have shown that the western New World taxa form a monophyletic group representing the majority of species in the genus, with evidence for high rates of recent diversification in South America following final uplift of the Andes 2–4 million years ago (Mya). For this study, three regions of rapidly evolving non-coding chloroplast DNA (trnL intron, trnS–trnG, and trnT–trnL) were examined to estimate the timing and rates of diversification in the western New World, and to infer ancestral states for geographic range, life history, and maximum elevation. The western New World species (5.0–9.3 Mya, 0.6–1.1 spp./My) comprise a basally branching assemblage of annual plants endemic to the lower elevations of western North America, from which two species-rich clades are recently derived: (i) the western North American perennials from the Rocky Mountains, Great Basin, and Pacific Slope (0.7–2.1 Mya, 2.0–5.9 spp./My) and (ii) the predominantly perennial species from the Andes Mountains of South America and highlands of Mexico (0.8–3.4 Mya, 1.4–5.7 spp./My). Bayesian posterior predictive tests for association between life history and maximum elevation demonstrate that perennials are positively correlated with higher elevations. These results are consistent with a series of one or more recent radiations in the western New World, and indicate that rapid diversification of Lupinus coincides with the derived evolution of perennial life history, colonization of montane habitats, and range expansion from North America to South America.     

Dispersal,vicariance, and the Late Cretaceous to early tertiary land mammal biogeography from South America to Australia

A review of paleontological, phyletic, geophysical, and climatic evidence leads to a new scenario of land mammal dispersal among South America, Antarctica, and Australia in the Late Cretaceous to early Tertiary epochs. New fossil land vertebrate material has been recovered from all three continents in recent years. As regards Gondwana, the present evidence suggests that monotreme mammals and ratite birds are of Mesozoic origin, based on both geochronological and phyletic grounds. The occurrence of monotremes in the early Paleocene (ca. 62 Ma) faunas of Patagonia and of ratites in late Eocene (ca. 41-37 m.y.) faunas of Seymour Island (Antarctic Peninsula) probably is an artifact of a much older and widespread Gondwana distribution prior to the Late Cretaceous Epoch. Except for South American microbiotheres being australidelphians, marsupial faunas of South America and Australia still are fundamentally disjunct. New material from Seymour Island (Microbiotheriidae) indicates the presence there of a derived taxon that resides in a group that is the sister taxon of most Australian marsupials. There is no compelling evidence that dispersal between Antarctica and Australia was as recent as ca. 41 Ma or later. In fact, the derived marsupial and placental land mammal fauna of Seymour Island shows its greatest affinity with Patagonian forms of Casamayoran age (ca. 51–54 m.y.). This suggests an earlier dispersal of more plesiomorphic marsupials from Patagonia to Australia via Antarctica, and vicariant disjunction subsequently. This is consistent with geophysical evidence that the South Tasman Rise was submerged by 64 Ma and with geological evidence that a shallow water marine barrier was present from then onward. The scenario above is consistent with molecular evidence suggesting that australidelphian bandicoots, dasyurids, and diprotodontians were distinct and present in Australia at least as early as the 63-Ma-old australidelphian microbiotheres and the ancient but not basal australidelphian,Andinodelphys, in the Tiupampa Fauna of Bolivia. Land mammal dispersal to Australia typically has been considered to be at a low level of probability (e.g., by sweepstakes dispersal). This study suggests that the marsupial colonizers of Australia included already recognizable members of the Peramelina, Dasyuromorphia, and Diprotodontia, at least, and entered via a filter route rather than by a sweepstakes dispersal.To whom correspondence should be addressed.     

The Leporid Datum: a late Miocene biotic marker

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Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline

Earth has experienced five major extinction events in the past 450 million years. Many scientists suggest we are now witnessing a sixth, driven by human impacts. However, it has been difficult to quantify the real extent of the current extinction episode, either for a given taxonomic group at the continental scale or for the worldwide biota, largely because comparisons of pre-anthropogenic and anthropogenic biodiversity baselines have been unavailable. Here, we compute those baselines for mammals of temperate North America, using a sampling-standardized rich fossil record to reconstruct species-area relationships for a series of time slices ranging from 30 million to 500 years ago. We show that shortly after humans first arrived in North America, mammalian diversity dropped to become at least 15%–42% too low compared to the “normal” diversity baseline that had existed for millions of years. While the Holocene reduction in North American mammal diversity has long been recognized qualitatively, our results provide a quantitative measure that clarifies how significant the diversity reduction actually was. If mass extinctions are defined as loss of at least 75% of species on a global scale, our data suggest that North American mammals had already progressed one-fifth to more than halfway (depending on biogeographic province) towards that benchmark, even before industrialized society began to affect them. Data currently are not available to make similar quantitative estimates for other continents, but qualitative declines in Holocene mammal diversity are also widely recognized in South America, Eurasia, and Australia. Extending our methodology to mammals in these areas, as well as to other taxa where possible, would provide a reasonable way to assess the magnitude of global extinction, the biodiversity impact of extinctions of currently threatened species, and the efficacy of conservation efforts into the future.     

Small Theropod Teeth from the Late Cretaceous of the San Juan Basin,Northwestern New Mexico and Their Implications for Understanding Latest Cretaceous Dinosaur Evolution

Studying the evolution and biogeographic distribution of dinosaurs during the latest Cretaceous is critical for better understanding the end-Cretaceous extinction event that killed off all non-avian dinosaurs. Western North America contains among the best records of Late Cretaceous terrestrial vertebrates in the world, but is biased against small-bodied dinosaurs. Isolated teeth are the primary evidence for understanding the diversity and evolution of small-bodied theropod dinosaurs during the Late Cretaceous, but few such specimens have been well documented from outside of the northern Rockies, making it difficult to assess Late Cretaceous dinosaur diversity and biogeographic patterns. We describe small theropod teeth from the San Juan Basin of northwestern New Mexico. These specimens were collected from strata spanning Santonian – Maastrichtian. We grouped isolated theropod teeth into several morphotypes, which we assigned to higher-level theropod clades based on possession of phylogenetic synapomorphies. We then used principal components analysis and discriminant function analyses to gauge whether the San Juan Basin teeth overlap with, or are quantitatively distinct from, similar tooth morphotypes from other geographic areas. The San Juan Basin contains a diverse record of small theropods. Late Campanian assemblages differ from approximately co-eval assemblages of the northern Rockies in being less diverse with only rare representatives of troodontids and a Dromaeosaurus-like taxon. We also provide evidence that erect and recurved morphs of a Richardoestesia-like taxon represent a single heterodont species. A late Maastrichtian assemblage is dominated by a distinct troodontid. The differences between northern and southern faunas based on isolated theropod teeth provide evidence for provinciality in the late Campanian and the late Maastrichtian of North America. However, there is no indication that major components of small-bodied theropod diversity were lost during the Maastrichtian in New Mexico. The same pattern seen in northern faunas, which may provide evidence for an abrupt dinosaur extinction.