[Show abstract][Hide abstract] ABSTRACT: The Eocene–Oligocene Boundary (,34 million years ago) marks one of the largest extinctions of marine invertebrates in the world oceans and of mammalian fauna in Europe and Asia in the Cenozoic era. A shift to a cooler climate across this boundary has been suggested as the cause of this extinction in the marine environment, but there is no manifold evidence for a synchronous turnover of flora, fauna and climate at the Eocene–Oligocene Boundary in a single terrestrial site in Asia to support this hypothesis. Here we report new data of magnetostratigraphy, pollen and climatic proxies in the Asian interior across the Eocene– Oligocene Boundary; our results show that climate change forced a turnover of flora and fauna, suggesting there was a change from large-size perissodactyl-dominant fauna in forests under a warm-temperate climate to small rodent/lagomorph-dominant fauna in forest-steppe in a dry-temperate climate across the Eocene– Oligocene Boundary. These data provide a new terrestrial record for this significant Cenozoic environmental event.
[Show abstract][Hide abstract] ABSTRACT: Mammalian bipedalism has long been thought to have arisen in response to arid and open environments. Here we tested whether bipedalism coevolved with environmental changes using molecular and paleontological data from the rodent superfamily Dipodoidea and statistical methods for reconstructing ancestral characteristics and past climates. Our results show that the post-Late Miocene aridification exerted selective pressures on tooth shape, but not on leg length of bipedal jerboas. Cheek tooth crown height has increased since the Late Miocene, but the hind limb/head-body length ratios remained stable and high despite the environmental change from humid and forested to arid and open conditions, rather than increasing from low to high as predicted by the arid-bipedalism hypothesis. The decoupling of locomotor and dental character evolution indicates that bipedalism evolved under selective pressure different from that of dental hypsodonty in jerboas. We reconstructed the habitats of early jerboas using floral and faunal data, and the results shows that the environments in which bipedalism evolved were forested. Our results suggest that bipedalism evolved as an adaptation to humid woodlands or forests for vertical jumping. Running at high speeds is likely a by-product of selection for jumping, which became advantageous in open environments later on. This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: Microtoid cricetids are widely considered to be the ancestral form of arvicoline rodents, a successful rodent group including voles, lemmings and muskrats. The earliest previously known microtoid cricetid is Microtocricetus molassicus Fahlbusch and Mayr 1975 from the MN9 (about 10-11 Ma) of Europe. Here we report a new microtoid cricetid, Primoprismus fejfari, gen. et sp. nov., discovered from locality XJ 200604 in the Junggar basin of Xinjiang, northwest China. The rodent assemblage found in association with this specimen indicates a late Early Miocene age, roughly estimated at 18–17 Ma, and thus more than 6 million years
earlier than the record of M. molassicus. While morphological comparisons suggest that the new taxon is most closely related to M. molassicus, it also presents a striking combination of primitive characters compared to the latter, including its relatively lower crown, smaller size, differentiated posterolophid and hypolophid, faint anterolophid, and the absence of an ectolophid, as well as the presence of a stylid on the labial border of the tooth. Arid conditions prevailing across the mid-latitude interior of Eurasia during the Early Miocene, enhanced by the combined effects of the Tibetan uplift and the gradual retreat of the Tethys Ocean, likely played a role in the appearance of grasslands which triggered the evolution of microtoid cricetids and, ultimately, the origin of arvicoline rodents.
[Show abstract][Hide abstract] ABSTRACT: The phylogenetic relationships of notoungulates, an extinct group of predominantly South American herbivores, remain poorly resolved with respect to both other placental mammals and among one another. Most previous phylogenetic analyses of notoungulates have not included characters of the internal cranium, not least because few such features, including the bony labyrinth, have been described for members of the group. Here we describe the inner ears of the notoungulates Altitypotherium chucalensis (Mesotheriidae), Pachyrukhos moyani (Hegetotheriidae) and Cochilius sp. (Interatheriidae) based on reconstructions of bony labyrinths obtained from computed tomography imagery. Comparisons of the bony labyrinths of these taxa with the basally diverging notoungulate Notostylops murinus (Notostylopidae), an isolated petrosal from Itaboraí, Brazil, referred to Notoungulata, and six therian outgroups, yielded an inner ear character matrix of 25 potentially phylogenetically informative characters, 14 of them novel to this study. Two equivocally optimized character states potentially support a pairing of Mesotheriidae and Hegetotheriidae, whereas four others may be diagnostic of Notoungulata. Three additional characters are potentially informative for diagnosing more inclusive clades: one for crown Placentalia; another for a clade containing Kulbeckia, Zalambdalestes, and Placentalia; and a third for Eutheria (crown Placentalia plus stem taxa). Several other characters are apomorphic for at least one notoungulate in our study and are of potential interest for broader taxonomic sampling within Notoungulata to clarify currently enigmatic interrelationships. Measures of the semicircular canals were used to infer agility (e.g. capable of quick movements vs. lethargic movements) of these taxa. Agility scores calculated from these data generally corroborate interpretations based on postcranial remains of these or closely related species. We provide estimates of the low-frequency hearing limits in notoungulates based on the ratio of radii of the apical and basal turns of the cochlea. These limits range from 15 Hz in Notostylops to 149 Hz in Pachyrukhos, values comparable to the Asian elephant (Elephas maximus) and the California sea lion (Zalophus californianus) when hearing in air, respectively.
Journal of Anatomy 09/2013; 223(5). DOI:10.1111/joa.12108 · 2.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tree-building with diverse data maximizes explanatory power. Application of molecular clock models to ancient speciation events risks a bias against detection of fast radiations subsequent to the Cretaceous-Paleogene (K-Pg) event. Contrary to Springer et al., post-K-Pg placental diversification does not require "virus-like" substitution rates. Even constraining clade ages to their model, the explosive model best explains placental evolution.
[Show abstract][Hide abstract] ABSTRACT: Reconstructing the earliest phases of primate evolution has been impeded by gaps in the fossil record, so that disagreements persist regarding the palaeobiology and phylogenetic relationships of the earliest primates. Here we report the discovery of a nearly complete and partly articulated skeleton of a primitive haplorhine primate from the early Eocene of China, about 55 million years ago, the oldest fossil primate of this quality ever recovered. Coupled with detailed morphological examination using propagation phase contrast X-ray synchrotron microtomography, our phylogenetic analysis based on total available evidence indicates that this fossil is the most basal known member of the tarsiiform clade. In addition to providing further support for an early dichotomy between the strepsirrhine and haplorhine clades, this new primate further constrains the age of divergence between tarsiiforms and anthropoids. It also strengthens the hypothesis that the earliest primates were probably diurnal, arboreal and primarily insectivorous mammals the size of modern pygmy mouse lemurs.
[Show abstract][Hide abstract] ABSTRACT: To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative
to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species.
Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that
crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data
are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon
of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera
(bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest
members of Afrotheria.
[Show abstract][Hide abstract] ABSTRACT: A new genus and species of aplodontid rodent, Proansomys dureensis, from the late Oligocene of the northern Junggar Basin of China is described. The new genus is referred to as Ansomyinae because the ectoloph on the upper cheek teeth, although not fully crested, has attained the same characteristic bucket-handle-shaped configuration as other members of the subfamily. It represents the earliest record of the subfamily yet discovered in Asia and is more plesiomorphic than species of the genus Ansomys in having a partly crested ectoloph, a lower degree of lophodonty, and less complex tooth basins (lacking accessory lophules). Proansomys has transitional features between Prosciurus and Ansomys, suggesting that the Ansomyinae derived from a group of aplodontids related to Prosciurus, as did other advanced aplodontid rodents. This provides new light on the paleobiogeography of the Ansomyinae.
PLoS ONE 01/2013; 8(1):e52625. DOI:10.1371/journal.pone.0052625 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we report a new early Oligocene cricetid, Paracricetops virgatoincisus, gen. et sp. nov., discovered from the Caijiachong locality in Yunnan Province, China. This new cricetid shows a peculiar combination of characters, such as massive and transversely positioned cusps, crenulated cheek tooth enamel, and a deep fossette enclosed between protocone and paracone. These characters are also present in Cricetops, a cricetid rodent of which the phylogenetic relationship with other cricetids remains debatable. Our phylogenetic analysis based on a data matrix including 37 taxa and 67 morphological characters reveals that Paracricetops and Cricetops are sister groups. Paracricetops, Cricetops, Deperetomys, Meteamys, Selenomys, Melissiodon, Mirrabella, Enginia, Muhsinia, and Aralocricetodon constitute a monophyletic group. This result suggests that these genera should all be grouped in the subfamily Cricetopinae. Our phylogenetic analysis also casts new lights on the origin and early radiation of the family Cricetidae. The subfamily Pappocricetodontinae is a polyphyletic group. Pappocricetodon and Raricricetodon, two basal cricetid genera, are also polyphyletic. A thorough systematic revision of these basal cricetids is needed. Chronological distribution of Eucricetodontinae, Paracricetodontinae, Pseudocricetodontinae, and Cricetopinae indicates that the establishment of these cricetid clades should be in the late Eocene at least. We therefore deduced that the first diversification and dispersal of the family Cricetidae across Eurasia must have occurred well before the Eocene-Oligocene transition.
[Show abstract][Hide abstract] ABSTRACT: The timing of the origin and diversification of rodents remains controversial, due to conflicting results from molecular clocks and paleontological data. The fossil record tends to support an early Cenozoic origin of crown-group rodents. In contrast, most molecular studies place the origin and initial diversification of crown-Rodentia deep in the Cretaceous, although some molecular analyses have recovered estimated divergence times that are more compatible with the fossil record. Here we attempt to resolve this conflict by carrying out a molecular clock investigation based on a nine-gene sequence dataset and a novel set of seven fossil constraints, including two new rodent records (the earliest known representatives of Cardiocraniinae and Dipodinae). Our results indicate that rodents originated around 61.7-62.4 Ma, shortly after the Cretaceous/Paleogene (K/Pg) boundary, and diversified at the intraordinal level around 57.7-58.9 Ma. These estimates are broadly consistent with the paleontological record, but challenge previous molecular studies that place the origin and early diversification of rodents in the Cretaceous. This study demonstrates that, with reliable fossil constraints, the incompatibility between paleontological and molecular estimates of rodent divergence times can be eliminated using currently available tools and genetic markers. Similar conflicts between molecular and paleontological evidence bedevil attempts to establish the origination times of other placental groups. The example of the present study suggests that more reliable fossil calibration points may represent the key to resolving these controversies.
PLoS ONE 10/2012; 7(10):e46445. DOI:10.1371/journal.pone.0046445 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The bony labyrinth of mammals, a delicate and complex cavity within the petrosal, houses the organs of hearing and equilibrium of the inner ear. Because this region is typically lodged deep within the skull, there have been few morphological studies of the bony labyrinth in fossils—where it frequently is completely enveloped by surrounding bone and sediment matrix. The recent development of high-resolution X-ray Computed Tomography (CT) scanning provides a powerful new tool for investigating such tiny, often inaccessible structures. Here we introduce a protocol for producing three-dimen-sional (3-D) virtual visualizations of the bony labyrinth from high-resolution CT images. As a case study, we scanned the skull of a basal platyrrhine primate, Chilecebus car-rascoensis, using the high-resolution CT facility at Pennsylvania State University, reconstructing the endocast of the bony labyrinth from the resulting data. Segmenting the original CT images is a vital step in producing accurate 3-D virtual reconstructions. We failed in efforts to isolate the bony labyrinth endocast through automated means, owing to the similar densities of the matrix filling the sinuses and spongy bone cavities of the specimen, and the bony labyrinth itself. Differing density contrasts across the fossil/matrix interface and overlapping grayscales of the fossil and matrix, required Half Maximum Height thresholds to be measured dynamically. Multiple thresholds are advantageous for processing the CT data of fossils that are inherently heterogeneous in material properties and densities. The iterative interaction between an operator and a computer offers the only means presently available for reliably discriminating the endocast of the bony labyrinth from the remainder of the specimen.
[Show abstract][Hide abstract] ABSTRACT: Tree shrew fossils are extremely rare. Here, we report two new tupaiine tree shrews, Prodendrogale engesseri sp. nov. and Tupaia storchi sp. nov., discovered from the late Miocene deposits of Yuanmou Lufengpithecus locality of Yunnan Province in China. P. engesseri is very close to the slightly younger species P. yunnanica Qiu (Vertebrata PalAsiatica, 24: 308–319, 1986) from the Lufeng Lufengpithecus locality of Yunnan Province. Relatively lower crowns and less trenchant tooth cusps of P. engesseri show that P. engesseri is more primitive than P. yunnanica. Tupaia storchi is a species larger than Palaeotupaia sivalicus Chopra and Vasishat, 1979, T. minor Günther, 1876 and T. javanica Horsfield, 1822, but smaller than all the other extant species of Tupaia and T. miocenica Mein and Ginsburg, 1997. The mesiobuccal side of the lower molar of this species develops a very strong cingulid. It should be interpreted as a primitive condition. Discovery of diverse tree shrew fossils in Yunnan suggests that multiple evolutionary lineages of tree shrews must have coexisted in a very large area in East Asia.