Latitudinal trends in genus richness of vascular plants in the Eocene and Oligocene of North America

The latitudinal richness gradient is a frequent topic of study on the modern landscape,but its history in deep time is much less well known.Here,we preliminarily evaluated the paleolatitudinal richness gradient of vascular plants for the Eocene(56-33.9 million years ago) and Oligocene(33.9-23 million years ago) epochs of North America north of IVlexico using 201 fossil floras.We calculated the direction and shape of the gradient using quadratic regression to detect linear and curvilinear trends.We performed regressions for the Eocene and Oligocene as well as for informal time intervals within the Eocene:early,middle,and middle + late.We found that quadratic models better explain the data than linear models for both epochs as well as for the early Eocene.A roughly linear trend in the middle and middle + late intervals may reflect limited sampling of high latitude floras for those times.The curvilinear relationship was weak for the Eocene and the model showed a peak in richness at 45.5°N.The curvilinear relationship was much stronger for the Oligocene and the peak occurred at 48.5°N.In the Eocene,the mid-latitude peak in richness may be explained by mean annual temperature,which was probably higher at some mid-latitudes than at lower ones.For the Oligocene,the peak in richness at midlatitudes may be explained by evolutionary diversification within the temperate zone or by increased aridity at low latitudes.We also assessed the latitudinal richness gradient of genera within modern floras in North America north of Mexico and we found a weak,curvilinear trend with a peak in richness at 31.5°N.Our results suggest that the latitudinal genus richness gradient of vascular plants in North America continued to develop into its modern structure following the Oligocene.

Paleoclimate evolution and aridification mechanism of the eastern Tethys during the Callovian–Oxfordian: evidence from geochemical records of the Qiangtang Basin, Tibetan Plateau

Global climate during the Jurassic has been commonly described as a uniform greenhouse climate for a long time.However,the climate scenario of a cool episode during the Callovian-Oxfordian transition following by a warming trend during the Oxfordian(163.53 to157.4 Ma)is documented in many localities of the western Tethys.It is still unclear if a correlatable climate scenario also occurred in the eastern Tethys during the same time interval.In this study,a detailed geochemical analysis on the 1060 m thick successions(the Xiali and Suowa formations)from the Yanshiping section of the Qiangtang Basin,located in the eastern Tethys margin during the Callovian-Oxfordian periods,was performed.To reveal the climate evolution of the basin,carbonate content and soluble salt concentrations(SO_(4)^(2-),Cl^(-))were chosen as climatic indices.The results show that the overall climate patterns during the deposition of the Xiali and Suowa formations can be divided into three stages:relatively humid(164.0 to 160.9 Ma),dry(160.9 to159.6 Ma),semi-dry(159.6 to 156.8 Ma).A similar warming climate scenario also occurred in eastern Tethys during the Callovian-Oxfordian transition(160.9 to159.6 Ma).Besides,we clarify that the Jurassic True polar wander(TPW),the motion of the lithosphere and mantle with respect to Earth’s spin axis,inducing climatic shifts were responsible for the aridification of the Qiangtang Basin during the Callovian-Oxfordian transition with a review of the paleolatitude of the Xiali formation(19.7+2.8/-2.6°N)and the Suowa formation(20.7+4.1/-3.7°N).It is because the TPW rotations shifted the East Asia blocks(the North and South China,Qiangtang,and Qaidam blocks)from the humid zone to the tropical/subtropical arid zone and triggered the remarkable aridification during the Middle-Late Jurassic(ca.165-155 Ma).

Movement History of the Microcontinents from the Tibetan Plateau Based on Paleomagnetic Results with Sufficient Sampling Units

Paleomagnetic results cannot be applied in global and regional tectonic reconstructions unless the paleosecular variation has been adequately averaged.However,how many sampling sites and samples are enough to calculate a reliable paleopole remains debated.Based on the relation among the sampling sites N,the precision parameter k,the virtual geomagnetic pole scatter s,and the confidence limit A_(95) of the paleopole,we find that 20 sites(samples)or more are required to yield a paleopole with an A_(95)≈5°based on a review of available paleomagnetic results from the Lhasa,Qiangtang and Tethyan Himalaya.Random samplings of Jurassic virtual geomagnetic poles from the Sangri area show that the Fisher mean pole with neglectable angle deviation can be obtained when sampling sites increase to 20.High-quality paleomagnetic results,with sites/samples number N/n≥~20–30,show that the Qiangtang,Lhasa,and Tethyan Himalaya moved northward in the Late Permian–Middle Triassic,Jurassic,and Cretaceous,respectively,and then accreted to Asia in the Late Triassic,Late Jurassic–Early Cretaceous and Paleocene–Early Eocene,respectively.