Sequence Thickness and its Response to Episodic Tectonic Evolution in Paleogene Qikou Sag, Bohaiwan Basin

The sequence stratigraphy framework of Paleogene of Qikou Sag has been built in this paper. The evolution law of episodic subsidence in Qikou Sag has been expounded, with the analysis of the characteristic of sequence thickness, as well as the study of consedimental tectonic activity, subsidence history, paleogeomorphology and the coupling relationship among them. The faults controlling action was the dominant mechanism during rifting stage I , as the depression was the dominant mechanism during rifting stage III, which embodies a process from intense disparity fault depression to integral subsidence. Meanwhile, under the control of regional faulting and later stage depression, subsidence rate centers and paleogeomorphic concave centers are always corresponding to the sequence thickness centers, and the changes of subsidence centers and paleogeomorphology concave centers are consistent with the migration of thickness centers, which presents the restriction, coupling and response among them and clarifies the law that tectonic activity controls the basin fill by means of controlling the subsidence rate and paleogeomorphology from contributing factor angle. On this basis, the dynamic evolution of the composite pattern of structure-subsidence-deposition in the fault controlling actic region during the three rifting stages and their petroleum geological significance has been discussed.

Evolutionary rates of mitochondrial sequences and gene orders in Spirurina(Nematoda)are episodic but synchronised

In contrast to highly conserved mitogenomic architecture in most metazoan lineages,which indicates that rearrangement events are generally strongly selected against,a limited number of often unrelated lineages exhibit highly elevated architectural evolution rates.The underlying reasons for this discontinuity in the mitogenomic evolution remain unknown.Previously we sequenced the mitochondrial genome of the first Camallanoidea species,Camallanus cotti(Nematoda:Chromadorea:Spirurina:Camallanidae),and found that it exhibited a highly disrupted architecture.We hypothesised that disrupted architecture might be a synapomorphic feature of the sister-clades Camallanoidea and Dracunculoidea.In this study,we sequenced mitogenomes of three freshwater fish-parasitic nematodes:Camallanus lacustris(Camallanidae),and two Philometridae(Dracunculoidea)species,Clavinema parasiluri,and Philometra sp.In partial agreement with the working hypothesis,both Camallanoidea species had exceptionally large mitogenomes of 18–19 Kbp,albeit the underlying reasons differed:in C.lacustris it was the existence of a single enlarged noncoding region of
5.5 Kbp.A segment of this region exhibited an inverted base composition skew,which is indicative of a sequence inversion or recombination event.Camallanidae is the second identified chromadorean(first for Spirurina)family that exhibits within-family protein-coding gene rearrangements,and the absence of trnL1 and trnF may be a synapomorphy for Camallanoidea.The underlying reason for the disrupted architecture of Camallanidae does not appear to be a particular event shared by their common ancestor,but rather an underlying mechanism that makes disruptive events more likely in this lineage.In disagreement with the working hypothesis,Spiruromorpha and Oxyuridomorpha exhibited even more highly rearranged gene orders and greater overall branch lengths than Camallanomorpha.However,withininfraorder architecture was highly conserved and leaf nodes very short.This indicates that common ancestors of Spiruromorpha and Oxyuridomorpha clades underwent a period of rapid mitochondrial evolution(both sequence and architecture),followed by a stabilisation after the taxonomic radiation.In contrast to this,Camallanomorpha,and particularly Camallanidae,appear to have entered a period of elevated evolutionary rates after the initial radiations of these two taxa.As a result of this evolutionary discontinuity,there was a strong correlation between the gene order rearrangement rate(GORR)and the overall branch length(0.81),but there was no correlation between the strength of purifying selection(ω?dN/dS)and the overall branch lengths(0.05)and GORR(0.04).These findings have important repercussions for future phylogenetic and other evolutionary studies of Spirurina.

Two episodic changes of trace fossils through the Permian-Triassic transition in the Meishan cores,Zhejiang Province

Six ichnogenera,Chondrites,Palaeophycus,Planolites,Rhizocorallium,Skolithos,and Thalassinoides,were identified from the drilling cores near the Global Stratotype Section and Point(GSSP) of Permian-Triassic boundary at Meishan,Zhejiang Province.The ichnological indices,including abundance,bioturbation index,and disturbed depth of trace fossils,show two episodes of intense changes near the Permian-Triassic Boundary(PTB).Episode I occurred in Beds 25-27b when the ecologically complicate forms such as Chondrites,Skolithos,Rhizocorallium,and Thalassinoides disappeared hereafter,the bioturbation index reduced from 1-5 to 1-3,and the disturbed depth declined from 5-66 to 2-5 cm.Episode II took place at the base of Bed 33 with the disappearance of Palaeophycus and Planolites,and subsequent absence of trace fossils and bioturbation till the middle-upper part of Bed 41 when the disturbed structures reoccurred,but they are only tiny Planolites and the bioturbation index was never higher than 3 and the disturbed depth less than 4 mm.Episode I shows an intense change,corresponding to the main stage of the end-Permian mass extinction,whereas Episode II is relatively weak,corresponding to the epilogue of the mass extinction of trace makers in the Early Triassic.Subsequently,ichnofossils were dominated by surface tracks in simple ecological habit and structures.This phenomenon indicates that the Early Triassic benthonic fauna is changed from sessile benthic system to mobile benthic system after the end-Permian mass extinction.In other words,the evolution of the trace fossils across the Permian-Triassic transition had an episodic process similar to the body fossils.In addition,the change of ichnofabrics is well coincided with the negative excursion of carbon isotopes and the expansion of cyanobacteria.As the results of physical and biogenic processes,trace fossils provided unique materials for the study of the biotic and environmental events,as well as their coupling evolution through the great Permian-Triassic transition.