Misplaced Pages

#389610

22-755: Using Argon–argon dating , Wang and colleagues in 2005 dated part of the Tiaojishan Formation to about 160 million years ago, the beginning of the Oxfordian stage , the first stage of the Upper Jurassic epoch. In 2006, a study by Liu and colleagues used U-Pb zircon dating to conclude that the Tiaojishan Formation correlates with the Daohugou Beds, and the complete chronological range of this shared biota dates to between 168 and 164/152 Ma ago. A subsequent study, published in 2008, refined

44-482: A partial skeleton with soft tissue impressions. Darwinopterus D. modularis Liaoning A wukongopterid named after Charles Darwin . The type species, D. modularis was the first known pterosaur to display features of both long-tailed rhamphorhynchoids and short-tailed pterodactyloids , and was described as a transitional fossil between the two groups. Darwinopterus specimens have also been reported to show several differences between males and females, with

66-547: A partial skull and lower jaw. Cascocauda C. rong Hebei Daohugou bed One specimen A long-tailed batrachognathine anurognathid known from a complete skeleton of a juvenile with extensive preservation of pycnofibres and wing membranes. Changchengopterus C. pani Hebei A pterodactyliform known only from a single specimen of a young juvenile, measuring 475 millimeters (18.7 in) in wingspan. Daohugoupterus D. delicatus Inner Mongolia One specimen A relatively basal pterosaur known from

88-495: A plate and counterplate. This specimen includes part of the torso and pectoral girdle , the left humerus (upper arm bone), both coracoids, right scapula, the sternal plate, neck vertebrae , back vertebrae, ribs, and the posterior part of the skull and lower jaws. Daohugoupterus was named and described in 2015 by Cheng Xin, Wang Xiaolin, Jiang Shunxing and Alexander Wilhelm Armin Kellner . The type and only known species

110-576: A rock retains all of its Ar after cooling past the closing temperature and that this was properly sampled during analysis. This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified. One problem with argon-argon dating has been

132-466: A sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by Ar/ Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is the conventional K/Ar technique . An alternative method of calibrating the used standard is astronomical tuning (also known as orbital tuning ), which arrives at a slightly different age. The primary use for Ar/ Ar geochronology

154-531: A slight discrepancy with other methods of dating. Work by Kuiper et al. reports that a correction of 0.65% is needed. Thus the Cretaceous–Paleogene extinction (when the dinosaurs died out)—previously dated at 65.0 or 65.5 million years ago—is more accurately dated to 66.0-66.1 Ma. Daohugoupterus Daohugoupterus is a genus of pterosaur from the Middle to Late Jurassic Daohugou Beds of

176-631: A strange skull morphology, at first believed to come from the Early Cretaceous . Liaoxitriton L. daohugouensis A little-known cryptobranchoid. Pangerpeton P. sinensis A cryptobranchoid characterized by its short trunk (only 14 presacrals) and short and wide head, giving a fat body shape, from which the genus name was derived ("Pang" means fat in Chinese). Archaeoistiodactylus A. linglongtaensis Liaoning A monofenestratan known from an incomplete skeleton with

198-481: Is Daohugoupterus delicatus . The generic name combines a reference to Daohugou with a Latinised Greek πτερόν, pteron , "wing". The specific name means "delicate" in Latin , in reference to the gracile build. Daohugoupterus is a small pterosaur. The humerus is 41.8 mm (1.65 in) long. Two distinguishing traits were indicated, which were considered possible autapomorphies : the nasal bones deeply invade

220-401: Is dating metamorphic and igneous minerals. Ar/ Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature . However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault systems is also possible with

242-646: Is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce Ar from K via the (n-p) reaction K(n,p) Ar. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and radiogenic gases like argon and helium, generated from regular radioactive decay over geologic time. The abundance of Ar* increases with

SECTION 10

#1732765443390

264-515: Is the Ar*/ Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of K to Ar than a less dense one. The Ar/ Ar method only measures relative dates. In order for an age to be calculated by the Ar/ Ar technique, the J parameter must be determined by irradiating the unknown sample along with

286-429: The Ar/ Ar method. Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing

308-552: The K/ Ar* ratio, and thus to calculate the age of the unknown sample. Ar* refers to the radiogenic Ar, i.e. the Ar produced from radioactive decay of K. Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the Ar (which is assumed to be of atmospheric origin) and assuming that Ar is found in a constant ratio to Ar in atmospheric gases. The sample

330-608: The Tiaojishan Formation in Inner Mongolia , China . Through field work by Zhou Zhonghe and Li Yan, the Institute of Vertebrate Paleontology and Paleoanthropology at Beijing acquired a pterosaur specimen found at the village of Daohugou near Linglongta . It was prepared by Li Yutong. The specimen is catalogued as IVPP  V12537. It is a partial skeleton with soft tissue impressions, compressed on

352-468: The frontal bones , and the sternal plate is broad , being 2.5 times wider than it is long. The short neck vertebrae, presence of cervical ribs, and the form of the humerus with a quadrangular deltopectoral crest indicate that Daohugoupterus was a relatively basal pterosaur, outside of the Pterodactyloidea . It was placed Pterosauria incertae sedis . Daohugoupterus is a member of

374-463: The age of the sample, though the rate of increase decays exponentially with the half-life of K, which is 1.248 billion years. The age of a sample is given by the age equation: where λ is the radioactive decay constant of K (approximately 5.5 x 10 year , corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R

396-705: The age range of the formation further, finding that the lower boundary of the Tiaojishan was formed 165 Ma ago, and the upper boundary somewhere between 156 and 153 Ma ago. Based on the plant life present in the Tiaojishan Formation, Wang Yongdong and colleagues determined that the climate in Liaoning during the mid Jurassic would have been subtropical to temperate , warm and humid . Beautifully preserved fossils of dinosaurs , pterosaurs , salamanders , insects , arachnids and other invertebrates , conifers , ginkgoes , cycads , horsetails , and ferns , and even

418-1302: The earliest known gliding mammal ( Volaticotherium ) have been discovered in these rocks.The tuffaceous composition of some rock layers show that this was a volcanic area, occasionally experiencing heavy ashfalls from eruptions. The landscape then was dominated by mountain streams and deep lakes surrounded by forests of gymnosperm trees. The forests of the Yanliao biota grew in a humid, warm - temperate climate and were dominated by gymnosperm trees. There were ginkgopsids like Ginkoites , Ginkgo , Baiera , Czekanowskia , and Phoenicopsis . There were also conifers like Pityophyllum , Rhipidiocladus , Elatocladus , Schizolepis , and Podozamites . Also, Lycopsids like Lycopodites and Sellaginellities , horsetails ( Sphenopsida ) like Equisetum , cycads like Anomozamites , and ferns ( Filicopsida ) like Todites and Coniopteris . Yanliaomyzon Y. occisor Liaoning A lamprey . Beiyanerpeton B. jianpingensis Liaoning A salamandroid known from an almost complete and articulated skeleton exposed in ventral view. Chunerpeton C. tianyiensis A cryptobranchoid measuring 18 centimeters in length. Jeholotriton J. paradoxus A cryptobranchoid with

440-449: The males having distinctive crests on their heads. They are known to have laid their eggs on the ground, and may have also not shown that much for parental care. Argon%E2%80%93argon dating Argon–argon (or Ar/ Ar ) dating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while

462-401: The newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes . Ar/ Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium ( K) into the radioactive Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate

SECTION 20

#1732765443390

484-428: The thermal history of the rock. Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. Ar/ Ar geochronology assumes that

#389610