Geological dating wiki

published their own study of the age of the Daohugou beds, this time using Zircon Uranium-lead dating on the volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used as index fossils). found that the beds formed between 164-158 million years ago, in the Middle to Late Jurassic.[12][6] A 2012 study by Gao and Shubin agreed with this assessment, and reported an Argon–argon date of 164 plus or minus 4 million years ago for the Daohugou horizon.

This clock representation shows some of the major units of geological time and definitive events of Earth history.

For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species.

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Each era on the scale is separated by a major and/or changing event.

The largest defined unit of time is the supereon, composed of eons.

In other words, despite the absence of Lycoptera, a typical fish of the Jehol Biota, the Daohugou vertebrate assemblage is closer to that of the Early Cretaceous Jehol Biota than to any other biota." Wang et al.

concluded that the Daohugou probably represents the earliest evolutionary stages of the Jehol Biota, and that it "belongs to the same cycle of volcanism and sedimentation as the Yixian Formation of the Jehol Group."[4] However, a later study by Ji et al.

found an overall similarity between the fossil animals found in the Daohugou Beds and those from the Yixian Formation.

The authors stated that "vertebrate fossils such as Liaoxitriton, Jeholopterus and feathered maniraptorans show much resemblance to those of the Yixian Formation.Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units, such as "Upper Jurassic", and "Middle Cambrian".Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales.The informal Hadean eon represents the time before fossil record of life on Earth, its upper boundary is now regarded as 4.0 Ga .Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Paleaozoic, Mesozoic and Cainozoic are eras of the Phanerozoic eon.Gao and Ren went on to defend a Middle Jurassic age for the beds based on biostratigraphy (the use of index fossils), and the bed's relationship to a layer that is known to mark the Middle Jurassic-Late Jurassic boundary.[10] Another study, published in 2006 by Wang et al., argued that the 159-164 million years old Tiaojishan Formation underlies, rather than overlies, the Daohugou Beds.

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