Name: Liaoning Province
Location: China

Age: Early Cretaceous

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The most important part of the Jehol group is the Yixian Formation. This is near the base, and contains many of the best preserved fossils. The formation consists of a series of intercalated lacustrine and volcanic sediments, which were deposited on the bottom of shallow lakes and rivers. The best fossils occur at the lower part in buff-coloured lacustrine sediments [3].

Exceptional preservation was due to rapid burial in these fine-grained sediments. Organisms are often preserved in their original position and with soft tissues such as wings, skin, eyes, feathers, gills and even parasites. The preservation is of such a high level that even veins of leaves and insect wings, and filaments of feathers are preserved [7]. The primitive bird Sinornis has collagen fibrils, barbs and feathers preserved, which provide information on the evolution of feathers [3]. Some specimens possess fossilised stomach contents and even patterns on the skin are preserved. The colours cannot be identified, but it is possible to see that insects, fish and small dinosaurs were spotted or striped, like their living counterparts [7].

The Jehol biota is spectacular not only in the preservation of the fossils, but also in the abundance and diversity. For example, approximately 800 fossil birds have been found to date, which means that population dynamics can be studied, usually impossible in fossil assemblages. Sexual dimorphism (in Confuciusornis – a primitive bird) and ontogeny can also be seen. For example, ontogenetic stages in development of Liaxiornis, another primitive bird, can be seen, as well as the whole life cycles of some insects. The Yixian Formation has yielded many important new fossils including feathered dinosaurs, pterosaurs with hair-like filaments and fossil plants that are new to science. However, there are no new groups of dinosaurs and no new major taxa. It also contains an unusual amount of relict taxa, which were thought to have disappeared at the end of the Jurassic. These include particular families and genera of pterosaurs, dinosaurs and mammals [3].

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