This fossil bed in the Liaoning Province of Eastern China is one of the most diverse Early Cretaceous sites anywhere. It is a key member of the now-famous Jehol biota and has seen plenty of strife in the recent past. It spent much of World War II under the thumb of the Empire of Japan, but once the invasion was thwarted and the war ended, proper scientific exploration could begin here.
In particular, the 1990’s were full of incredible discovery in the Yixian.
The formation itself has a massive span of seven million years and the huge proliferation of animal life is a little deceptive. They are all arranged into their own little bonebeds, and some of them are from undetermined beds in the formation.
The formation itself is mostly made up of basalt-based sediments. The basalt base is roughly 129 million years old and from here begins a climb upward into younger beds.
These are as follows:
Jianshangou (124 mya)
Lujiatun (123 mya)
Jinganshan (122 mya)
Among them, they preserve a lakeside forest ecosystem that had been buried under volcanic ash. The Yixian lake beds probably had very fine sand, and we know that the climate was cool and temperate. It was a landscape of mostly ginkgoes like the genera Sphenobareria, Ginkgoites and Ginkgo. There were also yew trees (genus Taxus and Torreya) as well as other conifers like the ubiquitous Brachyphyllum.
Horsetail beds (Equisetum) grew around the riverbank. There was also a colorful update to the flora of this time and place. Flowering plants had begun to spread out their shoots, and some of the earliest were all found here in the Yixian. Some of the best-known include Leefructus and Archaefructus, an aquatic plant. Another is Archaeamphora, the earliest-known pitcher plant. There were probably four seasons here, with a rather cold, snowy winter. In fact at the time, Eastern China was probably colder than Australia, which was stuck to the South Pole.
The most prolific animal of the Yixian biota though, is the small fish Lycoptera. This bony fish might have looked rather generic but is an important index fossil that helps to identify the formation as a whole. Lycoptera probably fed on small bits of plants and animals in the water, like insect larvae and other pieces of floating detritus.
The cold water of the lakes was high in nutrients and fueled the growth of planktonic organisms. Indeed, there are quite a few filter-feeders among the fish here. Some of these include relatives of today’s sturgeon and paddlefish, all of which filter their prey using specialized gill rakers. Crocodiles have not been found here. Instead, choristoderes are the most common reptilian predators.
These are also a group of diapsids.
The Yixian is most famous for two things though, and one of these is an impressive degree of preservation while the other is a proliferation of feathered dinosaurs.
The lakeside forests were teeming with primitive birds and stem-birds. Among the earliest found were small carnivores.
One of them, Sinosauropteryx, is known to have eaten early mammals. It was much more basal than the more birdlike theropods, and thus its feathers were limited to a fur-like coat. We also know the colors of a few Yixian taxa.
Sinosauropteryx was rusty-colored with a long, banded tail. The bands were white and it probably served as a display organ.
Caudipteryx was a herbivorous oviraptorid. One species, Caudipteryx zoui, is known to have been darkly-colored in places.
The dromaeosaur Sinornithosaurus was another dinosaur for which color tests have been done. These showed an animal with reddish-browns, blacks and yellows, resulting in an animal similar to a terrestrial hawk. Another feathered dinosaur known from its colors is the common primitive bird Confuciusornis.
This small creature is the most widespread in the Yixian and had a black and white color scheme. The males also had two long streamers stretching from their tails, likely for display. These birds were possibly omnivores, and we know this from the remains of both fish and seeds preserved in their stomachs. In this way, Confuciusornis was like a small species of crow, common and with a varied diet.
Other birds found here include Confuciusornis’ relative Chanchengornis, a much smaller more insectivorous bird, the kingfisher-like Longipteryx and the plover-like Hongshanornis. Not all these genera overlap, but the niches are diverse enough to allow peaceful coexistence much like lakeside birds today.
We also have fossils of Confuciusornis in the stomach of the 2.5-meter carnivore Sinocalliopteryx, a basal coelurosaur, a compsognathid.
This was the largest predatory dinosaur in the Yixian for many years, but in 2009, a large tyrannosauroid named Yutyrannus was described. It was the apex predator of its particular fossil bed, at 9 meters and 1.4 tonnes.
It is the first large member of its family for which feathers are known. Feathers formed a furry coat across its body, possibly keeping it warm against the chill of winter.
Of course not all the dinosaurs here were feathered coelurosaurs. The most common plant-eater was Psittacosaurus.
It is known for having had quills along its back, and is also the most species-rich dinosaur genus. These small horned dinosaurs lived from Russia to Thailand, and Mongolia and China in between.
Large and small iguanodonts have also been found here, especially the smaller Jeholosaurus and the horse-sized Jinzhousaurus and Bolong.
The biggest herbivores were medium-sized titanosaurs.
There was even diversity in mammals, with Repenomamus being a badger-sized carnivore that ate the young of dinosaurs.
Just as there were filter-feeding fish, there were abundant filter-feeding pterosaurs too. These were the boreopterids, named for the rather small taxon Boreopterus.
Many of the formation’s pterosaurs were from the Jianshangou and the Jingangshan Beds, with the big ctenochasmid Moganopterus hailing probably from the Lujiatun.