The Tiaojishan has often been regarded as a precursor to the more famous Early Cretaceous members of the Jehol Group but the actual dating of the site is rather unclear, and has been for a very long time. For one thing, it has been tentatively assigned dates between 169 and 122 million years old.
The Daohugou Bed for one was incredibly hard to date. It was once regarded as a separate fossil bed altogether and with a separate fauna to the Tiaojishan. In fact, it was this section that was dated to the Early Cretaceous via argon-argon dating in 2004. However, much later it was realized that they belonged to the Tiaojishan, therefore to the Middle Jurassic. This is roughly 165 to 156 million years ago, the Bathonian to the Oxfordian Stages of the Jurassic Period. These beds do not just form an area of the Liaoning Province but also Hebei and Inner Mongolia. This shows us that there was a massive community of plants and animals in this pre-Jehol formation.
The rocks are pyroclastic, made of volcanic and sedimentary elements and almost all fossils here are preserved quite excellently.
The environment did not have the winter of the Yixian, and neither was it a lakeside ecosystem despite the presence of obvious aquatic animals. Here the climate was subtropical and quite humid, with probably wet and dry seasons.
Most of the plants here were ginkgos like Ginkgo, Bareia, Ginkgoites and Phoenicopsis. These plants also carry over to the Jehol Group millions of years later.
These plants formed the majority of the canopy flora, while bennettites were also present in very large quantities. These include the ubiquitous and very well-documented Williamsonia and Zamites, as well as their relatives like Pterophyllum. Ferns, conifers and also horsetails existed in the Tiaojishan paleoenvironment, including the common horsetail genus Equisetum.
There are hardly any massive animals here, at least none more than a meter in length. One of the main plant-eaters in the forest was the small stem-bird Tianyulong. It was an ornithopod and the first known for the presence of some kind of feather-like soft tissue integument.
They might either be actual feathers or just convergently evolved structures but whatever the case, it confirmed the presence of such integument in dinosaurs of its kind. Tianyulong was 70 centimeters long and was probably a generalist. It took both plants and meat, and used its set of tusk-like canine teeth in defense like a wild boar.
All other stem-birds here were paravians of one kind or another. There is one family called the scansoriopterygids, all of them sparrow to pigeon-sized, and all of them living in the same time and at the same place. These three small creatures are limited to the Daohugou Beds of the Tiaojishan Formation. They all had an elongated third finger which might have been used to pick out insects and their larvae from underneath the tree bark. However, a new find has revealed something startling from the largest scansoriopterygid.
This is Yi qi, translated as ‘strange wing’, and it had a positively draconian look with its short body, possibly long streamer-like tail feathers and a wing membrane made of skin. It had a separate bone fixed to its wrist, a styliform element to further add support to this wing.
The others may have had a wing like this as well, or the feature might just be peculiar to Yi. It was most obviously a glider, somewhat like a flying squirrel. Like its two relatives it preyed on insects. Yi’s colors are known, with a golden-brown wing membrane as well as gold shades all over the head and body, with black lower legs.
Its smaller relative was the ribbon-tailed Epidexipteryx. This animal was just around the size of a small sparrow and it is best known for none other than its tail feathers. Whereas most other paravians had a fan of feathers, or stiff feathers along a long bony tail, this dinosaur had a short tail but its feathers were shaped like straps or thin ribbons.
These were ideal display features but we can only speculate at what these displays were like. All of these dinosaurs had slightly forward-pointing teeth and short skulls and somewhat curved lower jaws.
Yi qi has already taken up internet meme status and enjoys a position of power among the most famous and celebrated theropod discoveries in recent years.
Most of the animals in the Tiaojishan are indeed carnivorous or at least omnivorous. These include Xiaotingia, which at almost a meter in length was also the larger paravians in the formation. It is now thought to be closest to avialans although earlier evidence pointed to its relationship with dromaeosaurs and even the troodonts. It was likely an arboreal predator eating small mammals, reptiles and insects.
A proper basal avialan that rocked the world was Aurornis, the ‘dawn bird’. It was a 50-centimeter creature found to be even more primitive than the fabled Archaeopteryx itself, often celebrated as the so-called first bird.
Like almost all the taxa from Tiaojishan there were issues with classifying Aurornis, with plenty of disagreements among various researchers. Steve Brusatte for example, classified it as a troodontid while Lawrence Witmer adequately adds that all the small feathered creatures in the area were virtually similar. In fact, there is no difference between "birds" and "dinosaurs" in the traditional sense.
Another ancient avialan that revolutionized color studies is Anchiornis. This was once thought to have been a troodontid as well but is now a full, basal bird. However, it was clearly not a flying animal, with rounded wings and neither was it that much of a runner, with extremely long feathers projecting from its legs. Its legs were still incredibly long though, even if its feet were clothed in thick fluff. Whether or not it could at least glide is not known.
What is known about it though, is its coloration, which includes a gray body, red crest and cheeks, and black and white speckled feathers on its wings and legs. It may have been a slow, arboreal omnivore for all we know.
A proper ground-dweller here is Eosinopteryx. It was a short-armed avialan with very long legs and a short tail. It was also about 30 centimeters long, and probably a quail-like omnivore that scratched and pecked around the leaf litter in search of food. Strangely, for a basal avialan, or any maniraptoran for that matter, it had no feathers covering its legs.
Small mammals filled the Tiaojishan, including one of the largest of all Jurassic mammaliaforms, Castorocauda. It and its other relatives were not true mammals, as is the Theria, which includes marsupials and placentals.
Castorocauda was not yet a mammal proper. It was somewhat like a cross between a platypus and a beaver but in reality it was a docodont. Docodonts were relatively primitive mammals that lived throughout the Jurassic and Early Cretaceous.
They were mostly herbivorous or omnivorous. Castorocauda however, was very specialized to an aquatic life and probably took fish and small invertebrates in the water. It was a sizable animal at 42 centimeters. It was also able to burrow and probably had its young in burrows along the riverbank.
We also have a number of arboreal mammals and their ancestors in the Tiaojishan, including other docodonts. One is Agilodocodon, a small tree-climber and one of the earliest arboreal mammaliaforms. Then there is Docofossor, one that had evolved a properly mole-like lifestyle. Another mammaliaform group is the Eutriconodontia.
With the Yixian taxon Repenomamus they produced some of the biggest and most diverse of all Mesozoic mammaliaforms. In the Taiojishan there is Volaticotherium. It was virtually identical to a flying squirrel and probably lived like one too. It was definitely arboreal, and glided from tree to tree to find food or escape from predators.
Pterosaurs are also incredibly common here. All the pterosaurs here are non-pterodactyloids, including the crested rhamphorhynchid Pterorhynchus, known for its long, multi-vaned tail and impressions of its furry pycnofibers. The most interesting though, is the genus Darwinopterus.
Altogether three species have been discovered here. The type species, D. modularis was hailed as a missing link between pterodactyloids and non-pterodactyloids. For one, it sports the long tail of the latter group and the long neck vertebrae of the former.
The species name "modularis" comes from the genus having undergone modular or mosaic evolution. This means that pterodactyloid features evolved with no order whatsoever rather than gradually.
Darwinopterus was thus still clearly a non-pterodactyloid pterosaur though. Aside from D. modularis, we have D. linglongtaensis with its short jaws and cone-like teeth and the larger D. robustodens.
They are separated in their habitat by dietary preferences, among other things. D. linglongtaensis had the weakest and thinnest teeth of the lot while D. robustodens had the largest, and the best adapted for crushing. Given their size, which is often no bigger than a robin, all of them would have fed on insects.
The large D. robustodens would have tackled hard-shelled beetles. We also have a record for sexual variation in the genus, with a specimen named "Mrs. T" having been found with broad hips for laying eggs. "Mrs. T" also preserves an unhatched egg. This egg is very small and soft-shelled like that of most sauropsids.
The young hatched fully developed and able to fly within minutes of birth, so parental care was limited or nonexistent. The males showed evidence of having big soft tissue crests on their heads.
A pterosaur that shows clear evidence of its furry coat is the Daohugou taxon Jeholopterus. This is the largest anurognathid, although with a 90-centimeter wingspan, this is hardly a giant. It had the same nocturnal adaptations as its relatives, but the fossil actually shows the extent of its pycnofibers.
The skin of the wing membrane has also been found. Thanks to fossils like this, we know that pterosaur wings were not just formed out of a single thin membrane but actually of multiple layers of fibers called actinofibrils. These were light and incredibly strong and resistant.
The animal was actually able to perform slight shifts of mere millimeters of the wing profile while in flight, a remarkable adaptation.