The Lower and Upper Xiashaximiao Formations are part of the much larger Dashanpu Formation, and these form the best and most prolific Jurassic beds in China, dating back to the Bathonian to the Callovian Stages. This is about 168 to 161 million years ago.
In fact these are some of the best Jurassic beds in the world. They are located in the Sichuan Region, near the small town of Dashanpu. The area is known for mining natural gas, and conversely one of the top predators of the area is named after this.
The pterosaur fauna here is limited to a sole rhamphorhynchid named Angustinaripterus. This animal is just a typical non-pterodactyloid, similar in size and shape to its relative Rhamphorhynchus.
It might have fished from the lakes and rivers of the area.
There are plenty of aquatic animals here, including stem-crocodiles. These are almost all goniopholidids, a group of crocodylomorphs which superficially resemble modern forms in size and shape.
However they differ primarily in having much simpler osteoderms, with just two rows across their back, and not as ornate as those of modern crocs. Also their skulls were somewhat flatter and the osteoderms on their bellies were hexagonal and not rectangular.
Two are known from this formation. One of these is the predatory Hsisosuchus. It was around 3 meters long, similar in size to an American alligator and probably in terms of habits too.
The other was Sunosuchus. Sunosuchus is actually very widespread, living all over Asia, from Kyrgyzstan to Thailand, and China in between. However many species in this massive genus have been reassigned.
For example the Kyrgyzstani species is unnamed, and the Thai species was once known as Goniopholis.
However in another twist, a second Thai species called S. thailandicus was renamed as Chalawan, a completely new genus.
These two goniopholidids and the discovery of a large labyrinthodont amphibian called Sinobrachyops suggest a well-watered environment. In fact the Dashanpu Formation preserves an area of lakes, floodplains and riparian forests.
The area was explored most widely by paleontologist Dong Zhiming from 1975 onward. Much of it dates back to the Middle Jurassic,
Here the non-avialan dinosaur fauna was incredibly diverse, especially when concerning the sauropods. These were not the diplodocoids and macronarians of Jurassic North America but instead a group known as mamenchisaurs.
These were pretty much limited to East Asia of the time, and their claim to fame lies in their grossly elongated necks. They lived their lives vertically, with their heads stuck high up in the trees and a small body many meters below.
Both the Lower and Upper Xiashaximiao Formations had their own unique sauropod fauna. The Lower Xiashaximiao contains the well-known Shunosaurus-Omeisaurus Assemblage. This consists mainly of two sauropods that could not be more different from one another. Omeisaurus is probably among the most graceful of all sauropods.
It is a mamenchisaur with the characteristically massive neck and tiny body and rather short tail. The neck was curved, almost like that of a swan, while everything about this genus was slim and almost winsome. A number of species have been found but the best-known is probably O. tianfuensis.
This one had the longest neck of the genus, at over 9 meters, while the whole animal was 15 meters long. It was still virtually a tiddler in comparison with North American titans like Brachiosaurus and Supersaurus but was still tall thanks to the length of its neck. It probably browsed from the very tops of the conifer trees of its lakeside forest environment.
Other mamenchisaurs include the large-headed, probably long-necked Datousaurus, and Protognathosaurus, and they all lived in the same place. They had different neck lengths and tooth shapes in order to avoid competition.
A very similar thing happened with almost all sauropod-dominated faunas, like the Morrison especially, where erect-necked genera coexisted peacefully with lower, longer types. Of course some went extinct while others continued.
Omeisaurus species, for example, are also known from the Upper Xiashaximiao, or the Mamenchisaurus Assemblage where they existed alongside the even larger, longer-necked Mamenchisaurus. A little lower to the ground but arguably the commonest sauropod here was Shunosaurus.
Made somewhat famous by the critically panned show Dinosaur Revolution, and shown as an unfortunate victim of shroom addiction, this genus is actually one of the best-known sauropods from Dashanpu.
It is also the smallest and shortest, at 9 meters in length. It also weighs no more that 3 tonnes. Its neck is very short but its body is quite compact. Fossils from both juveniles and adults are known, with about twenty individuals.
In fact skulls are known, which is a rarity for sauropods in general. In fact 94 per cent of skeletal remains are known from Shunosaurus. There is just one species, Shunosaurus lii. It coexists nicely with its long-necked mamenchisaur brethren, but does not continue to the Upper Dashanpu.
The main predators in the Lower Shaximiao Formation are rather small, far smaller and lighter than their contemporary sauropods in fact. One of the most famous is Gasosaurus, named not for flatulence but instead for the natural gas mining operations here. It is just 4 meters long and probably preyed on smaller ornithopods and baby sauropods, instead of tackling mature adults.
While there are some beautiful skeletal mounts present, we are still unsure what exactly it is.
It has been classified as various things at different times. These include a carnosaur, specifically a metriacanthosaurid, a basal coelurosaur and even a common ancestor lying somewhere between the carnosaurs and coelurosaurs.
Whatever it is, it is still the oldest known member of the Tetanurae. These are all theropods exclusive of Ceratosauria. The Carnosauria and Coelurosauria form their own clade, the Avetheropoda. Another theropod in need of classification is Kaijiangosaurus.
This is also quite similar to Gasosaurus in terms of size and difficulty in classification. It might be just another primitive tetanuran of some sort, probably a megalosauroid.
An actual metriacanthosaurid from the formation is Xuanhanosaurus, also very similar in dimensions to the aforementioned genera. It was once thought to be a quadruped thanks to its long forelimbs.
However theropods were unable to pronate their hands, so it is now thought that Xuanhanosaurus simply used these for catching prey.
The smaller ornithopods in the formation are represented by the best-known Chinese Jurassic ornithopod, the genus Agilisaurus.
Agilisaurus is known to have been a 1.2-meter diurnal herbivore or possibly omnivore.
The best-represented armored dinosaur from this formation is the medium-sized stegosaur Huayangosaurus.
Its larger and better-known relative Tuojiangosaurus is part of the Mamenchisaurus fauna of the Upper Dashanpu. Huayangosaurus itself is known from very good remains, including of the skull, limbs and plates on its back.
Thus it can be restored with confidence. It was about 4.5 meters long, half as much as its larger and more famous relative Stegosaurus from North America’s Morrison Formation.
