Xiaowa Formation

The Xiaowa Formation is a special group of rocks from a long time ago, found in southern China. These rocks are made mostly of limestone and marls and come from a time called the Carnian stage of the Triassic period, which was about 234 million years ago.

Scientists first called these rocks the Wayao Formation, but later they changed the name to Xiaowa Formation to avoid mixing it up with another rock layer from a different time. This rock layer is very important because it holds many well-preserved fossils, forming something called the Guanling biota. You can find many interesting sea creatures here, like crinoids and marine reptiles.

Among the fossils are ichthyosaurs, thalattosaurs, placodonts, and Odontochelys, which is an early relative of modern turtles. These fossils help scientists understand what life was like in the ancient seas during the Triassic period. The rocks also tell us about big changes in the Earth's climate that happened around that time, known as the Carnian Pluvial Event.

Geology

The Xiaowa Formation is made up of three parts. The lower part is thin but full of fossils. It starts with thick layers of grey biomicrite mixed with greenish shale. Common fossils here include bivalves and pieces of crinoids. The lower part changes to darker layers showing conditions with less oxygen and less reef activity. It ends with dark marls and black shale containing more fossils like ammonoids and conodont remains.

The middle part is the thickest and has layers of grey limestone and marl mixed together. These layers often show signs of movement and contain occasional fossils of bivalves and ammonoids. This part formed in deep water affected by tectonic events. The upper part mostly has layers of limestone with common bits of sand and silt, showing a shallower ocean environment.

Paleobiota

The Xiaowa Formation contains many different types of ancient sea creatures. In the lower part of the formation, scientists find fossils from the Protrachyceras costulatum ammonoid zone, which continues from an older rock layer called the Zhuganpo Formation. The middle part of the Xiaowa Formation has fossils from the Sirenites cf. senticosus ammonoid zone.

Bivalves

Brachiopods

Cephalopods

Conodonts

Echinoderms

Fish

Reptiles