Hornstones have a hard shell, and as the name suggests, the hornstone shell is shaped like the horns of a cow or sheep, generally straight, or curved or coiled. From the beginning of the development to the final growth, the diameter of the shell gradually becomes larger, the flesh grows and constantly moves forward and secretes a calcareous shell, and finally grows in the front part of the shell, forming a living room. The rear side of the chamber facing the tip of the shell forms a series of air chambers, and the lifting and balancing of the diagonal stones of the gas chamber plays an important role. After the death of the hornstone, the flesh is usually difficult to preserve, and only the hard shell can be preserved as a fossil. The appearance of the horned stone shell is not necessarily smooth, and the surface of many types of shells has different ornaments, such as nodules, tumors, various horizontal stripes, vertical stripes, etc., and the structure of the next door and body tube in the body is also very different, which are important identification basis. China's horn fossil resources are very rich, the Northern Ordovician Formation Ordos Horn Stone, Amen Corner Stone, Gray Horn Stone, the Southern Ordovician Formation Aurora Horn Stone, Pan Horn Stone, Michlin Horn Stone, etc. are representative species, they have long been effectively used to divide the comparative strata.
Hornstones
The shell of the hornstone is conical or cylindrical, and some species have a distinct spiral line – proving that it is evolutionarily related to the Mesozoic ammonite.
Horned stones are the most widely distributed cephalopods in the Ordovician oceans, and were still widely distributed in the oceans until the Jurassic; they were still distributed in the Paleocene to the Miocene oceans, but have shown an evolutionary trend of shelllessness.
The extant Nautilus is one of the descendants of the horned stone animals.
Aurora Horn Stone