A shell is not a shell, left and right are always relative

Since the Cambrian Period of the Paleozoic Era, brachiopods have lived in a low-key manner at the bottom of the ocean. They rose in the Ordovician Period, began to flourish in the Devonian Period, reached a low point in the Carboniferous Period, and gradually declined in the Permian Period. Despite several ups and downs, they are still alive in the world, not just active in movies, TV shows or books.

They are often considered to be shellfish because of their shells. In fact, the shellfish we often talk about belong to the phylum Mollusca. There are also many unclear points about their research. Not only are many brachiopods named "XX shellfish", such as owl-headed shellfish, banana leaf shellfish, long-bodied shellfish, etc., but even their "arms" were mistakenly considered to be equivalent to the "feet" of the locomotor organs of mollusks in early studies, so they were called "brachiopods". In fact, the "arms" are used for breathing and feeding, and have nothing to do with movement.

Owlhead Clam

Banana Leaf

In biological classification, they belong to the phylum Brachiopoda. The sizes of adult individuals of different species vary greatly. For example, the small ones such as the top-hole clams and the tongue-shaped clams are only 3-6 mm, while the large ones such as the giant hypertrophic owl-headed clams can reach 10-15 cm and weigh up to 2 kg. The general length of the body is between 2-4 cm.

Tongue-shaped shell

Their shells are made up of two parts, large and small, mostly made of calcium, with a few being horny or apatite. The larger part is the ventral shell, and the smaller part is the dorsal shell. Although they vary in size and the sizes of the dorsal and ventral shells, all brachiopods have bilateral symmetry on the same shell.

The two shells are connected at the junction of the twist line and the base surface. The ventral shell has a bird-like ventral beak with a round or triangular hole on the inside or at the end. The fleshy stem inside the body extends from the hole to fix it to the seabed. There are also complex "carpal bones" or spiral "carpal spirals" inside the shell to support the carpal soft body.

On the surface of the shell, there are patterns centered on the "beak", which are divided into concentric layers, concentric folds, concentric lines, concentric patterns, as well as radial folds, radial lines and radial patterns from strong to weak.

Since their appearance in the early Cambrian period, brachiopods have evolved very rapidly, and the characteristics of different eras and different biotas are extremely different. For example, those with thin shells composed of chitin and phosphate are adapted to slightly deeper still water environments, while those with thick calcium shells are adapted to shallow high-energy zone environments. But in general, most of them are adapted to coastal environments with normal salinity (3.3%).

Shi Yan

Their fossils also show obvious characteristics of the times: at the beginning of the Cambrian period, they were extremely small thin shells, mainly distributed in southern my country; the Ordovician period was dominated by small-sized protoliths and ortholiths; the Silurian period was dominated by five-chambered clams and small-mouthed clams; in the early Devonian period, due to marine invasion, the climate in Guangxi and parts of Yunnan was warm and shallow seas were widespread, and the moon shells and stone clams flourished.

In the middle period, large owl-headed clams and Xiangzhou clams appeared in large numbers in southwestern my country. In the late period, due to the continued expansion of marine invasion, the brachiopod community represented by stone swallows and Yunnan clams was extremely prosperous in central and southern Hunan. The most common in the early Carboniferous period was the long-bodied clams; the most important in the Permian period were banana leaf clams and Oham clams; in the Mesozoic Era, brachiopods declined sharply, and only a few genera and species are left in modern times. Among them, the tongue-shaped clams commonly known as sea bean sprouts can be called cockroach-level. They have not evolved significantly from the Cambrian period to the present, but they can be found in strata of any era.

Due to multiple prosperity, there are simply too many brachiopod fossils, but this does not mean that they are not valuable. The price of individual fossils is not expensive, but treating them as a whole and collecting as many symbiotic fossils as possible can highlight their precious value - becoming the "golden key" to accurately compare and divide eras and restore the ancient environment.