Looks like a little "towboy", but it turns out to be a Devonian coral

When the weather is hot, I can't help missing the sea, especially for people like me who live in inland provinces. Wearing beach slippers (in Yunnan, our slippers are called "slippers") and strolling on the beach is really something we yearn for!

When talking about the sea, corals are naturally indispensable. Whether it is the charming coral trees or the beautiful coral reef islands, as an important group of marine invertebrates, these beautiful creatures are known as the elves in the ocean.

But even among these beautiful creatures, there have been some ugly creatures that can be called mudslides. I would like to call them the little drag kids of the Devonian period. What kind of coral is this? Let's find out with me.

Let’s go find the little “drag kid” of the Devonian period

Time goes back to the middle and late Devonian period. At that time, the ocean was not only the world of fish, but also the warm home of tetracorals. Tetracorals are a subclass of the class Anthozoa. The calcareous hard skeleton of each of their individuals is composed of "partitions" arranged radially longitudinally. In the early stages of growth, four "primary septa" grow in sequence, namely the main septa, the opposite septa, and the left and right opposite septa. Then, on this basis, they repeatedly and densely grow first-level long septa and second-level short septa in multiples of 4, hence the name tetracorals.

Schematic diagram of the partition wall formation method

Four-ray coral

Like modern corals, most tetracorals are only adapted to shallow sea environments with a normal salinity of 3.3%, turbulent seawater, sufficient oxygen, clear water and plenty of sunshine. Some of them look like horns, some are cylindrical, and some are like branches. They grow fixedly like plants. Today's protagonist is an extremely specialized group of tetracorals - slipper corals. In my country, their fossils are mainly found in Yunnan, Sichuan, Guangxi, Guizhou and other southwestern regions. These discovery sites tacitly call slippers "slipper children".

Why is it called such a strange name?

Why is the slipper coral called such a strange name? You can tell from its strange appearance: they are small single corals, with a pointed beginning, a triangular outline, a semicircular cross section, a dorsal protrusion, the main septum located in the center of the back, arranged in a feather-like manner with the other septa, and a protruding antiseptum located in the center of the ventral surface, arranged parallel to the other septa, with a flat ventral surface, a deep calyx, and a calyx cover, just like a small slipper child.

This appearance makes their lifestyle very different from that of other corals. The flat ventral surface (i.e. the sole surface) allows them to lie flat in the quieter muddy soft bottom environment of the shallow sea. When they are free, they will open the calyx cover, extend their tentacles to stir the water to prey on microorganisms. Once they encounter enemies, they will quickly close the calyx cover to protect themselves. In addition, in order to avoid being buried alive by the mud and turbidity stirred up by the sea water, they will also move slowly with their tentacles, unlike other corals that live a fixed life.

So for a long time they were mistaken for some kind of bivalve or brachiopod until the end of the 19th century, when paleontologists identified them as corals.

After experiencing two booms in the early and middle Devonian period, the slipper coral gradually declined and became extinct. Therefore, it has become an important index fossil of the Devonian period. At the same time, their pointed tips often point in the direction of ocean currents, which also makes their fossils have finger-pointing significance, making them good finger-pointing fossils (that is, fossils that can clearly indicate the environment in which they lived).

The above is what we shared in this issue. See you next time if you are interested.