Why could trilobite become the first "overlord" of the Earth's biosphere?

Produced by: Science Popularization China

Author: Worm Words Worm Words

Producer: China Science Expo

The earth has existed for 4.6 billion years, but the first major development of the animal kingdom was in the Cambrian period 600 million years ago. The most common animals in the Cambrian period were trilobites, accounting for about 60% of the total number of preserved fossils. Therefore, some people call the Cambrian period the "Age of Trilobites."

(Photo source: veer photo gallery)

In animal taxonomy, trilobites belong to the phylum Vertebrata and the class Trilobites. They lived in the ancient oceans, mainly appearing in the Cambrian period, and reached their peak in the late Cambrian period. After that, trilobites declined from their peak and continued to decline until the end of the Permian period, and did not enter the Mesozoic era. Trilobites continued to reproduce throughout the long geological history of more than 300 million years in the Paleozoic era, and gave rise to numerous groups and huge numbers, totaling more than 1,500 genera and more than 10,000 species, of which about 500 genera were found in my country.

Most trilobites are oval or elliptical in shape, but their sizes vary greatly. Common trilobites are generally 3 to 10 cm long and 1 to 3 cm wide, and those over 20 cm are considered large.

Trilobite fossils are widely distributed around the world, so they are very important for dividing strata. But at the same time, many trilobite genera and species have local characteristics, so they are also important for dividing the sea areas at that time and restoring the biogeographical system at that time.

(Photo source: veer photo gallery)

Although trilobites lived a long time ago, scientists have a fairly comprehensive understanding of their morphology, structure and other characteristics. The main reasons are as follows: first, trilobites have a strong shell on their body surface and undergo multiple molting processes during their individual development, so the number of fossils they leave in the strata is greater than that of other organisms; second, there were few animals larger and more ferocious than trilobites living with them in the Cambrian oceans, so they were able to reproduce rapidly and be widely distributed; in addition, most trilobite fossils are preserved in fine-textured limestone or shale, so not only can the characteristics of the outer shell be observed clearly, but sometimes its internal structure can also be seen clearly.

The main characteristics of trilobites are reflected in the structure of their back shells. There is a protruding "head saddle" in the center of its head, which may be the place where the brain is placed. The surface of the head saddle is sometimes smooth and undecorated, sometimes dotted with tumor spots, and sometimes with varying numbers of transverse grooves. These transverse grooves are called "head saddle grooves". There are usually pairs of eyes on both sides of the head saddle. There is a groove along the front and back of the eyes, called the "face line", which is where the trilobite uses to shed its shell and drill out of the body during its growth. There is a pair of segmented antennae at the front end of the ventral side of the head, which are both organs of movement and sensory organs. Behind the antennae is the feeding mouth, which is usually covered with a "lip". There are many small and segmented organs of movement - appendages on both sides of the mouth. The appendages have fine cilia, which probably play a role in breathing.

(Photo source: veer photo gallery)

The thorax of trilobites is segmented, with as many as a dozen or as few as two. The segments are connected in a shingle-like manner (i.e., one tile is overlapped on top of another tile on a roof) to facilitate curling and movement. There are numerous segmented appendages on both sides of the ventral surface of trilobites, which have cilia, so these appendages are also used for movement and breathing. The tail of trilobites is the same as the thorax, which is divided into a central axis and the ribs on both sides in the longitudinal direction, and its shape is diverse; some of the edges of the tail are thorny, while others are not.

The life habits of trilobites are varied. The most common type of fossils are preserved in limestone or shale, which shows that most of them lived on the shallow seabed or swam on the mud. Some of them can swim a little, and some drift with the water. The tooth insects in the middle Silurian period were almost surrounded by dense long spines. These long spines were a powerful propeller for them to swim in the water, so it can be inferred that they were good swimmers; at the same time, these long spines were also effective weapons to resist natural enemies. This type of trilobite mainly appeared in the Ordovician to Devonian period. At that time, the nautilus, horseshoe crabs and fish that coexisted with it were all strong enemies of trilobites. If trilobites did not enhance their swimming ability and weapons to defend against enemies, how could they continue to survive and reproduce in that fiercely competitive environment?

Trilobites were the "kings" of the Cambrian oceans and survived on Earth for nearly 300 million years before becoming extinct. The study of trilobites has always been a hot topic in paleontological research. From the study of trilobites, we can learn more about our Earth.