Trigonotarbida

Long ago, tiny creatures lived on Earth.

They looked a lot like spiders. But these animals did not make silk webs. They were hunters that ate other small bugs.

These creatures had hard shells on their backs. Some shells had sharp spines to keep them safe. Their bodies had many small plates. These plates helped protect their soft parts.

They could be very small or a few centimeters long. Some lived in places like Europe and North America. Scientists found many of them in old coal rocks. These tiny hunters are now gone forever.

Trigonotarbids were a group of extinct arachnids.

They lived a long time ago. Their fossils come from the Silurian to the Permian periods. We find them in Europe and North America. Some fossils even come from Argentina.

These animals looked like spiders. However, they did not have spinnerets to make silk. They were likely predators that hunted other small bugs. Their bodies had two main parts. The front part is called the prosoma. The back part is the opisthosoma, or abdomen.

The opisthosoma had hard plates on its back. These are called tergites. In many species, these plates were divided into three or five parts. Some species had spines to stay safe.

Trigonotarbids breathed air using book lungs. These are special parts used for breathing on land. They also had mouthparts called chelicerae. These fangs worked like a pocket knife. Scientists think they could only eat liquid food. They likely used hairs in their mouths to filter it.

Trigonotarbids were a group of extinct arachnids that lived a very long time ago.

Their fossils date from the late Silurian to the early Permian periods. These creatures lived in places like Europe and North America. Scientists even found a single record from Argentina. They were likely predators that hunted other small arthropods. Many species were heavily armoured with spines and tubercles to protect them.

They ranged in size from just a few millimetres to a few centimetres long.

These animals had bodies divided into two main parts. The front part is called the prosoma, and the back part is the opisthosoma. On the back of the opisthosoma, they had hard plates called tergites. These plates were often divided into three or five separate pieces. They also had book lungs to help them breathe air on land. These lungs were made of thin layers called lamellae. This helped them live a life out of the water.

Finding these fossils has been a long journey for science. In 1837, Dean William Buckland found the first one in England. He thought it was a beetle and gave it a different name. Later, in 1871, Henry Woodward correctly identified it as an arachnid. He named it Eophrynus prestvicii. In 1882, Ferdinand Karsch described fossils from Silesia. He named a new order Anthracomarti after the coal where they were found.

In 1884, Samuel Hubbard Scudder found the first North American species in Arkansas.

Scientists have worked hard to understand how they are related to living animals. For a long time, people confused them with harvestmen. In the 1980s, Bill Shear and his team studied fossils from New York. They found that trigonotarbids are closely related to a group called Tetrapulmonata. This group includes modern spiders, whip spiders, and whip scorpions.

They all share similar mouthparts and two pairs of book lungs. Some scientists even think they are related to strange living animals called ricinuleids.

Trigonotarbids might remind you of the spiders you see in your garden. However, they were different because they lacked spinnerets to make silk. Their mouthparts, called chelicerae, worked like a pocket knife. These fangs hung downwards, similar to some modern spiders. They likely used hairs inside their mouths to filter liquid food. This means they had to turn their prey into liquid before eating. It is a very different way of eating than many other animals.

Trigonotarbida is an extinct order of arachnids that lived during a vast stretch of Earth's history. Their fossil record spans from the late Silurian to the early Permian periods. These creatures were likely predators that hunted other small arthropods. While they looked similar to modern spiders, they were a distinct group. One major difference is that they lacked spinnerets, which are the organs spiders use to produce silk.

The anatomy of a trigonotarbid was divided into two primary sections. The front section is the prosoma, or cephalothorax, which bears the eyes and mouthparts. The rear section is the opisthosoma, or abdomen. A defining feature of the opisthosoma was its dorsal exoskeleton, made of plates called tergites. These tergites were characteristically divided into three or five separate plates. This division is a key way scientists distinguish them from other arachnids.

Trigonotarbid mouthparts, known as chelicerae, functioned like a pocket knife. They consisted of a basal segment and a sharp, curving fang. These fangs were paleognathic, meaning they were held parallel to each other. However, they hung downwards, similar to the mouthparts of modern araneomorph spiders. Scientists have found no evidence of venom glands in well-preserved fossils. Instead, they likely used a filtering system of hairs or platelets inside their mouths. This allowed them to consume prey that had been digested into a liquid form.

To breathe on land, these animals used a specialized respiratory system. They possessed two pairs of book lungs located on the ventral side of the opisthosoma. These lungs were composed of thin layers called lamellae, which are supported by structures called trabeculae. This setup was an adaptation for a terrestrial, air-breathing lifestyle. The opisthosoma itself consisted of 12 segments, though some segments were fused or reduced. The final two segments were often constricted into a tiny ring called a pygidium.

The history of discovering these animals involves many different scientists and locations. In 1837, Dean William Buckland described the first specimen from England. He initially mistook it for a beetle and named it *Curculoides prestvicii*. It was not until 1871 that Henry Woodward correctly identified it as an arachnid. He renamed the species *Eophrynus prestvicii*. In 1882, Ferdinand Karsch described fossils from Silesia and established the order Anthracomarti.

Research into their evolutionary relationships has changed significantly over time. Early studies often confused trigonotarbid fossils with harvestmen. In the 1980s, Bill Shear and his colleagues performed a cladistic analysis of fossils from New York. They discovered that trigonotarbid are closely related to the Tetrapulmonata group. This group includes modern spiders, whip spiders, and whip scorpions. All these animals share features like two pairs of book lungs and similar fangs.

Some scientists have even suggested a connection to a living group called ricinuleids. Both trigonotarbid and ricinuleids have segmented tergites and a specific way their body halves lock together. They also both possess a small claw at the tip of their pedipalps. While modern studies often place ricinuleids closer to mites and ticks, the relationship remains a topic of interest. As of 2020, scientists have identified approximately 70 valid species within the order Trigonotarbida. These fossils continue to help us map the complex history of arachnid life.