Cambrian chordates

The Cambrian chordates are an extinct group of animals that belonged to the phylum Chordata. They lived during the Cambrian period, between 538 and 485 million years ago.

The first Cambrian chordate ever discovered is called Pikaia gracilens. It looked like a small lancelet. This fossil was found in the Burgess Shale in British Columbia, Canada.

When the fossil was first found in 1911 by Charles Doolittle Walcott, he thought it was a kind of worm called an annelid. But later, scientists realized it was actually a chordate. Since then, more fossils of possible chordates have been found in the Burgess Shale and in the Chengjiang biota of China.

These ancient chordates had special features like segmented muscle blocks called myomeres and a flexible rod called a notochord. These are key traits of all chordates. Their existence helped shape new ideas about how life evolved.

Discovery

The first known Cambrian chordate was Pikaia gracilens. It was found in the Burgess Shale in the mountains of British Columbia, Canada, by Charles Doolittle Walcott in 1911. At first, Walcott thought it looked like a worm. Later, in 1977, researchers Harry B. Whittington and Simon Conway Morris studied the fossils again and decided it was a chordate. They found features in its body that are typical of chordates.

More chordates from the same time were found later. Another one, Metaspriggina walcotti, was also discovered by Walcott. In the 1990s, scientists found more fossils in China, including one called Yunnanozoon lividum. These discoveries help scientists learn about early chordates and how they lived over 500 million years ago.

Burgess Shale chordates

Pikaia gracilens

Pikaia gracilens was an early chordate with a body shape similar to a modern lancelet. It did not have a well-defined head and was about 3.8 centimetres long. On each side of its head, it had large, tentacle-like structures. Its pharynx had six pairs of slits.

Scientists once thought a hollow tube in its body was a notochord, but later discovered it was a different organ. They believe the real notochord was a thin rod under this organ. Pikaia had body segments made of skeletal muscles called myomeres, which are also found in vertebrates. It did not have eyes.

The way its muscles were arranged suggests that Pikaia could swim through the water in S-shaped curves, but it was not a fast swimmer.

Metaspriggina walcotti

Metaspriggina walcotti looked somewhat like a fish and measured up to 6 centimetres long. It had a notochord and seven pairs of pharyngeal bars, possibly made of cartilage. The first two pairs of bars were larger. It did not have fins and had a weakly developed cranium, but it had two well-developed eyes with nostrils behind them. Unlike Pikaia, its body segments were arranged in a W-shape.

Maotianshan Shale chordates

Yunnanozoon lividum

Yunnanozoon lividum was a small animal, measuring between 2.5 to 4 centimetres long. Scientists think it might be one of the earliest animals related to groups that include humans. It looked similar to another animal called Pikaia. Special body parts like a heart or gills were not found in the fossils.

Myllokunmingia fengjiaoa

Myllokunmingia fengjiaoa was about 2.8 centimetres long and had a small head and body. It is one of the oldest known animals with a skull-like structure. It had a special rod-like structure along its back, and pockets near its head that might have been used for breathing. The body had segments arranged in a pattern.

Zhongjianichthys rostratus

Zhongjianichthys rostratus is thought to be one of the first fish. It had a long, eel-like body about 1.1 centimetres long and was covered with a thick outer layer. It did not have many body segments and likely stayed close to the bottom of the water.

Zhongxiniscus intermedius

Zhongxiniscus intermedius had a small, short body, about 10 millimetres long, and looked a bit like a fish. It had curved body segments and fins on its back and belly. Scientists think it might show features between some early fish-like animals and true fish.

Haikouichthys ercaicunensis

Haikouichthys ercaicunensis looked similar to Myllokunmingia, which was found in the same place. Its body was about 2.5 centimetres long and narrower. It had a clear head and tail, and parts that might have been used for breathing. It also had a long rod-like structure along its body and a fin on its back.

Marjum Formation

Nuucichthys

Main article: Nuucichthys

Nuucichthys was found in the Marjum Formation in the House Range in Utah. It was small, about 32.4 millimeters (1.28 inches) long and 7.9 millimeters (0.31 inches) tall. It had eyes with special lenses and a large space inside its body for important organs.