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, which 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, both from around 1991.

These ancient chordates had special features like segmented muscle blocks called myomeres and a flexible rod called a notochord, which are key traits of all chordates. Before these discoveries, many scientists thought chordates appeared much later in Earth’s history. But finding these early chordates changed our understanding of how life evolved, as noted by scientist Stephen Jay Gould. Their existence helped shape new ideas about evolution, ecology, and how living things develop.

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 and classified it as such. Later, in 1977, researchers Harry B. Whittington and Simon Conway Morris studied the fossils again and decided it was actually a chordate. They noticed features in its body that are typical of chordates, like a rod-like structure that may have been a notochord.

More chordates from the same time period were found later. Another one, Metaspriggina walcotti, was also discovered by Walcott but not examined until much later. In the 1990s, scientists found more fossils in China, including one called Yunnanozoon lividum. At first, they were unsure what kind of animal it was, but later studies suggested it might also have been a chordate. These discoveries help scientists understand what early chordates were like 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 similar to those found on snails. There were also small appendages near its mouth, but their purpose is still unknown. Its pharynx had six pairs of slits with tiny filaments that may have helped it breathe.

Scientists once thought a hollow tube running through its body was a notochord, a key feature of chordates. But in 2012, they changed their minds and said this tube was actually a dorsal organ, perhaps used for storage. They now believe the real notochord was a thin rod underneath 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 and its flat body suggest that Pikaia was an active swimmer, moving through the water in S-shaped curves like eels. However, its muscle arrangement also suggests it was not a fast swimmer.

Metaspriggina walcotti

Metaspriggina walcotti looked somewhat like a fish and measured up to 6 centimetres long and 1 centimetre wide. It had a notochord and seven pairs of pharyngeal bars, possibly made of cartilage. These bars were made of several separate pairs of bones. The first two pairs were larger and did not support gills, which hints at a distant connection to gnathostomatans (jawed vertebrates, from fish to humans). It did not have fins and had a weakly developed cranium, but it had two well-developed eyes facing upward, with nostrils behind them. Unlike Pikaia, its body segments (myomeres) were arranged in a W-shape and there were 40 of them.

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 members of the group that includes animals like 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 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 with important organs inside its body.