Haikouichthys ercaicunensis

Last updated: November 22, 2022
Verified by: AZ Animals Staff
© Talifero/Wikimedia Commons – License / Original

Haikouichthys was the first animal to develop a well-defined head


Haikouichthys Scientific Classification

Scientific Name
Haikouichthys ercaicunensis

Read our Complete Guide to Classification of Animals.

Haikouichthys Conservation Status

Haikouichthys Locations

Haikouichthys Locations

Haikouichthys Facts

Group Behavior
  • shoal
Fun Fact
Haikouichthys was the first animal to develop a well-defined head
Most Distinctive Feature
Haikouichthys had a fully-defined skull
Distinctive Feature
Haikouichthys had a primitive spinal cord
Shallow seas of Asia
  • Sociable
Favorite Food
Flesh of injured or dead creatures on the sea floor
Number Of Species

Haikouichthys Physical Characteristics

less than a ounce
1 inch

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The Haikouichthys is an extinct animal closely related to present-day hagfishes and lampreys. It was first identified in 1999 and is a member of the Myllokunmingiidae family, a group of prehistoric jawless fish known to have existed during the Cambrian period (518 million years ago). Well-preserved fossils of this animal are hard to come by because they had no bones. The Haikouichthys was a scavenger that probably fed on dead or injured aquatic creatures.

Description and Size

Haikouichthys 3D Illustration

A 3D illustration of Haikouichthys, based on fossil evidence of their ostensible appearance.

©Talifero/Wikimedia Commons – License

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Haikouichthys is an extinct genus of craniate animals that lived 518 million years ago during the Cambrian. Its fossil was found near Haikou at Ercaicun in China. The name of this creature is a reference to the location close to where it was found, and it translates as “Haikou fish from Ercaicun.”

Haikouichthys is often described as one of the earliest fish and an ancestor to modern-day fish species. This is because it was one of the first animals with a clearly-defined skull. It was a craniate, a relative of modern-day lamprey and hagfishes. Many scientists consider this fish as the earliest vertebrate with a primitive backbone.

It measures only around one inch (2.5 centimeters) in length. This is about the size of an average human thumb, making it one of the tiniest known creatures. Haikouichthys was a tiny, transparent fish with a jelly-like body. The small fish had a mouth and two eyes on its head.

The base of this creature’s head had about six to nine gills used for breathing. In addition to having a clearly defined head, Haikouichthys also had a segmented tail. Thirteen circular structures are located at the base of the Haikouichthys body. Some scientists think they may be gonads or slime organs. However, they could also be entirely different organs with unique functions. Haikouichthys had general cranial traits similar to adult lampreys and hagfishes which is why they are classified as craniates.

There was a sequence of W-shaped muscles, also known as myotomes, attached to the dorsal section of this animal, and they were necessary for controlling the animal’s many motor functions. They lacked the sophisticated neural system that vertebrates developed over time.

Evolution and History

Haikouichthys was one of the first animals to evolve a predefined skull. This is why it is often classified as the first true craniate, with relatives in the lamprey family. Because of this characteristic, paleontologists also consider it an ancestor to modern-day fishes. 

Haikouichthys was also one of the first organisms to develop a spinal cord. The primitive spinal cord of this animal is not as advanced as that of modern-day organisms. Still, scientists often classify it as a basal chordate. It’s difficult to conclusively put this animal in the chordate or craniate stem group.

Diet — What Did Haikouichthys Eat?

Haikouchthys was probably one the most advanced life forms that lived during the period of its existence. There’s evidence of a mouth, but it didn’t have a jaw structure strong enough to kill prey. This suggests that it was probably a scavenger lurking in the shadows to wait for an opportunity to feed on wounded or dead aquatic creatures in the Cambrian seas.

Because of their unusual anatomy—a shrunken head and no jaw—Haikouichthys was more prey than predator in the aquatic environment, feeding on the tiniest organisms that floated by. 

Habitat — When and Where It Lived

Haikouichthys existed 518 million years ago, at the height of the Cambrian Period, when multicellular organisms began to increase. Haikouichthys fossils have only been discovered in present-day China. It is hypothesized that they resided in the warmer, shallower coastal Chinese waters. Scientists speculate that Haikouichthys were sociable animals that swam in shoals. However, it is impossible to tell from the ancient, little fossils they have been able to retrieve. 

Haikouichthys — Threats and Predators

Due to the small size of the Haikouichthys, they probably faced threats from other apex predators in the ocean. Fortunately, there weren’t a lot of massive oceanic monsters shortly after the Cambrian explosion. Only a few, such as the Anomalocaris, existed. Swimming in shoals may have protected them from these predators.

Discoveries and Fossils — Where Haikouichthys Was Found

These fish-ancestor fossils are only found in the Yanshan region of China. In 1999, Chinese paleontologists Yu Liu and Hou Lianhai introduced the first Haikouichthys fossils. The name translates to “Haikou fish,” after Haikou, a place close to the location the first two specimens were discovered. 

The fossil was discovered in one of the several Lagerstätten sites in Chengjiang, China, where thousands of similarly well-preserved fossils of soft-bodied organisms have already been found. Additional fossils of Haikouichthys ercaicunensis from the Lower Cambrian have been discovered since the holotype’s discovery.

Only two fossils were shown at the original presentation; since then, however, more than 500 have been unearthed and studied. Its compact body and few internal organs were arranged along its main axis like beads on a string, making it incredibly simple compared to the many fish known today. Skeletal structure was predominantly cartilage rather than bone. 

Extinction — When Did Haikouichthys Die Out?

Haikouichthys might not have gone extinct if the proposed role they played in the development of life on Earth was right. They probably underwent a variety of transformations over millions of years, eventually giving rise to some aquatic animals seen today. Although the specific period of the Haikouichthys extinction is not specified, the oldest fossils date back to about 500 million years ago. 

Similar Animals to the Haikouichthys

Similar animals to the Haikouichthys include:

  • Meyllokunmingia — This is a genus of basal chordate similar to the Haikouichthys. Myllokunmingia fengijaoa, the only species in the genus, is among the oldest possible craniate. The genus has also been reported to be synonymous with the Haikouichthys.
  • AnomalocarisAnomalocaris looked like a shrimp-jellyfish hybrid measuring three feet in length. It lived about 500 million years ago during the Cambrian Period. The name Anomalocaris translates to “unusual shrimp.” It had a segmented trunk, at least eleven gill-bearing lateral swimming flaps, and a tail fan made up of three pairs of elongated fins.  
  • CephalaspisThe prehistoric scavenger fish of the Early Devonian was about the size of a trout, and its name means “head shield.” This creature may be a direct descendant of the Haikouichthys. It had the ability to vacuum up its prey from bottom sediments.  
  • DunkleosteusDunkleosteus (meaning “Dunkle’s bone”) was a huge placoderm that existed in the late Devonian Period, around 382 to 358 million years ago. Being the largest predator of the Devonian, it reached a length of 33 feet (10 meters).  

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Haikouichthys FAQs (Frequently Asked Questions) 

Was Haikouichthys the first fish?

Yes, Haikouichthys was the first fish. Although not a fish in the strictest sense, it was the first aquatic animal to have a clearly-defined head and body.  


When was Haikouichthys found?

Haikouichthys was discovered for the first time in China’s Qiongzhusi Formation, where it was identified as a member of the Chengjiang fauna. The species, named Haikouichthys ercaicunensis, was officially recognized in 1999.


Is Haikouichthys the first vertebrate?

Not only was the Haikouichthys the first fish ever discovered, but it was also the oldest animal ever found with a primitive backbone. The first vertebrates were publicly shown to the public in 1999. Two fish fossils were found, one of which was identified as Haikouichthys


How big was the Haikouichthys? 

Haikouichthys was a relatively small marine animal. The entire length of this fish-like creature was about one inch, meaning it was one of the smallest creatures ever found.


When was Haikouichthys alive? 

Haikouichthys lived during the Cambrian Period, about 513 million years ago. It was one of the creatures that evolved shortly after the Cambrian explosion

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  1. Thought Co., Available here: https://www.thoughtco.com/overview-of-haikouichthys-1093670
  2. Wikipedia, Available here: https://en.wikipedia.org/wiki/Haikouichthys
  3. Prehistoric World and Monsters Wiki, Available here: https://prehistoric-world-and-monsters.fandom.com/wiki/Haikouichthys#:~:text=during%20it's%20time.-,Food,bite%20on%20anything%20at%20all.

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