The fish of prehistory

  • Name: Haikouichthys
  • Diet: Scavenger and Detritivore
  • Weight: 8 grams
  • Period: Cambrian 
  • Found In: China

In order to understand the great marine diversity that the planet earth currently possesses, it is necessary to first look back at the evolution of vertebrates, which leads us to make an obligatory stop at the Haikouichthys species.

This name comes from the Greek and could be translated as “Haikou fish”, making a clear reference to the place where its fossils were found, since this small prehistoric animal of only 2.5 cm long was found in China in 1999, specifically in a geological formation near the city of Haikou.

Experts consider that it inhabited this planet approximately 530 million years ago, in the Lower Cambrian period.

This period was also known as “The Cambrian explosion or evolutionary radiation of the Cambrian“, to which can be attributed the sudden appearance of multiple macroscopic and multicellular organisms, which allowed a very rapid increase in biodiversity.

Taxonomy and Classification of Haikouichthys

  • The specimen belonged to the Animalia Kingdom.
  • Within the Filo or Chordata Division.
  • Classified within the Vertebrata subdivision
  • It belonged to the superclass Agnatha.
  • She belonged to the order Myllokunmingiida
  • Within the family called Myllokunmingiidae.
  • Its genus was the Haikouichthys.

The exact classification of the Haikouichthys is that of skull or basal cord, that is to say, a category of the animal kingdom whose main characteristic is to present 5 important evolutionary novelties.

Among these qualities we could say that the most outstanding is that of a structure called notochord or dorsal cord, which is very similar to what we currently know as cartilage.

History of Haikouichthys

We could catalogue Haikouichthys as the first vertebrate fish; although it does not have all the necessary characteristics to be included in the group of fish.

Nevertheless, it has been presented on numerous occasions as the ancestor of all vertebrates. For example, it has already been presented in a documentary broadcast by the BBC.

In fact, it has been concluded that the Haikouichthys originated in the sea, not only because it possessed strong musculature against the tides, but also because the main sources of fresh water (rain, fog and snow) were scarce in the climate characteristic of the Cambrian period.

Structure and characteristics of the first vertebrate

The main characteristic of this fish is that it is an agnatus, that is, it has a bony skull but no jaw, so it was more of a prey than a predator.

But then, how could it become an essential piece for the evolution of vertebrates?

As you will see below, Haikouichthys’ anatomical structure allowed him to be a very fast and agile animal. Thanks to this characteristic, he was able to face the numerous predators that lived in the ocean at that time, such as slimy worms, sponges and large arthropods.

Another peculiarity of the Haikouichthys is that they had a series of W muscles called myotomes, which allowed them to have a greater impulse and better control of their movements, since these muscle fibers were attached to the bones located in their dorsal region.

Together with the formation of myotomes and a skeleton that supported them, we can differentiate their body structure in two regions: trunk and caudal. In other words, the head is clearly differentiated from the body.


This type of structure favours a better distribution of the organs for their protection (the bones begin to develop as a method of protection and not just as a method of support).

Contrary to what many may believe, the Haikouichthys had a bony and not a cartilaginous skeleton.

However, the anatomical rigidity observed in the Haikouichthys was not only due to the developed bone tissue, since in this sense the notocordia being formed by vacuolized cells also provided greater strength to the body.

Vacuolized cells are those with cellular organelles capable of retaining fluids.

The muscular structure it presented also required an adaptation, such as a more capable nervous development to be able to control its movements and perceive the environment around it better.

Here appears another of the great changes with respect to the invertebrates, and it is that a dorsal nervous cord is generated, which being a vertebrate animal was protected by the skull

Although it must be specified that this dorsal nerve cord is an element inherited from their protochordic ancestors.

The last great evolutionary change that it makes, and not only with respect to invertebrates and protochordic ancestors, is the development of gills, which as you know are the main respiratory organs of aquatic animals, which allow them to extract oxygen from the water and return carbon dioxide to the external environment.

Although the invertebrates already possessed these structures they did not obtain them in the same way as the Haikouichthys.

The proto-stringed animals had a pharyngeal basket that served both as a respiratory and digestive organ, but due to the nutritional and energetic needs of the Haikouichthys this structure had to be specialized.

This process of specialization that took place in the pharyngeal basket consisted mainly of the adaptive evolution that allowed the transformation of a cartilaginous pharyngeal arch into a gill arch.

The pharyngeal basket was formed by several cartilaginous rings also known as pharyngeal arches, in the case of the Haikouichthys, one of these cartilaginous pharyngeal arches was transformed into a gill arch, which is the primary structure on which the gills are based.

In this way, the pharyngeal basket could focus more on feeding and reinforcing the absorption of nutrients, which allowed Haikouichthys to have an anatomical structure that was perfectly in line with its nutritional demand.

This adaptive differentiation takes on greater importance beyond the existence of this prehistoric animal, since it was later observed in other species as the modification of the gill arches could give rise to pharyngeal jaws.

However, this fact, although of great importance, is part of the long evolutionary history that the biodiversity of our planet has suffered.

Therefore, if you want to know what came before and after Haikouichthys and how it originated, we suggest that you pay attention to the articles that we update and introduce every month referring to the evolution of vertebrates.