Hallucigenia: Characteristics, Habitat And Morphology

Hallucigenia is an extinct marine genus that inhabited the Earth approximately 500 million years ago. Its shape was similar to that of a small worm, but with 14 spines arranged in pairs on its back. In the belly it had seven pairs of tentacles that it used to move.

The vast majority of the fossil records of this animal come from a paleontological site located in Canada, the Burgess Shela. Although its discovery occurred at the beginning of the 20th century, some subsequent investigations were those that managed to elucidate certain unknowns about the enigmatic structures of its body.

Given the very particular characteristics of its body, initially there were proposals that suggested that it was a unique taxon that was already extinct, so it had no relationship with modern animals.

After this, some organs were identified that were tentatively related to the species of the phylum Lobopodia, which led to Hallucigenia being located within that taxonomic group.

Recently a team of specialists discovered a relevant link between Hallucigenia and modern worms, belonging to the superphylum Ecdysozoa. Both species share morphological structures (such as small claws), which suggests that these could be an evolutionary trace that hints at the origin of the Ecdysozoa group.


In the early 1900s the scientist Walcott found a fossil record in the Burgess Shale, in the Canadian mountains. It was about 30 millimeters long; He described it as a spiny worm and named it Canadia sparsa .

Later, in 1977, the paleontologist Conway-Morris reviewed this fossil again. He characterized it with seven pairs of spines, located on a body that had tubules on its back.

At one end, he observed a spot, which he identified as the animal’s head. The scientist changed the name of the species, calling it Hallucigenia.

This model was maintained until 1991, when a group of researchers discovered an error in the description made by Conway-Morris, since he had observed the fossil upside down. The spines were not on the belly but on the back of the animal and the tubes were actually the legs.

In 1992 the researcher Ramskold proposed the idea that the stain at one end could be some fluid product of the decomposition of the animal’s body.

It was not until 2014 when the animal’s head could be identified thanks to the use of the electron microscope. The eyes and a plate with the mouthparts stood out.


Hallucigenia was a tubular organism that was between 10 and 35 millimeters long. It had a small, elongated head with two eyes and an opening surrounded by radial teeth. In addition to these dental structures in his mouth, he also had pharyngeal teeth.

The head was located at a rounded end of the animal and extended towards the legs. The researchers suggest that this position made it easier for them to reach the food in the substrate where they were located.

On its back there are 14 rigid spines and the belly has 7 pairs of soft tentacles ending in a kind of strong nails. The caudal end ends in an open tube slightly curved downwards; there are three small pairs of tentacles.


There are different hypotheses related to the type of food that formed the diet of this animal. Some think that it fed on animal carrion; This is based on the fact that several Hallucigenia fossils were found together with the remains of larger animals.

On the other hand, they are also represented clinging to sponges. Their legs were too thin, long, and flimsy enough to walk long distances; Due to this, it is estimated that they held tightly with their claws to a sponge, in order to suck pieces and digest them.


The largest fossil deposit of this species is in the Burgess Shale, in Canada. There are also some fossil reservoirs in China.

Hallucigenia inhabited the shallow seabed. Due to the characteristics of its legs – which would imply a slow movement – it was possibly among the rocks frequently.

It lived during the evolutionary period known as the Cambrian outbreak. This natural event implied not only an evolution towards more complex living beings, but also a remarkable change in the nature of marine ecosystems.

Cambrian radiation occurred mainly in the huge ocean that made up the Earth in the Cambrian period. The large amount of nutrients and chemical conditions, as well as the presence of oxygen, favored the development of the species in this aquatic environment.

Increased oxygen

Thanks to photosynthesis carried out by algae and marine cyanobacteria, atmospheric oxygen reached levels suitable for the development of multicellular animals.

In addition to this, the rise in sea level brought as a consequence the flooding of the lowlands. In this way, shallow habitats were created with bottoms covered with calcareous and siliceous sediments, bacteria and algae.

These frotic areas and the continental shelves met the ideal conditions for the development of Hallucigenia.


The head was located at one end of the body, it was rounded and there were the eyes. This pair of sense organs lacked a complex structure, implying that perhaps they could only distinguish light and shadow.

Hallucigenia sparsa had a double dental structure. One of these was located in the mouth, it was circular and surrounded by numerous teeth.

In the area of ​​the neck (which could have been the throat) also had several rows of small and sharp teeth, oriented towards the intestine of the animal. This morphological characteristic probably had the function of preventing the food being returned to the mouth.

In this way, the teeth contributed to the digestive process, ensuring that food reached the intestine.

It is presumed that the teeth around the mouth were not used to chew food. Rather, it functioned as a suction valve, allowing the animal to ingest water and capture its prey.

Once in the mouth, the food was transported to a primitive intestine that ended in an anus, in the posterior region of the body.

Tentacles and spines

In the upper part of the trunk it had seven pairs of spines, and on the sides of the ventral area it had seven pairs of tentacles. The spines were made up of one or four ringed elements and were covered by tiny triangular-shaped scales.

These structures had plates at the base that make them inflexible. Due to this, it is estimated that they were used as defense organs against the attack of any predator that was in the area.

The ventral tentacles were thin and soft; each had a small retractable claw at its distal end. It is thought that these tubular appendages were used to move, for which they were helped with the claws.

The space between the spines and the legs does not show any significant variation. Those found in the spinal column are displaced forward, so that the pair of hind legs did not have a corresponding pair of spines on it.

In the anterior ventral area, in the upper part of the thorax, it had other pairs of tentacles. These were smaller and thinner than the legs, in addition to lacking claws. 

The Hallucigenia probably used them to grab food or other particles and put them in the mouth. It was also hypothesized that they served to fix her body to the soft surfaces where she lived.


  1. Smith, Martin (2011). Fossil Focus – Hallucigenia and the evolution of animal body plans. Palaeontology Online. Recovered from palaeontologyonline.com.
  2. Becky Ferreira (2015). Massive Spikes, Neck Tentacles, and Two Mouths: Hallucigenia, Everybody. Motherboard. Recovered from motherboard.vice.com
  3. Martin R. Smith, Javier Ortega-Hernández (2014). Hallucigenia’s onychophoran-like claws and the case for Tactopoda. Recovered from core.ac.uk.
  4. Burgess shale (2011). Hallucigenia sparsa. Royal Ontario Museum. Recovered from burgess-shale.rom.on.ca.
  5. Arielle Duhaume-Ross (2015). After 50 years, scientists discover head of the insane Hallucigenia ‘worm’. Recovered from theverge.com
  6. Stephanie Pappas (2015). 500-Million-Year-Old ‘Smiling’ Worm Rears Its Head. Lives cience. Recovered from livescience.com.
  7. Cienna Lyon (2015). Paleontology’s Strangest Fossil Finally Explained.The evolution institute. Recovered from evolution-institute.org.

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