Hallucigenia sparsa doesn’t sound like an organism you’d be familiar with. In fact, even upon seeing this sea-dwelling creature, many people, scientists included, couldn’t tell its top from its bottom. This small, worm-like creature is a 500 million-year-old Cambrian Explosion fossil originally described in 1977 by Conway Morris, a paleontologist best known for his work in the Burgess Shale. Finally, decades later in 2014, scientists figured out which end was the head and on what side it stood. Then they started the process of discovering the worm’s descendants. As it turns out, this funny looking fossil isn’t really a worm at all, and neither are its closest relatives of today!

Hallucigenia sparsa from the Burgess Shale. Image by  Jean-Bernard Caron

Hallucigenia sparsa from the Burgess Shale. Image by Jean-Bernard Caron.

The history of this creature is just as strange as the worm itself. Upon original discovery by Charles Doolittle Walcott in 1911, it was classified in the same group as today’s common earthworms, which turned out to be quite incorrect. It was then shelved away until more specimens of this species could be found later. This “later” time turned out to be nearly 70 years later in 1977 when Morris rediscovered the worm in museum collections. He recognized that it belonged to its own unique group, but still had trouble identifying how it fit together anatomically . The jumble of parts and confusing nature of the anatomy actually gave the creature its name; coming from the word “hallucination.”  The bulbous shape seen at one end was originally thought to be the head. The long spikes were thought to be the legs, and the tentacles were thought to be on the back. Of course, once paleontologists gathered more and more of these tiny one-inch fossils (currently about 100 now exist), it was evident that the accepted picture was quite incorrect. In fact, another similar species was uncovered without the spikes, making it clear which end it walked on, and this helped clarify the situation with Hallucigenia. The spikes, unsurprisingly, belonged on the back, and the tentacles were the legs. What was thought to be the head was actually a gathering of fluids that had been literally squashed out of the organism upon being preserved or upon death.  Once this was sorted out, they had another problem; which end was the head?

Original concept of the Hallucigenia worm with spikes for legs and a large round head (right). On the left side of the worm was a tubular apparatus originally thought to be the tail. Image from Michel Vuijlsteke's weblog.

Original concept of the Hallucigenia worm with spikes for legs and a large round head (left). On the right side of the worm was a tubular apparatus originally thought to be the tail. Image from Michel Vuijlsteke’s weblog.

Thanks to the help of an electron microscope this question was solved! This microscope is capable of scanning fossils and the like through the rock without heavily damaging the sample like other methods might. Scientists also waited until several specimens were available before attempting this. Once under the scope, researchers were able to see a shocking number of teeth on one end of this animal along with a pair of basic eyes. The eyes are simple, only enough to tell light from dark. The teeth on the contrary are much more advanced. They are located in a couple places on the worm’s face. One ring is around the mouth and the other inside the neck (where the throat would have been). Here, there are several rows of the tiny teeth that scientists believe are responsible for aiding in digestion. Also found under the microscope were several pairs of smaller fore-tentacles most likely used for grasping food items or other objects. It was even hypothesized that these most-likely-weak legs were used to grip onto soft surfaces such as sea sponges in its underwater environment. Most importantly of all was the discovery of small retractable claws at the end of some of these feet. These claws were very similar to those of a few select small invertebrates today. These teeth and the tiny claws led the researchers to begin unraveling the mystery of the Hallucigenia family tree.

New concept of Hallucigenia. Tentacles are now placed underneath the creature and the head is the long tube where the eyes were found. Artwork by Danielle Dufault.

The new concept of Hallucigenia. Tentacles are now placed underneath the creature and the head is the long tube where the eyes were found. Artwork by Danielle Dufault.

The most recent research into this fossil, published in October 2014 in Nature magazine, actually suggests that this strange and spiky creature is most closely related to the velvet worm which, contrary to its name, is not a worm either. The velvet worm belongs to a strange group of animals called the panarthropods. Among these animals are Hallucigenia and all its subspecies as well as the velvet worm and the more famous tardigrade, or “water bear,” known for being able to survive in the vacuum of space. The research team, led by Martin Smith of Cambridge, Jean-Bernard Caron of the University of Toronto, as well as Javier Ortega-Hernández of Emmanuel College, figured out the key that gave away its relation to the velvet worms, which was the earlier-mentioned claws. These unusual claws are stacked on top of each other much like cups, and are shed when they become dull or broken. The only family of animals bearing such claws besides the velvet worm is the strange Hallucigenia fossils. The Cambrian worm could finally be linked together with some extant animal–but that’s not all. Upon further cladogram reconstruction, it was found that the velvet worm and its close relatives are actually more closely related to all arthropods than originally thought. This means that animals like Hallucigenia could actually be the ancestors of today’s spiders, insects, crustaceans, and more. Even more interesting is that spiders and other arthropods do not have mouth parts nearly as complicated as those of Hallucigenia. This suggests that the teeth evolved independently in the Hallucigenia line and then were lost in the arthropod line.

The discovery of this weird worm was overlooked decades ago, but once rediscovered in modern times, it became vastly more interesting. The odd shape and confusing anatomy of the Hallucigenia worm made it a favorite topic for many paleontologists and a challenge in itself. The struggle of recreating an accurate depiction of the worm was only completely solved with the help of modern technology like the electron scanning microscope. Without it, we might have never been able to unravel the mysteries of this worm. Its link to the velvet worm through claws and mouth parts also made it possible to connect two lineages never thought to have been related: the arthropods and the panarthropods. This common ancestor unlocks yet another answer to how animals came to be today and what they might have come from along their journey of evolution. Hopefully in the coming years more will be discovered about Hallucigenia and more ancestors of today’s strange animals.


Duheim-Ross A. After 50 years, scientists discover head of the insane Hallucigenia ‘worm.’ The Verge [Internet]. 2015 Jun 24; [21 paragraphs].

Pappas S. 500-Million-Year-Old ‘Smiling’ Worm Rears Its Head. LiveScience [Internet]. 2015 Jun 24; [16 paragraphs].

Pappas S. Weirdest Worm Ever? Clawed Creature Finds Its Family Tree. LiveScience [Internet]. 2015 Aug 18; [12 paragraphs].

Prehistoric Wildlife [Internet]. c2011-2015. Hallucigenia; [cited 2015 Aug 3]; [8 paragraphs].

Prostak S. Hallucigenia: Paleontologists Reconstruct Cambrian Worm-Like Creature. Sci-News [Internet]. 2015 Jun 24. ; [16 paragraphs].


Published On: August 24, 2015

Cienna Lyon

Cienna Lyon

Cienna Lyon is a student at Ithaca College in New York studying Biology and German and a volunteer science writer for the Paleontological Research Institute. She works in a lab with Te-Wen Lo on genetic research and at the Whalen Center for Music which are both on the Ithaca College campus. She spends her free time playing the french horn in several on campus musical ensembles or getting up to date with all the new paleontological discoveries as becoming a theropod paleontologist is her end goal.

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