Life › The Cambrian explosion › Burgess Shale (Canada)
Hallucigenia
c. 508 million years ago· Fossil or site· Animals
Well establishedStrong fossil, genetic or geological evidence.
A tiny Cambrian legged worm with spines on its back, famous for first being reconstructed upside down and back to front.
See it on the Life map →What happened
Hallucigenia sparsa, a few centimeters long, had a tube-shaped body, pairs of soft clawed legs below and seven pairs of stiff spines on its back. Walcott first called it a worm in 1911; Simon Conway Morris renamed it in 1977 for its dreamlike look and drew it walking on its spines. In 1991 a Chinese relative showed it was upside down, and in 2015 its head, with simple eyes and a toothed mouth, was found at the other end.
Why it matters
It belongs to the lobopodians, legged worms related to the ancestors of arthropods and velvet worms, and it is a famous lesson in how fossils get misread.
The evidence (fossils, genes, rocks)
Many Burgess Shale specimens, related species from Chengjiang, and electron-microscope studies in 2014–2015 linking its claws to those of living velvet worms.
Still debated
Exactly where it fits among early relatives of arthropods and velvet worms, and what it ate, are still debated.
Connections (2)
▸Burgess Shale (Canada) preserves HallucigeniaWell established
The connection
Hallucigenia was tiny and soft, with only its claws and spines hardened. The Burgess Shale preserved whole bodies, but often bent, squashed or partly decayed, which is why early researchers misread its top and bottom and its front and back.
When and how
Burgess Shale, British Columbia, about 508 million years ago; first described by Charles Walcott in 1911 as a worm, renamed Hallucigenia by Simon Conway Morris in 1977.
The evidence
Many complete specimens, plus 2015 electron-microscope work by Martin Smith and Jean-Bernard Caron that revealed its head, eyes and teeth.
Debates and open questions
Even with good fossils, details of its throat, gut and legs are hard to read from flattened films, and interpretations keep changing.
▸Hallucigenia is a distant cousin of TrilobitesStrong evidence
The connection
Hallucigenia belongs to the lobopodians, legged worms near the base of the group that includes arthropods and velvet worms. Trilobites are true arthropods with jointed legs and hard exoskeletons. Both descend from a common ancestor that molted its outer skin, so lobopodians hint at what arthropod ancestors looked like before legs became jointed.
When and how
Both lived in Cambrian seas around 520–508 million years ago; Hallucigenia at the Burgess Shale and Chengjiang, trilobites worldwide.
How sure are we?
Shared features such as claws, molting and leg structure, plus family-tree studies; a 2014 claw study placed Hallucigenia close to velvet worms.
Debates and open questions
Lobopodians are a mixed group, and exactly how they relate to each other and to true arthropods is still debated.