What Is Hallucigenia? A Fossil Reconstructed Upside Down and Back to Front

The fossils show a worm-like body a few centimetres long, bearing seven or eight pairs of long rigid spines along one side and the same number of slender flexible legs along the other, each leg ending in a pair of small claws. A head sits at one end with simple eyes and a mouth ringed with teeth, and a throat lined with further teeth extends inward. The animal lived on the sea floor, walking on its legs with the spines projecting upward as a defence, which is the arrangement now accepted. It comes from the Burgess Shale and from comparable deposits elsewhere, which preserve soft-bodied animals in extraordinary detail and which document the period when most major animal groups first appear in the record.

The reconstruction changed three times and each change had a reason:

• •The original description in the 1970s placed the spines downward as stilts and interpreted the legs as tentacles on the back, since only one row was visible in the available specimens
• •The name was chosen to reflect how bizarre the result appeared, describing it as a dream-like apparition
• •Better specimens from China in the early 1990s showed two rows of legs rather than one, which required flipping the animal over
• •A dark blob at one end had been read as the head, and improved preparation showed it was decayed gut contents squeezed out of the body
• •The head was therefore at the other end, and specimens prepared in 2015 revealed eyes and teeth confirming it
• •Each correction followed from new material or better preparation rather than from reinterpretation of the same evidence

Placing the animal in the tree of life took as long as reconstructing it. It was initially treated as a member of no known group, part of a set of Cambrian animals described as evolutionary dead ends representing body plans that no longer exist, an interpretation popularised in a widely read book and subsequently revised. The current placement is within the lobopodians, a group of soft-bodied legged animals that sit near the origin of the arthropods and the velvet worms, and the claws on the legs are the feature that anchors that placement, since they match the claws of living velvet worms in construction. That makes the animal a relative of a living group rather than an isolated experiment, which is the general pattern for the Cambrian oddities once enough evidence accumulated, and it changed how that whole fauna is interpreted.

The Burgess Shale is worth describing because the animal is unintelligible without it. The deposit formed on a Cambrian sea floor at the base of a submarine cliff, where periodic slides of fine mud buried whole communities rapidly and in conditions with little oxygen, preserving soft tissue as dark carbon films on the rock. That preservation is exceptional, since the overwhelming majority of the fossil record consists of shells and bones, and it means the deposit records the animals that had no hard parts, which is most of them. Quarrying since its discovery in 1909 has produced tens of thousands of specimens representing a substantial community rather than isolated oddities. Comparable deposits elsewhere, particularly in China and Greenland, preserve the same interval and have supplied the better specimens that resolved several disputes, including this one.

The story is told frequently as an amusing series of mistakes and it is better read as a demonstration of the method. At each stage the reconstruction was the best available reading of the evidence then in hand, and the errors were corrected by new evidence rather than by anyone deciding differently. The specimens were genuinely difficult, being flattened films a few centimetres long in dark rock, and the preparation techniques that eventually resolved the head did not exist when the animal was first described. Publishing an interpretation that later proves wrong is how the process works, since a described specimen enters the literature where others can examine it, and the alternative of withholding until certainty is available would mean publishing nothing. The animal is now among the better-understood Cambrian forms precisely because so many people worked on it.

A few centimetres long with rigid spines along the back and clawed legs below, walking on the sea floor. It was described upside down because only one row of legs was visible, then flipped when better specimens appeared, then reversed when the supposed head turned out to be squeezed-out gut contents. The claws place it near the ancestry of arthropods and velvet worms.