The Upside-Down-and-Backwards History of Hallucigenia
The Cambrian period, which lasted from 541 to 485 million years ago, was the earliest phase of complex life on the planet Earth.  After a massive diversification of multicellular life in a relatively short time (commonly dubbed the “Cambrian explosion”), the Cambrian seas were packed with weird and wild life forms - many of which were so successful that their ancestors are still with us today.
One of the weirdest and wildest Cambrian animals was a creature called Hallucigenia. Â With a name like that, you can already tell that the story of its discovery and classification is a strange one.
Hallucigenia was discovered in 1977, in the famous Burgess Shale fossil formation of British Columbia. Â Numerous specimens were found within the shale, clearly illustrating an animal with a worm-like body with a round, featureless bulb at one end, a row of tentacles along one side, a cluster of smaller tentacles next to them, and twin rows of spines along the opposite side. Â However, the fossil was so unlike any living animal that nobody was sure which way it was supposed to be facing.
Paleontologist Simon Conway Morris saw the animal’s spines as walking legs, and interpreted the tentacles as food-collecting graspers that passed bits of food towards the round head.
However, some paleontologists disagreed, noting that the spines looked quite stiff, and would be of limited use as walking legs. Â A different interpretation of Hallucigenia was offered: one with twin rows of tentacle-like legs, and a spiny back for protection. Â (At this point, the paired nature of the tentacles was purely speculative; no fossil had been found preserving both sets.)
Eventually, some scientists began questioning whether the featureless bulb was even a “head” at all.  Perhaps the animal’s “tail” was in fact a worm-like head, and the “head” was a bulbous posterior.
The paleontological community was unable to satisfyingly answer these questions for almost 40 years, until new evidence - and new analytical technology - revealed the answer.
The blob often interpreted as a bulbous head was not, in fact, a part of the animal at all, but rather a fossilized blot of fluid produced during the decomposition process. Â In addition, microscopic analysis of the opposite end revealed a face - and what a face it was!
Hallucigenia sported a pair of beady eyes and a round mouth lined with dozens of rows of tiny teeth.  This shined a tremendous light on Hallucigenia’s place in the tree of life.  It is now believed to be an early member of Ecdysozoa - a group comprising numerous phylums of invertebrate life, including arthropods, nematodes, and tardigrades.  Its mouth closely resembles those of other primitive ecdysozoans, including those alive today.
In addition, microscopic analysis revealed that Hallucigenia’s tentacles (now conclusively determined to be legs) were tipped with twin claws.  The structure of these claws is unique, shared only by one group of animals - the onychophorans.
Onychophora is a small phylum comprised of about 180 species of velvet worms.  These strange and primitive ecdysozoans have a known fossil range of only 40 million years, but recent discoveries have classified Hallucigenia as a primitive onychophoran - extending their time on earth by some 500 million years, all the way back to the Cambrian.  These humble little panarthropods have had a long, strange trip, and it all began with one of the world’s most confusing fossil animals.
