Hallucigenia
Most fossils preserve the hard parts of an animal’s anatomy: shells, bones, carapaces, ossicles, spicules, and the like. The rest of their biomass, the soft parts, is seldom preserved. But not never preserved. A handful of localities feature fossils with soft tissue preservation thanks to unusual depositional conditions that inhibited decay and scavenging, as well as a not insubstantial amount of luck. These fossil lagerstätten (from the German for “storage places”) are rare glimpses into the full breadth of ancient ecosystems. Not only do they show the soft parts of otherwise hard-shelled organisms, such as the legs, gills, and antennae of trilobites, but they also record soft-bodied animals that are completely missing at other, more typical localities.
One of the most well-known lagerstätten is the Burgess Shale, a middle Cambrian locality high in the Canadian Rockies of British Columbia. Roughly 508 million years ago, the Burgess Shale was deposited along the base of the Cathedral Escarpment, a prominent underwater cliff of older (but still Cambrian) limestone. Unlucky animals were entombed in sediments flowing down from this cliff and preserved in exquisite detail. While the precise geochemical mechanism for this preservation is still unclear and subject to debate, the results are unquestionable: dozens upon dozens of species with soft parts preserved, ranging from well-known creatures, such as trilobites, to mysterious and exotic forms that have defied categorization.
One iconic member of the Burgess Shale fauna is known for both its unique form and memorable name: Hallucigenia. Only a few centimeters long, this perplexing animal had a tube-like body with pairs of prominent spines on one side and tentacles on the other. Originally considered a polychaete worm, it was later reevaluated by the paleontologist Simon Conway Morris as an animal with uncertain affinities, which he dubbed Hallucigenia for its “bizarre and dream-like appearance”. As its spines were disproportionately large relative to the overall body size, Conway Morris interpreted them as legs. Many older reconstructions show Hallucigenia teetering around on the spines, with noodly tentacles sticking upward. The head end of the animal was also uncertain, due to the lack of clear anatomical features.
Thanks to a growing collection of specimens as well as discoveries at other localities, researchers now interpret Hallucigenia as a lobopodian. This group of legged soft-bodied animals includes the ancestors of modern velvet worms (onycophorans; found in tropical terrestrial environments in the Southern Hemisphere) and is the sister group to the arthropods. Whereas arthropods are characterized by their hard jointed legs, lobopodians have soft lobe-like legs (lobopods, which give the group its name). Seen through this lens, the anatomy of Hallucigenia instantly becomes more clear: the “tentacles” were actually pairs of ventral bendy legs and the spines were dorsal, well, spines. And thanks to specimens that preserve eyes and orifices, the question of the head end has also been resolved. Thus, Hallucigenia is now somewhat less mysterious than before, though it will forever remain a bizarre little beast.