“It was as if the fossil was grinning at us at the secrets it had been hiding,” said Dr. Martin Smith, but he remembered the time when a 500-million-year-old riddle all at once became clear. Hallucigenia was a highly enigmatic creature whose anatomy had been long a puzzle to pin. Its fossils were first identified more than a hundred years ago in the Burgess Shale of Canada, and so confusing that initially it was reconstructed inverted, with the spines taken to be legs and the legs taken to be tentacles. Scientists never determined the location and structure of its head even after adjusting the orientation of the head.
This was followed by meticulous preparation of fossils and high-resolution electron microscopy, which gave a clear picture of a spoon-shaped head with two small eyes and a curvy semi-circular shape which gave the animal a rather comical look. Under those eyes a mouth was bordered with a ring of teeth, then again there were other rows of teeth like a needle running round the neck. It is an odd arrangement that hints at Hallucigenia having used a suction-feeding behavior in which the teeth of the mouth served as a sort of valve to pressure food into the stomach and the teeth of the throat served as a ratchet to keep it from dropping out. This type of configuration, found in a few Cambrian animals, allows an important connection between Hallucigenia and modern velvet worms which, presumably, had this feeding apparatus in common among their ancestors and subsequently lost it throughout evolution.
No less significant was the explanation of a mistake that had been present long enough. One end of many specimens, which had been supposed to be the head of the animal, was a dark, rounded mass, which, as revealed by chemical examination, was a fossilized decay fluid, forced out of the gut when it was dead. This misunderstanding highlights the difficult nature of interpreting the fossil record of soft-bodied organisms, which may be distorted by decay processes to resemble an anatomical structure. The excellent preservation of the Burgess Shale a feature of Konservat-Lagerstatten deposits was critical here. The fine-grained sediments quickly encased organisms in low oxygen environments and stopped the decay process preserving details of external spines and microscopic tooth formations.
The electron microscopy was essential in the clarification of the facial structure of Hallucigenia. The researchers were able to identify the exact orientation of teeth and eyes and a mineralized structure (not the surrounding matrix) by scanning specimens of collections in the Royal Ontario Museum and the Smithsonian Institution. This type of imaging has revolutionized paleontology, enabling scientists to see minute structures in the sub-millimeter that would have been missed in other light microscopy methods, and to replicate feeding structures in extinct life with greater precision than ever before.
This anatomical discovery also contributes to more general evolutionary arguments. Hallucigenia is a lobopodian (worm) with soft legs that thrived during the Cambrian explosion – a time when the majority of large animal groups were first found. Comparison Lobopodians, such as Hallucigenia, are related to modern onychophorans, with their paired claws and non-jointed limbs, ectoderm-based arthropods and tardigrades, all members of the ecdysozoan supergroup. The find of a toothy mouth and throat in Hallucigenia reinforces the argument of an individual with a toothy mouth and throat and a pharynx that will be simplified or lost in numerous homodimetalline lineages.
Even the Burgess Shale itself is the focus of such evolutionary detective work. Its fossils, which are well preserved and with soft tissues, provide some of the rare insights into the ancient ecologies. The spines of Hallucigenia, such as these, probably acted as antipredatory signals to herbivores such as Anomalocaris, whereas its suction-feeding apparatus suggests a benthic life, taking nutrients out of the sediment in the water column or the biomass. Further general conservation at localities such as Chengjiang and Qingjiang deposits in China have added to the known diversity of lobopodians, showing that these animals were more diverse and widespread than previously believed.
Not only have scientists corrected an anatomical puzzle that has persisted in the century but they have also added to the knowledge of the evolution of complex feeding systems in early animals by untangling the real head structure of Hallucigenia. The smiling fossil, with its microscopic tooth set, is now a bright memory of the way the advanced technology can open the veil of the obscure history of the earth.
