Head of Hallucigenia Revealed
Hallucigenia sparsa is a 508 million year old animal from the mid-Cambrian, shortly after the evolution of multi-celled animals. It is a bizarre-looking creature, with spikes along its back and many legs. Since it was first described from fossil specimens taken from the Burgess shale, paleontologists have been unsure which end was its head.
Now an analysis of new Hallucigenia fossils reveals the “face” of the creature, with a ring of teeth and two simple eyes. The study authors are Martin R. Smith and Jean-Bernard Caron from the University of Toronto, Canada.
Hallucigenia is a stem group onychophoran, an early member of the group Ecdysozoa, which is ancestral to panarthropods (including arthropods, onychophorans, and tardigrades) and cycloneuralian worms (Nematoda, Nematomorpha, Priapulida, Loricifera and Kinorhyncha). This clade has in common the ability to molt.
The researchers used electron microscopy on the fossils to identify an elongated head with a pair of simple eyes (what you would expect from such an early multicelled animal). Further, it has a ring of teeth in its mouth, and another ring of teeth in its pharynx, and the front of its foregut. This is similar to oral and pharyngeal teeth found in tardigrades and cycloneuralian worms, but not modern onychophorans.
This new evidence suggests that these oral and pharyngeal teeth in all these groups are homologous, meaning they derive from a common Ecdysozoa ancestor, and they were subsequently lost in modern onychophora (including velvet worms).
It is amazing how much we can learn about these early creatures, part of the explosion of multi-cellular life in the Cambrian over 500 million years ago. This knowledge is largely due to the well-preserved fossils in the Burgess shale dating to that time, and the years of painstaking reconstruction by many paleontologists.