Hallucigenia -- a thumb-sized worm that went extinct over 400 million years ago -- left behind several bizarre fossils that have, for the longest time, left scientists scratching their heads. The fossils, which were first discovered in the early 1900s, show a creature about 5 millimeters to 35 millimeters long, with a row of rigid spines along its back and seven or eight pairs of legs ending in claws. 

Due to its bizarre appearance, scientists had a hard time classifying Hallucigenia and it was initially placed both backwards and upside down so that its head was mistaken for its tail, the spines were originally thought to have been legs, and its actual legs were believed to have been tentacles along its back.

New specimens unearthed from the Burgess Shale in Canada have finally provided scientists a complete picture of what this tiny sea creature looked like, including details of its face and mouth that have long remained elusive.  

“Prior to our study there was still some uncertainty as to which end of the animal represented the head, and which the tail,” Martin Smith, a postdoctoral researcher at the University of Cambridge and lead author of the paper, detailing the findings, published Wednesday in the journal Nature, said, in a statement, referring to the creature from which all modern velvet worms and arthropods, the group which contains modern insects, spiders and crustaceans, descended. “We previously thought that neither velvet worms nor their ancestors had teeth. But Hallucigenia tells us that actually, velvet worm ancestors had them, and living forms just lost their teeth over time.”

The new images, obtained by studying the fossils under an electron microscope, show an elongated head with a pair of simple eyes above a mouth with a ring of teeth. Additionally, Hallucigenia's throat was lined with needle-shaped teeth. 

"When we put the fossils in the electron microscope, we were initially hoping that we might find eyes, and were astonished when we also found the teeth smiling back at us," Jean-Bernard Caron, curator of invertebrate paleontology at the Royal Ontario Museum and an associate professor at the University of Toronto, said in the statement. “These teeth resemble those we see in many early moulting animals, suggesting that a tooth-lined throat was present in a common ancestor. ... Hallucigenia tells us that arthropods and velvet worms did ancestrally have round-the-mouth plates and down-the-throat teeth.”

The discovery is not only helping scientists fill the existing gaps in the evolutionary tree of life, but also helping them better understand the Cambrian period -- which marks a turning point in the history of life on Earth.

No matter what new details emerge, one thing remains abundantly clear -- Hallucigenia couldn’t have been more aptly named.