Scientists have spotted a previously unknown ring of teeth in one of the most strange looking fossils ever discovered, helping them distinguish its head from its tail.
The ancient animal was a worm-like creature called Hallucigenia that had spikes and a simple set of eyes, and walked the Earth during the Cambrian Explosion, the University of Cambridge reported. The findings are the first to identify the animal's head, and could help researchers pinpoint the common ancestor of creatures such as roundworms and giant lobsters.
Arthropods, velvet worms (onychophorans) and water bears (tardigrades) belong to the same group, dubbed ecdysozoans. Although Hallucigenia is not a common ancestor of all ecdysozoans, its teeth-lined throat could provide clues to the origins of the velvet worm.
"It turns out that the ancestors of [molting] animals were much more anatomically advanced than we ever could have imagined: ring-like, plate-bearing worms with an [armored] throat and a mouth surrounded by spines," said Jean-Bernard Caron, Curator of Invertebrate Palaeontology at the Royal Ontario Museum and Associate Professor in the Departments of Earth Sciences and Ecology & Evolutionary Biology at the University of Toronto. "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."
Past confusion over Hallucigenia's body structure made it difficult to link it to modern animal groups, but a study of its claws conducted in 2014 suggested it was related to the velvet worm. In this new study, Hallucigenia was definitively linked to the modern worm using electron microscopy.
"Prior to our study there was still some uncertainty as to which end of the animal represented the head, and which the tail," said Martin Smith, a postdoctoral researcher in Cambridge's Department of Earth Sciences, and the paper's lead author. "A large balloon-like orb at one end of the specimen was originally thought to be the head, but we can now demonstrate that this actually wasn't part of the body at all, but a dark stain representing decay fluids or gut contents that oozed out as the animal was flattened during burial."
The new model suggests an elongated head with a pair of simple eyes that were placed above a terrifying mouth with a ring of teeth. The teeth, which extended down the animal's throat, most likely were used to generate suction to move food into its body and keeping it from slipping back out.
"These teeth resemble those we see in many early [molting] animals, suggesting that a tooth-lined throat was present in a common ancestor," said Caron. "So where previously there was little reason to think that arthropod mouths had much in common with the mouths of animals such as penis worms, Hallucigenia tells us that arthropods and velvet worms did ancestrally have round-the-mouth plates and down-the-throat teeth - they just lost or simplified them later."
The findings were published in a recent edition of the journal Nature.
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