Paleontologists from the University of Toronto and the University of Cambridge, UK, have found that a weird creature from 505 million years ago, known as Hallucigenia due to its strange appearance, had a throat lined with needle-like teeth – a previously unidentified feature which could help connect the dots between it, modern velvet worms and arthropods.
Reconstruction of Hallucigenia sparsa. Image credit: Danielle Dufault.
Arthropods, velvet worms and water bears all belong to a large group of animals known as ecdysozoans. Though Hallucigenia is not the common ancestor of all ecdysozoans, it is a precursor to velvet worms.
“While we know that the animals in this group are united by the fact that they molt, we haven’t been able to find many physical characteristics that unite them,” said Dr Martin Smith of the University of Cambridge, lead author of the paper reporting the results in the journal Nature.
Finding a previously unknown throat arrangement in Hallucigenia helped Dr Smith and his colleague, Dr Jean-Bernard Caron, determine that velvet worms originally had the same configuration – but it was eventually lost through evolution.
“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. 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,” Dr Caron said.
Hallucigenia was 10 – 50 mm in length and lived 505 million years ago during the Cambrian Explosion, a period of rapid evolution when most major animal groups first appear in the fossil record.
Hallucigenia sparsa from the Burgess Shale. The fossil is 15 mm long. Image credit: Jean-Bernard Caron.
At first, the animal threw scientists for a bit of a loop. When it was identified in the 1970s, it was reconstructed both backwards and upside down: the spines along its back were originally thought to be legs, its legs were thought to be tentacles along its back, and its head was mistaken for its tail.
Right side up and right way round, Hallucigenia still looks pretty strange: it had pairs of lengthy spines along its back, seven pairs of legs ending in claws, and three pairs of tentacles along its neck.
More significantly, Hallucigenia’s unearthly appearance has made it difficult to link it to modern animal groups and to find its home in the Tree of Life.
A 2014 study by University of Cambridge paleontologists partially solved this problem by studying the structure of Hallucigenia’s claws, which helped definitively link it to modern velvet worms.
In the new study, Drs Smith and Caron used electron microscopy to examine fossils from the collections of the Royal Ontario Museum and the Smithsonian Institution.
“Prior to our study there was still some uncertainty as to which end of the animal represented the head, and which the tail. 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,” Dr Smith said.
Identifying this end as the tail led the team to revisit the fossils and dig away the sediment that was covering the head: the animals died as they were buried in a mudslide, and their floppy head often ended up pointing down into the mud.
“This let us get the new images of the head. 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,” Dr Caron said.
The new images show an elongated head with a pair of simple eyes, which sat above a mouth with a ring of teeth. In addition, Hallucigenia’s throat was lined with needle-shaped teeth.
The ring of teeth that surrounded the animal’s mouth probably helped to generate suction, flexing in and out, like a valve or a plunger, in order to suck its food into its throat.
The scientists speculate that the teeth in the throat worked like a ratchet, keeping food from slipping out of the mouth each time it took another suck at its food.
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Martin R. Smith & Jean-Bernard Caron. Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature, published online June 24, 2015; doi: 10.1038/nature14573
