By Helen Briggs BBC News
11 January 2017
A strange animal that lived on the ocean floor 500 million years ago has been assigned to the tree of life, solving a long-held mystery.
The creature has eluded scientific classification since the first fossil was discovered 175 years ago.
The extinct hyolith has a cone-shaped shell, tentacles for feeding and appendages that acted as "feet".
It belongs to an invertebrate group that includes animals such as the horseshoe worm, say scientists.
Joseph Moysiuk, of the University of Toronto, made the discovery after analysing more than 1,500 specimens dug out of rocks in Canada and the US.
"Hyoliths are small cone-shaped sea dwelling animals. They are known from all around the world, mostly from fossils of their shells," he told BBC News.
"They appear in the fossil record about 530 million years ago and survived until about 250 million years ago.
"But the question of where hyoliths actually fit into the tree of life has been somewhat of a mystery for the last 175 years, since they were first described."
The research, published in the journal Nature, analysed soft tissue preserved in "very special fossils" from a site in Canada known as the Burgess Shale.
In the past, hyoliths have been interpreted as being related to molluscs, which are common today and include squid, clams and snails.
The new research suggests the animals are in fact more closely related to a different group of shell-bearing organisms, known as lophophorata, which includes brachipods (lamp shells), among others.