By Julianna Kettlewell
BBC News science reporter
A strange 525 million-year-old fossil creature is baffling scientists because it does not fit neatly into any existing animal groups.
The animal, from the early Cambrian Period, might have belonged to a now extinct mollusc-like phylum, academics from America and China say.
Other researchers have suggested the creature could represent an early annelid or arthropod.
Details are published in Proceedings of the Royal Society B.
The 5-10cm-long (2-4 inch) fossil, from Anning in China, had a flattened body and horizontal fins which, researchers think, could have been used to support it as it moved along the sea floor. It also had well developed senses, including a pair of eyes on stalks.
The trouble is the animal, named Vetustovermis planus, did not possess a set of features, or characters, which placed it clearly within any known group.
When it was first described in 1979, Vetustovermis was included in the annelid category. Later researchers argued against this classification, saying it was, in fact, either an arthropod or a mollusc.
According to the latest study, the weird creature seems closest to molluscs, primarily because it had a snail or slug-like flat foot. However, the researchers say, it does not sit happily in this group.
"Phyla are defined by an organism having a set of features called characters, and currently there are no animals that we know of which contain the set of characters that Vetustovermis has," co-author David Bottjer, of the University of Southern California, US, told the BBC News website.
"The phylum with which it shares the most characters is the Mollusca, but squeezing Vetustovermis into the mollusca is a somewhat messy job."
Vetustodermis planus does not fit comfortably within any known phylum
Since Vetustovermis requires some "pushing and pulling" to force it into any known phylum, Professor Bottjer and his colleagues are tempted to speculate it belonged to a different group entirely; one which flourished and faded within the Cambrian.
"We have always been intrigued by the many molluscan features of these fossils, but in the great menagerie of organisms that have inhabited Earth through life's long history, we may come to conclude that Vetustovermis indeed represents a new phylum," he said.
Jonathan Todd, a palaeontologist from the Natural History Museum, London, UK, is also mystified by the baffling animal.
"It is an intriguing beast," he told the BBC News website. "It is another strange thing from the Cambrian. It doesn't look much like an arthropod and I don't find its molluscan affinities particularly convincing."
However, Dr Todd is reluctant to create a whole new phylum to accommodate Vetustovermis; that, he thinks, would be premature.
"Some scientists have thought that there were so many distinct phyla in the Cambrian," he said. "They came to that conclusion because they were not thinking in the phylogenetic sense, they were thinking 'hey, that is a unique set of features - it must be a distinct phylum'."
So rather than creating new phyla every time something doesn't fit an existing one, the really interesting exercise, Dr Todd thinks, is to establish just how Vetustovermis slotted into the greater evolutionary tree.
If, indeed, it did belong to a different phylum, how did that group connect to the molluscs, annelids and arthropods?
"We don't really know the phylo-genetic relationships between the extant phyla," he said. "Molecular genetics has only gone so far. But recent phyla have got to connect somehow. These fossils really offer the opportunity to tie together recent phyla."