Cambrian Acorn Worm Fills Key Gap in Evolutionary Understanding

Canada's 505 million year-old Burgess Shale fossil beds, located in Yoho National Park, have yielded yet another major scientific discovery – this time with the unearthing of a strange phallus-shaped creature.

A study to be published online in the journal Nature on March 13 confirms Spartobranchus tenuis is a member of the acorn worms group which are seldom-seen animals that thrive today in the fine sands and mud of shallow and deeper waters. Acorn worms are themselves part of the hemichordates, a group of marine animals closely related to today's sea stars and sea urchins.

"Unlike animals with teeth and bones, these spaghetti-shaped creatures were soft-bodied, so the fossil record for them is extremely scarce," said lead author Dr. Jean-Bernard Caron, associate professor of earth sciences and ecology & evolutionary biology at the University of Toronto and curator of invertebrate palaeontology at the Royal Ontario Museum. "Our analysis of Spartobranchus tenuis, a creature previously unknown to science, pushes the fossil record of the enteropneusts back by 200 million years and fundamentally changes our understanding of evolution from this period."

Since their discovery in the 19th-century, some of the biggest questions in hemichordate evolution have focused on the group's origins and the relationship between its two main branches: the enteropneusts and pterobranchs. Enteropneusts and pterobranchs look very different, yet share many genetic and developmental characteristics that reveal an otherwise unexpected close relationship.

"Spartobranchus tenuis represents a crucial missing link that serves not only to connect the two main hemichordate groups but helps to explain how an important evolutionary transformation was achieved," added Caron. "Our study suggests that primitive enteropneusts developed a tubular structure – the smoking gun – which has been retained over time in modern pterobranchs."

Hemichordates also share many of the same characteristics as chordates – a group of animals that includes humans – with the name hemichordate roughly translating to 'half a chordate.'

Spartobranchus tenuis probably fed on small particles of matter at the bottom of the oceans. "There are literally thousands of specimens at the Walcott Quarry in Yoho National Park, so it's possible Spartobranchus tenuis may have played an important role in recycling organic matter in the early Burgess Shale environment, similar to the ecological service provided by earth worms today on land," said Caron.

Detailed analysis suggests Spartobranchus tenuis had a flexible body consisting of a short proboscis, collar and narrow elongate trunk terminating in a bulbous structure, which may have served as an anchor. The largest complete specimens examined were 10 centimetres long with the proboscis accounting for about half a centimetre. A large proportion of these worms was preserved in tubes, of which some were branched, suggesting the tubes were used as a dwelling structure.