The strange phallus-shaped creature which scientists say provides the crucial missing link to marine evolution Fossilised remains were unearthed in Canada's Burgess Shale fossil beds, which date back to the Cambrian Explosion 542million years ago Scientists say the worm is a common ancestor of two groups of marine invertebrates whose link was previously a mystery A curiously phallus-shaped prehistoric worm has provided a 'crucial missing link' to understanding the evolution of certain marine creatures, scientists say. Fossilised remains of the creature dubbed Spartobranchus tenuis were unearthed in Canada's Burgess Shale fossil beds, in the Yoho National Park. The area is one of the most important fossil deposits for understanding the early evolution of life during an era of increasing biodiversity on Earth known as the Cambrian Explosion, which started about 542million years ago. Phallic: Scientists say this newly described creature, dubbed Spartobranchus tenuis, is a common ancestor to two related groups of marine invertebrates whose evolutionary link was previously a mystery Researchers from the University of Montreal today published the first full description of the creature, found in an area thought to be about 505million years old, in the journal Nature The strange-looking invertebrate is thought to be a previously unknown ancestor of the acorn worm, or enteropneust, a seldom seen animal that thrives today in the fine sands and mud of shallow and deeper waters. 'Unlike animals with hard parts including teeth, scales and bones, these worms were soft-bodied, so their fossil record is extremely rare,' said study author Dr Chris Cameron. Our description of Spartobranchus tenuis, a creature previously unknown to science, pushes the fossil record of the enteropneusts back 200million years to the Cambrian period, fundamentally changing our understanding of biodiversity from this period. 'Crucial missing link': The strange-looking invertebrate is thought to be a previously unknown ancestor of the acorn worm, or enteropneust, and the sea stars and sea urchins, known as pterobranchs 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 10cm long with the proboscis accounting for about half a centimetre. It is thought they probably fed on small particles filtered from seawater. Enteropneusts, Spartobranchus's modern day descendants, are themselves part of the hemichordates, a group of marine animals closely related to today's sea stars and sea urchins, known as pterobranchs. 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 these two branches. Enteropneusts and pterobranchs look very different, yet share many genetic and developmental characteristics that reveal an otherwise unexpected close relationship. Modern day descendant: Enteropneusts (pictured) and pterobranchs look very different, yet share many genetic and developmental characteristics that reveal an otherwise unexpected close relationship The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 530million years ago, of most major animal phyla, as demonstrated in the fossil record. Before about 580million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80million years, the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today. It ended with the Cambrian-Ordovician extinction event, approximately 488million years ago. Professor Simon Conway Morris from the University of Cambridge, who also worked on the study, told the BBC: 'Enteropneusts look very different from pterobranchs in as much as the former are worms, whereas the latter are tiny animals, live in tubes, are colonial and have feeding tentacles.' The new insights into Spartobranchus tenuis has shown that it was a common ancestor of both groups, finally providing the missing evolutionary link between these two groups, the researchers say. 'One of the big punchlines from my graduate work, was molecular evidence that enteropneusts and pterobranchs are closely related,' added Dr Cameron, who specialises in the taxonomy, evolution and biogeography of hemichordates. 'It's astonishing how similar Spartobranchus tenuis fossils are to modern day acorn worms, except that they also formed fibrous tubes.' Ancient: The fossilised remains of Spartobranchus tenuis were unearthed in Canada's Burgess Shale fossil beds, which date back to the Cambrian Explosion 542million years ago A large proportion of these worms was preserved in tubes, of which some were branched, suggesting the tubes were used as a dwelling structure. 'The explosive radiation of graptolites in the Paleozoic planktonic ecosystems is known only from the diversity of their tubes,' said Dr Cameron. Our findings suggest that the tubes were lost in the lineage leading to modern day enteropneusts, but elaborated on in graptolites and retained to the present day in 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.'