Trace the vertebrate family tree back far enough, and you'll find some interesting, um, members. Like the worm in the photograph above (bottom) called an enteropneust, or an acorn worm, it burrows into the sand and mud at the bottom of underwater environments. Solitary and highly mobile, it doesn't appear to have much in common with its close cousin the pterobranch, a much smaller marine worm that spends its life in stationary colonies, anchored to the sea floor by rigid tubes. Scientists have spent decades searching for the missing link between the two creatures, and now it seems they finally found it in the form of a 505-million-year-old fossil from Canada's Burgess Shale: Spartobranchus tenuis (pictured in the illustrated artist's conception). WhileS. tenuis moved independently around the sea floor like today's enteropneusts, it was capable of constructing a fibrous tube around its soft body, like the stationary pterobranchs, the researchers report online today in Nature. If S. tenuis is indeed the common ancestor of these two types of worms, it's likely that it's also a common ancestor of all vertebrates. So if your co-workers are wondering what you're looking at right now, just tell them it's a particularly embarrassing distant relative.