A fossil long considered the "weirdest" ever found and thought to be possibly an evolutionary dead end has finally been linked to a descendant species living on the Earth today, paleontologists say.
After almost 40 years of being baffled by where the odd spiny, walking worm should be placed in the history of life on Earth, researchers have linked the fossil so odd scientists in the 1970s dubbed it "Hallucigenia", to a present-day species of worms that inhabit the globe's tropical rainforests.
The odd fossil has puzzled scientists since it was discovered in the 1970s in the Burgess Shale formations in Canada's Rocky Mountains.
When first found, the fossil was so unexpectedly strange that scientists attempting to describe it got things backwards, identifying its head as the tail and believing legs on its bottom surface to be spines on its back.
However, a recent study focusing on the strange creature's spines has tagged Hallucigenia sparsa as an ancestor of today's velvet worms, bizarre, sluglike creatures with multiple legs similar to a centipede.
The spines were the "smoking gun" that confirms the link and "solves a long and heated debate in evolutionary biology," says Martin Smith from the University of Cambridge in England, noting layers of cuticle stacked one atop another in the fossil's spines is the same as seen in the claws of velvet worms.
For one part of a creature's anatomy to become a different part in a descendant species is a common evolutionary path, the researchers say.
The jaws or mandibles of arthropods including crustaceans, insects and spiders have also evolved from adapted legs, and the same process has created bird and insect wings, they say.
"It's often thought that modern animal groups arose fully formed during the Cambrian Explosion," says Smith, lead author of a paper published in Nature. "But evolution is a gradual process: today's complex anatomies emerged step by step, one feature at a time."
The period known as the Cambrian Explosion saw the appearance of most of the major groups of animals in the fossil record.
Looking at "in-between" or intermediate fossils such as Hallucigenia, which lived 500 million years ago, provides insights into how different groups of animals evolved their current body plans, he says.
With its seven pairs of stiff, spiny legs and soft tentacles in a row on its back, Hallucigenia was among the most common inhabitants of the Cambrian period, and fossils of the creature have been found in many parts of the world.
