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Phallus-like animal is a missing link

modern-day enteropneust

Two Harrimania planktophilus, a modern enteropneust worm. This proboscis is to the left. The length of an uncoiled animal is about 32mm.(Source: C.B. Cameron, Universitde Montrl)

Fossilised forms of a phallus-shaped invertebrate have shed light on a dramatic spurt in Earth's biodiversity that occurred half a billion years ago, a new study says.

The claim is based on the fossilised remains of 10-centimetre, worm-like creatures found in shale beds in Yoho National Park in the Canadian Rocky Mountains.

The sediment in which the fossils were found dates back about 505 million years ago to the mid-Cambrian, a period when the number of species exploded and life forms became more complex.

The long-dead animal, named Spartobranchus tenuis, is a member of the acorn worms group , a seldom-seen marine animal that thrives in the mud and sand of shallow and deeper waters.

In a paper published today in Nature, senior author Dr Chris Cameron, a biologist at the University of Montreal, says the find reveals a crucial evolutionary link between two distinct living groups of animals: enteropneusts and pterobranchs.

Enteropneusts and pterobranchs are branches of the hemichordates phylum, a group of marine animals closely related to today's sea stars and sea urchins. However while enteropneusts are worms, pterobranchs are tiny animlas that live in tubes and have feeding tentacles.

Cameron says since the discovery of hemichordates in the 19th century, the inter-relationship of enteropneusts and pterobranchs has been problematic because scientists have failed to uncover any "transitional form" that might link the "anatomically disparate" relatives.

"Unlike animals with hard parts including teeth, scales and bones, these worms were soft-bodied, so their fossil record is extremely rare," Cameron says.

"Our description of S. tenuis, a creature previously unknown to science, pushes the fossil record of the enteropneusts back 200 million years to the Cambrian period, fundamentally changing our understanding of biodiversity from this period."

Previously, the oldest specimens found were from the later Triassic period (248 million to 205 million years ago).

Cameron says S. tenuis appears to be the missing common ancestor.

"It's astonishing how similar S. tenuis fossils are to modern-day acorn worms, except that they also formed fibrous tubes," he says.

It is the tubes, he adds, that provide a key missing link that connects the two hemichordate groups.

"Our findings sugges the tubes were lost in the lineage leading to modern-day enterpneusts, but ... retained to the present day in pterobranchs."

If the fossil evidence is right, S. tenuis was so plentiful that its feeding may have helped store carbon - a factor in the complex equations for global warming and ocean acidity - on the ocean floor.

Enteropneusts feed by filtering small particles of food through gill slits.

"It's possible Spartobranchus tenuis may have played an important role in moving carbon from the water column to the sediment in the early Burgess Shale environment," says Cameron.

Tags: animals, earth-sciences, evolution, palaeontology