Burgess Shale yields wormy ‘missing link’

U. TORONTO (CAN) — The discovery of a strange ancient worm sets the fossil record back 200 million years for a group of creatures known as enteropneusts.

“Unlike animals with teeth and bones, these spaghetti-shaped creatures were soft-bodied, so the fossil record for them is extremely scarce,” says 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.

Above: a colony of Rhabdopleura normani, a modern tubular pterobranch. Below: Spartobranchus tenuis from the Burgess Shale. (Credit: C.B. Cameron, JB Caron/Smithsonian Institution)

“Our analysis of Spartobranchus tenuis, a creature previously unknown to science, pushes the fossil record of the enteropneusts back by 200 million years and illuminates our understanding of the early evolution of this group of organisms,” Caron says, lead author of the study.

The study, published in the journal Nature, finds that Spartobranchus tenuis is a member of the acorn worms group. Acorn worms are marine animals that belong to the phylum hemichordates, a group that is closely related to today’s sea stars and sea urchins. While Spartobranchus tenuis is long extinct, other species of acorn worms thrive in the fine sands and mud of deep and shallow waters in present-day ecosystems.

Since the discovery of hemichordates 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,” says 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,” says 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 centimeters long with the proboscis accounting for about half a centimeter. A large proportion of these worms were preserved in tubes, of which some were branched, suggesting the tubes were used as a dwelling structure.

Researchers from the University of Cambridge and the Université de Montréal also contributed to the study.

Source: University of Toronto

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