Burgess Shale Fossil Provides Crucial Missing Link
March 14, 2013

Phallus-Shaped Creature Found In The Burgess Shale Fossil Beds

April Flowers for redOrbit.com - Your Universe Online

Located in Yoho National Park, Canada's Burgess Shale fossil beds have yielded yet another major scientific discovery. Scientists from the University of Toronto, the University of Cambridge, and the University of Montreal have unearthed a strange phallus-shaped creature from the 505 million year-old rock layers.

The study, published in a recent issue of Nature, confirms Spartobranchus tenuis as a member of the acorn worms group. Acorn worms are seldom-seen animals that currently thrive in the fine sands and mud of shallow and deeper waters as part of the hemichordates, a group of marine animals closely related to modern sea stars and sea urchins.

"Unlike animals with teeth and bones, these spaghetti-shaped creatures were soft-bodied, so the fossil record for them is extremely scarce," said Dr. Jean-Bernard Caron, associate professor of earth sciences and ecology & evolutionary biology at the University of Toronto and curator of invertebrate paleontology at the Royal Ontario Museum. "Our analysis of Spartobranchus tenuis, a creature previously unknown to science, pushes the fossil record of the enteropneusts back by 200 million years and fundamentally changes our understanding of evolution from this period."

Hemichordates were discovered in the 19th-century, and since then some of the biggest questions concerning their evolution has focused on the group's origins and the relationship between its two main branches: the enteropneusts and pterobranchs. Even though they share many genetic and developmental characteristics that reveal an otherwise unexpected close relationship, the two look very different.

"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," added Caron. "Our study suggests that primitive enteropneusts developed a tubular structure — the smoking gun — which has been retained over time in modern pterobranchs."

"It's astonishing how similar Spartobranchus tenuis fossils are to modern day acorn worms, except that they also formed fibrous tubes." The tubes provide a critical missing link that connects the two main hemichordate branches. "The explosive radiation of graptolites in the Paleozoic planktonic ecosystems is known only from the diversity of their tubes. 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" added Dr. Chris Cameron of the University of Montreal, a specialist on the taxonomy, evolution and biogeography of hemichordates.

Hemichordates, whose name roughly translates to "half a chordate," share many of the same characteristics as chordates — a group of animals that includes humans.

"Work from my lab has shown that enteropneusts filter feed using a pharynx perforated with gill slits, just like the invertebrate chordates" added Cameron. Small particles of matter at the bottom of the oceans were most likely the main food source for Spartobranchus tenuis.

"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," said Caron.

S. tenuis may have 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 study analysis reveals. Of the complete specimens examined, the longest were just less than 4 inches long with the proboscis accounting for approximately 0.2 inches. The study also suggests that tubes were used as dwelling structures, as a large proportion of the fossils were preserved inside of tubes, many of them branched.