In a case of mistaken identity, a 505-million-year-old fossil from the Burgess Shale was identified as an ancient seaweed in the 1930s and given the name Margaretia dorus.
In a case of mistaken identity, a 505-million-year-old fossil from the Burgess Shale was identified as an ancient seaweed in the 1930s and given the name Margaretia dorus.
Some 80 years later, scientific sleuths with the Royal Ontario Museum and the universities of Toronto, Cambridge and Montréal now understand that this ancient ‘seaweed' is, in fact, the cylindrical, porous home of the equally ancient worm Oesia disjuncta.
The 2012 discovery of the Burgess Shale fossil site located high on a mountain overlooking Marble Canyon in Kootenay National Park made this new understanding of Oesia possible. It was at Marble Canyon that researchers found exceptionally preserved fossils that clearly showed Oesia in the tube.
“It is two fossils, really, that are being joined together here as a single,” said Karma Nanglu, a PhD student at the University of Toronto and lead author of the study re-describing Oesia based on these new fossils. “One is the housing and the other is the actual body fossil.”
The confusion that led Oesia's home to be described as a green algae seaweed is that the flattened tubes do indeed look like seaweed fronds. Burgess Shale creatures were buried by underwater mudslides that packed the creatures in silt while sweeping their bodies down into an oxygen-poor environment deep in the ocean. While the mud, along with the low oxygen, created the requirements to preserve soft-bodied animals, it also makes seeing a worm inside a tube extremely difficult.
The initial specimen that led to the misinterpretation does in fact have a worm present, but the worm is so badly decayed Nanglu said it appears only as a dark line of reflective carbon.
“It is hard to make a description based on that shape alone as it didn't really have the shape of a worm,” Nanglu said. “You are usually going to get just a view of the tube itself or just the worm, or like that original specimen, you might find a tube with a really badly decayed worm inside of it. So if the tube is decayed enough before fossilization that you can see the worm inside of it, the worm is going to be badly decayed as well.”
Many Marble Canyon fossils, however, fortuitously cracked along the axis of the tube, allowing the researchers to see worms inside the tubes.
“Some of the most critical specimens that we found at Marble Canyon, their value was immediately apparent,” Nanglu said. “You could pull it out of the rock, and even when it was dirty and dusty and you haven't had the chance to get it under a microscope, you could tell already it was a promising specimen. A lot of those specimens, when we pulled them out, we knew it was going to be an important specimen. As soon as we got them back to the museum, their worth was apparent,” said Nanglu.
And where Oesia is rare at the original Burgess Shale site, the Walcott Quarry, which is located in Yoho National Park, the worm is very abundant at the Marble Canyon site, which sits in Kootenay National Park.
“The big advantage of Marble Canyon, as far as the worm itself is concerned, is it is hyper-abundant, so we've observed over 1,000 of these animals at Marble Canyon, whereas before we had something on the order of 50 or 60,” said Nanglu.
Oesia was a filter feeder with primitive gill bars located along the length of its body. The worm, which grew to 12 centimetres, also had a grasping claw located at the end of its body. Oesia likely used the claw to move up and down its tube. The tube itself, reaching lengths of 50 cm, appears to have been made from collagen fibres the worm secreted and braided into structures, much like modern tube worms.
First described in 1911, Oesia has been moved over the years into different groups of animals, including annelids (segmented worms), tunicates (sea squirts) and chaetognaths (arrow worms), as researchers worked to understand this enigmatic creature. The Marble Canyon specimens, however, confirmed Oesia is a primitive hemichordate, making it an ancestor of everything with a spinal column, including humans.
As a hemichordate, Oesia is a forerunner of acorn worms and pterobranchs (corals and tube worms). And where pterobranchs continue to build tubes and still have a posterior grasping claw, they are generally much smaller in size than Oesia. They also developed tentacles to gather food.
Acorn worms, however, kept the appearance of Oesia and a similar Cambrian hemichordate worm, Spartobranchus tenuis, discovered in 2013. But unlike Oesia, acorn worms moved away from tube-building and filter feeding to instead live in the ocean sediment, drawing nutrition from the muck it consumes with its primitive mouth.
“From the perspective of evolution of this group (hemichordates), and also understanding evolution on a larger scale, there are the acorn worms and the pterobranchs; their evolutionary relationship is that of sister groups,” said Nanglu. “They are more closely related to each other than anything else. They don't look like each other at all, and if we didn't have embryonic information, development information, molecular information, it is hard to reconcile why these two would be so closely related. And what Oesia does, is it helps fill in the story not of how they might be related, but why that relationship makes sense.”
Along with providing greater understanding of the relationship between hemichordates, acorn worms and pterobranchs, the study also provides some insights into what the common ancestor of a group known as deutrosomes would have looked like. Deutrosomes includes hemichordates, chordates (animals with backbones) and echinoderms (starfish). This recent study, Nanglu said, helps to show that suspension feeding, also known as filter feeding, was important to hemichordates, chordates and echinoderms.
“So adding that third line of evidence is adding to a growing body of literature that suggests that suspension feeding was massively important for this group very early in evolutionary time,” he said.
In terms of evolution, Nanglu said he believes acorn worms stopped making tubes and instead moved into the ocean floor as part of an evolutionary “arms race” in a bid to outsmart predators.
The study has also helped researchers to better understand that hemichordates were likely more numerous during the Cambrian than once thought.
“Hemichordates in particular might be a more important component of Cambrian ecosystems than originally thought. (The study) alters their bio-geographical distribution by a huge amount and it also changes the scope in terms of how long they are present in evolutionary time at this point,” said Nanglu.
“In the last three to four years, two hemichordate forms have emerged, one of which, Spartobranchus, is quite abundant by raw numbers as well, but this other one, Oesia, makes up a major ecological component of the new Marble Canyon site as well,” said Nanglu.
“If you find other sites like Marble Canyon, you're getting the ability compare across sites and see whether or not trends in biodiversity are similar, the types of animals, and if they fulfill the same ecosystem functions - that is the story that begins to emerge that is really exciting for the future.”
The study, Cambrian suspension-feeding tubicolous hemichordates, was published in the journal BMC Biology.
