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An extinct ribbonlike sea creature about the size of a human hand was one of the earliest animals to evolve a precursor of a backbone. Scientists recently identified the animal’s nerve cord by using a topsy-turvy twist. They turned its fossils upside down.
Paleontologist Charles Doolittle Wolcott first encountered fossils of Pikaia in the Burgess Shale deposits of British Columbia, dating to 508 million years ago, and described them in a 1911 treatise. The animal measured roughly 6.3 inches (16 centimeters) long and had a flattened, sinuous body and a tiny head, tipped with two tentacles and fringed with external gills. These were originally thought to be rudimentary legs, so the animal was positioned with these structures facing downward.
In 2012, after decades of studying Pikaia fossils, researchers described its fossilized internal structures in great detail. They identified a long strand near the belly as a blood vessel and named a sausage-shaped 3D structure running below the animal’s back as a dorsal organ, possibly used for internal support, though such an organ was anatomically unlike anything seen in fossils or in living animals.
However, recent analysis of Pikaia fossils by another team of scientists, published June 11 in the journal Current Biology, has upended this view and all other earlier studies about Pikaia.
According to the researchers, earlier anatomical interpretations positioned the animal wrong side up. The so-called dorsal organ was actually located in the belly and was Pikaia’s gut. The presumed blood vessel was a dorsal nerve cord, a feature associated with the animal group known as chordates, in the phylum Chordata.
All chordates, such as vertebrates, eel-like lancelets, and tunicates, or sea squirts, at some point in their lives have a flexible, rod-shaped nerve structure called a notochord in their backs. A dorsal tubular nerve cord is also a feature in chordates.
Pikaia was initially thought to be a worm, then was later upgraded to an early type of chordate, based on features such as shapes of certain muscles and the position of its anus. But experts were uncertain about where exactly Pikaia belonged on the chordate family tree.
With the description of a dorsal nerve cord, Pikaia can now be considered part of the foundational lineage of all chordates, even though it has no direct descendants that are alive today, the study authors reported.
Inverting Pikaia “clarifies things a lot,” said evolutionary biologist Dr. Jon Mallatt, a clinical professor at the University of Idaho. Mallatt, who was not involved in the new research, published a paper on Pikaia in 2013, working from the established (and upside-down) body position.
In retrospect, the truth was “hiding in plain sight,” and the reversal in orientation resolves questions about why Pikaia’s purported blood vessel and dorsal structure clashed with established anatomical features in other chordates, Mallatt said.
“Pikaia’s suddenly become a lot less weird,” he said.
New orientation
Reevaluating which way was up for Pikaia originated years ago with a coauthor of the new study, Dr. Jakob Vinther, a lecturer in macroevolution at the University of Bristol in the United Kingdom, said lead study author Giovanni Mussini, a researcher and doctoral candidate in the department of Earth sciences at the University of Cambridge in the UK.
There were a number of reasons for revisiting earlier interpretations of the fossils, Mussini told CNN. For one, there was the enigma of what scientists had believed was the dorsal organ’s position. Its placement — near what was supposedly Pikaia’s back — seemingly ruled out the possibility that the organ could be a gut.
Once Pikaia was flipped upside down, however, the organ’s location and features made more sense anatomically. It broadened and extended into the animal’s pharynx, the throat region where a gut typically connects to a mouth. Its 3D status could be explained by the presence of chemically reactive tissues — hallmarks of a gut. In other Burgess Shale fossils, abundant ions and reactive compounds that are typically found in gut tissue cause digestive structures to mineralize more quickly than the rest of the body, and thereby retain more of their original shapes. Structures inside Pikaia’s organ were possibly remnants of swallowed food, according to the study.
In an inverted Pikaia, the external gills that formerly pointed down were now angled upward, as are external gills in modern mudskippers and axolotls.
Flipping Pikaia also changed the orientation of muscle groups that bunch together in a wave formation. These muscles, called myomeres, are a key feature in vertebrates. In Pikaia’s new position, the strongest flex point of these muscles is along its back, which is also true for the arrangement of myomeres in other animals with backbones.
“It makes Pikaia’s movement consistent with what we see in modern chordates,” Mussini said.
Finding the nerve
Pikaia’s presumed blood vessel was also anatomically puzzling, as it lacked the branches typically found in vertebrate blood vessels.
“It’s a single line going through most of the body up until the head, where it bifurcates into those two strands into the tentacles,” Mussini said.
An important part of recognizing the structure as a nerve cord was fossilized nervous systems in other animals from the Cambrian Period (541 million to 485.4 million years ago) that were discovered over the past decade, Mussini added.
“We have a better understanding of how nerve cords and other tissues fossilize because we’ve been lucky enough to find quite a few Cambrian nervous systems preserved in other deposits,” he said, “mostly from Chinese fossils that came to light in the last few years.”
Many of these fossils were arthropods — invertebrates with exoskeletons — with living relatives such as insects, arachnids and crustaceans; comparing the fossils with modern arthropods helped paleontologists to identify preserved internal tissues. One example is a fossil specimen of the Cambrian arthropod Mollisonia, which showed brain organization comparable with that of living spiders, scorpions and horseshoe crabs, Mussini said.
While there are no living analogues for Pikaia, the fossil arthropod data gave the scientists a more detailed frame of reference for Pikaia’s nerve cord. Like other fossilized nervous tissue, the nerve cord in Pikaia was dark, rich in carbon and relatively brittle compared with other fossilized tissues.
This dorsal nerve cord solidifies Pikaia’s status as a chordate, placing it “pretty much at the base of what we would consider traditional chordates,” Mallatt said.
Much about Pikaia’s anatomy remains a mystery, but looking at it from a new angle could offer fresh insights into its puzzling array of features, Mussini said.
“A lot of these details have come to light only in the last 10 or 12 years,” Mussini added. “The authors of the 2012 paper can certainly be forgiven for not bringing these details to the conversation, because it’s a work in progress.”
Clarification: This story has been updated to reflect that the dorsal nerve cord is a feature associated with chordates and the notochord is a feature in chordates that’s distinct from the dorsal tubular nerve cord.
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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