Scientists say flipping over a famous fossil reveals our earliest vertebrate ancestor

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An extinct ribbon-shaped marine creature the size of a human hand was one of the first animals to develop a precursor to a spinal column. Scientists recently identified the animal’s nerve cord by twisting it upside down. They flipped its fossils over.

Paleontologist Charles Doolittle Wolcott first came across fossils of Pikaia in the Burgess Shale deposits of British Columbia, dated to be 508 million years old, and described them in a 1911 paper. The animal was about 16 centimeters long and had a flattened, coiled body and a tiny head lined with two tentacles at the tip and 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 the animal’s fossilized internal structures in great detail. They identified a long cord near the abdomen as a blood vessel and labeled a sausage-shaped three-dimensional structure below the animal’s back as a dorsal organ that may have served for internal support, although such an organ was anatomically unlike anything found in fossils or living animals.

However, a recent analysis of Pikaia fossils by another team of scientists, published June 11 in the journal Current Biology, has turned this view and all other previous studies on Pikaia on their head.

According to the researchers, previous anatomical interpretations placed the animal with the wrong side up. The so-called dorsal organ was actually in the abdomen and was Pikaia’s intestine. The suspected blood vessel was a nerve cord, a feature associated with the chordate group of animals in the phylum Chordates.

All chordates, such as vertebrates, eel-like lancelets and tunicates or sea squirts, have a flexible, rod-shaped nerve structure in their back called the notochord at some point in their lives.

Pikaia was initially thought to be a worm, but was later upgraded to an early chordate type based on characteristics such as the shape of certain muscles and the position of the anus. However, experts were unsure where exactly Pikaia should be placed in the chordate family tree.

With the description of a nerve cord, Pikaia can now be considered part of the original lineage of all chordates, even if there are no direct descendants still alive today, the study authors reported.

Pikaia’s reversal “clears up 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 2013 paper on Pikaia that assumed the established (and upside-down) body position.

In retrospect, the truth was “obviously” hidden, and the reversal of the orientation clarifies why Pikaia’s supposed blood vessel and back structure clashed with known anatomical features of other chordates, Mallatt said.

“Pikaia suddenly became a lot less strange,” he said.

The reassessment of which direction Pikaia should evolve began years ago with a co-author 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 student in the department of earth sciences at the University of Cambridge in the United Kingdom.

There are several reasons to rethink previous interpretations of the fossils, Mussini told CNN. First, there is the mystery of what scientists thought the dorsal organ’s location was. Its placement – near Pikaia’s supposed back – ruled out the possibility that the organ could be an intestine.

However, after Pikaia was turned upside down, the organ’s location and features made more anatomical sense. It widened and extended into the animal’s pharynx, the neck region where the intestines normally connect to the mouth. Its three-dimensional status could be explained by the presence of chemically reactive tissues – hallmarks of a gut. In other fossils from the Burgess Shale, abundant ions and reactive compounds typically found in gut tissue cause digestive structures to mineralize faster than the rest of the body, thereby retaining more of their original shape. Structures inside Pikaia’s organ may have been remnants of swallowed food, according to the study.

In an inverted Pikaia, the external gills, which previously pointed downward, were now directed upward, as is the case with the external gills of modern mudskippers and axolotls.

Turning Pikaia over also changed the orientation of the groups of muscles that cluster together in a wave-like pattern. These muscles, called myomeres, are a key feature of vertebrates. In Pikaia’s new position, the strongest flexure point of these muscles is along his back, which is also true of the arrangement of myomeres in other animals with backbones.

“This makes Pikaia’s movements consistent with those of modern chordates,” Mussini said.

Pikaia’s presumed blood vessel was also an anatomical mystery because it lacked the branches typical of vertebrate blood vessels.

“It’s a single line that runs through most of the body to the head, where it splits into the two strands that lead to the tentacles,” Mussini said.

An important reason for recognizing the structure as a nerve cord is the discovery in the last decade of fossilized nervous systems of other animals from the Cambrian period (541 to 485.4 million years ago), Mussini added.

“We now have a better understanding of how nerve cords and other tissues fossilize because we have been fortunate enough to find a number of Cambrian nervous systems in other deposits,” he said. “Most of these come from Chinese fossils that have come to light in recent years.”

Many of these fossils were arthropods – invertebrates with exoskeletons – with living relatives such as insects, arachnids and crustaceans. Comparing the fossils to modern arthropods helped paleontologists identify preserved internal tissues. One example is a fossil specimen of the Cambrian arthropod Mollisonia, whose brain structure is comparable to that of living spiders, scorpions and horseshoe crabs, Mussini said.

Although there are no living analogues to Pikaia, the fossil arthropod data provided scientists with a more detailed frame of reference for Pikaia’s nerve cord. Like other fossilized nerve tissue, Pikaia’s nerve cord was dark, carbon-rich, and relatively brittle compared to other fossilized tissues.

This nerve cord cements Pikaia’s status as a chordate and places it “pretty much at the base of what we would consider traditional chordates,” Mallatt said.

Much of Pikaia’s anatomy remains a mystery, but looking at it from a new angle could shed new light on the enigmatic diversity of its features, Mussini says.

“Many of these details have only come to light in the last ten or twelve years,” Mussini added. “The authors of the 2012 paper can certainly be forgiven for not bringing these details up for discussion, because this is an ongoing project.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works.