Flipping Famous Fossil Reveals Our First Vertebrate Ancestor, Scientists Say | CNN

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An extinct, ribbon-like sea creature the size of a human hand was one of the first animals to develop a precursor to a spine. Scientists recently identified the animal’s nerve cord using a reverse twist. They upset his fossils.

Paleontologist Charles Doolittle Wolcott first discovered Pikaia fossils in British Columbia’s Burgess Shale deposits, dating to 508 million years ago, and described them in a 1911 treatise. The animal was about 6.3 inches (16 centimeters) long and had a flattened, sinuous body and a small head, tipped with two tentacles and lined with external gills. These were originally thought to be rudimentary legs, which is why the animal was positioned with these structures facing downward.

In 2012, after decades of studying Pikaia’s 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 3D sausage-shaped structure running under the animal’s back as a dorsal organ, perhaps used for internal support, although such organ is anatomically different from anything seen in fossils or in living things. animals.

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

According to the researchers, previous anatomical interpretations positioned the animal upside down. The so-called dorsal organ was actually located in the belly and was the intestine of Pikaia. The suspected blood vessel was a 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 ascidians, have at some point in their lives a flexible, rod-shaped nerve structure called a notochord in their backs.

Pikaia was initially thought to be a worm, but was later upgraded to an early type of chordate, based on features such as the shape of certain muscles and the position of its anus. But experts weren’t sure where Pikaia belonged in the chordate family tree.

With the description of a nerve cord, Pikaia can now be considered part of the fundamental lineage of all chordates, even though it has no direct descendants living today, the study authors reported .

Reversing 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 in the established body position (and upside down).

In retrospect, the truth was “hidden in plain sight”, and the orientation reversal resolves questions about why Pikaia’s purported blood vessels and dorsal structure conflicted with established anatomical features of others agreements, Mallatt said.

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

The reassessment of the path forward for Pikaia 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. The lead author of the study, Giovanni Mussini, researcher and doctoral student in the Department of Earth Sciences at the University of Cambridge in the United Kingdom.

There were a number of reasons to revisit previous interpretations of the fossils, Mussini told CNN. On the one hand, there was the conundrum of what scientists thought was the position of the dorsal organ. Its location – near what was believed to be Pikaia’s back – apparently ruled out the possibility that the organ could be an intestine.

However, once Pikaia was turned over, the location and characteristics of the organ made more sense anatomically. It widened and extended into the animal’s pharynx, the area of ​​the throat where the intestine usually connects to the mouth. Its 3D status could be explained by the presence of chemically reactive tissues, characteristic of an intestine. In other Burgess Shale fossils, the abundant ions and reactive compounds typically found in intestinal tissues cause digestive structures to mineralize more quickly than the rest of the body, and thus retain more of their original shape. According to the study, the structures inside Pikaia’s organ were possibly remnants of swallowed food.

In an inverted Pikaia, the external gills that once pointed downward were now angled upward, just like the external gills of modern mudskippers and axolotls.

Pikaia’s flip also changed the orientation of muscle groups that come together in a wave formation. These muscles, called myomeres, are a key element in vertebrates. In Pikaia’s new position, the strongest flexion point for these muscles is along its back, which is also true for the arrangement of myomeres in other animals with a spine.

“This makes Pikaia’s movement consistent with what we see in modern agreements,” Mussini said.

Pikaia’s presumed blood vessel was also anatomically confusing, as it lacked the branches typically found in vertebrate blood vessels.

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

An important part of recognizing the structure as a nerve cord lies in the fossilized nervous systems of other animals from the Cambrian period (541 million to 485.4 million years ago) discovered over the past decade , added Mussini.

“We understand better how nerve cords and other tissues fossilize because we have been fortunate to find a number of preserved Cambrian nervous systems in other deposits,” he said, “primarily from of Chinese fossils discovered in recent years. a few years.”

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

Although no living analogues of Pikaia exist, data on fossil arthropods have provided scientists with a more detailed frame of reference for Pikaia’s nerve cord. Like other fossilized nerve tissues, Pikaia’s nerve cord was dark, carbon-rich, and relatively fragile compared to other fossilized tissues.

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

Much of Pikaia’s anatomy remains a mystery, but examining it from a new perspective could offer new insights into its puzzling set of features, Mussini said.

“A lot of these details have only come to light 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, as this is a work in progress.”

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