Flipping a famous fossil reveals our oldest vertebrate ancestor, scientists say

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An extinct armadillo sea creature about the size of a human hand was one of the first animals to develop a precursor to a backbone. Scientists recently identified the animal’s nerve cord using a twist at the top. They turned his fossils upside down.

Paleontologist Charles Doolittle Wolcott first encountered Pikaia fossils in the 508-million-year-old Burgess Shale deposits of British Columbia and described them in a 1911 treatise. The animal measured roughly 6.3 inches (16 centimeters) in length and had a flattened, wavy body and a tiny head ending in two tentacles and lined with external gills. These were originally thought to be rudimentary legs, so the animal was positioned with these structures pointing downwards.

In 2012, after decades of studying Pikaia fossils, scientists described its fossilized internal structures in great detail. They identified a long strand near the abdomen as a blood vessel and named a 3D sausage-shaped structure running under the animal’s back as a dorsal organ, possibly used for internal support, although such an organ was anatomically unlike anything seen in fossils or in living organisms. animals.

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

According to the researchers, earlier anatomical interpretations placed the animal wrong side up. The so-called dorsal organ was actually located in the abdomen and was the Pikai intestine. The presumed blood vessel was a nerve cord, a feature associated with a group of animals known as chordates in the phylum Chordata.

All chordates, such as vertebrates, spear eels, and tunicates or sea squirts, at some point in their lives have a flexible, rod-like nerve structure called the notochord on their backs.

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

With the description of the neural cord, Pikaia can now be considered part of the basic lineage of all chordates, although it has no direct descendants alive today, the study authors said.

The inversion of Pikaia “clarifies a lot of things,” said evolutionary biologist Dr. Jon Mallatt, clinical professor at the University of Idaho. Mallatt, who was not involved in the new research, published a paper in 2013 about Pikaia working from a fixed (and inverted) body position.

In hindsight, the truth was “hidden in plain sight,” and the reversal of orientation resolves questions about why the purported blood vessel and dorsal structure of Pikaia clashed with established anatomical features of other chordates, Mallatt said.

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

A reconsideration of the direction Pikaia took was created years ago by the co-author of a 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 PhD candidate in the Department of Earth Sciences at the University of Cambridge in the United Kingdom.

There are a number of reasons for reconsidering earlier interpretations of the fossils, Mussini told CNN. For one, there was the mystery of what scientists believed was the position of the spinal organ. Its location—near what was believed to be Pikachu’s back—seemed to rule out the possibility that the organ could be an intestine.

Once Pikaia was turned upside down, the placement and function of the organ made more anatomical sense. It spread and spread into the animal’s pharynx, the throat area where the intestine typically joins the mouth. Its 3D state could be explained by the presence of chemically reactive tissues – characteristic features of the intestine. In other Burgess Shale fossils, the abundant ions and reactive compounds typically found in intestinal tissue cause the digestive structures to mineralize faster than the rest of the body, thus retaining more of their original shapes. The structures inside Pikai’s organ were likely the remains of ingested food, according to the study.

In the inverted Pikaia, the outer gills, which used to point downwards, now slanted upwards, just like the outer gills of modern mudslides and axolotls.

Flipping the Pikaia also changed the orientation of the muscle groups that bunch up in a wave formation. These muscles, called myomeres, are a key feature of vertebrates. In Pikaia’s new position, the strongest point of flexion of these muscles is along its back, which also applies to the arrangement of myomeres in other animals with backbones.

“This makes the movement of Pikaia consistent with what we see in modern chordates,” Mussini said.

The presumed blood vessel of Pikaia was also anatomically puzzling because it lacked the branches usually found in vertebrate blood vessels.

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

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

“We can better understand how nerve cords and other tissues fossilize because we’ve been fortunate enough to find several Cambrian nervous systems preserved in other sites,” he said, “mostly from Chinese fossils that have come up recently.” 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 helped paleontologists identify preserved internal tissues. One example is a fossil specimen of the Cambrian arthropod Mollisonia, which showed a brain organization comparable to that of living spiders, scorpions and horseshoe crabs, Mussini said.

While there are no living analogues for Pikaia, data from fossil arthropods has provided scientists with a more detailed frame of reference for Pikaia’s nerve cord. Like other fossilized nervous tissue, the nerve cord in Pikaia was dark, carbon-rich, and relatively fragile compared to other fossilized tissues.

This neural cord cements the pikaia’s status as a chordate, putting it “very much at the base of what we would consider a traditional chordate,” Mallatt said.

Much of Pikaia’s anatomy remains a mystery, but looking at it from a new angle could offer new insights into its mysterious array of functions, Mussini said.

“Many 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 including these details in the interview, as it 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.