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A ribbon-like marine creature about the size of a human hand was one of the earliest animals to develop a precursor to a backbone. Scientists have recently identified the animal’s nerve cord using the spin of a shellfish. They turned his fossils upside down.
Paleontologist Charles Doolittle Wolcott first discovered Pikaia fossils in the Burgess Shale deposits in British Columbia, dating back 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, oblong body and a tiny head, two tentacles with external gills on the edge. These were originally thought to be rudimentary legs, so the animal was positioned with these structures facing down.
In 2012, after many years of studying Pikaia fossils, researchers detailed its fossilized internal structures. 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 that is anatomically unlike anything seen in fossils or life. animals.
However, a recent analysis of Pikaia fossils by another team of scientists, published on June 11 in the journal Current Biology, has discarded this view and all other previous studies of Pikaia.
According to the researchers, earlier anatomical interpretations showed the wrong side of animals. The so-called dorsal organ was actually located in the gut and was the gut of Pikaia. The presumed blood vessel was a nerve cord, a feature belonging to the group of animals known as chordates, in the phylum Chordata.
All chordates, like vertebrates, lancelets like eels, and tunicates, or sea squirrels, have a flexible rod-shaped nerve structure called a notochord in their back at some point in their lives.
Pikaia was originally thought to be a worm, then upgraded to an early chordate, based on features such as certain muscle shapes and the location of its anus. But experts weren’t sure exactly where Pikaia belonged on the chordate genesis tree.
With the description of a nerve cord, Pikaia can now be considered part of the original lineage of all cords, although it has no direct descendants living today, the study authors reported.
Inverting 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 on Pikaia in 2013, working from the position of the established body (and upside down).
In retrospect, the truth was “hidden in plain sight,” and the reversal in orientation resolves questions about why Pikaia’s purported blood vessels and dorsal structure clashed with established anatomical features in other chordates, Mallatt said.
“All of a sudden Pikaia gets a lot weirder,” he said.
New orientation
Reassessing the way in which Pikaia arrived years ago, co-author of the new study, Dr Jakob Vinther, a lecturer in macroevolution at the University of Bristol in the UK, said lead study author Giovanni Mussini, researcher and a doctoral candidate in. Department of Earth Sciences at the University of Cambridge in the UK.
There were several reasons to revisit earlier interpretations of the fossils, Mussini told CNN. In one case, there was an enigma of what scientists believed was the location of the spinal organ. Due to its placement – apparently near Pikaia’s back – there was no chance that the organ could be inside him.
When Pikaia was turned upside down, however, the location and features of the organ made more anatomical sense. It expanded and expanded into the animal’s pharynx, the throat region where gut normally connects to mouth. Their 3D status could be explained by chemically reactive tissues — guttural hallmarks. In other Burgess Shale fossils, abundant ions and reactive compounds typically found in gut tissue cause the digestive structures to mineralize faster than the rest of the body, so they retain more of their original shapes. Structures inside Pikaia’s organ may be the remains of swallowed food, according to the study.
In inverted Pikaia, the previously downward-pointing external gills were now angled upwards, as are external gills in modern mudslides and stilts.
Pikaia flipping also changed the orientation of the muscle groups that come together in a wave formation. These muscles, called muscles, are a key feature in vertebrates. In Pikaia’s new position, the strongest point of flexion of these muscles is along its back, which is also true of the arrangement of myomeres in other animals with backbones.
“It makes the movement of Pikaia consistent with what we see in modern chordates,” Mussini said.
Find the nerve
The putative blood vessels of Pikaia were also highly anatomical, lacking the branches normally found in vertebrate blood vessels.
“It’s a single line that goes through most of the body up to the skull, where it expands into those two strands into the tentacles,” Mussini said.
The nervous systems in other animals from the Cambrian Period (541 million to 485.4 million years ago) discovered in the past decade were an important part of identifying the structure as a nerve cord, Mussini added.
“We have a better understanding of how nerve cords and other tissues fossilize because we have been fortunate to find quite a few Cambrian nervous systems preserved in other deposits,” he said, “primarily from the fossils of China emerged in the end. a few years.”
Many of these fossils were arthropods – invertebrates with fish skeletons – with living relatives such as insects, arachnids and crustaceans; comparison of the fossils with modern arthropods helped paleontologists identify preserved internal tissues. One example is a fossil specimen of the Cambrian arthropod Mollisonia, which showed brain organization comparable to living spiders, scorpions and horseshoe crabs, Mussini said.
Although there are no living analogues for Pikaia, the fossil arthropod data gave 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 to other fossilized tissues.
This nerve cord cements Pikaia’s status as a chordate, putting it “at the very bottom of what we would consider a traditional chordate,” Mallatt said.
Much about Pikaia’s anatomy remains a mystery, but his view of Pikaia’s anatomy could provide new insights into its amazing array of features, Mussini said.
“A lot of this data has 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 data to the conversation, 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 magazine.
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