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An extinct ribbon-like marine creature about the size of a human hand was one of the first animals to develop the precursor of a spine. Scientists recently identified the animal’s nerve cord using an inverted twist. They turned their fossils upside down.
Paleontologist Charles Doolittle Wolcott first encountered Pikaia fossils dating back 508 million years in the Burgess Shale deposits in British Columbia and described them in a review dated 1911. The animal appeared to be roughly 6.3 inches (16 centimeters) long and had a flattened, curved body and a small head tipped with two tentacles and surrounded by external gills. It was originally thought that these were primitive legs, so the animal was positioned with these structures facing downwards.
In 2012, after decades of studying Pikaia fossils, researchers described Pikaia’s fossilized internal structures in great detail. They determined that a long string near the abdomen was a blood vessel and called the sausage-shaped 3-D structure running under the animal’s back a dorsal organ, possibly used for internal support; However, such an organ was anatomically unlike anything seen in fossils or living things. animals.
But a recent analysis of Pikaia fossils by another team of scientists, published June 11 in the journal Current Biology, overturned this view and all previous studies on Pikaia.
According to the researchers, previous anatomical interpretations had the animal positioned with the wrong side facing up. The so-called dorsal organ was actually located in its abdomen and was Pikaia’s intestine. The putative blood vessel is predicted to be a nerve cord, a feature associated with the group of animals known as chordates in the phylum Chordata.
All chordates, such as vertebrates, eel-like lancelets, and tunicates or sea squirts, have a flexible, rod-shaped nerve structure on their backs called a notochord at some point in their lives.
Pikaia was originally thought to be a worm, later upgraded to an early chordate species based on features such as the shapes of certain muscles and the location of the anus. But experts were uncertain about where exactly Pikaia belonged in the chordate family tree.
With the identification of the nerve cord, Pikaia can now be considered part of the basal lineage of all chordates, although it has no direct descendants alive today, the study authors reported.
Evolutionary biologist Dr. D., clinical professor at the University of Idaho. Jon Mallatt said reversing Pikaia “clarified everything so much”. Mallatt, who was not involved in the new research, published a paper in 2013 about Pikaia, working from a seated (and upside-down) body position.
Mallatt said that in retrospect the truth was “hiding in plain sight” and that the change in direction resolved questions about why Pikaia’s alleged blood vessel and dorsal structure clashed with established anatomical features in other chordates.
“Pikaia suddenly became a lot less weird,” he said.
new direction
Dr. D., a lecturer in macroevolution at the University of Bristol in the United Kingdom. Jakob Vinther, co-author of the new study that reevaluates which pathway for Pikaia emerged years ago. Jakob Vinther, lead author of the study, researcher and PhD candidate Giovanni Mussini, said: Department of Earth Sciences at the University of Cambridge in the UK.
Mussini told CNN there are many reasons to reconsider previous interpretations of the fossils. First, there was a mystery that scientists believed was the location of the dorsal organ. Pikaia’s supposed placement near its back apparently ruled out the possibility that the organ was an intestine.
However, when Pikaia was turned upside down, the organ’s location and features became more anatomically meaningful. It expanded and extended into the animal’s pharynx, the throat area where the intestine typically connects to the mouth. Its 3D state can be explained by the presence of chemically reactive tissues, which are distinctive features of the intestine. In other Burgess Shale fossils, abundant ions and reactive compounds, often found in intestinal tissue, cause the digestive structures to mineralize faster than the rest of the body, thus retaining more of their original shape. According to the research, the structures inside Pikaia’s organ were probably the remains of swallowed food.
In an inverted Pikaia, the external gills that previously pointed downward are now angled upward, just like the external gills in modern mudskippers and axolotls.
Turning Pikaia also changed the direction of the muscle groups coming together in a wave formation. These muscles, called myomeres, are an important feature in vertebrates. In Pikaia’s new position, the strongest flexion point of these muscles is located along the back. This also applies to the sequence of myomeres in other vertebrates.
“This makes Pikaia’s movement consistent with what we see in modern chordates,” Mussini said.
find the nerve
Pikaia’s putative blood vessel was also anatomically confusing, as it lacked the branches found in vertebrate blood vessels.
“This is a single line that runs through most of the body up to the head, where it bifurcates into two strands,” Mussini said.
Mussini added that a key part of understanding that the structure was a nerve cord was fossilized nervous systems in other animals from the Cambrian Period (541 million to 485.4 million years ago) discovered in the last decade.
“We have a better understanding of how nerve cords and other tissues fossilize because we have been fortunate enough to find a large number of Cambrian nervous systems preserved in other deposits,” he said, “mostly from Chinese fossils that have come to light over the last few years.”
Most of these fossils consisted of arthropods (invertebrates with exoskeletons) that had living relatives such as insects, spiders, and crustaceans; Comparing fossils with modern arthropods helped paleontologists identify preserved internal tissues. One example of this is the fossil specimen of the Cambrian arthropod Mollisonia, which shows brain organization comparable to that of living spiders, scorpions and horseshoe crabs, Mussini said.
While Pikaia has no living analogue, arthropod fossil data gave scientists a more detailed frame of reference for Pikaia’s nerve cord. Like other fossilized nerve tissues, the nerve cord in Pikaia was dark, carbon-rich, and relatively fragile compared to other fossilized tissues.
This nerve cord solidifies Pikaia’s status as a chordate, placing it “almost at the foundation of what we consider traditional chordates,” Mallatt said.
Mussini said much about Pikaia’s anatomy remains a mystery, but looking at it from a new angle could offer new information about its puzzling features.
“Most of these details have only come to light in the last 10 or 12 years,” Mussini added. “The authors of the 2012 paper could certainly be forgiven for not including these details in the conversation, because 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.
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