An extinct ribbon-like sea creature 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 by using a reverse twist. They turned the fossils upside down.
Paleontologist Charles Doolittle Wolcott first encountered fossils of Pikaia in the Burgess Shale deposits of British Columbia, dating to 508 million years ago, and described them in a 1911 paper. The animal was about 16 centimeters long and had a flattened , wavy body and a small head, provided with 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 research on Pikaia fossils, researchers described its fossilized internal structures in detail. They identified a long cord near the abdomen as a blood vessel and named a sausage-shaped 3D structure running under the animal's back as a dorsal organ, possibly used for internal support, although such an organ was anatomically different from anything else seen in fossils or in living things. animals.
However, 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 research on Pikaia on its head.
According to the researchers, previous anatomical interpretations positioned the animal with the wrong side up. The so-called dorsal organ was actually located in the abdomen and was the intestine of Pikaia. The supposed blood vessel was a nerve cord, a feature associated with the animal group known as Chordates, in the phylum Chordata.
All chordate animals, such as vertebrates, eel-like lancets and tunicates, or sea squirts, have at some point in their lives a flexible, rod-shaped nerve structure in their back called a notochord.
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 the anus. But experts weren't sure where exactly Pikaia belonged in the Chordate family tree.
With the description of a nerve cord, Pikaia can now be considered part of the basic lineage of all chordates, even though it has no direct descendants alive today, the study authors reported.
Reversing Pikaia "clarifies things a lot," says 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 from the established (and upside-down) body position.
In retrospect, the truth was "hidden in plain sight," and the reversal of orientation resolves questions about why Pikaia's putative blood vessel and dorsal structure collided with established anatomical features in other chordates, Mallatt said.
"Pikaia suddenly became a lot less weird," he said.
New orientation
Reevaluating the future 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 lead study author Giovanni Mussini, a researcher and PhD candidate in the Department of Earth Sciences at the University of Cambridge in Britain.
There were a number of reasons to revise previous interpretations of the fossils, Mussini told CNN. First, there was the mystery of what scientists thought was the position of the dorsal organ. Its placement - near what was supposedly Pikaia's back - seemingly ruled out the possibility that the organ could be an intestine.
Once 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 throat area where an intestine normally connects to a mouth. The 3D status can be explained by the presence of chemically reactive tissues - hallmarks of the intestines. In other Burgess Shale fossils, abundant ions and reactive compounds typically found in intestinal tissue cause digestive structures to mineralize faster than the rest of the body, thereby retaining more of their original shapes. According to the study, the structures in Pikaia's organ may have been remnants of swallowed food.
In an inverted Pikaia, the external gills that previously pointed downward now pointed upward, much like the external gills of modern mudskippers and axolotls.
Flipping Pikaia also changed the orientation of muscle groups that come together in a wave formation. These muscles, called myomeres, are an important feature of vertebrates. In Pikaia's new position, the strongest flexion point of these muscles is along the 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 chords," Mussini said.
Finding the nerve
Pikaia's supposed 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 up to the head, where it splits into the two cords and the tentacles," Mussini said.
A key part of recognizing the structure as a nerve cord were fossilized nervous systems in other animals from the Cambrian period (541 million to 485.4 million years ago) discovered in the past decade, Mussini added.
"We have a better understanding of how nerve cords and other tissues became fossils because we've been fortunate enough to find quite a few Cambrian nervous systems preserved in other deposits," he said, "mainly from Chinese fossils that have come to light in the last few 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, which showed brain organization similar to that of living spiders, scorpions and horseshoe crabs, Mussini said.
Although no living analogues for Pikaia exist, the fossil arthropod data gave scientists a more detailed frame of reference for Pikaia's nerve cord. Like other fossilized nerve tissue, the nerve cord in Pikaia was dark, rich in carbon, and relatively brittle compared to other fossilized tissues.
This nerve cord solidifies Pikaia's status as a chord type and puts it "pretty much at the root of what we would consider traditional chords," Mallatt said.
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.Much about Pikaia's anatomy remains a mystery, but looking at it from a new angle could provide new insights into the enigmatic set of features, Mussini said.
"Many of these details have only come to light in the last 10 to 12 years," Mussini added. "The authors of the 2012 article can certainly be forgiven for not including these details in the conversation, as it is a work in progress."
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