Scientists have discovered a set of “extraordinary” microfossils that preserve muscle tissue from a mysterious creature that lived 535 million years ago.
The tiny fossils were recovered from the Kuanchuanpu geological formation in southern Shaanxi Province, China, according to a study published in the Royal Society journal Proceedings of the Royal Society B.
This formation is rich in fossils that have helped to shed light on a period of Earth’s history known as the Cambrian explosion, which began around 540 million years ago.
Occurring at the beginning of the Cambrian (the period that spanned roughly 539-485 million years ago), the explosion was characterized by an unparalleled emergence of organisms over the course of several million years, including the first appearance of most of the major animal groups we know today.
The remarkable microfossils documented in the latest study date back to the earliest part of the Cambrian period, known as the Fortunian Age.
It is not clear exactly what animals the fossils belong to. But the researchers have proposed that they preserve muscle tissue from an animal group known as the cycloneuralians.
While fossils representing this animal group from the Fortunian are relatively abundant, finding preserved muscle or nerve tissue among them is almost unheard of.
“The finding is significant because it sheds unprecedented light on the musculature systems of early animals, features that are typically not preserved in the fossil record but are critical in understanding the behaviors of early animals. Such fossils are extremely rare—literally a needle in a haystack,” Shuhai Xiao, a researcher in the Department of Geosciences at Virginia Tech university and an author of the study, told Newsweek.
The cycloneuralian group includes animals such as roundworms and mud dragons that are characterized by a worm-like body. The group first appears in the fossil record at the beginning of the Cambrian period, and there are several species alive today, spread out across marine, freshwater and terrestrial environments.
In the latest study, the researchers describe three fossil specimens, measuring just a few millimeters across, which they say represent preserved cycloneuralian muscle tissue from a body part known as the proboscis—a kind of sucking organ.
“I found them in late 2021. When I first saw them under my binocular microscope, I was of course surprised by their conical shape and grid-like morphology. I soon realized that they may represent some muscle tissues,” Huaqiao Zhang, another author of the study with the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, told Newsweek.
In early 2022, Zhang showed some scanning electron microscope images of the fossils to co-author Shuhai Xiao.
“He also felt excited by their extraordinary preservation, and he immediately concluded that they should be muscle tissues,” Zhang said.
Because the researchers have only obtained part of the proboscis muscle tissue, they have “no idea” about what the host animal actually looks like or its behavior, Zhang said.
Based on the documented specimens, “it can be inferred that their host animals may have been at least millimeter-sized,” he said. “They may have a proboscis… and they may have lived on the soft sea floor, if they were crawlers, or within the sands, if they were burrowers.”
“It is unclear what they fed on. Our analysis shows that they may have very limited ability to retract their proboscis. The partially retractable proboscis would have prevented them from being fierce predators, and, thus, it is likely that they may feed on small organic particles on the sea floor.”
Among the specimens, one labelled NIGP179459, is better preserved, consisting of five successively larger rings with interconnecting structures, which the researchers interpreted to represent preserved proboscis muscle tissue. The latest study is the first to document the proboscis muscle tissue of a cycloneuralian animal from the Fortunian Age.
“The muscles that control the body wall and the proboscis are preserved,” Xiao told Newsweek. “The coordinated contraction and relaxation of these muscles controls the locomotion and feeding activities of the animal.”
It is unclear why the muscle tissue has been preserved in isolation from the host animal, according to the researchers.
“Generally, the muscles, if preserved, should have been preserved together with their host animals, as in most previously reported cases in younger rocks,” Zhang said.
One potential explanation is that after the creatures died, gut contents, including phosphorus and microbes, were forced out of the body that led to the preferential fossilization of the muscle tissues above others.
Muscle and nerve tissues from these animals are usually among the first to decay after death, hence why they are rarely preserved as fossils.
The current fossils come from part of the geological formation where Zhang and his team have been working for over 10 years, during which time they have processed more than 12 tons of rock samples.
“Indeed, we have recovered abundant cycloneuralian body fossils, but only three musculature fossils were obtained. So, the current fossils are indeed very rare,” Zhang said.