About 550 million years ago a small creature crawled a few centimeters down the ocean floor shortly before dying. His body was quickly covered by sediments, fossilized over time and preserved through the different geological ages, to end up being found by a team of scientists in present-day China. These are the oldest remains of an animal with motor capacity from the Ediacárico period (635-539 million years ago) and shed new light on a key moment in the evolution of life on the planet.
The fossils were discovered at the Dengying site, in the Three Gorges region of the Yangtze River, and belong to a species that has been baptized as Yilingia spiciformis . "The rocks that are preserved from the old surface of the seabed appeared," says Shuhai Xiao, a professor in the geosciences department at Virginia Tech University and responsible for research, whose details appear Thursday in the journal Nature . "These are carbonated rocks that were deposited in shallow waters between 550 and 540 million years ago: twice as old as the first dinosaurs."
The authors describe an elongated and bilaterally symmetrical (or bilaterian) animal, with a body structure divided into repeating units. "It is similar to a trilobite, but it is not a trilobite," says Xiao. The appearance of symmetrical beings is one of the crucial points in the genesis of vertebrates , since it allows the definition of a body axis in the direction of movement, which in turn favored the formation of a central nervous system.
Footprints before dying
The fossil footprints that occur when an animal takes its last steps before dying, called mortichnia , provide valuable additional information to experts. "This mortichnium is a bridge that allows us to interpret other remains," Xiao notes, "because, although the marks of holes and footprints are very abundant in the rocks of the late Ediacárido, it is often not possible to determine what type of animals produced them."
In this case, the segmentation tests on the body and directional locomotion confirm that Y. spiciformis was, in effect, a bilaterian animal with the ability to move . However, its position in the evolutionary family tree is more uncertain; The authors assume that this species could be related to arthropods (eg centipede) and annelids (such as earthworm).
Strange organisms
The strange organisms of the Ediacárido period - which begins 635 million years ago and ends 539 million years ago - have baffled researchers for a long time. At this point large living beings appear for the first time, but mostly they were mass reports such as fronds and sponges, totally different from today's animals. Based on the available fossils, it was long believed that these beings had suddenly disappeared when the Ediacárico gave way to the Cambrian, an era in which more complex and recognizable animals appear.
An abrupt transition that led some researchers to consider Ediacárido's biota as a failed evolutionary experiment, unviable creatures that would be replaced by the ancestors of modern animals. In the later period, the so-called Cambrian explosion, the animal kingdom would undergo a dramatic transformation. A world dominated by almost motionless creatures that roamed the seabed in search of food gave way to a new fauna of predators and prey, with eyes, jaws, clamps, legs and shells.
But new fossils found in recent years show that some features that define the Cambrian had already appeared in the Ediacárido and the lines between the two stages are no longer as clear. "We used to think that the first animals had emerged quickly after a single evolutionary event, but the new findings suggest that it actually happened in stages," explains Rachel Wood, Professor of Geoscience.
Wood and other scientists published last April an article in Nature Ecology & Evolution arguing that the Cambrian explosion was rather a series of evolutionary impulses. Differentiated stages in which existing creatures gradually evolved into new ones, instead of being replaced. For example, the footprints and fossilized holes found in recent years - such as Yilingia spiciformis itself - suggest that animals already moved at least 25 million years before the Cambrian and that the ecosystems of this era may have been even more complicated than previously thought
Also, animals with shells and skeletons would also have appeared for the first time in the Ediacárico and some of the fossils even point out that their owners died because of predators. Although scientists were already speculating that the mobile and segmented bilaterians had evolved millions of years before that Cambrian explosion, until these last years there was hardly any fossil evidence to support this claim. "We had suspected that these complex animals were already present in the Ediacárico, now the new findings provide the first evidence," says Wood.
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