Trace Fossils Showcase Creatures Possibly Didn't Get Evolutionary 'Burst' During Cambrian Explosion, Studies Reveal

Many major groups of animals that have become a mainstay in modern times first appeared during the Cambrian Explosion. However, a new study undertaken by a team of researchers claims that though the creatures may have appeared during the event, their key characteristics developed long before that, as reported by Phys. The assertion was made through the examination of trace fossils dating back to the Late Ediacaran Period. The insights shed more light on the evolutionary history of several prominent members of numerous ecosystems spread worldwide. Findings regarding this investigation have been published in Proceedings of the Royal Society B and Geology.

Cambrian Explosion

The Cambrian explosion lasted from around 539 to 519 million years ago. During this event, tardigrades, mollusks, and arthropods appeared in the fossil record. For a long time, researchers have assumed that a "burst" of evolution occurred during this phase, as the fossil record showcased a rapid expansion in different life forms amidst this "explosion." Animals did exist before, but the presumption has been that they diversified and evolved during the 20-million-year event. The new findings do not align with this assertion. 

The evidence of animals before the Cambrian Explosion typically came from trace fossils, according to the Natural History Museum. Trace fossils are evidence of a creature's actions, rather than their remains. For the study, researchers analyzed trace fossils' tracks and burrows from the Late Ediacaran Period (635 to 539 million years ago). The tracks can reveal the shape of the animal that made them, while their trajectory can showcase the senses they applied while moving across the environment. The results indicated that complex features had already begun to develop in creatures. The team believes that the outcome showcased complex features, did not develop during the Cambrian explosion, but instead, evolved to some degree. The study claims that the "explosion" brought more of a final flourish, rather than a "burst." 

Complexity in the Late Ediacaran Period

Dr Zekun Wang, a scientist from the Natural History Museum, led the examination. He shared that life was not microscopic during the Ediacaran period. Past examinations indicate that life was still unable to move on the seafloor; however, by the time the Cambrian arrived, the fossils suggest that they could explore the seabed. It meant that a change occurred between the Ediacaran and Cambrian. Researchers have yet to learn about these changes, as fossils reflecting this transition do not have adequate anatomical details. In the study, researchers have turned to trace fossils to give them some insights into the transitional features. 

In total, 170 different trace fossils from the Ediacaran–Cambrian transition were analyzed and then compared with tracks created by modern worms, arthropods, and gastropods. The trace fossils indicate that the Ediacaran animals had already acquired advanced senses and patterns of movement. The creatures also exhibited a more slender profile than their previous counterparts. "This sets the stage for the Cambrian Substrate Revolution and the Cambrian Explosion," Wang claimed. 

Results of Investigation

Researchers noted that the trace fossils from the transition began as simple horizontal trails and burrows but later transformed into complex trajectories. The team concentrated on the smoothness and tightness of the tracks to figure out the features of its creators as well as its senses. "Think of a snake, for example," explained Wang. "While these animals didn't exist during the Cambrian, a snake will always make a long, curving path behind it because of its slender shape. As a result, there are certain turns that can never be made by this body shape because they can only bend a certain amount. If there are abrupt changes in the angle of a track, then it must have been made by a shorter, even round, body shape," Wang noted. 

The smoothness will indicate whether the senses were advanced enough for a direct path towards food, or if the animals stumbled towards food, and made random turns, Wang added. Experts found three groups of trace fossils during the investigation. The earliest ones, dating back to 550 million years ago, are simple tracks with abrupt turns. These were made by simple animals with short and round bodies, exhibiting limited senses. The second group dates back to 545 million years ago, which displayed smoother tracks with less abruptness in turns. These tracks implied that animals were more in control of their bodies and coordinated in their approach.

The last group showcases entirely smooth trails around the same period. These tracks are similar to ones left by modern worms in the fossil record. These animals cannot make abrupt turns; they have longer bodies, which are streamlined, that considerably reduce the resistance faced by these beings while moving. They also had more surface area, which covered more sensory organs. The tracks show that during this "transition," animals evolved from an ovoid shape towards more worm-like bodies.

The results indicate several key characteristics witnessed in the fossil record of the Cambrian period were developing millions of years before in the Ediacaran period. These features were probably the reason certain species survived the end of the Cambrian, while many mysteriously disappeared, according to past examinations. It gave animals a good platform to diversify during the explosion and establish many of the modern life forms that continue to exist today.