Researchers have discovered that the evolutionary origins of arachnids, such as spiders and scorpions, may lie in the sea rather than on land. According to Live Science, they found that certain features of the brain of the 500-million-year-old fossil Mollisonia symmetrica are arranged backward compared with those of most modern arthropods. This opposite arrangement is exclusive to arachnid brains in modern animals, indicating that Mollisonia symmetrica was an early ancestor of modern arachnids.
In a study published in the journal Current Biology, researchers investigated the fossilized brain and central nervous system of a Mollisonia symmetrica specimen from the Burgess Shale formation of the Canadian Rockies. The team, including Nicholas Strausfeld, a neuroscientist at the University of Arizona, used optical microscopy to examine the central nervous system of the fossil. For the first time, they were able to take a detailed look at the internal structures of the ancient marine inhabitant.
"The nervous system of Mollisonia symmetrica does not resemble that of a horseshoe crab, nor that of crustaceans or insects," said Strausfeld, according to Live Science. "It's as if the Limulus-type brain seen in Cambrian fossils, or the brains of ancestral and present-day crustaceans and insects, have been flipped backwards, which is what we see in modern spiders."
The researchers found that certain regions of Mollisonia symmetrica's brain appear to be arranged in the opposite direction compared with those of other arthropods, similar to how modern spiders' brains are laid out. The finding suggests that arachnids may have begun to diverge from other chelicerates much earlier than previously thought, with some estimates placing their origins around 500 million years ago.
Previously, it was believed that arachnids first appeared on land, with the oldest recognized fossil remains dating back to a scorpion from 430 million years ago. However, the findings indicate that the evolutionary origin of arachnids could lie in the sea. The discovery contributes to an ongoing scientific debate regarding the evolutionary origin of spiders and their closest relatives, overturning previous teachings on the origin of arachnids.
Terrestrial arachnids are closely related to other groups of chelicerates that inhabit the oceans, such as sea spiders and horseshoe crabs. Understanding the transition is fundamental to our knowledge of evolution. The fossil record is extremely fragmentary, making it difficult to establish with certainty where and when arachnids first appeared.
To determine whether the similarities between Mollisonia symmetrica's brain and those of modern spiders came from a common lineage or mere coincidence, the researchers used a computer program to estimate the likelihood that the two were related. They compared brain and body traits of several living and extinct arthropods. The analysis suggested that the Mollisonia lineage eventually evolved into the arachnid group.
"This means that Mollisonia symmetrica may have led to 'the planet's most successful arthropodan predators'," the researchers wrote in the study. Studies of existing spider brains suggest that the inverted setup allows spiders to coordinate many aspects of predatory movement, including their stealth, speed, and dexterity. Neural shortcuts for controlling legs and claws, for example, could facilitate complex movements such as walking or spinning webs.
"Being able to fly gives you a serious advantage when you're being pursued by a spider," added Strausfeld, according to Live Science. He also said, "Yet, despite their aerial mobility, insects are still caught in their millions in exquisite silken webs spun by spiders."
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