Ancient Fossilized Brains Prompt a Reevaluation of Spider Evolution

Research indicates that the brains of ancient sea creatures, dating back over 500 million years, were structured similarly to those of spiders. This challenges past theories that arachnids originated on land.

Morrison reflects a time of significant biological diversity increase, known as the Cambrian Explosion, when various animal groups began appearing in fossil records. These creatures possessed chelicerae, pincer-like mouthparts likely used for tearing into small prey.

Previous beliefs suggested that modern relatives of Morrison, which include horseshoe crabs, were connected to spiders. However, Nicholas Strausfeld and his team at the University of Arizona propose otherwise.

The researchers reexamined specimens of Mollisonia symmetrica, collected in 1925 from British Columbia, Canada, and now housed at Harvard University’s Comparative Zoology Museum. Strausfeld and his colleagues identified a brain structure that had previously been overlooked.

In horseshoe crabs, the chelicerae exhibit a neural connection at the back of the brain; however, in Morrison, this structure was inverted, with chelicerae linked to two neural regions that offered a perspective on the forefront of the nervous system.

Strausfeld notes that this orientation is “characteristic of arachnid brains.” Unlike the brains of crustaceans and insects, which are folded inward, arachnids have crucial areas for planning agile movements situated at the back. This architecture likely contributes to the remarkable agility and speed seen in spiders.

While it was previously thought that arachnids evolved on land, the earliest existing land fossils of obvious arachnids will not appear for millions of years later, according to Strausfeld. “Perhaps the first arachnids inhabited tidal environments, like Morrison, in search of prey,” he mentions.

Mike Lee, a researcher at Flinders University in Adelaide, Australia, who was not involved in the study, suggests that Morrison may now be viewed as a primitive arachnid. “We now recognize it possessed a brain akin to that of a spider, indicating it was an aquatic relative of the early spiders and scorpions,” Lee states.

Nonetheless, he cautions that while researchers strive to extract as much insight as possible from a single fossil, there remains a degree of ambiguity in interpretation. “It’s akin to attempting to piece together a unique Pavlova after it has been dropped,” he explains.