New research suggests spiders and other arachnids may have originated in the sea, based on analysis of a 500-million-year-old fossil.

The “exquisitely preserved” specimen supports the idea that these creatures swam before adapting to life on land, according to a study published Tuesday in Current Biology.

Researchers at the University of Arizona analyzed the fossilized brain of Mollisonia symmetrica, an extinct Cambrian-period species once thought to be an ancestor of horseshoe crabs. However, the study revealed that its neural structure more closely resembles that of modern spiders and their relatives, suggesting a closer evolutionary link to arachnids than previously believed.

The front part of Mollisonia’s body, called the prosoma, has a radiating pattern of nerve clusters that control five pairs of appendages. Additionally, its unsegmented brain sends short nerves to a pair of pincer-like “claws,” resembling the fangs found in spiders and other arachnids.

The key feature identifying the fossil as an early arachnid is its brain’s unique organization, which is the reverse of the front-to-back arrangement seen in modern crustaceans, insects, centipedes and horseshoe crabs, researchers said.

In a statement, Nick Strausfeld, lead author and professor at the University of Arizona, said the fossil’s brain appears “flipped backwards,” similar to modern spiders.

This back-to-front brain arrangement may be a key evolutionary adaptation, providing neural shortcuts that enhance movement control.

According to the study, this discovery questions the common belief that diversification happened only after a common ancestor transitioned to land. Earlier fossil evidence suggested that arachnids lived and evolved solely on land.

The prosoma, or front section of Mollisonia’s body, features a radiating arrangement of nerve clusters that manage five pairs of appendages. (Nick Strausfeld/Dept. of Neuroscience, University of Arizona)

“It is still vigorously debated where and when arachnids first appeared, and what kind of chelicerates were their ancestors, and whether these were marine or semi-aquatic like horseshoe crabs,” Strausfeld said.

As they adapted to life on land, Mollisonia-like arachnids likely fed on early insects and millipedes. These early arachnids may have also influenced the evolution of insect wings, an important defense mechanism.

Researchers say the Mollisonia’s ancestry likely led to spiders, scorpions, sun spiders, vinegaroons and whip scorpions.

A new analysis of an exquisitely preserved fossil that lived half a billion years ago suggests that arachnids – spiders and their close kin – evolved in the ocean, challenging the widely held belief that their diversification happened only after their common ancestor had conquered the land.

Spiders and scorpions have existed for some 400 million years, with little change. Along with closely related arthropods grouped together as arachnids, they have dominated the Earth as the most successful group of arthropodan predators. Based on their fossil record, arachnids appeared to have lived and diversified exclusively on land.

In a study led by Nicholas Strausfeld at the University of Arizona and published in Current Biology, researchers from the U.S. and United Kingdom undertook a detailed analysis of the fossilized features of the brain and central nervous system of an extinct animal called Mollisonia symmetrica. Until now, it was thought to represent an ancestral member of a specific group of arthropods known as chelicerates, which lived during the Cambrian (between 540 and 485 million years ago) and included ancestors of today’s horseshoe crabs. To their surprise, the researchers found that the neural arrangements in Mollisonia’s fossilized brain are not organized like those in horseshoe crabs, as could be expected, but instead are organized the same way as they are in modern spiders and their relatives.

“It is still vigorously debated where and when arachnids first appeared, and what kind of chelicerates were their ancestors,” said Strausfeld, a Regents Professor in the U of A Department of Neuroscience, “and whether these were marine or semi-aquatic like horseshoe crabs.”

Mollisonia outwardly resembles some other early chelicerates from the lower and mid-Cambrian in that its body was composed of two parts: a broad rounded “carapace” in the front and a sturdy segmented trunk ending in a broad, tail-like structure. Some scientists have referred to the organization of a carapace in front, followed by a segmented trunk as similar to the body plan of a scorpion. But nobody had claimed that Mollisonia was anything more exotic than a basal chelicerate, even more primitive than the ancestor of the horseshoe crab, for example.

What Strausfeld and his colleagues found indicating Mollisonia’s status as an arachnid is its fossilized brain and nervous system. As in spiders and other present-day arachnids, the anterior part of Mollisonia’s body (called the prosoma) contains a radiating pattern of segmental ganglia that control the movements of five pairs of segmental appendages. In addition to those arachnid-like features, Mollisonia also revealed an unsegmented brain extending short nerves to a pair of pincer-like “claws,” reminiscent of the fangs of spiders and other arachnids.

But the decisive feature demonstrating arachnid identity is the unique organization of the mollisoniid brain, which is the reverse of the front-to-back arrangement found in present-day crustaceans, insects and centipedes, and even horseshoe crabs, such as the genus Limulus.

“It’s as if the Limulus-type brain seen in Cambrian fossils, or the brains of ancestral and present days crustaceans and insects, have been flipped backwards, which is what we see in modern spiders,” he said.

According to co-author Frank Hirth from King’s College London, the latter finding may be a crucial evolutionary development, because studies of existing spider brains suggest that this back-to-front arrangement provides shortcuts from neuronal control centers to underlying circuits that coordinate a spider’s (or its relative’s) amazing repertoire of movements. This arrangement likely confers stealth in hunting, rapidity in pursuit and in the case of spiders, an exquisite dexterity for the spinning of webs to entrap prey.

“This is a major step in evolution, which appears to be exclusive to arachnids,” Hirth said. “Yet already in Mollisonia, we identified brain domains that correspond to living species with which we can predict the underlying genetic makeup that is common to all arthropods.”

“The arachnid brain is unlike any other brain on this planet,” Strausfeld added, “and it suggests that its organization has something to do with computational speed and the control of motor actions.”

The first creatures to come onto land were probably millipede-like arthropods and probably some ancestral, insect-like creatures, an evolutionary branch of crustaceans, according to Strausfeld.

“We might imagine that a Mollisonia-like arachnid also became adapted to terrestrial life making early insects and millipedes their daily diet,” he said, adding that the first arachnids on land may have contributed to the evolution of a critical defense mechanism: insect wings, hence flight and escape.

“Being able to fly gives you a serious advantage when you’re being pursued by a spider,” Strausfeld said. “Yet, despite their aerial mobility, insects are still caught in their millions in exquisite silken webs spun by spiders.”

For the study, Strausfeld spent time at the Museum of Comparative Zoology at Harvard University, where the Mollisonia specimen is housed, taking scores of photographs under various directions of illumination, light intensities and polarization light, and magnifications.

To rule out the possibility that the congruence between Mollisonia’s brain and that of spiders was the result of parallel evolution – in other words, coincidence rather than derived by a common lineage – co-author David Andrew, a former graduate student in the Strausfeld laboratory who is now at Lycoming College in Pennsylvania, performed a statistical analysis comparing 115 neuronal and related anatomical traits across arthropods, both extinct and living. The results placed Mollisonia as a sister group of modern arachnids, lending further weight to the idea that Mollisonia’s lineage gave rise to the clade that today includes spiders, scorpions, sun spiders, vinegarroons and whip scorpions, amongst many others.

Unfortunately, other Mollisonia-like arthropods are not preserved in a way that allows for a detailed analysis of their nervous system. But if they shared the same unique kind of brain, the authors suggest, their descendants might have established diverging terrestrial lineages that today account for the various branches of the arachnid tree of life.

In a twist worthy of a detective novel, a long-misidentified fossil at Harvard’s Museum of Comparative Zoology (MCZ) has emerged as a key discovery in early animal evolution. Originally described in 1865 as a caterpillar, Palaeocampa anthrax shuffled between classifications — worm, millipede, and eventually a marine polychaete — until 130 years later, when researchers realized its true identity: the first-known nonmarine lobopodian and the earliest one ever discovered.

Lobopodians are extinct, soft-bodied creatures that bridge the evolutionary gap between a primitive worm-like ancestor and modern arthropods like insects and crustaceans. Known mostly from Cambrian marine deposits such as Canada’s Burgess Shale, they include iconic fossils like Hallucigenia and Aysheaia pedunculata discovered in 1911, and were thought to be exclusively marine — until now.

A new study published in Communications Biology led by Richard Knecht, a former graduate student (PhD ’25) in Harvard’s Department of Organismic and Evolutionary Biology (OEB), redescribes Palaeocampa anthrax as the first nonmarine and youngest lobopodian discovered; predating the famous Burgess Shale lobopodians by nearly fifty years.

“Lobopodians were likely a common sight on Paleozoic sea beds,” said Knecht, “but apart from microscopic tardigrades and terrestrial velvet worms, we thought they were confined to the ocean.”

Knecht, currently a postdoctoral fellow at the University of Michigan and an associate of the MCZ, discovered Palaeocampa while examining fossil millipedes in the MCZ collection. He noted legs on every trunk — ruling out caterpillar or worm — and recognized it as a lobopodian.

To confirm this, the team analyzed 43 specimens from two Carboniferous Lagerstätten — Mazon Creek (USA) and Montceau-les-Mines (France) — using advanced imaging, including backscatter scanning electron microscopy (SEM) and energy-dispersive spectroscopy. They revealed exquisite anatomical features — most notably, nearly 1,000 bristle-like spines covering the body.

Co-author Nanfang Yu, associate professor of physics at Columbia University, used Fourier-transform infrared spectroscopy (FTIR) to detect chemical residues at the spine tips — suggesting the spines secreted toxins to deter predators in its swampy habitat.

“What amazed me is that fragments of biomacromolecules could be exceptionally preserved or altered to geomacromolecules in fossils,” Yu said. ” I’m thrilled this technique possessed the sensitivity and specificity to differentiate fossilized remains from the rocky substrate.”

Palaeocampa’s closest relative is Hadranax, a Cambrian lobopodian from Greenland, nearly 200 million years older. Both had ten pairs of legs, no claws andwere blind. But while Hadranax was unarmored and navigated the deep sea using elongated frontal appendages, Palaeocampa, at just four centimeters long, bore a dense coat of spines — arranged above each pair of legs, giving it a fuzzy caterpillar-like appearance — and inhabited freshwater, possibly amphibious, environments.

Palaeocampa’s discovery also resolves the mystery of France’s Montceau-les-Mines fossil site, once considered as marine. “Mazon Creek is a mix of terrestrial, freshwater, and marine animals,” Knecht explained. “But, Montceau-les-Mines, where half of the specimens come from, was hundreds of kilometers inland, with no ocean present.” Its reclassification confirms the site’s nonmarine setting, offering a rare glimpse into ancient freshwater ecosystems.

This discovery broadens our understanding of lobopodian diversity and raises new evolutionary questions: How many others made the leap from marine to freshwater and could more be hiding, misidentified, in museum drawers?

“The conditions required to fossilize soft-bodied creatures like lobopodians are rare,” Knecht noted. “Most of our insights come from Cambrian Lagerstätten, but the Carboniferous period — when Palaeocampa lived — offers far fewer such windows, making every new find incredibly valuable.”

This breakthrough came from reexamining century-old specimens from museums including the MCZ, Yale Peabody Museum, the Smithsonian National Museum of Natural History, France’s Muséum d’histoire naturelle d’Autun, the Chicago Field Museum, and the University of Illinois Urbana-Champaign — highlighting the ongoing scientific value of museum collections.

Ironically, Palaeocampa sat for decades in a drawer just feet from the office of Stephen Jay Gould’s office — MCZ curator and author who popularized the Cambrian oddities in Wonderful Life. “It was literally hiding in plain sight,” Knecht said. “Sometimes, the biggest discoveries are the ones waiting to be looked at again.”

Your average backyard spider may owe its creepy life to an ancient marine critter that looked like a trilobite.

Mollisonia symmetrica was a 500-million-year-old arthropod that scientists now think could be the original arachnid, setting the stage for countless creepy-crawly spiders to come. Or, at least, from the neck up, that is.

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In a new study published in Current Biology, detailed by Science Alert, researchers from the University of Arizona, Lycoming College, and King’s College London re-examined a fossilized Mollisonia. They discovered a brain that looks an awful lot like the kind you’d find in a modern-day spider.

Its brain structure looked an awful lot like a flipped and rewired arachnid brain more than it did a horseshoe crab brain, which is what the Mollisonia had been previously grouped with.

For as much as we know about arachnids, we don’t really know where they came from. The prevailing theory is that arachnids, such as spiders, scorpions, mites, and ticks, evolved on land from a common land-based ancestor. This discovery suggests that arachnid evolution may have originated in the ocean, long before their first known land appearance 430 million years ago.

This Ancient Sea Bug May Have Been the Blueprint for Every Spider Ever

According to neuroscientist Nicholas Strausfeld, arachnid brains are unlike any other in the animal kingdom. And Mollisonia’s fossilized neural map shows early versions of the same brain regions we see in today’s eight-legged freaks (have I made it obvious enough that I really dislike spiders?).

This wasn’t a quirk of evolution, a fluke that appeared one day and stuck all these millions of years. That’s evolution working as intended, stemming from an ancestor that shatters our conception of the common spider.

This bug-eyed sea creature with pincers and a neural control center built for multitasking may have handed down the original arachnid brain blueprint. The neural connections between its brain and limbs could explain why arachnids became such fearsome land predators.

Strausfeld thinks the Mollisonia’s influence extends further beyond spiders. He theorizes that something like a Mollisonia could have shimmied its way onto land and started feeding on insects, thus eventually, over an enormous amount of time, forcing those insects to adapt by learning to fly. T

his forced the Mollisonia to adapt by spinning webs to catch the food that used to be terrestrial but now is airborne. One creature’s evolution forces another creature’s evolution, thus necessitating a secondary evolution if it wants to survive.

Mollisonia might not look much like a spider, but its internal wiring suggests that it may have been a precursor to the phase that would eventually become the thing that made you shriek when you saw it in the top corner of your bathroom.

Did spiders first appear on land or in the sea?

That’s a question that has puzzled scientists for many years.

A team of researchers in America think they might have found the answer.

They’ve discovered a tiny 500-million-year-old marine fossil that suggests that arachnids (a group of animals which includes spiders, scorpions and ticks) evolved in the ocean, before moving out of the water and adapting to land.