More than half a billion years ago, a tiny ancestor of today’s arthropods scuttled along the floor of a shallow sea in what’s now Yunnan, China. It had large grasping arms, stalked eyes, and a surprisingly modern-looking brain tucked into a head barely wider than the tip of a pencil.
Now, a new look at this ancient fossil – Jianfengia multisegmentalis – is reshaping what scientists thought they knew about how these animals first branched into two major evolutionary paths.
Arthropods rule the world
Insects, spiders, crabs, centipedes – all of them are arthropods. This massive group includes more than a million known species.
Arthropods live everywhere: in the ocean, on land, in the air, and even inside of other animals. Their tough exoskeletons, segmented bodies, and jointed legs have helped them survive and thrive for over 500 million years.
But one big puzzle has remained: where exactly did the two main branches of arthropods come from?
On one side, you have mandibulates – creatures like insects, crustaceans, and millipedes, which have antennae and use jaws (or mandibles) to eat. On the other hand, chelicerates – spiders, scorpions, and horseshoe crabs – have fangs or pincers instead of jaws.
Until now, scientists weren’t sure when or how these two groups split. That’s where Jianfengia comes in.
A fossil that tells a different story
At first, Jianfengia was thought to belong to a group called megacheirans, meaning “large hands” in Greek.
These extinct animals had long, claw-like limbs sticking out from their heads – similar to the claws of modern horseshoe crabs. Because of that, scientists had placed them in the chelicerate group.
But when researchers took a closer look at the fossilized brain of Jianfengia, they noticed something didn’t add up.
The research team, led by the University of Arizona, reconstructed the nervous system from four well-preserved specimens.
What they found was unexpected: a brain that looked a lot like that of a modern shrimp or crayfish. It even had features seen in simple freshwater crustaceans, like brine shrimp – the kind sold as “sea monkeys.”
The ancestors of antennae
“These megacheirans didn’t have antennules, which are antenna-like appendages that are common to crustaceans, insects and centipedes,” said lead researcher Nicholas Strausfeld, a professor in the U of A Department of Neuroscience.
“Instead we see these strange, quite sturdy head appendages that were specialized for reaching and clasping things.”
Turns out, those big “claws” weren’t early versions of spider fangs after all. Instead, they were likely the ancestors of antennae – the kind found in today’s insects and crustaceans.
Seeing with ancient eyes
Jianfengia’s head was only about 0.08 inches wide, but it packed in a lot of sensory power. It had stalked compound eyes, like those of insects and crabs, plus at least three single-lens eyes – the simple kind also seen in many modern arthropods.
In one fossil, the researchers even looked inside the compound eyes and saw fossilized cone cells that once supported light-sensitive parts of the eye.
“What we saw was unexpected: the brain looks really modern, comparable to that of a living crustacean,” Strausfeld said.
This level of detail is almost never seen in fossils this old. Soft tissues like brains usually break down long before they can fossilize. But thanks to the fine-grained rock and lucky conditions in this ancient seabed, a few rare specimens kept their delicate neural features.
Ancient cousins go separate ways
Another fossil – Alalcomenaeus – also belonged to the megacheiran group and had similar body parts. However, it turned out to be quite different on the inside. Its brain looked more like that of Limulus, the horseshoe crab, which is a chelicerate.
So even though these two creatures looked similar on the outside, their brains revealed that they were actually early members of different arthropod groups. One led to modern crustaceans and insects. The other led to spiders and scorpions.
“Many repeats of these comparisons revealed that in the arthropod tree of life, Jianfengia sat at or near the root of all mandibulates, whereas its putative cousin, Alalcomenaeus, has the same status, but within the chelicerate branch of the tree of life,” said co-author David Andrew of Lycoming College.
The key body part in all of this are the large claws sticking out of the head. Over hundreds of millions of years, evolution shaped them into very different things.
“In chelicerates, these ‘great appendages’ shrunk, so they eventually became the spider fangs,” said Strausfeld. “In mandibulates, evolution modified them into segmented antennules.”
Some tiny marine crustaceans alive today – called ostracods – still have antennules tipped with claspers, offering living proof of this evolutionary transition.
Arthropods from the inside out
Looking at a creature’s shell or limbs can tell scientists part of the story. But to really understand where an animal fits in the family tree, the brain may offer better clues.
“Our results demonstrate that close examination of fossilized neural remains can provide powerful data indicating evolutionary relationships impossible to obtain just from features of the exoskeleton,” Strausfeld said.
“One needs to know what to look for in the fossil brain because it tells us a lot about a fossil’s identity.”
Frank Hirth, a co-author from King’s College London, noted that the organization of their fossilized brains perfectly aligns with that of living arthropods, suggesting that their ancient constituents are extraordinarily robust, yet diverse.
This may explain why arthropods are the most successful inhabitants of this planet, said Hirth.
Ultimately, the tiny brain of a long-dead sea creature has helped scientists solve one of the oldest mysteries in animal evolution – where the arthropod family tree began to branch.
The full study was published in the journal Nature Communications.
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