This is your brain on arthropod evolution

Reference: Moysiuk, J., Caron, J.B. (2022). A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation. Current Biology. 32(15), 3302-3316.e2. https://10.1016/j.cub.2022.06.027.

Cover image credit: Junnn11 on Wikimedia Commons. Licensed under CC BY-SA 4.0

It’s an arthropod’s world, and we just live in it: from spiders to crabs to insects, this group of scuttling, hard-shelled creatures spans an incredible range of diversity. United by their segmented bodies and jointed limbs, arthropods have adapted to almost every habitat type on Earth, and they’ve been doing it for a very, very long time. Emerging in the seas of the Cambrian period 500 million years ago, ancient arthropods were just as strange and complex as their contemporaries. New research on a cache of arthropod fossils from the Royal Ontario Museum discusses one such bizarre arthropod ancestor— a three eyed marine predator with grasping appendages and blade-like lobes that extended down its body.

An inside insight into sight

The fossils in question belong to a now-extinct group of arthropods called radiodonts. Perhaps the most striking feature of these newly-discovered radiodonts was a large third eye in the middle of its head. The central third eye is framed by a pair of stalked eyes, which are characteristic for other radiodont species. Researchers think that this supports the idea that radiodonts had a complex visual system that allowed them to actively capture and pursue prey.

An illustration of the radiodont discovered, Stanleycaris hirpex.
An illustration of the three-eyed radiodont. Image credit: Junnn11 on Wikimedia Commons. Licensed under CC BY-SA 4.0.

But this hypothesis is based on more than the radiodont’s extra eye. Most radiodont fossils capture their external anatomy—their hard outer shell, or appendages for catching prey. However, these specimens were extraordinarily well-preserved, detailing the neuroanatomy of these radiodonts. Not only was the brain preserved, but the structure and connections of nerves were visible as well. The radiodont’s trio of eyes were preserved, along with the parts of the brain that supported them in life. Researchers were thus able to conclude that radiodonts had dedicated visual processing centers in their brain to accompany their sizable eyes.

Divided on the possibilities

The fossils also suggested radiodont brains were divided into two parts. While still a subject of debate within the field, this conclusion implies that the oldest arthropods had a two-part brain as well. In contrast, modern arthropods have a three-part brain. This difference of a segment is more than superficial— it has major implications for how arthropods evolved their amazing array of adaptations.

Researchers can easily determine which arthropod body parts share an evolutionary origin because they are visibly segmented. Heads are trickier—they’re only segmented on the inside, with the arthropod’s three part brain. Structures associated with the head, such as mouthparts, are specifically connected to one of the segments of the arthropod brain. For instance, the radiodont’s eyes and claws were connected to the first and second segment respectively.

Top and side view of radiodont.
Top and side view of the radiodont, displaying its eyes and frontal claws respectively. Image credit: Junnn11 on Wikimedia Commons. Licensed under CC BY-SA 4.0.

Comparing body parts with shared evolutionary origins is key to understanding how arthropods evolved such an amazing range of diversity. A scorpion’s claw might look like a lobster’s claw, but nerves connect it to the third brain segment. This makes it distinct from a lobster’s claw— it likely evolved from a mouthpart instead of a limb!

An intellectual connection

The ancient nervous system in these fossils may provide an important key to understanding arthropod evolution— they were so well-preserved that individual nerves connecting various appendages to the brain were visible. Being able to map various head-derived structures to specific brain segments will help to link body parts in ancient arthropods like radiodonts to their contemporary counterparts. By following the changes in structures that share an evolutionary origin, we’ll have a clearer idea of how arthropods have diversified and specialized parts of their bodies. Which parts share a common origin? How has half a billion years of evolution changed them?

These radiodont fossils offer a rare insight into the insides of an animal that lived hundreds of millions of years ago, a crucial piece of evidence that will help us to understand arthropod evolutionary history. Like following nerves extending throughout the body, who knows what connections we’ll be able to make?

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