A new study led by researchers at Durham University have uncovered an incredibly rare and detailed fossil, named Youti yuanshi, that gives a peek inside one of the earliest ancestors of modern insects, spiders, crabs and centipedes.

This fossil dates back over 520 million years to the Cambrian period, when the major animal groups we know today were first evolving.

This fossil belongs to a group called the euarthropods, which includes modern insects, spiders and crabs. What makes this fossil so special is that the tiny larva, no bigger than a poppy seed, has its internal organs preserved in exceptional quality.

Using advanced scanning techniques of synchrotron X-ray tomography at Diamond Light Source, the UK's national synchrotron science facility, researchers generated 3D images of miniature brain regions, digestive glands, a primitive circulatory system and even traces of the nerves supplying the larva’s simple legs and eyes.

https://www.eurekalert.org/news-releases/1052858

The creature died while still in its early development, or larval stage, and belongs to a new species named Youti yuanshi, which combines the standard Chinese words "yòutǐ," meaning "larva," and "yuánshǐ," meaning "primitive," according to a new study published Wednesday (July 31) in the journal Nature.

Y. yuanshi inhabited the Cambrian seas and helped give rise to living arthropods like insects, spiders and crabs. Despite being about the size of a grain of sand, the fossil is exceptionally well-preserved, revealing never-before-seen details that help explain how arthropods developed complex brains.
...
The exceptional preservation allowed researchers to see more of an early arthropod's anatomy than they normally would. This included signs that arthropod brains were becoming more sophisticated during Y. yuanshi's time. For example, the larva had an ancestral brain region that would become the nub of a more specialized arthropod head with antennae and various other appendages, according to the statement. These specialized heads meant the creatures could adopt a variety of lifestyles, including becoming sophisticated predators.

"As the arthropod brain has developed, that's allowed arthropods to exhibit much more complicated behaviors and even more complicated morphologies," Smith said.

https://www.livescience.com/animals/my-jaw-just-dropped-500-million-year-old-larva-fossil-found-with-brain-preserved

Organ systems of a Cambrian euarthropod larva

The Cambrian radiation of euarthropods can be attributed to an adaptable body plan. Sophisticated brains and specialized feeding appendages, which are elaborations of serially repeated organ systems and jointed appendages, underpin the dominance of Euarthropoda in a broad suite of ecological settings. The origin of the euarthropod body plan from a grade of vermiform taxa with hydrostatic lobopodous appendages (‘lobopodian worms’) is founded on data from Burgess Shale-type fossils. However, the compaction associated with such preservation obscures internal anatomy. Phosphatized microfossils provide a complementary three-dimensional perspective on early crown group euarthropods, but few lobopodians. Here we describe the internal and external anatomy of a three-dimensionally preserved euarthropod larva with lobopods, midgut glands and a sophisticated head. The architecture of the nervous system informs the early configuration of the euarthropod brain and its associated appendages and sensory organs, clarifying homologies across Panarthropoda. The deep evolutionary position of Youti yuanshi gen. et sp. nov. informs the sequence of character acquisition during arthropod evolution, demonstrating a deep origin of sophisticated haemolymph circulatory systems, and illuminating the internal anatomical changes that propelled the rise and diversification of this enduringly successful group.

https://www.nature.com/articles/s41586-024-07756-8