Scientists have uncovered the oldest cardiovascular system they've ever found in a fossil, in the form of a shrimp-like animal that once roamed the turbulent ancient seas. The finding, described in the journal Nature Communications, shows that the internal systems in the ancestors of modern crustaceans may have been much more complicated than scientists might have thought.

The 520-million-year-old fossil of an ancient arthropod (the group that today includes crustaceans, insects and arachnids) called Fuxianhuia protensa was discovered in southwest China. This creature would have lived in the early Cambrian Period, when life was diversifying into increasingly complex forms.

"The addition of its vascular system to documented digestive and nervous systems resolves the internal organization of F. protensa as the most completely understood of any Cambrian arthropod, emphasizing complexity that had evolved by the early Cambrian," the study authors wrote.

Fossils are typically made of mineralized shell or bone, or are the rocky imprints left by those hard skeletal elements. Soft tissue-like skin, muscle or arteries quickly decompose without a chance to leave an impression in the surrounding rock. But in a Pompeii-like natural disaster, this animal was trapped, killed and flattened in fine-grained soil in a relative flash, giving it a better shot at preservation.

While the soft tissues in this 3-inch fossil were long gone, the international team of researchers was able to trace the dark carbon trails left along all the blood vessel paths inside the body. The researchers found that while F. protensa's simple body looked primitive on the outside, it was remarkably complex on the inside, with long arteries leading out of the heart and several vessels leading to the brain. This system was more complicated than those seen in some modern crustaceans today, the scientists said.

Understanding how this animal was wired internally will help researchers to get a handle on how it must have behaved while it was alive, the researchers said.

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