In the fossil, the creature's organs were preserved like a carbon 'copy.' Its hard exoskeleton is extremely faint, but the soft, internal organs became a dark-brown carbon imprint on fine-grained rock called mudstone. [Fabulous Fossils: Gallery of Earliest Animal Organs]
The animal had a tube-shaped heart positioned near its back, rather than toward the front. The blood vessels extended from the heart along its body segments and clustered near the eyes and brain, which suggests these organs required a rich oxygen supply. The fossil also has eyestalks, antennae, legs and a brain, the researchers reported.
The heart and blood vessels were identified in a fossil in the collection at the Yunnan Key Laboratory for Palaeobiology in China, by an international team of researchers led by London Natural History Museum paleontologist Xiaoya Ma. The findings were published in the April 7 issue of the journal Nature Communications.
In 2012, the same team also reported the oldest example of an arthropod brain, in a different Chengjiang F. protensa fossil.
Strausfeld described the Chengjiang fossil depositas a seafloor "Pompeii" — akin to the Roman city buried in volcanic ash — because of its remarkable preservation of soft body parts such as eyes, guts and brains. The abundance of fossil species in Chengjiang rivals that of the Burgess Shale in Canada, and provides the oldest glimpse into the Cambrian Explosion, when life rapidly diversified into the wide range of body plans known today.
"520 million years ago, we had these basic patterns appear that have been maintained over time," Strausfeld said. "The search is on for the ancestors of these early animals. The question is, what came before?"
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