By Will Dunham
WASHINGTON, April 7 (Reuters) - You've heard of having a heart of stone, but this old guy takes it literally.
Scientists said on Monday they have found a fossil of a shrimp-like creature that lived 520 million years ago with an exquisitely preserved heart and blood vessels that represent the oldest-known cardiovascular system.
Named Fuxianhuia protensa, the creature was a primitive arthropod, a group of invertebrates with external skeletons that includes crustaceans like crabs, lobsters and shrimp as well as insects, spiders and millipedes.
The remarkable fossil, unearthed in Yunnan province in southwestern China, dates from the "Cambrian Explosion," a pivotal juncture in the history of life on Earth when many major animal groups first appeared more than half a billion years ago.
"It is an extremely rare and unusual case that such a delicate organ system can be preserved in one of the oldest fossils and in exquisite detail," said paleontologist Xiaoya Ma of the Natural History Museum in London, one of the researchers in the study published in the journal Nature Communications.
The soft parts of an animal's body tend to decay after death, meaning that fossils typically preserve only the hard parts like bones, teeth and shells. "However, under very exceptionally circumstances, soft tissue and anatomical organ systems can also be preserved in fossils," Ma said.
In the case of Fuxianhuia protensa, the fossil showed a tubular heart in the middle of the body with a rich and elaborate system of blood vessels leading to the creature's eyes, antennae, brain and legs.
The cardiovascular system, including the heart and blood vessels, is an important organ system that permits blood to circulate around the body and to deliver oxygen and nutrients. Most animals have such a system, although those without a real body cavity like jellyfish and flatworms do not.
This fossil sheds new light on the evolution of animal body organization and shows that even some of the earliest creatures resembled their relatives alive today, the researchers said.
"It shows that already 520 million years ago, such a system had evolved considerable complexity, particularly with respect to the rich vascularization in the head. This suggests that the brain of this species required a good supply of oxygen for its performance," said University of Arizona neuroscientist Nicholas Strausfeld, another of the researchers.
Fuxianhuia protensa measured up to about 4-1/2 inches long (11 cm), was covered in an exoskeleton, possessed numerous pairs of legs, had a "head shield" similar to those seen in shrimp. It had pairs of antennae and stalked eyes that could be rotated to enable it to see in different directions, the scientists said.
Flourishing in shallow seas, it probably both swam and walked along seabed, they said. It is not clear whether it was an active predator or a scavenger.
'A SORT OF POMPEII EVENT'
The beautifully preserved internal structures in the creature likely resulted from a calamity that claimed its life.
"These fossils are likely to be a consequence of sudden entombment - a sort of Pompeii event, though not of lava but an underwater mudslide or massive and sudden dust fall-out," Strausfeld said.
Fossils of Fuxianhuia protensa have proven to be relatively common in the area where it was found. In fact, another fossilized specimen of this animal that was previously described by scientists showed the oldest-known brain.
"Its gut, nervous system and vascular system are indeed unmistakably similar to that of some shrimp-like crustaceans alive today," Strausfeld said.
Many innovations related to animal anatomy occurred during the Cambrian period, although it is unclear when key structures like the heart and brain first appeared.
The researchers said that creatures with cardiovascular systems presumably lived earlier than this creature, but evidence is simply lacking in the fossil record.
This creature's genus name, Fuxianhuia, comes from a lake in the region where it was unearthed. Its species name, protensa, means "elongated," referring to its body's trunk region. (Reporting by Will Dunham; Editing by Grant McCool)