Half-Billion-Year-Old Heart Found More Complex than Today’s

520 million years ago, the first known animal heart was formed.

It was the heart of an ancient shrimp, and quite a heart it was. For it, and its vascular system, have been found to be more complex than that of modern shrimp, researchers reported in a recent Nature Communications. Its cardiovascular system was apparently one of evolution’s first templates for modern cardiovascular systems. Significant streamlining has occurred since.

The find comes on the heels of an equally important one by the same group: that of a stunningly intact nervous system from the same breed of primordial shrimp: Fuxianhuia protensa.

“This is only the second case of the description of a cardiovascular system in a Cambrian arthropod, the first one being that of the inch-long Marrella from Burgess Shale,” emailed Diego Garcia-Bellido of the University of Adelaide, who co-discovered that first arthropod while at the University of Cambridge. Garcia-Bellido was not involved in the new study. “This new finding of a cardiovascular system in a larger animal (Fuxianhuia is about two to three times as large, thus more detail), together with a fantastically preserved, and very complex, nervous system, unknown in Marrella, and the gut, make it probably the most complete arthropod internal anatomy known in the fossil record.”

The main conclusion drawn, said Garcia-Bellido: “The level of complexity of the Fuxianhuia was extremely high, considering that we are studying some of the oldest animals on Earth.”

The recent discovery of the first known, preserved vascular system (it clearly had predecessors, but this was the first unearthed) was made by a US, UK, and China team led by University of Arizona neuroscientist Nicholas Strausfeld. For the London Natural History Museum’s Xiaoya Ma, who did much of the technical work, the “biggest surprise” in unearthing creatures existing before life on land was “the exquisite preservation of this delicate organ system, which is an extremely rare and unusual case.”

Said the paleontologist via email: “As we know, most soft tissue of animals tends to decay away once the animals died, so often only the hard parts of animal body (bones, shells, teeth, etc.) are preserved in fossils. However, under very exceptional circumstances, soft tissue and anatomical organ system can also be preserved in fossils.”

Fossils from Chengjiang in China, such as these, “are famous for such exceptional preservation. Even so, there was a wide assumption that only the digestive system can withstand fossilization. This view has now been challenged by our recent discovery of both neural and cardiovascular structures from the Cambrian arthropod F. protensa,” Ma said.

Underwater Pompeii

Chengjiang is now being called an area that underwent a kind of underwater Pompeii, rife with sandstorms that did not allow plant life to take hold, but buried animal life in such a way as to uniquely preserve it.

“This particular specimen was preserved in a very unusual way even among Chengjiang fossils,” said Ma. “It was collected from a mudstone layer that is normally poor in fossil preservation, so the external structures and other anatomical features were more or less decayed away and only left an imprint of the body outline. But for some yet unknown reason the single organ system (cardiovascular system) was preserved intact. Those exceptional preservations broaden our horizon about fossilization potentials, and provide rare opportunity for furthering our understanding of those ancient animals.”

The group carefully employed some standard techniques to get their highly unusual results. “The main imaging technique employed was the SEM (Scanning Electron Microscope) and EDX (energy-dispersive X-ray spectroscopy),” said Ma. “We used the facility based at the Imaging and Analysis Centre of the Natural History Museum in London to carry out element mapping on the fossil specimen, and the results show that the cardiovascular system of Cambrian arthropod Fuxianhuia protensa was preserved as a thin layer of carbon film, structurally and chemically distinct from its surrounding matrix and other tissues. We didn’t preselect what the elements might be in the structure, as the SEM and EDX machine would automatically detect all chemical signals on the fossil specimen and would reveal how they distributed and related to different structures or organs.”

She continued:  “We also used the Adobe Photoshop CS5 to manipulate the images in order to extract further information. For example, in our Fig. 3c, we overlapped the image of the cardiovascular system with the image of another specimen that has better preserved neural structures. The result revealed that the cardiovascular system in this animal was closely linked with the brain, eyes and antennae, indicating a high oxygen and nutrients demands in those key active organs.”

Noted Garcia-Bellido: “The fossil was collected like most fossils in these localities with exceptional preservation of soft parts: splitting the shale or mudstones with hammers. The techniques are also standard in the rest of the study, with the usual light microscopy/photography and the increasingly common EDX, which indicates differences in elemental composition between the rock matrix and the fossil (enhanced levels of carbon and iron in this case).”

In the end, however, noted Garcia-Bellido, “what this study shows is that the rock beds which generally don't exhibit good preservation, and where carcasses experienced more decay before fossilization, might, paradoxically, have helped in the cardiovascular system being fossilized in this case, and not in the hundreds of other specimens of Fuxianhuia known from the ‘better’ beds.”

More Ancient Hearts

Other ancient hearts need to be unearthed, concluded Garcia-Bellido. “As scientists, we compare what we observe with what is already known, and in that respect, presently we can only compare Fuxianhuia with modern arthropods, 500 million years younger. We need to find nervous and cardiovascular systems in a number of early arthropods and compare them. That will be when even more significant conclusions might be drawn about the speed and complexity of early animal evolution.”

Update: In a statement, senior author Strausfeld said: "Over the course of evolution, certain segments of the animals' body became specialized for certain things, while others became less important and correspondingly certain parts of the vascular system became less elaborate."