When Animals First Flexed Their Muscles: Earliest Evidence of Muscular Creature Discovered

Muscles weren't always in the fossil record. In fact, animals once went without this feature. Now, though, scientists have uncovered the earliest evidence for animals with muscles, which may also provide the oldest evidence of muscle tissue.

The fossil itself is dated from 560 million years ago. Discovered in Canada, it possesses four-fold symmetry, morphological characteristics and what appear to be some of the earliest impressions of muscular tissue. The animal was likely a cnidarian, which is the group of animals that include corals, sea anemones and jellyfish.

In the past, researchers believed that the evolution and spread of animals really geared up during the Cambrian Explosion, a period of rapid evolutionary development that started about 541 million years ago. Yet recent discovers of preserved trackways and chemical evidence in older rocks have suggested that animals may have a much earlier origin.

"The problem is that although animals are now widely expected to have been present before the Cambrian Explosion, very few of those fossils found in older rocks possess features that can be used to convincingly identify them as animals," said Alex Liu, lead author of the new paper, in a news release. "Instead, we study aspects of their ecology, feeding or reproduction, in order to understand what they might have been."

In this case, the new fossil, called Haootia quadriformis, dates to the Ediacaran Period, which occurred 635 to 541 million years ago. It actually differs from any previously described Ediacaran fossil since its body plan is similar to modern cnidarians. Not only that, but it had bundles of fibers that were likely muscular tissue.

"The evolution of muscular animals, in possession of muscle tissues that enabled them to precisely control their movements, paved the way for the exploration of a vast range of feeding strategies, environments, and ecological niches, allowing animals to become the dominant force in global ecosystems," said Liu.

The findings are published in the journal Proceedings of the Royal Society B.