A microscopic, bag-like marine creature that lived approximately 540 million years ago (Fortunian stage of the Cambrian period) has been identified from microfossils found in Shaanxi Province, China.

The ancient animal, named Saccorhytus coronarius, is the most primitive example of a so-called deuterostome.

The creature is thought to be the common ancestor of a huge range of species, and the earliest step yet discovered on the evolutionary path that eventually led to humans, hundreds of millions of years later.

Forty-five phosphatized specimens of Saccorhytus coronaries were collected from the Kuanchuanpu Formation, Hexi, Xixiang County, Shaanxi Province, central China.

They were analyzed by experts from the University of Cambridge in the UK, the University of Kassel in Germany, Northwest University, China University of Geosciences and Xi’an Shiyou University in China. The research was published in the journal Nature on January 30, 2017.

“To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail is jaw-dropping. All deuterostomes had a common ancestor, and we think that is what we are looking at here,” said lead co-author Prof. Simon Conway Morris, from the University of Cambridge.

“Our team has notched up some important discoveries in the past, including the earliest fish and a remarkable variety of other early deuterostomes,” added lead co-author Dr. Degan Shu, from Northwest University.

“Saccorhytus coronarius now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us.”

Most other early deuterostome groups are from about 510 to 520 million years ago, when they had already begun to diversify into not just the vertebrates, but the sea squirts, echinoderms and hemichordates.

This level of diversity has made it extremely difficult to work out what an earlier, common ancestor might have looked like.

By isolating the microfossils from the surrounding rock, and then studying them both under an electron microscope and using a CT scan, the paleontologists were able to build up a picture of how Saccorhytus coronarius might have looked and lived.

This revealed features and characteristics consistent with current assumptions about primitive deuterostomes.

Saccorhytus coronarius was about a millimeter in size, and probably lived between grains of sand on the seabed.

Its body was bilaterally symmetrical — a characteristic inherited by many of its descendants, including humans — and was covered with a thin, relatively flexible skin.

“The body is hemi-ellipsoidal, with a maximum length of 1,300 μm, width 800 μm and height 900 μm. Most material is crushed but several specimens confirm original bilateral symmetry,” the authors said.

This in turn suggests that it had some sort of musculature, leading the team to conclude that it could have made contractile movements, and got around by wriggling.

Perhaps its most striking feature, however, was its rather primitive means of eating food and then dispensing with the resulting waste.

The animal had a large mouth, relative to the rest of its body, and probably ate by engulfing food particles, or even other creatures.

A crucial observation is small conical structures on its body. These may have allowed the water that it swallowed to escape and so were perhaps the evolutionary precursor of the gills we now see in fish.

But the scientists were unable to find any evidence that the creature had an anus.

“If that was the case, then any waste material would simply have been taken out back through the mouth, which from our perspective sounds rather unappealing,” Prof. Conway Morris explained.

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Jian Han et al. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature, published online January 30, 2017; doi: 10.1038/nature21072