An extraordinary find allowing scientists to see through the head of the 'fuxianhuiid' arthropod has revealed one of the earliest evolutionary examples of limbs used for feeding, along with the oldest nervous system to stretch beyond the head in fossil record.

Until now, all fossils found of this extremely early soft-bodied animal featured heads covered by a wide shell or 'carapace', obscuring underlying contents from detailed study.

But a new fossil-rich site in South China has been found to contain arthropod examples where the carapace has literally been 'flipped' over before fossilisation - allowing scientists to examine the fuxianhuiid head to an unprecedented extent.

The study, published in Nature, highlights the discovery of previously controversial limbs under the head, used to shovel sediment into the mouth as the fuxianhuiid crawled across the seabed, millions of years before creatures emerged from the oceans.

Scientists say that this could be the earliest and simplest example of manipulative limbs used for feeding purposes, hinting at the adaptive ability that made arthropods so successful and abundant - evolving into the insects, spiders and crustaceans we know today.

Using a feeding technique scientist's call 'detritus sweep-feeding', fuxianhuiids developed the limbs to push seafloor sediment into the mouth in order to filter it for organic matter - such as traces of decomposed seaweed - which constituted the creatures' food.

Fossils also revealed the oldest nervous system on record that is 'post-cephalic' - or beyond the head - consisting of only a single stark string in what was a very basic form of early life compared to today.

"Since biologists rely heavily on organisation of head appendages to classify arthropod groups, such as insects and spiders, our study provides a crucial reference point for reconstructing the evolutionary history and relationships of the most diverse and abundant animals on Earth," said Javier Ortega-Hernandez, from Cambridge's Department of Earth Sciences, who produced the research with Dr Nicholas Butterfield and colleagues from Yunnan University in Kunming, South China. "This is as early as we can currently see into arthropod limb development."

Fuxianhuiids existed around 520 million years ago, roughly 50 million years before primordial land animals crawled from the sea, and would have been one of the first examples of complex animal life - likely to have evolved from creatures resembling worms with legs. Arthropods were the first jointed animals, enabling them to crawl.

Fuxianhuiid arthropods would have spent most of their time grazing on the sea floor, using these newly discovered limbs to plow sediment into their mouths. They could probably also use their bodies to swim for short distances, like tadpole shrimps.

The fossils date from the early part of the event known as the 'Cambrian explosion', when life on Earth went from multi-cellular organisms we know very little about to a relatively sudden and wide spread explosion of diverse marine animals - the first recognisable evolutionary step for the animal kingdom we know today.

"These fossils are our best window to see the most primitive state of animals as we know them - including us," said Ortega-Hernandez. "Before that there is no clear indication in the fossil record of whether something was an animal or a plant - but we are still filling in the details, of which this is an important one."

While still a mystery, theories about the cause of the 'Cambrian Explosion' include possible correlations with oxygen rises, spikes in oceanic nutrient concentration, and genetic complexity reaching critical mass.

But the new site in South China where these fossils were found could prove to be key in uncovering ever more information about this pivotal period in the history of life on Earth. The Xiaoshiba 'biota' - that is the collection of all organisms preserved in the new locality - in China's Yunnan Province is similar to the world-famous Chengjiang biota, which provided many of the best arthropod fossil records to date.

"The Xiaoshiba biota is amazingly rich in such extraordinary fossils of early organisms," said Ortega-Hernandez. "Over 50 specimens of fuxianhuiids have been found in just over a year, whereas previous areas considered fossil rich such as Chengjiang it took years - even decades - to build up such a collection."

"So much material is so well preserved. There's massive potential for Xiaoshiba to become a huge deal for new discoveries in early animal evolution".