Around 600 million years ago, the first complex life forms appeared, known as the Ediacaran biota. Ever since their discovery, scientists have wondered what the connection was between the Ediacaran organisms and those that came afterward, including families that survive to this day. The discovery of astonishingly preserved soft tissues in rocks from Namibia has led the finders to conclude one famous Ediacaran life form was the ancestor of many surviving marine invertebrates.
Despite many gaps, paleontologists can trace the ancestry of most surviving animals back to the sudden diversification of life known as the Cambrian Explosion 540 million years ago. Before that, however, things get much murkier. The puzzlement over where the Cambrian animals came from is called “Darwin's dilemma” because the great man was so bemused as to what preceded these forms. When the remains of even older organisms dating to the Ediacaran were found, scientists spent years debating whether these were even animals, and if so what their relationship might be to more modern versions.
Namacalathus hermanastes represents the oldest animal we have found with a complex skeleton. We haven't known much about them, however, since only the calcium carbonate skeleton fossilized, leaving us to guess about the surrounding soft tissues. Without these, there was little way of knowing whether Namacalathus was a predecessor of Cambrian species or an evolutionary dead end. The discovery of a 547 million-year-old fossil bed in Namibia, where some soft tissues were preserved in pyrite, may have changed that.
Namacalathus is described in Science Advances as having a “goblet-like” skeleton shape. In at least some cases, spines extended from the stem and cup. Examples have been found on four continents, but the body shape around the goblet was previously unknown. The paper announces the use of X-rays to reveal the body wall and gut of some of the specimens preserved within the rock.
The structures revealed indicate a relationship to certain Cambrian worms that are considered to be the ancestors of modern lines.
"These are exceptional fossils, which give us a glimpse into the biological affinity of some of the oldest animals,” said Professor Rachel Wood of the University of Edinburgh in a statement. "They help us trace the roots of the Cambrian Explosion and the origin of modern animal groups. Such preservation opens up many new avenues of research into the history of life which was previously not possible."
The paper proposes an “affinity” between Namacalathus and the lophotrochozoa, an enormous group of invertebrates that includes many worms and filter feeders, obscure other than to marine biologists. More famous lophotrochozoa are the mollusks, the phylum that includes the humble garden snail and the fearsomely intelligent octopus.
The fossils described in the paper were just 4-12 millimeters (0.16-0.5 inches) across. Although other Namacalathus have been found to reach 25 millimeters (1 inch), it is humble beginnings indeed for the branch of the tree of life that today includes the giant squid.
The Namacalathus studied here were living on borrowed time. Around two million years later, other Namibian fossil beds reveal a decline in species diversity, followed by the displacement for Ediacaran organisms with Cambrian species.