547 million year-old fossils show Cambrian animals had roots in earlier epoch

Discovery of the first fully preserved three-dimensional fossils of tiny animals 547 million years old is helping scientists better understand the evolutionary link between the Ediacaran Period and the beginning of the Cambrian, 541 million years ago.

The Cambrian is known for the evolution of a huge diversity of animal groups, called the Cambrian Explosion. That rapid expansion of animal diversity has puzzled scientists.

The exquisite preservation of these specimens allowed researchers to observe a bilateral body plan and the ability to biomineralize. Biomineralization is the process by which living organisms produce minerals to harden or stiffen existing tissues.

The fossils are exceptional because the animals' soft tissues are preserved in pyrite, a metallic mineral inside the skeletal remains. Previous fossils from this period did not have soft tissue preserved, so their features could not so easily be determined.

The fossils are of the Ediacaran metazoan Namacalathus hermanastes, a goblet-shaped organism with a cup about 3 to 35 millimeters in diameter. A research team from the University of Edinburgh found the remains in a region in Namibia called the Nama Group.

The research was published in the journal Science Advances on Jan. 1, 2021.

The researchers used X-ray micro-computed tomography (CT) and backscatter electron microscopy to image the Namacalathus in detail. The features include a hollow stalk shaped like a cup at the top, with six lobes projecting into the top opening. The outside stem and cup of some of the Namacalathus fossils has spines, making it look like a pincushion.

From the discernible features, the scientists ruled out Namacalathus affinities to protists (a type of eukaryotic organisms) and cnidarians (aquatic invertebrates like jelly fish or coral). Instead they found affinities with animals that evolved later in the Cambrian, the first geological period of the Paleozoic Era. 

In particular the researchers identified affinities with lophotrochozoan phyla, such as annelids, mollusks and others. They write: "... we are now able to reconstruct Namacalathus as a total group lophotrochozoan, capable of asexual budding with an organic-rich, foliated calcareous skeleton and an open, apical J- or U-shaped gastric cavity within the apical opening potentially accommodating a retractable collar of tentacles and with brood chambers around the lumens."

The analysis of the Namacalathus fossils confirms that animals that emerged in the Cambrian can be traced back to the Ediacaran. It was previously thought that lophotrochozoans originated only in the Cambrian explosion.

"However, now we can extend the origin of these modern lophotrochozoan phyla further back still into the terminal Ediacaran," the scientists write. "In so doing we establish a phylogenetic connection between Ediacaran and early Cambrian taxa, faunas that were previously thought distinct. We hence extend the roots of the Cambrian Explosion itself into the Ediacaran, where total group lophotrochozoans such as Namacalathus show a combination of features that became typical of both later lophophorates and representatives of the entoproctan-molluscan-annelidan branch."