Possible ancestor of sponges found

Exquisite details of 600-million-year old fossil similar to cell structure seen in today’s marine animals

sponge fossil

TINY BUT EXQUISITE  A scanning electron micrograph of a fossil just 1.2 millimeters wide, found in southern China, has a shape and some details that look like sponges.

Image courtesy of Zongjun Yin.

A beautifully preserved 600-million-year-old fossil shows actual cells that make it the best candidate yet for an ancestor of sponge animals, researchers propose.

The new find, now named Eocyathispongia qiania, is just a single fossil barely as big as a pinhead. Yet its three tubular chambers arising from a base and its visible parts of cells resemble sponges, Zongjun Yin of the Chinese Academy of Sciences in Nanjing and colleagues report March 9 in the Proceedings of the National Academy of Sciences.

Scrutinizing the fossil with X-rays and scanning electron microscopy, the researchers can see cells that resemble modern sponges’ outer structural elements, called pinacocytes. Among these surface cells are signs of pores, like those that let water swoosh into modern sponges. And a patch inside one of the tubes has pits encircled by raised collars. These could be an early version of the cells called choanocytes, distinctive cells in modern sponges that move water through the animal.

The search for fossils from such ancient times, 60 million years before the Cambrian period and its burst of multicellular evolution, has yielded tantalizing bits but nothing with details like this fossil, showing actual cells. The fossil, found in the Doushantuo Formation in southern China, “is the oldest and best” of the sponge ancestor candidates so far, study coauthor Maoyan Zhu says, but for that claim to be conclusive, the researchers need more examples.

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