A skeleton and a shell? Ancient fossil finally finds home on the tree of life

Skeleton season may be just around the corner, but the skeleton age dawned with the early Cambrian Period, about 538 million to 506 million years ago.  

In this time span, most major animal groups independently evolved methods to build mineral skeletons or shells, usually in one of two ways: They either built up mineral tissues using an organic scaffolding, like how we grow our bones and teeth, or they gathered materials from their environment and “glued” them together in a protective coating.

Then they stuck with that technique for the next 540 million-plus years — hey, if it ain’t broke...

One notable exception can be found in the fossilized remains of Salterella, a tiny creature that thrived in the early Cambrian and is so common in rocks from that time that paleontologists use it as an index fossil to orient themselves in time.

Salterella bucked the “either-or” trend by growing a conical shell around its body and then packing the shell’s cavity full of carefully selected minerals to form a snug inner lining. Scientists have rarely observed this type of doubling up in any other animal group.

“It makes Salterella difficult to place on the tree of life,” said Prescott Vayda, a geosciences graduate student who authored a study of the enigmatic Salterella published in the Journal of Paleontology.

Scientists first classified Salterella with squids and octopuses, said Vayda. Then they were categorized with creatures closely related to sea slugs. Later, they were grouped with ancestors of jellyfish. Then with worms. Finally, in the 1970s, a researcher created a new classification for Salterella, along with a slightly older fossil with similar construction called Volborthella.

And there they languished, disconnected and misunderstood.

Until Vayda, working with University Distinguished Professor Shuhai Xiao, started tracking down connections.

“Finding the right place for these fossils is important for our understanding of how animals evolved skeletons and shells,” Vayda said.

Vayda spent the past four years collecting fossil samples from places such as Death Valley and Yukon, Canada, as well as from much closer to home in Wythe County, Virginia.

Working with colleagues at Virginia Tech, Johns Hopkins University, Dartmouth College, the University of Missouri, and the Denver Museum of Nature and Science, he studied the shape, mineral composition, and crystal structure of these organisms in hopes of finding them a context.

Salterella was a scrappy creature, as Vayda found, but was selective about its building materials.

No clays, for instance, ew. Quartz was acceptable, but not ideal. Titanium was choice, of course. Who wouldn’t like a Titanium skeleton?

The variety of minerals selected led researchers to believe that the internal structure served a distinct purpose, likely something to do with feeding or enhanced stability. The findings also imply that the creatures must have had some sort of appendage to pick and grab.

“We’re starting to get an image of their biology and where they fit in the larger web of life,” Vayda said.

Based on the combined evidence of morphology, ecology, and shell structure, the research team suggested that Volborthella and Salterella belong with cnidarians, a group made up of more than 9,000 living species, including corals, jellyfish, and sea anemones.

Reconnecting this unique and long missing fragment of evolutionary lineage may lead to new answers about why and how creatures formed shells and skeletons.

And for Vayda?  It’s all about “truly learning where we come from and the history of life on Earth, which is an amazing and beautiful thing.”

Original study: DOI: 10.1017/jpa.2025.10164

---

---