A skeleton and a shell? Ancient fossils eventually find a home on the tree of life

The age of skeletons may be right around the corner, but the age of skeletons ended with the early Cambrian period, about 538 million to 506 million years ago.

During this time, most large animal groups independently developed methods of building mineral skeletons or shells, usually in one of two ways: they either built mineralized tissues using organic scaffolds, like how we grow our bones and teeth, or they collected materials from their environment and assembled them together into a protective coating.

Then they stuck with this technique for the next 540 million plus years. A notable exception can be found in the fossil remains of Salterella, a tiny creature that flourished in the early Cambrian and has been so common in rocks since then that paleontologists use it as an index fossil to date itself back in time.

Salterella grew a conical shell around its body and then packed the shell cavity to form an inner lining filled with carefully selected minerals. Scientists have rarely observed this type of doubling in any other animal group.

“This makes it difficult to place Saltarella on the tree of life,” said Persket Vida, a geosciences graduate student. Journal of Paleontology.

Scientists first classified Saltarella with squids and octopuses, Vaida said. They were then classified with creatures closely related to sea slugs. Later, they were grouped with the ancestors of jellyfish. Then with the worm

Finally, in the 1970s, a researcher created a new taxon for Salterella, along with a slightly older fossil with a similar structure called Wolberthella.

And there they became ungrounded, disconnected and misunderstood.

It wasn’t until working with Shuhai Zhao, a prominent university professor, that Wayda began tracing the connections.

“Finding the right place for these fossils is critical to our understanding of how animals developed skeletons and shells,” Vaida said.

Vida has spent the past four years collecting fossil samples from places like Death Valley and the Yukon, Canada, as well as much closer to home in Withee 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 morphology, mineral composition, and crystal structure of these organisms in hopes of finding context.

Salterella was a scraping creature, as Vyda found, but it was selective about its building materials.

For example there is no soil. Quartz was acceptable, but not ideal. Of course Tit Titanium was the choice. Who wouldn’t love a titanium skeleton?

The variety of minerals selected led researchers to believe that the internal structures served a distinct purpose, possibly something to do with feeding or better stability. These results also indicate that the creature must have some kind of appendage to pick up and hold.

“We’re starting to get a picture of their biology and where they fit into the larger web of life,” Wyda said.

Based on combined evidence from morphology, ecology, and shell structure, the research team suggested that Volbertella and Salterella belong to Cnidarians, a group of more than 9,000 living species, including corals, jellyfish, and sea anemones.

Reconnecting this unique and long-lost piece of evolutionary lineage could lead to new answers about why and how creatures developed shells and skeletons.

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